Control of vector-borne diseases, (virtually all forms of trypanosomosis are in this category, except dourine), can be based on:
The accent in this manual will be on drug control of the causal agent, but the other approaches will be briefly reviewed, in order to place drug control in a wider perspective and indicate its place in integrated control.
Chemotherapy: generalities. Chemotherapy is the treatment of disease by the use of chemical drugs. Such drugs are curative. They disrupt or block one or more of the vital processes which are essential to the invading micro-organism. Certain compounds have specific effects on some enzyme system or block essential metabolic pathways, but the exact way in which they work is often not known or only incompletely understood, and this is true of most of the trypanocides (chemotherapeutic drugs which kill trypanosomes or inhibit their development).
It is important to realize that drugs alone will not cure trypanosomosis. Trypanosomes overwhelm the immune system of the host; they are immunosuppressive. Chemotherapy, by stopping the multiplication of the trypanosomes, helps the immune system to overcome the infection. Treatment will be more effective in a well-fed and rested animal, in which the immune system is not adversely affected by stress and lack of food.
Chemotherapeutic drugs are toxic to the trypanosome, because they interfere with one or more of its vital processes. An ideal drug is one which kills the parasite but, at the same time, causes no or minimal harm to host cells. However, cells of all living organisms have much in common and a drug which affects a certain metabolic process in a parasite often has a similar disruptive effect on the cells of the host. If this side effect is too severe, the drug is too toxic for use. When the minimum effective treatment dosage is close to the toxic level, the margin of safety is low and the drug must be used with great care. When the toxic dosage is many times the therapeutic level, the margin of safety is high and the dangers attendant upon its use are correspondingly less. But it should always be remembered that all drugs have a disruptive effect on one or another living process and, accordingly, they should always be used with care and at the recommended dose level only. The efficiency of the vital detoxifying processes of animals that are ill may be reduced and a drug may be much more toxic for an animal with damaged liver or kidneys than for a healthy animal. It should also be remembered that the few trypanocides which are still commercially available have been developed before the stringent tests for absence of chronic toxicity and carcinogenicity that nowadays are required before new drugs can be released on the market.27
One should distinguish between local toxicity, the irritant action at the site of injection, and systemic toxicity, the toxic effect on the whole system (usually because of the effect on one or more internal organs).
The toxicity of drugs differs in different species of animals. For example, the margin of toxicity to quinapyramine in dogs is quite low; in pigs it is many times higher. Horses are known to be the most sensitive of the domestic animals to drugs which are irritant at the site of inoculation, and one must always be careful of the subcutaneous route in horses because of this sensitivity. Diminazene aceturate has given fatal reactions in camels, horses, donkeys and dogs at doses which are considered to be normal and harmless in cattle.
One of the earlier trypanocides, dimidium bromide, had been used throughout Africa in many thousands of cattle until signs of acute toxicity were reported in several countries. They were caused by an acquired sensitivity to light (photosensitivity), causing necrosis and sloughing of extensive areas of unpigmented skin, followed by infection and death.
In certain instances a combination of drugs may prove to be toxic whereas either may be used safely if administered alone. On the other hand, some compounds used at the correct dosage are non-toxic, but are excreted so slowly from the organism that repeated administration at relatively short intervals may result in accumulation until a level is reached which will result in the appearance of toxicity.
It is essential to read thoroughly all available information in leaflets issued by the manufacturers on possible toxic reactions, before administering a drug.
Chemoprophylaxis: generalities. Chemoprophylaxis, or chemo-prevention, is the prevention of disease by the use of chemical drugs. Logically, this implies a residual effect, as prevention depends on the persistence of the drug in the system of the animal. A chemoprophylactic drug is also curative, but a curative drug is not necessarily prophylactic. There is no essential differerence between the two categories of drugs, drugs that can be used for Chemoprophylaxis just persist longer, and what has been said above about the toxicity of curative drugs also applies to the preventive ones. Preventive drugs are administered at defined intervals, and care must be taken not to administer them more frequently than prescribed. A combination with another drug can be dangerous. For example, the most commonly used chemoprophylactic drug, isometamidium, will cause weight loss in poorly nourished cattle if administered at short (monthly) intervals. Cattle treated with diminazene aceturate after several isometamidium treatments suffer from hepatic damage and may even die.
Often, a depot of the drug is formed at the site of the injection where it is retained and slowly released into the circulation to maintain a concentration in the blood at a level at which no trypanosomes can exist. Other drugs are not maintained in the form of such a local depot, but are found loosely attached to blood proteins and become slowly available to act on the parasite.
There is another way to create a drug depot, even if the drug is normally only curative, by incorporating it into a suitable support. After subcutaneous implantation the drug will be gradually released so that an adequate prophylactic (and curative) level is maintained in the blood and tissue fluid.
The administration of drugs. Drugs are normally sold in powder form or compressed tablets, and are stable over long periods when kept dry. Some become unstable when dissolved and must be used within a short period. The manufacturer's instructions should always be followed and it is a good practice to use solutions only on the day they are made.
The drug is dissolved, or suspended when it is insoluble, in a suitable liquid (mostly sterile water) and administered by injection with a syringe. Depending on the drug, the injection will be given by the subcutaneous, intramuscular or intravenous route.
The subcutaneous route. Absorption after subcutaneous (SC) administration is the slowest of all three methods of injection; the drug enters the circulation slowly from the subcutaneous site. This can be an advantage for chemoprophylactic drugs.
Compounds which are systemically toxic are also given in this way, so that they do not enter the blood stream and exert their poisonous reaction in large quantities at a time. However, drugs which are locally irritant at the site of the injection are not given SC since they remain in a concentrated form long enough at the site to cause skin necrosis (death of the skin). The skin sloughs away and a raw area prone to infection remains and may take weeks to heal, leaving a hairless scar. The skin of horses is particularly prone to such damage.
If the drug presentation injected causes pain, this will be most pronounced when the SC route is used, because the subcutis is well supplied with sensory nerves.
A short needle can be used for SC injections. It is inserted into the pouch formed by picking up a fold of loose skin. The drug is absorbed from the subcutaneous depot into the circulation via the extensive network of lymph vessels and blood capillaries which occur there.
SC injections can be made in cattle under the skin of the dewlap, on the side of the neck, or over the ribs behind the shoulder. In sheep and goats the best site is just behind the point of the elbow where there is less hair or wool. In pigs the most suitable site is under the skin behind the ear flap where it joins the neck. SC injections in horses and camels are best given on the side of the neck. Dogs and cats may be injected in a skin fold over the ribs behind the shoulder.
The intramuscular route. Muscles are richly supplied with blood vessels and absorption is much more rapid than from the subcutis and the curative effect faster. Intramuscular (IM) injections are also more suitable for locally irritant drugs and also for painful drugs, as there are fewer sensory nerves in muscles. Nevertheless, the mechanical pressure exerted by a large quantity of injected liquid can be quite painful.
Any accessible mass of muscle can be used, but one should avoid as much as possible the heavily muscled areas which make up the prime joints of meat and are of the greatest value in food animals. Not only are concentrated drug residues in such sites highly undesirable (but this is true for any part of the animal that is eaten), but muscles may show local damage after the injection. Very irritant drugs can cause destruction of the muscle (necrosis) and its replacement by fibrous tissue. This process may be quite extensive after repeated injections and parts of the choicest meat cuts may be rendered inedible and unsuitable for sale. Muscles concerned with walking should also be avoided as the irritant effects may cause lameness.
For these reasons, the neck is the best site in all species (except for pigs, which are normally injected by the SC route only, as indicated above), although absorption of drugs from the neck muscles may be somewhat slower than from the hip or thigh. The neck is an area which is one of the least valuable in the beef carcass and even a great deal of trimming and wastage will not reduce its value too much. The injection should be made into the middle third of the neck, halfway between the cervical vertebrae (which can be felt beneath the skin) and the crest of the neck. In the case of irritant drugs, or a large volume to be injected, the dose may be divided into two equal amounts and injected on both sides of the neck. Although less desirable as far as carcass value is concerned, in animals pressed together in a crush it is not always practical to inject in the neck (and moreover inject in the correct site of the neck), and the rump is then commonly used.
A 4-cm (16 G) needle is recommended. The needle is thrust deeply into the muscle mass and must therefore be strong with a thick shaft. This may result in leakage of the drug along the track of the needle into the subcutaneous tissues when the needle is withdrawn. This should be avoided if the material is irritant by applying pressure and rubbing to the area immediately after the needle is withdrawn.
The intravenous route. The needle is passed into a vein and the drug is thus introduced directly into the circulation. This route is used for drugs which are highly irritant at the injection site, as drugs applied intravenously (IV) are rapidly diluted in the circulating blood. It is also used in urgent situations, when the action of the drug must be rapidly achieved. This is not normally the case in trypanosomosis, but in diseases with a very rapid course, heart failure, induction of anaesthesia, etc. It may also be used when large volumes of liquids are administered as in shock or blood transfusions The method requires a certain level of skill and practice. It is relatively time consuming and only rarely used in the treatment of trypanosomosis, apart from individual suramin treatment of surra, and certainly not for mass inoculation of cattle.
It should also be realized that drugs with a marked systemic reaction will have almost immediate access to the various organs and may cause a dangerous toxic effect; the likelihood of this happening is lessened by carrying out the injection slowly.
It is obvious that IV treatment should be carried out by a practised operator and only if the manufacturer's leaflet states that the drug is recommended for intravenous use. Shock (sometimes fatal) may exceptionally occur even when all precautions have been taken, although the likelihood of this is much less if the injection is given slowly.
The jugular vein in the neck is normally used for IV injections in ruminants (including the camel) and horses. It is a large vein, conveniently located just beneath the skin, in a groove (the jugular groove) running between the neck muscles from the angle of the jaw down to the front of the shoulder. The vein carries blood from the head towards the heart, and when the jugular vein blood flow is obstructed by placing one's thumb in the groove, or by using a thin rope or specially designed tongs, the blood accumulates above the obstruction and distends the vein. It thus becomes clearly visible beneath the skin, making it easy to thrust a needle into its interior. It is essential to have needles with a very sharp tip (disposable needles are best), and to push it into the vein with a decisive movement, otherwise the vein is likely to “roll” away from the needle. The needle is pushed well into the vein, so that it does not come out at every movement of the animal, but care must be taken that it does not pass right through the vein. To avoid this it is best to push the needle obliquely forward (towards the head of the animal) inside the vein, not perpendicularly in relation to it. Effective restraint of the animal is important. The tissues around the vein, and its walls, are very sensitive to irritant substances and leakage may result in necrosis and rupture of the vein, often with a fatal result.
Once the needle is in place and blood flows out, the syringe is attached and the plunger is first withdrawn slightly to check whether blood enters the syringe, a confirmation that the needle is correctly positioned in the vein. After releasing the pressure on the vein, the injection is then given slowly and carefully. After emptying the syringe, a little blood is sucked back into the syringe by slightly withdrawing the plunger and pushed out again, washing any (irritant) fluid out of the needle into the circulation. A 5-cm long needle, with a relatively large bore, is required for large animals such as cattle, camels and horses.
In dogs IV injections are usually made into the radial vein on the inner foreleg.
As stated above, IV administration of trypanocides is not commonly carried out in cattle and is mainly used for injecting suramin in camels.
Dosage of drugs. The correct dosage of a drug eliminates or controls the trypanosome, with no or acceptable toxic side effects to the host.
It is not only useless to give a drug at ineffectively low levels, but underdosing may have extremely serious consequences, as it may contribute to the development of drug resistance (see Drug resistance, p. 110).
Effective, safe dosage rates of drugs have been established in long series of experiments and trials in laboratory and domestic animals. The dosage is normally indicated as the amount of the active ingredient to be given per unit of body weight, usually in milligrams (mg) of the active compound per kilogram (kg) of live body weight (mg/kg). The drug is usually issued in powder or tablet form and dissolved or suspended in a suitable liquid (usually water). The prescribed concentration is expressed in grams (g) per litre or in milligrams per millilitre (mg/ml). Things would be simple if the commercial preparation consisted entirely of the active compound, but this is usually not the case as manufacturers add other ingredients (see Names of drugs, p. 97), which may cause confusion. The percentage of active ingredient in the commercial presentation has to be known as the dosage rate is based on it.
The dosage can now be expressed in millilitres (ml) (= cubic centimetres, cc) of this liquid per kg (ml/kg), and the volume to be injected into an animal of estimated weight can be calculated.
The procedure is illustrated with the two following examples.
There are also sachets containing 23.6 g of granules, containing 10.5 g of active compound. To obtain a dosage rate of 3.5 mg/kg, one sachet has to be dissolved in 150 ml of water to obtain a 7 percent solution. The dosage rate of 3.5 mg/kg is obtained as above, by using the solutions at 1 ml/20 kg.
All dosages are based on the live body weight. The weight of the animal to be treated should therefore be determined or estimated as accurately as possible.
The most exact method is by the use of a weighing-scale or balance, but in the field and in most treatment centres these are not available.
The use of a “weigh-band” is more practical, but less accurate, for cattle. This is a flexible tape graduated to read the weight directly when put around the chest behind the elbows. These tapes are designed and calibrated for modern breeds of cattle, but the conformation of the many different breeds in Africa varies greatly and weigh-bands should be recalibrated locally against the real weight (determined on a scale) of a number of animals of the local breed or type. Care must be taken that the tape is placed around the chest at the exact recommended point. In the case of zebu cattle the tape should be behind the hump. Weighing by weigh-band can be undertaken even by inexperienced operators. However, it slows down mass inoculations.
Weight is most often estimated by eye, but this is prone to inaccuracy and perhaps the greatest cause of errors in drug dosage. Skill is required, which can only be acquired by experience, to be gained for instance by attending sales where the animals are weighed on a scale.
In fat animals the proportion of body fluid (blood and tissue fluid) to total body weight is somewhat less than in the average animal. Because drugs are carried in the blood plasma and the tissue fluid, fat animals will have a slightly higher concentration of the drug per ml of body fluid. The tendency in such animals will thus be towards slight overdosing, which is not a serious error.
One should not underestimate the possible errors of weight estimates. Within 10 percent accuracy levels such errors are acceptable. Considerable underdosing results in ineffective treatment and the development of drug resistance, while considerable overdosing may be associated with toxic effects.
Names of drugs. To the user of drugs, two kinds of names are important:
Use of drugs. The first successful attempts at the treatment of cattle suffering from “nagana” were made with tartar emetic (= antimony potassium tartrate) early in the twentieth century. The drug is extremely irritant at the site of injection and had to be given intravenously. Another disadvantage was that several injections were needed to achieve a cure. The drug suramin is also one of the earliest, but is only active against trypanosomes of the brucei group (subgenus Trypanozoon). For a long period, few other drugs were available and it was only after the Second World War that a few new compounds reached the market stage that were both effective and practical, able to effect a cure by a single injection.
In the past, pharmaceutical companies routinely screened large numbers of chemical compounds for their trypanocidal action (mostly against trypanosomes of the brucei group, easiest to maintain in laboratory animals, but of less importance in the context of AAT). Few of the promising ones however were developed up to the market stage, either because they were found to be too toxic in domestic animals, or because they did not show the expected efficacy in practical field situations. Sometimes the trypanosomes quickly developed resistance to the drug and some have disappeared from the market for this reason. However, the fact that at present there are very few effective products against African trypanosomosis commercially available is to a large extent because of economic reasons. It should be realized that the chemical industry is not actively seeking to develop new compounds, because the market is rather uncertain and not very profitable.29 Several drugs have disappeared from the market, and it is significant that since the first edition of this manual, in 1985, only one novel type of trypanocide has been added to the list of commercially available veterinary products, and then only for use against surra in camels. For the control of African bovine trypanosomosis the only drugs currently in use are diminazene aceturate, homidium salts and isometamidium chloride, and all have been in use for 35 years or more. In this field guide we will consider only those compounds that are at present available, although they may not be available in each and every country concerned. It has recently been estimated that about 35 million doses of trypanocides are at present used each year in Africa.
Curative drugs. Curative drugs are mainly used where disease incidence is low and only a limited number of animals in a herd contract the disease during the course of a year. This is often the case when the threat mainly occurs seasonally. Examples are the higher risk situations that may occur during the rains, as in the case of T. evansi in camels, of mechanically transmitted T. vivax, or in marginal tsetse areas. The trypanosomosis risk may also be higher during the dry season, for instance when herds enter tsetse belts during transhumance, either accidentally or deliberately (see Epidemiology, p. 34). Curative drugs are meant to cure individual infected animals, not to protect the whole herd or group for a longer period.
Curative treatment is most effective in herds that are inspected and treated at regular intervals. The more frequently the visits are carried out, the more effective the treatment regimen. For instance, if loss of condition and some mortality occur when cattle are visited and treated on a three-monthly basis, the situation may be corrected if the visits are carried out on a monthly or two-monthly basis.
Block treatment of all animals in a herd should not be carried out with curative drugs, at least not with one that possesses a residual effect, because of the high risk of drug resistance developing during the period that the drug reaches a subcurative level (see Drug resistance, p. 110). Table 4 gives the few curative drugs and the only prophylactic one that are available at present. All curative drugs also possess some residual effect, but in the case of diminazene aceturate and mel cy this is practically negligeable.
Prophylactic drugs. These are used where the risk is so high that the health of the herds cannot be maintained by individual application of curative compounds. In such a situation too many animals contract the disease too frequently. This is particularly the case in tsetse-infested areas. Although curative treatments may prevent a high death rate, the overall health of the affected herds is not satisfactory, with loss of condition, poor (re)productive performance, many clinical cases and some deaths, despite a relatively large number of treatments at each regular visit. Other circumstances which might warrant the use of prophylaxis may occur when the infected animals cannot be reached (for instance, inaccessibility during the rainy season, transhumant herds, trade cattle moving to distant markets and passing through tsetse belts, etc.).
TABLE 4
Commercial trypanocides*
| Generic name | Trade names** | Solution for use | Dosage rate*** | Route | Remarks |
| Suramin | Naganol | 10% | 10 mg/kg (1 ml/1O kg) | IV | Mainly used against T. evansi in camels |
| Diminazene aceturate | Berenil, Ganaseg, Trypazen, Veriben | 7% | 3.5–7 mg/kg (1–2 ml/20 kg) | IM | Mainly used in cattle and small ruminants |
| Homidium bromide | Ethidium bromide | 2.5% | 1 mg/kg (1 ml/25 kg) | IM | Mainly used in cattle and small ruminants. Should be dissolved in hot water. Potentially carcinogenic |
| Homidium chloride | Ethidium C, Novidium | 2.5% | 1 mg/kg (1 ml/25 kg) | IM | See above, but soluble in cold water |
| Quinapyramine methyl sulphate | Antrycide, Trypacide, Noroquin, Quintrycide | 10% | 5 mg/kg (1 ml/20 kg) | SC | Now mainly used against T. evansi and T. brucei in camels and horses |
| Mel cy | Cymelarsan | 0.5% | 0.25–0.5 mg/kg (1–2 ml/20 kg) | IM or SC | Registered only for use against T. evansi in camels |
| Isometamidium chloride | Samorin, Trypamidium | 1% 2% | 0.25–0.5 mg/kg (1.25–2.5 ml/50 kg) 1 .0 mg/kg (2.5 ml/50 kg) | IM | Used mainly in cattle, as a curative at lower rates, as a prophylactic at higher rates. Also contains homidium, and is therefore to be considered as potentially carcinogenic as well |
** The list of trade names is not complete, and names listed do not imply a qualitative judgement
*** Dosage rate of solutions for use are given in brackets.
As has been said earlier, there is no fundamental difference between curative and prophylactic drugs, the latter just persist and protect long enough in the organism of the host to be of use in prevention. Figure 15 shows the difference for two imaginary drugs: drug A has a curative concentration lasting one month only, too short to be of use in chemoprophylaxis, while drug B, with a curative concentration lasting three months, can be used in prevention.
When considering prophylactic drugs, the notion of challenge or trypanosome risk is important. This means the amount or rate of infection (number of infected insect bites) to which the average animal in a herd is exposed in a given period of time. The challenge somehow affects the period of protection conferred by a prophylactic drug. While there is no adequate explanation for this phenomenon, it is a fact that the period of protection is shorter when the challenge is higher. It is as though each trypanosome “consumes” a minute amount of the drug and the total sum of these very small amounts of drug reduces the amount of active compound in the circulation. The reduction of the level of injected drug in the blood plasma and tissue fluid appears to result from the combined effect of the natural excretion of the drug and the level of challenge.
Figure 15
Plasma concentration curves for two trypanocides with different excretion rates
How can we measure challenge? When we consider tsetse-transmitted trypanosomosis and cattle, the challenge depends on the density of tsetse flies in the area and on the proportion of tsetse flies which are infected (the so-called infection rate in the fly population). The (apparent) fly density can be roughly estimated by counting flies caught on a screen during so-called fly rounds or by using traps. But this number is influenced by many factors. Also, the species of tsetse influences challenge because there are different host preferences. The infection rate is established by dissecting a large number of flies and recording the percentage that are infected. This is also not a simple matter; young (so-called teneral) flies have to be excluded as they have not yet had the chance to infect themselves and their inclusion would result in too low an infection rate. Non-pathogenic trypanosomes (for example, from reptiles) have to be distinguished in the fly from those causing AAT. The figure for challenge obtained by these methods at best gives a very rough, comparative indication, and because it is so inaccurate in absolute terms it is not commonly used any more.
Another way to determine challenge in an area is by using the so-called Berenil index (BI). Diminazene aceturate (of which Berenil is a trade name30) is a commonly used curative drug that is rapidly excreted. Its preventive effect is short (although some authors have claimed that it may exert a prophylactic effect lasting for up to three weeks). Blood samples of no fewer than ten cattle in the area where the level of challenge is to be established are examined over a year at weekly intervals by the most efficient method which can be practically used, and the infections are recorded. Cattle found to be infected are treated with diminazene aceturate. The number of treatments (= infections recorded) over a year reflects the challenge in the area, and is expressed as the average number of infections each animal contracts over a year. The following example explains the principle.
In a herd of 25 head of cattle a total of 29 cases was detected in the course of a year, an average number of infections per animal of 1.16 (29 divided by 25). The BI therefore is 1.16.
The method is realistic and practical since the animals graze naturally throughout the area and the results are not influenced by various experimental conditions. Nevertheless, the BI found should not be taken as the absolute number of infections contracted, which is underestimated. The BI is influenced not only by the (relatively short) persistence of an effective level of the drug after treatment, but also by the susceptibility of the cattle concerned. It has been found that in mixed herds of susceptible zebu cattle and trypanotolerant N'Dama cattle the BI found in the former is significantly higher than in the latter. Thus, a BI determined in one breed of cattle does not necessarily apply to a different breed in the same area. Also, the sensitivity of the detection method used is of influence; none will detect 100 percent of the infections, but some are more sensitive than others (see Laboratory methods, p. 60).
A disadvantage of the method is certainly also that it is slow and labour intensive. The longer the period of observation, the greater the accuracy. A year is the minimum, so that any seasonal variation is taken into account, but a period of two years provides a much more reliable picture.
A Berenil index of three or less is regarded (in cattle that are not trypanotolerant) as relatively low, requiring only curative treatment of infected cases as they are recorded. For this purpose, in an ideal situation, monthly or at least two-monthly inspections should be carried out. A BI of four to six indicates medium to high challenge and the disease may theoretically still be controlled by curative drugs if inspections can be regularly undertaken at monthly intervals throughout the year. However, a prophylactic regimen may be more indicated. A higher Berenil index reflects high to very high challenge and will definitely require prophylactic treatment if the disease is to be controlled efficiently. However, at that level it may not always make economic sense to continue to keep cattle.
In the first edition of this field guide reference was still made to three drugs for the prevention of animal trypanosomosis, but two have since been discontinued (because of drug resistance and other problems). The only prophylactic drug that remains available is isometamidium chloride (Samorin®, Trypamidium®) at doses of 0.5 to 1 mg/kg (see Table 4). The drugs that have been discontinued are pyrithidium bromide (Prothidiurn®) and quinapyramine chloride. The latter was an almost insoluble salt which, mixed with quinapyramine methyl sulphate (curative), was marketed as Antrycide Prosalt. Although both drugs were initially very promising, serious problems of resistance and to some extent important local or systemic reactions have led to their abandonment.31
When using chemoprophylaxis the concentration of the drug in the body fluids should ideally not drop below the curative level in any of the animals in the treated herd, so that the establishment of trypanosomes and the development of disease are prevented at all times. This objective is achieved by a planned series of treatments at appropriate regular intervals.
Figure 16 is a diagrammatic representation of a prophylactic regimen. The intervals between treatments are equal and so spaced that the concentration of the drug in the plasma is never permitted to drop to a level which is ineffective in controlling the parasite. The interval is chosen so that the drug level in the plasma is boosted by the next treatment before it reaches the limit of the effective concentration. This principle is of utmost importance if the drug is to remain effective, that is to say if the development of drug resistance is to be prevented.
Figure 16
Diagrammatic representation of a prophylactic situation
The aim should be to maintain the concentration of the drug above the subcurative level by means of a series of treatments at planned regular intervals
The chosen interval between treatments may vary with the existing situation, depending on the persistence of the drug (on its rate of transformation or excretion) and on the challenge. Higher levels of isometamidium give longer periods of protection but, as stated above, the level of challenge also interferes in a manner which is not completely clear. In the face of a high challenge the dosage of the drug should be increased or the intervals between treatments shortened. However, short interval (monthly) administration of isometamidium is dangerous, particularly in cattle under nutritional stress. The concurrent administration of diminazene aceturate is particularly dangerous in this situation (see Chemoprophylaxis: generalities, p. 89).
Recent laboratory methods of determining the concentration of isometamidium in the blood with an ELISA, using antibodies to the drug conjugated with a protein, (see Drug resistance, p. 110) may assist in establishing suitable treatment intervals.
In recent years research has been carried out on slow release devices (= sustained release devices) (SRD) containing homidium or isometamidium in polymers which are implanted subcutaneously. The intention is to maintain an adequate and more regular level of the drug in the blood and tissues for longer periods than can be attained by depot-forming injections. In laboratory experiments involving cattle, SRDs containing homidium or isometamidium have given protection against tsetse-transmitted infection with T. congolense lasting about three times as long as protection afforded by IM injections, using identical quantities of these drugs. It should however be remembered that these drugs have mutagenic properties (i.e. they cause gene mutations), making them potentially carcinogenic.
Notes on the various drugs. See also Table 4. Names in brackets are some of the trade names of commercial formulations that are, or have been, commonly available. No attempt is made to be exhaustive.
Suramin (Naganol). A white powder, very hygroscopic (= readily taking up water from the air), which must be kept in tightly closed containers.
Suramin is effective against trypanosomes of the subgenus Trypanozoon (T. evansi, T. brucei), not against T. congolense and T. vivax. It is irritant at the site of injection and is normally given intravenously. Toxicity may sometimes be evident in horses, even at recommended dosage rates, while other animal species are much more tolerant. Resistance to suramin of T. evansi is widespread.
Diminazene aceturate (Berenil, Ganaseg, Trypazen, Veriben). This is a yellow powder that produces a clear yellow solution in water.
It is normally injected intramuscularly, as some swelling may occur after subcutaneous administration. It is not normally used in horses, because even after IM injection the reaction is sometimes severe. Toxicity and even deaths have been reported in camels, donkeys and dogs treated at the recommended dosage.
The tissues at the site of treatment are stained by the drug and 14 days should elapse after treatment before slaughtering animals. Diminazene aceturate is less effective against trypanosomes of the subgenus Trypanozoon (such as T. evansi and T. brucei) than against T. congolense and T. vivax.
Diminazene is also active against Babesia infections (babesiosis or redwater). This is sometimes an advantage in situations where one does not have the opportunity to diagnose and differentiate between possible causes of a febrile disease associated with anaemia. For a long time resistance to diminazene has been less of a problem than for other drugs, but it is no longer unusual.
Quinapyramine methyl sulphate (Antrycide, Trypacide, Noroquin, Quintrycide). Production of this compound has been taken up again, after an interruption towards the mid-1970s because at that time it had been found to cause serious drug resistance problems in cattle trypanosomosis in Africa. Its use in cattle is now strongly advised against, because drug resistance to it develops readily and can be associated with cros resistance to all the other trypanocidal drugs in use. Moreover, there are toxicity problems in cattle. Quinapyramine is now produced mainly for the treatment of surra in camels and horses, in particular where there is resistance of T. evansi to suramin.
It is dispensed as a pale cream powder, producing a clear solution in water. It is administered by subcutaneous injection.
Under certain conditions, which have never been fully explained, the drug causes systemic toxic effects in cattle. Heat, fatigue, fear, etc. aggravate this. Toxicity in horses and dogs is also well known. Acute toxicity in dogs may be avoided if the drug is preceded by a dose of a tranquillizing drug some 20 minutes before treatment and dogs should only be treated when cool and rested.
The activity of quinapyramine against T. vivax is less pronounced than that against other species. T. simiae infection in pigs may also be treated, using very high dosage rates (20–40 mg/kg) but the intervention has to be very fast, as the course of the disease is so rapid.
Homidium bromide (Ethidium bromide) and Homidium chloride (Ethidium C, Novidium). Both are crimson powders dispensed as tablets that produce a deep red solution. Contrary to the chloride, the bromide requires warm water to ensure that it is completely dissolved. At normal dosage rates no toxicity problems have been reported. Deep intramuscular injection is recommended as both drugs are irritant at the site of injection. Horses are the most susceptible to the irritant effect and should be treated only by the intravenous route, taking great care to avoid leakage into the tissues surrounding the jugular vein. The dosage rate of 1 mg/kg indicated in Table 4 is for the treatment of T. congolense and T. vivax. T. brucei is less susceptible and the dosage for this species should be increased to 1.5 or even 2 mg/kg.
As stated before, molecular biologists use Ethidium bromide in their colorations with great care because it is highly mutagenic, and therefore potentially carcinogenic; this fact appears to be as yet little known in the veterinary world. It is not uncommon to see the hands of inoculators, unprotected by gloves, deeply stained with the drug.
Resistance to homidium is also well known.
Isometamidium chloride (Samorin, Trypamidium). This drug belongs to the same chemical group as homidium, the group of the phenanthridinium compounds.32 It is a dark red powder, producing a brownish red solution in water.
It is normally administered by deep intramuscular injection as it is irritant at the site of injection. Preferably, the neck should be used to avoid damage to the more valuable areas of the carcass. Pressure should be applied to the site immediately after the needle is withdrawn to prevent leakage into the subcutaneous tissue. It should not be administered by the subcutaneous route. As far as systemic toxicity is concerned, isometamidium has a wide margin of safety. In camels intramuscular administration may give a severe local reaction, especially at the high dosage rate required for the cure of T. evansi infection (1–2 mg/kg). In horses it is not advisable to exceed a dosage of 0.5 mg/kg.
Depending on the dosage rate isometamidium can be used (in cattle) as a curative drug (0.25–0.5 mg/kg) or as a prophylactic (0.5–1 mg/kg). The protective period after prophylactic doses varies from two to more than four months depending on dosage and challenge. In situations of medium challenge 0.5 mg/kg given every two months may protect as well as 1 mg/kg applied every three or four months, but under high challenge the higher dosage rate should be used. When challenge is intense, 1 mg/kg may have to be applied every three or even two months, but such intensive treatment is not recommended for extended periods.
Isometamidium should never be used at the same time as other trypanocidal drugs and at least a fortnight should elapse between treatment with isometamidium and any other drug, particularly diminazene aceturate. Also, after diminazene treatment, there has to be an interval of at least two weeks before isometamidium can be administered.
Mel cy (Cymelarsan). This is an arsenical drug, related to melarsen (which is used in human sleeping sickness). The name mel cy is derived from the chemical formula Melarsen oxide, with cysteamine as a lateral chain.
It is dispensed as a dry powder, highly soluble in water. The drug was developed in the 1980s for use against surra (T. evansi infection) in camels, as the other compounds in use against this disease are either frequently associated with resistance problems (suramin, quinapyramine) or are not always sufficiently effective against this trypanosome species (isometamidium). Mel cy is administered preferably intramuscularly, but the subcutaneous route is also possible. There are often transient local and systemic reactions. It has also been tested against T. evansi in domestic buffaloes and T. evansi and T. brucei in other animals. The dosage rate originally recommended by the manufacturer for camel surra (0.25 mg/kg) proved to be too low in these other animals and even for camels a dose of 0.5 mg/kg may be more realistic. Mel cy so far appears to have been registered only for use in camels.
Drug resistance. Drug resistance, also called drug fastness, may be defined as a loss of sensitivity by a strain of an organism to a compound to which it had previously been susceptible. It implies failure of treatment or prevention, and if no other active drugs are available the animal has to rely on its immune defences alone to combat the disease.
Drug resistance is known in many pathogenic micro-organisms, as well as in larger parasites. The results of its appearance can be catastrophic where an animal (or human) population has come to depend on the regular administration (or external application in the case of ectoparasites) of the compound, and no other effective treatment is available. Prolonged, regular use of a drug (or pesticide) apparently inevitably leads in the end to the development of resistance by the micro-organism or parasite in question.
The discovery of the first compound active against T. brucei rhodesiense was rapidly followed by reports of resistance to that drug. Sooner or later, newly developed trypanocidal drugs have failed to cure some cases of human or animal trypanosomosis after a period of use. Every time that new drugs are released by the manufacturers and used in the field, drug resistance has appeared. It is especially by misuse of trypanocides, by their mistaken and inefficient application, that resistance develops.
Resistance to chemicals used against other micro-organisms, helminths, insects and ticks is believed to be caused by the selection of “naturally” resistant individual organisms, which were present in small numbers before the compound was used, or which arise by continually occurring natural mutations during its use. Susceptible organisms are killed by the chemical, but resistant natural mutants survive and multiply, giving rise to a resistant population.
In the case of trypanosomes the evolution of drug resistance may not always follow this process. Trypanosomes often appear to actively develop resistance; it is as though they are capable of learning the trick of avoiding the toxic effects of the compound. Thus, the process may not always be one of selection of pre-existing resistant trypanosomes. We have already seen that trypanosomes have an amazing ability to change in the face of the successive defensive reactions of the host's immune system, and chemoresistance appears to be another example of the ability of the trypanosome to adapt in the face of unfavourable changes in its environment. The level of drug resistance may vary considerably among different isolates, and the sensitivity of trypanosomes to drugs may also vary at different times in the course of an infection.
Drug resistance in trypanosomes may be a combination of such adaptation and of selection.
In addition to classical drug resistance in trypanosomes, a change in behaviour of the parasites may produce the same practical result. It is commonly observed that trypanosomes, belonging to a so-called resistant strain, disappear quickly from the blood stream following treatment, and the only (but very important) difference with a non-resistant strain is that they reappear after a period of varying length. They have apparently acquired not so much a classical drug resistance but the ability to hide in parts of the organism of the host in which the drug cannot penetrate sufficiently to rid the animal of all the trypanosomes in its body (for example the chambre of the eye or parts of the brain). It is unknown how this “behavioural” resistance arises.
In this section we shall only discuss classical drug resistance further, but the result of behavioural resistance is similar.
The exact mechanism of drug resistance is insufficiently known and this is not surprising when one remembers that the precise mode of action of the trypanocides is also unclear. It is possible that sometimes penetration of the drug into the trypanosome is decreased because of changes in the surface of the parasite cell or that the enzyme process that is disrupted by the drug in susceptible trypanosomes has become insusceptible to the action of the drug. Whatever the mechanism involved, the fundamental fact of extreme practical importance is that drug resistance in trypanosomes often arises or is accelerated as a result of their exposure to a sublethal level of the compound used. It is therefore clear that such an event, so often the result of the careless use of drugs, must be avoided. Naturally resistant strains (as opposed to the exceptional naturally resistant individual parasite) of a pathogenic trypanosome apparently do not exist in an area where drugs have never previously been used. This apparently indicates that non-resistant trypanosomes have a (perhaps only slightly) better chance of surviving than resistant ones (but this changes of course when a trypanocide is used). Indeed, resistance may in the long run spontaneously disappear or at least greatly decrease after the use of the trypanocide is stopped, as the non-resistant individuals (which may for example have survived in domestic or wild animals that were not treated) in the end gain the upper hand.
As resistance often appears to be a direct result of the exposure of parasites to a sublethal (subcurative) dose of the drug, every attempt must be made to avoid that prime cause. Figure 15 is a graphic diagram of the plasma concentration curves of two drugs. A is a curative drug and B a prophylactic one. The horizontal line indicates the lowest effective (curative) level of the drug. (For purposes of convenience this level is represented as being the same for both drugs.) It is now obvious why the use of prophylactic drugs for purely curative treatment is to be avoided. The length of time during which a subcurative level occurs in the blood is much longer with such compounds than with purely curative drugs with a short residual effect; prophylactic drugs thus give occasion to new infections to become established and develop resistance during this long period of a subcurative concentration. When drug B is used in mass prophylaxis, a subcurative level can be avoided by retreating the animals at appropriate intervals, so that the level always remains above a sublethal one (see Figure 16).
If we accept the fact that drug-resistant strains are most likely to arise during the course of a regime that permits subcurative exposure to occur, and that the greater the fraction of the cattle population in which subcurative drug concentrations exist, the greater the likelihood of this unwanted contact between parasite and drug, the most favourable circumstances for the development of resistance are:
In order to delay the development of resistance (complete avoidance of its development is probably impossible), the following are recommended.
When resistance first appears in the course of a drug regime it is confined to the parasites present in relatively small numbers of animals, but the numbers will increase rapidly, particularly if it is not detected and subcurative levels persist or increase. The trypanosomes become more and more efficient at dealing with the trypanocide and each retreatment will lead to an increased level of resistance.
As long as drug fastness remains confined to the domestic host it is called an individual resistance. Resistance becomes firmly established in the blood forms of the parasites, as a genetical characteristic, and will survive direct passages through a series of the same or different hosts. It has also been shown that drug fastness survives cyclical passage through the tsetse fly and is passed on by the infected insect during subsequent blood meals. In this manner resistance becomes disseminated throughout an area, and is present in as many host species as are favoured by the tsetse fly. It is then known as area resistance. This is a most serious event presenting a different problem. It must also be remembered that the hosts of tsetse are not only domestic animals but also various wild game species which will in turn infect future generations of tsetse.
Individual resistance can be dealt with by the use of an alternative drug to which the parasite remains susceptible. Once the resistance is widely disseminated (area resistance) it will be necessary to use in that area, for at least a year, a drug to which the trypanosomes remain sensitive. During that time the resistant strain becomes “diluted” as it were and is replaced by the normal susceptible strains; it thus tends to disappear.33 As stated above, non-resistant trypanosomes may have a biological advantage over resistant ones, and some scientists have indeed found that resistant strains are less virulent and. less fit, so that they tend to disappear if put in competition with normal susceptible strains. However, it has also been shown that drug resistance may persist in the field for at least four years.
The secret of the management of drug resistance is prevention. Signs of resistance must be immediately dealt with to prevent the spread of the drug fast strain and the change from individual to area resistance. This requires efficient monitoring and the regular examination of blood preparations of all clinical cases and, if none are found, of at least 10 percent of the animals of herds selected at random.
The phenomenon of cross-resistance is of utmost importance. Cross-resistance is resistance to a drug that has arisen as a result of previous exposure of the trypanosome strain to a different drug. Cross-resistant drugs are often members of the same chemical family. The resistance that appears to the drug used is known as primary resistance, that which appears to a different compound is called secondary resistance.
In general, continued exposure to a drug that has caused resistance, or an increase of its dose, boosts the resistance to a higher level and this mistake is to be avoided. Also, exposure to a different drug, to which there is secondary resistance, has the same effect and it is therefore necessary to know the resistance relationships of all the available compounds. It is also important to know that secondary resistance may even, in turn, lead to a lack of sensitivity to a third drug. This may be illustrated by the following example.
A strain of T. congolense that has acquired primary resistance to homidium compounds may show secondary resistance to quinapyramine methyl sulphate. If treated with the latter compound this secondary resistance will become enhanced and may reach a level that could create resistance to diminazene aceturate, a drug that is commonly used in sanative actions. A drug that will eliminate parasites that have developed resistance to another compound is known as a sanative drug.
Table 5 shows the cross-resistance among the five compounds in use for the treatment of tsetse-transmitted trypanosomosis in livestock. The five columns in the left half of the table show the resistance score when the drugs are given at the basic curative rates, while the four columns on the right record the score when the drugs are used at higher dosage rates (except for quinapyramine which is too toxic). The sanative action of some of the drugs when administered at increased doses can be read in this right half of the table. However, once resistance to a drug is present it is as a general rule not a good strategy to increase its dose; even when there may be some short-term benefits, the selection pressure is increased and the level of resistance will certainly rise sooner or later.
TABLE 5
Cross-resistance between trypanocidal drugs*
| Cross resistance to | |||||||||
| Trypanosomes resistant to | At curative doses | At increased doses** | |||||||
| QP | M | PB | IM | DA | HM | PB | IM | DA | |
| QP | + | + | + | + | + | + | ± | - | - |
| HM | + | + | + | + | - | + | + | - | - |
| PB | + | + | + | + | - | + | + | - | - |
| IM | + | + | + | + | - | + | + | - | - |
| DA | + | - | - | - | + | - | - | - | + |
QP = Quinapyramine;
HM = Homidium;
PB = Pyrithidium bromide;
IM = Isometamidium;
DA = Diminazene aceturate.
+ = resistant;
- = not resistant;
± = some strains resistant.
A careful examination of Table 5 reveals several important facts.
It is known that resistance can also develop against the new drug mel cy (Cymelarsan), which in turn may result in cross-resistance to some other arsenical drugs, and, as has been shown in at least in one experiment, to partial cross-resistance to diminazene aceturate.
Sanative pairs of drugs are two drugs between which there is no cross-resistance and either may therefore be used to treat trypanosomes that have become resistant to the other. The adoption of this principle allows to make in advance a contingency arrangement so that if resistant strains appear after a period of use of one drug, the second drug of the curative pair can be used. However, such a “sanative” treatment regimen should not be applied haphazardly, with a change of drug at every visit, as this may well result in strains resistant to both drugs. Before the drug switch is made there must be good evidence of the presence of resistance and the second drug should be used for a year or so before a switch in the reverse direction can be considered. It is also important to integrate other control measures in order to reduce the number of treatments with the second member of the sanative pair and thus delay the development of resistance to it.
How can one detect drug resistance? The manufacturer of a newly developed compound should run a series of tests before it is commercialized. One of the aims is to check its tendency towards developing drug-resistant strains of trypanosomes, by deliberately encouraging the process. The classical experimental method is to infect a laboratory animal with the trypanosome species to be examined; once the parasite is established the animal is treated with the drug, given deliberately at a very low level, insufficient to eliminate the parasite. The experiment is meant to imitate the classical situation of subcurative exposure conducive to the development of resistance. The process is enhanced by transferring the strain to another susceptible animal which is also treated as soon as the parasites are established. Retreatment at higher dosage rates and further repetition of the process will accelerate the development of resistance. Any compound to which resistance appears quickly in the laboratory is obviously not suitable for the field.
When drug-fast strains first appear in the field, further treatment with the drug will cause the resistance to increase and there is a danger of it reaching a level which results in cross-reaction to drugs that would have been effective if they had been used directly at the first appearance of the drug-fast strain. It is therefore of prime importance to detect drug-fast strains as early as possible, before resistance generalizes and develops into an area resistance. As stated before, there is some evidence suggesting that drug-fast strains of trypanosomes may be somewhat milder in their pathogenicity and may not produce pronounced clinical signs of the disease. A somewhat benign form of trypanosomosis as is sometimes caused by drug-resistant trypanosomes may initially complicate the early detection of developing resistance.
T. congolense is perhaps most likely to develop resistance, but there are also numerous instances of resistance in T. vivax, even to the “sanative” drug diminazene aceturate. Many strains of members of the brucei-group (subgenus Trypanozoori) also have developed resistance to one or more compounds; the veterinary world is particularly concerned with the widespread resistance to drugs against surra (T. evansi) in camels and other animals, in Africa as well as in Asia and Central and South America. The appearance of unexpected cases of trypanosomosis during the course of a drug regime should always be considered with suspicion.
The identification of samples of individual animals is invaluable for the rapid detection and confirmation of drug resistance. An identified individual animal may thus be found infected within the period during which it should have been protected. In order to confirm or to refute the presence of resistance, the parasite must once again be exposed to the drug and its blood be examined within a short period, say 10–20 days after this renewed treatment. If at all possible, it is best to transport such suspect animals to a tsetse-free centre, where they can be monitored without risking to induce an area resistance. However, much valuable time may thus be lost, and efficient field monitoring should detect resistance at an early stage, before it is widespread.
To confirm suspected resistance at an early stage, positive blood preparations may be taken from as many cattle as possible, pooled, and inoculated into a susceptible calf. The animal is then monitored regularly and treated with the trypanocide at the recommended dose as soon as the parasitaemia starts. Persistence or reappearance of the infection indicates that there were drug-resistant trypanosomes in the initial pooled inoculum. A group of susceptible ruminants, preferably from outside the tsetse area, is then infected with the breakthrough trypanosome strain and, on becoming parasitaemic, treated with various dosages of the trypanocide in order to assess the drug sensitivity of the strain. The animals should be examined regularly at short intervals and monitoring should continue over a prolonged period (at least three months, unless the animals become positive before) in order to determine the curative dose which will provide a permanent cure.
Instead of ruminants, laboratory rodents (usually mice) can be used to screen for drug resistance. They offer the advantage of being far cheaper, but they can only be used for those species and strains of trypanosomes that will grow in them. Also, infections in mice require far higher dosages of trypanocide than in cattle. The advantage of ruminants is that all isolates from cattle will grow in them and that the results are directly applicable to the field.
In recent years some progress has been made with the development of assays to determine drug sensitivity of trypanosomes grown in in vitro cultures, but this is not yet applicable to field isolates, and it is definitely not a routine procedure, even in a well-equipped laboratory. As indicated in Prophylactic drugs, p. 101, other laboratory tests can determine the concentration of trypanocide in the blood, and if it is above a certain level and the animal is nevertheless parasitaemic, this helps in confirming resistance. Drug detection is carried out with an ELISA using antibodies to the drug conjugated with a protein (as drugs alone do not induce the formation of antibodies.
All these methods are time consuming and expensive, and the appearance of drug-fast strains calls for urgent remedial action. If drug resistance is strongly suspected, sanative measures should be commenced in the field even while experimental confirmation takes place at the same time.
Once drug resistance has been confirmed, knowledge of the range of possible cross-reactions between the drug (or drugs) in use and other compounds is required. If resistance occurs in an area where curative drugs such as homidium or isometamidium at 0.25 mg/kg are used, diminazene aceturate often controls the situation. For this, it is used to block inoculate the herds in the area, in order to eliminate the developing drug-fast strains. It must be remembered that block inoculation with other curative drugs that have an appreciable persistence may encourage resistance and should be avoided under any circumstance.
If the trypanosome risk is low and a curative regime is in use, one may use the alternative member of a curative pair of drugs (see above).
Where resistance appears in a group of cattle in a high risk area and a prophylactic isometamidium regime is in force, block inoculation with diminazene aceturate is practised if the cases breaking through the drug cover are still few. This is when resistance has been discovered in its early stages and can still be regarded as an individual resistance. Diminazene aceturate will not control the situation where challenge is high and the resistant strain is widely distributed and transmitted by tsetse flies (area resistance), because the rate of reinfection with resistant trypanosomes is too great.
Diminazene aceturate may sometimes be used to prevent the development of drug-fast strains. In some cases where challenge is high and prophylaxis is practised, poor presentation of the animals for treatment or the influence of some other factor may make the appearance of drug-fast strains almost inevitable. In such cases a block treatment with diminazene aceturate may be carried out at regular intervals, usually every six months, to eliminate any strains that are likely to have developed resistance to the prophylactic drug. For example if in such an area isometamidium is administered every three months at 1 mg/kg, a diminazene aceturate block inoculation would be carried out every six months, one month before a routine isometamidium treatment: day 0 isometamidium — after three months isometamidium — after five months (from day 0) diminazene aceturate — after six months isometamidium — after nine months isometamidium — after 11 months diminazene aceturate — after 12 months isometamidium — and so on. The diminazene aceturate is given two months after the isometamidium because the concentration of the latter will be fairly low and the former will be eliminated before the next isometamidium treatment, thus removing any danger of combined toxicity.
There are situations where trypanosomes are resistant to all commercially available drugs, including diminazene aceturate, homidium and isometamidium. In such a case of multiple drug resistance, trypanocides cannot offer a solution and other measures should be considered (see p. 125–133). The use of integrated control should help to prevent such situations from occurring in the first place.
Other causes of apparent drug failure. Often, apparent drug failure in the course of chemotherapy under field conditions is wrongly blamed on drug resistance and the real cause is to be found in incorrect procedures, such as:
All the above deficiencies in drug administration will of course eventually lead to genuinely resistant strains if allowed to proceed unchecked. Thus, it is of paramount importance to involve personnel fully in the planning and implementation of drug regimes. Animal health assistants should be familiar with the theory and the principles that ensure the successful control of trypanosomosis.
The importance of regular monitoring of the disease situation cannot be sufficiently stressed. Examination of blood preparations should be a matter of routine, irrespective of the trypanosomosis risk and the treatment conditions.
So far all attempts at developing a vaccine against trypanosomosis have failed. With the rapid advances in molecular biology the situation may change at some point in the future, but so far this approach has been stranded by the almost unlimited ability of trypanosomes in the host to change their surface antigens frequently. With new antigens appearing, antibodies elicited against previous types of antigens are no longer effective and the immune system has to start all over again to produce new antibodies, until it becomes exhausted. Also, the antigen repertoire is different between different strains, types and subspecies of the same trypanosome species. Moreover, African tsetse-transmitted trypanosomosis is often a mixed infection of two or even three different species. The subject of vaccination will therefore not be discussed any further in this guide.
Although the subject of tsetse control is addressed extensively in the FAO publication series “Training manuals for tsetse control personnel” (Vols 4 and 5), the control of trypanosomosis cannot be considered in isolation of the control of its vectors, and is part of an integrated approach towards control. The general principles and possibilities of tsetse control, which have also been mentioned in the Introduction, are therefore briefly reviewed.
Vector control has traditionally been based on specialized large-scale operations (insects recognize no boundaries between farms, districts, or even countries). Recently, there has been a tendency towards smaller-scale methods which can be applied by farmers themselves, but this approach is still in its infancy. This section is limited to tsetse flies; the control of mechanical vectors such as stable flies and tabanids is beyond the scope of this field guide.
Some methods have been based on ecological control. Ecology is the science of animals or plants in relation to their environment.
Tsetse flies require rather precise conditions of temperature and relative humidity, which are determined by the presence of woody vegetation. The different tsetse species may conveniently be grouped into three broad categories according to preferred vegetation type: forest, riverine and savanna type of tsetse. Bush clearing may deprive savannah species from the resting places and shade needed to maintain these conditions. The method has been used to diminish the density of fly, to stop fly advances, or to isolate an area from the main tsetse belt before clearing it from the fly. The crossing of such a barrier can only be prevented by ruthless clearing of all trees and bush over a width of several kilometres. However, some risk remains as tsetse flies may be carried over large distances in or on vehicles, and may also follow animal herds. Such barriers require substantial maintenance as vegetation regrowth occurs relatively quickly. Bush clearing with the aim of destroying tsetse habit is no longer practised, but the barrier concept still exists and involves the use of bait devices in the form of odour baited, insecticide treated targets, to which dipped or pour-on treated animal populations may contribute.
Another ecological method was to starve tsetse flies by depriving them of the hosts they feed on. The high mortality among wild ruminants and wild pigs provoked by the rinderpest panzootic around the turn of the century was followed in many areas by a considerable retraction of fly belts, at least where savannah species of tsetse were concerned. The shooting of game has met with some success in the control of savannah tsetse species, but it proved difficult to eliminate wart hogs, which are often the preferred hosts of the flies, and also the smaller antelopes. Today, the method has been abandoned, to a large extent on ethical and environmental grounds. Nevertheless, where game disappears because of human settlement and expanding agriculture, savannah species tend to disappear, with a few exceptions where they may adapt to domestic animals. The source of food of riverine species, which even includes reptiles, is too varied for the method to be effective.
The most effective ecological control method is to avoid contact with tsetse flies.
Tsetse flies are highly susceptible to the action of insecticides, and many different products, starting with DDT and dieldrin up to the more recently introduced and less harmful pyrethroids, have been used over the past 50 years to control and eradicate tsetse. Initially the insecticide, mostly DDT, was applied by handspraying on the vegetation, in particular on the favourite resting sites on tree stems, and also under fallen logs and other potential pupae sites. This ground spraying has historically been the most prominent weapon against tsetse. Today, with DDT banned in most countries and the general shift towards a more sophisticated use of insecticide, ground spraying has practically been abolished.
Later, extensive use has been made of spraying insecticides from aircraft, either fixed wing or helicopters (the latter being more expensive to use but more suitable for covering difficult terrain). It is beyond the scope of this book to go into details of aerial insecticide spraying (Sequential Airspray Technique, SAT) against tsetse flies, which is a specialist's job. SAT efficacy depends on factors such as droplet size, wind and inversion conditions.
A particular concern with large-scale insecticide application is the pollution it may cause, as most insecticides are harmful to aquatic life (including fish, frogs and other amphibia), while the earlier compounds, such as DDT, also affect terrestrial animals, including birds. The effect of pollution caused by a one-time application is usually quite temporary, but repeated use at frequent intervals of insecticides with a long residual activity is more harmful.
A different way of using insecticides is to apply these on domestic animals, so that flies settling on such animals are killed. Synthetic pyrethroid formulations are applied by spraying, dipping or used as a “pour-on” formulation, which is more expensive but does not need any pump, spray-race or dip. When such live bait animals are used without any other form of tsetse control, difficulties arise with the persistence of flies in areas where the treated animals do not go. Nevertheless, locally the flies may be reduced in density to a tolerable level. The frequency of application depends on how long the insecticide remains active on the animal (the length of its residual activity); the effect is shorter during the rains. Depending on the particular insecticide used, ticks (and other ectoparasites) may also be reduced (although the frequency of application needed for the effective control of ticks is greater than that required for tsetse control). 34 This method of insecticide application is much less polluting than spraying the environment.
So far, the use of insecticides has not produced insecticide resistance in tsetse flies, as far as is known. This can presumably be attributed to the low selection pressure for resistance and the fact that each female tsetse fly only produces few descendants during her life span.
It has been known for a long time that male tsetse flies that have been rendered sterile by gamma irradiation or by certain chemical compounds will mate with females, but these will not produce offspring, as females normally mate once only after hatching, contrary to males. With the continuous release of sterile males in large numbers it is possible to eradicate tsetse flies from a particular area. For the recent successful campaign on Zanzibar (see Introduction) a ratio of more than 100 sterile males against one wild one was used at some stage.
The method is very specific and not polluting in itself,35 but the effect on the population only becomes apparent after a period, as opposed to control by instantly killing insecticides. Because of this and to allow the sterile males to be competitive, a substantial fly suppression has to precede the application of SIT (Sterile Insect Technique), which is reserved for the final “mopping up” of the remaining population. The released males will seek out the females even in places where the insecticide cannot penetrate.
The males to be sterilized have to be mass reared in the laboratory. Also, the numbers of males that can be obtained are limited by the low rate of reproduction of tsetse flies and the fact that they have to be fed at least twice a week on blood. Where tsetse eradication is envisaged in places where several tsetse species occur in association, as is often the case, mass rearing has to be carried out for each individual species separately.
The application of insecticide on livestock has been mentioned in Use of insecticides, p. 126. Another approach is the use of traps or insecticide-impregnated targets (screens). The attractivity of traps and targets for tsetse flies depends on their shape, size, colour and colour pattern, and this differs from species to species. The catch can be increased, in many cases considerably, by certain substances which have an attractive odour for the flies, e.g. acetone, phenolic molecules, bovine or buffalo urine. This type of attractivity is also called olfactory or olfactive. The choice for the best substance depends very much on the species of tsetse fly one is dealing with, and can even be seasonally different. Savannah tsetse flies are generally more attracted by the odours studied so far than riverine species.
Some of the drawbacks of traps and screens are:
Preliminary observations indicate that certain types of traps and odour substances will also attract mechanical vectors of trypanosomosis, such as tabanids and stable flies. This may be of help in reducing mechanical transmission of the disease. More research on this point is needed.
What is meant here by methods of control are methods that reduce the fly density, and therefore the trypanosomosis challenge, to a “tolerable” level, a level at which cost-effective animal production is possible, but it does not necessarily aim at eradication of tsetse. Indeed, for a long time the various ways of controlling tsetse flies were intended to eradicate them from land that was destined for human and/or livestock settlement. Especially the application of insecticide from aircraft opened the possibility of large-scale eradication campaigns and the sterile male technique was also very promising in this respect. However, at present it is more and more recognized that eradication is often not sustainable and that expensive large-scale campaigns can only be justified in certain circumstances:
It should be said that human settlement in the freed area is sometimes enough to discourage the fly from coming back (permanent bush clearing for agriculture, elimination of wild host animals). This is valid where savannah species of tsetse are concerned, which usually cannot maintain themselves in a densely settled area (but there are exceptions where they have turned to domestic animals and humans as their sole source of food).36 Riverine species have a wide range of hosts and live in dense vegetation along rivers, so that they are not eliminated just by human activities.
There are no universal answers that will apply to all the various situations that exist, depending on species of tsetse, type and value of livestock, wild hosts, veterinary infrastructure, foreign exchange situation of the country, etc. Extension services have an important role to play, for initial information and motivation and then, as fly numbers go down and the owners lose motivation and interest, for continuously reminding them that any let-up in control will inevitably lead to a return of the old situation.
It goes without saying that wherever control of tsetse flies is attempted in areas to be settled, without prior human occupation, land-use plans are just as essential as in the case of eradication.
It is well-known that genetically determined innate resistance to many diseases occurs in animal populations that have been subject to natural selection by exposure to disease pressure over many generations. This is also true for trypanosomosis. Taurine (humpless) breeds of cattle were the first to be introduced into Africa. They populated what is now the Sahara, but were pushed back further south when this area became a desert thousands of years ago. At present, they persist in the subhumid and humid northern parts of sub-Saharan Africa where they live and produce in tsetse areas. Such taurine breeds are now mainly confined to West Africa, from Senegal to Nigeria, but they used to occur as far to the east as the central Sudan (Nuba Mountains) and even western Ethiopia. N'Dama cattle (which originate from Guinea) have rather long horns, while breeds with short horns comprise, for example, the Baoulé (Burkina Faso and northern Côte d'lvoire) and the Muturu (Nigeria). They are “dwarf” cattle (although a N'Dama cow can weigh as much as 200 kg, similar to the size of many of the smaller zebu breeds).
It should be added that trypanotolerance is not limited to taurine cattle, as it has been found that some of the zebu breeds in East Africa, such as the Orma Boran and the Masai zebu, also have a higher resistance to trypanosomosis than other breeds such as the “normal” Boran zebu, Indian breeds of zebu and other “exotic” (i.e. introduced) breeds such as European taurine breeds. However, the resistance of West African taurine breeds appears to be considerably more pronounced.
Apart from cattle, breeds of sheep and goats (and even of horses) living in tsetse areas are also relatively trypanotolerant. This is particularly true of the Djallonke sheep and dwarf goats in West Africa. Knowledge of the resistance of small ruminants is still fragmentary and far more is known about resistance in cattle.
There have been attempts to introduce West African trypanotolerant cattle to human populations who had no cattle previously, and to other parts of Africa, with some success, notably in the Central African Republic, Gabon and the Democratic Republic of Congo (ex-Zaire). However, livestock owners who are used to larger cattle, are not readily attracted to the smaller trypanotolerant breeds.37 There are also limits to their trypanotolerance and when challenge is high even such animals may show clinical trypanosomosis. Their resistance is particularly effective in the face of riverine species of tsetse, which usually occur in lesser numbers and have a lower infection rate with pathogenic trypanosomes than the savannah species. Nevertheless, there are situations where such cattle live under continuous, but moderate, exposure to savannah species as well. Animals of such breeds not exposed to infection before and introduced as adults to tsetse-infested areas, may well become ill and even die. Cattle that stand up to challenge in a particular region may suffer from disease when introduced into another area, and any such move has to be well monitored and supervised.
Trypanotolerant breeds are certainly of great benefit, particularly where riverine species of tsetse flies are concerned, and where people are used to them.38
Drug prices and drug and pesticide resistance are on the increase, and we should use all available methods that can be applied in each particular situation and are cost-effective. Combining different control methods against a parasitic disease is called integrated disease control or integrated disease management and is generally not intended to achieve eradication of the parasite in question.39 Such a cost-effective combination of techniques, adapted to each particular set of circumstances, is very relevant for the control of African animal trypanosomosis. It cannot be sufficiently stressed that the cost-benefit principle is an essential aspect of all methods. Apart from this crucial point, all of the available methods have advantages and disadvantages and the various techniques act in a complementary way; an advantage of one may offset a disadvantage of another.
The sequential use of insecticidal spraying or of insecticide-impregnated targets, followed by the sterile male technique, has already been discussed.
A combination of chemoprophylaxis against the disease and insecticidal application on the cattle against the vector may greatly improve the trypanosomosis situation.
Where drug resistance is a problem, the accent will be put more on vector control and/or on the use of trypanotolerant livestock.
Many more examples of integrating two or more control measures could be given. It is essential to realize that drugs alone are not the answer to AAT. Flexibility and adaptation to each situation are important For instance, the association of trypanotolerant livestock and systematic chemoprevention is likely to be uneconomical; it is more logical to reserve curative trypanocides for the limited number of animals that may require treatment.
Finally, one must always keep in mind the necessity for gaining the confidence of the owners. Right from the beginning, before drawing up a plan, the control options should be thoroughly explained and the stockowners should be given a full opportunity to discuss the situation. Gaining and keeping their full confidence in this manner will enable extension and/or control personnel to work in a more relaxed and efficient manner.
