African animal trypanosomiasis

PART I. DISEASE AND CHEMOTHERAPY

P. FlNELLE ^*

This is the first of three articles on African animal trypanosomiasis by Dr. Pierre Finelle, who has spent many years in Africa studying this parasitic disease. This first part describes the disease and its occur­rence, and the drugs that have been introduced to combat it and their relative merits.

The second part will deal with chemoprophylaxis and the raising of trypano-tolerant livestock, while the third and last article will review vector control as a means of over­coming trypanosomiasis.

Trypanosomiasis is a parasitic dis­ease caused by species of flagellate protozoa belonging to the genus Try-panosoma which inhabit the blood plasma and various body tissues and fluids. These parasites are found in many animals but seem to be patho­genic only for mammals, including man.

Animal trypanosomiasis occurs in most of the tropical regions, but only in equatorial Africa does it constitute a major obstacle to the development of animal production. The consider­able economic and social repercussions make control of this disease a priority operation for the develop­ment of a large part of the African continent.

Trypanosomes

African animal trypanosomiasis can be caused by several species of try­panosomes:

Trypanosoma congolense is found in most domestic mammals: cattle, sheep, goats, horses, pigs, camels and dogs; and also in many wild animals (Figure 1).

T. vivax is a parasite of domestic and wild ruminants and of horses.

T. simiae is found mainly in do­mestic and wild pigs.

T. brucei is a parasite very close to T. gambiense and T. rhodesiense, which are the causes of human sleep­ing sickness. It can be found in practically all domestic and wild an­imals.

T. evansi is found in Africa only in the Saharan and Sahelian regions where it is primarily a camel parasite, but it may be a parasite of horses, cattle and dogs as well. It also occurs in Asia — where it commonly causes disease in camels and horses, and less commonly in cattle, water buffaloes, elephants and dogs — and in Central and South America. Thus it has a very wide distribution.

Figure 1. Photomicrograph of a film of blood showing three specimens of Trypano­soma congolense.

T. equiperdum is the causal agent of dourine, a contagious equine dis­ease transmitted by coitus, which in Africa occurs only in the north African region and in South Africa. As control of dourine is an entirely different problem from that presented by other forms of trypanosomiasis, it will not be discussed in the present review, which deals only with the African trypanosomiasis transmitted by insects.

Transmission of trypanosomes

Transmission of trypanosomes by insects may be effected by widely different means.

Cyclical transmission, during which the trypanosomes actively multiply in the vectors, occurs through the in­termediary of Glossina or tsetse flies (Figures 3 and 4). This form of transmission occurs with T congo-lense, T. vivax, T. simiae, T. brucei, and the trypanosomes responsible for human sleeping sickness, T. gam-biense and T. rhodesiense. Glossina spp. are strictly blood feeders living exclusively in tropical Africa. There are about thirty species or subspecies, classified in three groups: palpalis, morsitans and fusca. Each species has distinct biological characteristics, but in general it may be said that the palpalis group consists basically of the species living in forest galleries or in the marginal areas of forests; the fusca group consists of large-sized species whose habitat is generally as­sociated with equatorial forests; and the morsitans group consists mainly of species living in wooded savanna.

Mechanical transmission is effect­ed by various blood-sucking insects such as flies of the family Tabanidae (horse flies) and Stomoxys spp. In the course of a blood meal begun on an infected animal and ended on a healthy one, these insects may car­ry trypanosomes provided that the interval between the two meals is short. This form of transmission is the rule for T. evansi, but may also occur with trypanosomes habitually transmitted cyclically by Glossina, particularly T. vivax which may therefore be found in regions far from the Glossina distribution area (such as Latin America).

Figure 2. Demonstration of chemotherapeutic treatment against trypanosomiasis by inoculation with a trypanocidal drug in the dewlap.

Figure 3. Close-up of a tsetse fly, the insect vector of African trypanosomiasis.

Trypanosomiasis

Trypanosomiasis is generally a chron­ic evolving disease which is usually fatal if appropriate treatment is not established. It leads to considerable loss of weight and anaemia. Various symptoms are exhibited, including fever, oedema, adenitis, dermatitis and nervous disorders. Because of its protean symptomatology the dis­ease cannot be diagnosed with cer­tainty except through detection of parasites by microscopic examination of blood or by various serological reactions.

The evolution of trypanosomiasis varies widely according to the try-panosome involved and the animal species or breed affected. Trypano­somiasis caused by T. simiae in pigs usually assumes a highly acute form leading to rapid death, at least in improved pig strains. T. brucei is highly pathogenic for horses and dogs, but in cattle this trypanosome usually causes asymptomatic infec­tion. Zebu cattle are extremely sus­ceptible to infections caused by T. congolense and T. vivax, but the humpless cattle of west Africa and the Guinean strain of goats show remarkable resistance, enabling these animals to live in areas where other breeds cannot exist.

Biologically-based control of animal trypanosomiasis

In the control of animal trypano­somiasis action is possible on various aspects of the epizootiological cycle of the infection: parasites, host ani­mals and vectors.

ACTION ON PARASITES

This consists of the use of trypano-dal drugs on infected animals. The method aims first at limiting losses caused by the disease, and second at eliminating trypanosome reservoirs. Thus, detection and treatment of in­fected animals can be considered to be both a curative and a prophylactic procedure.

ACTION ON HOST ANIMALSs

Although immunological responses occur in trypanosomiasis, it has not yet been possible to develop a prac­tical method for immunization. Short of such a method, the use of prophy­lactic trypanocidal drugs makes it possible in certain conditions to pro­tect animals for several months. Another method consists in raising animals showing natural resistance to trypanosomiasis, such as the humpless cattle of west Africa.

ACTION ON VECTORS

This method applies primarily to Glossina. Attempts may be made to (a) destroy the insects, particular­ly through the use of insecticides; (6) make the environment unsuitable as a habitat, either by altering the vegetation or by eliminating the ani­mal species which constitute the pre­ferred hosts of these insects; (c) re­duce their reproductive capacity by the release of sterile males; {d) limit their number by using biological con­trol methods. The two latter tech­niques are still only in the research stage and have not been used so far as a practical control method for Glossina.

The various methods will now be considered which can be used in the control of African animal trypano­somiasis, excluding dourine; and the account will be confined to measures for the treatment and protection of cattle, small ruminants, pigs, horses and camels. The measures reviewed include (a) chemotherapy, (b) chem-oprophylaxis, (c) breeding of try-panosome-tolerant animals and {d) vector control.

Chemotherapy

Since 1938, the date of the discovery of the trypanocidal properties of the phenanthridines, the chemotherapy of animal trypanosomiasis has made great progress and there are several highly active drugs now available which are easy to use. The use of trypanocides has consequently be­come widespread, and the number of trypanocidal treatments carried out every year in Africa can be estimated at over 6 million, the great majority of them for combating bovine try­panosomiasis.

The trypanocides currently em­ployed are: homidium salts (Ethi-dium-Novidium); quinapyramine sul-fate (Antrycide); diminazene acetu-rate (Berenil); isometamidium (Samo-rin-Trypamidium) and suramin so­dium.

Table 1, which gives the data con­cerning the use of these products, shows that the action of the different trypanocides varies according to the animal species infected and the try-panosomes involved.

Figure 4. Different stages of evolution of the tsetse fly: larva, pupa and adult fly.

CATTLE TRYPANOSOMIASIS

T. congolense and T. vivax

Cattle infections caused by T. con­golense and T. vivax are by far the most serious, both for frequency and for economic influence. The first really effective trypanocides were the dimidium salts which were widely used during the 1950s. However, their toxic effects, the difficulties in­volved in their adoption and the frequent appearance of drug-resistant trypanosomes have made the use of more recent trypanocides preferable. Homidium salts have been and are widely used, but a considerable num­ber of cases of drug resistance to homidium have been reported and in many countries it has been neces­sary to suspend their use. Drug re­sistance has also been a serious hand­icap in the employment of quinapy­ramine sulfate (Antrycide) which is no longer extensively used in cattle for the treatment of either T. con­golense or T. vivax trypanosomiasis.

Diminazene aceturate (Berenil) offers numerous advantages: its high activity against T. congolense and T. vivax, particularly on those strains resistant to other trypanocides, its very low toxic effects in cattle and its easy utilization make it a prac­tical and safe trypanocide, at least for cattle. Although some cases of resistance were observed early in the use of the trypanocide, it was the accepted view at the time that this was the result of cross-resistance with quinapyramine, and that dimin­azene did not directly cause resis­tance because of its rapid elimination through the kidneys, which prevents accumulation of residual subcurative doses. Since 1967, however, strains of trypanosomes directly resistant to diminazene have been found in var-ious countries, notably in the Central African Republic, Chad, Kenya, Ni­geria and Uganda, primarily with regard to T. vivax but also to T. congolense. These strains are for­tunately still vulnerable to the phen-anthridine group of trypanocides, particularly isometamidium, leading to the conclusion that in case of failure of a diminazene treatment it is preferable to use another trypano-cide such as isometamidium rather than give further treatment with an increased dose of diminazene.

Table 1. Use of trypanocidal drugs

¹ im = intramuscular injection: sc = subcutaneous injection.

²Boots Pure Drug Co. Ltd.

³ May & Baker Ltd.

⁴ Farbwerke Hoechst A.G.

⁵ Imperial Chemical (Pharmaceutical) Ltd.

⁶ Specia.

Isometamidium (Samorin, Trypa-midium) is the most recent of the commonly employed trypanocides. Its main advantage is its effectiveness on trypanosomes resistant to other drugs. At the same time it has the disadvantage of easily creating drug-resistant strains itself; however, these trypanosomes show no cross-resis­tance with diminazene, which there­fore retains its effectiveness on such strains. The isometamidium deposit at the injection site can cause a persistent local reaction which may be invisible from outside if deep in-tramuscular injection has been given, as is recommended. This reaction makes the surrounding flesh unfit for consumption and partial confiscation of the carcass is necessary. It is therefore advisable to choose an in­oculation site on a part of the body where the meat is inexpensive; the neck muscles are usually recommend­ed.

The two foregoing drugs, dimin­azene and isometamidium, are cur­rently the preferred treatments for T. congolense and T. vivax trypano-somiasis in cattle.

T. brucei and T. evansi

Trypanosomiasis in cattle caused by T. brucei is of secondary importance as this trypanosome is only slightly pathogenic for cattle. The most ac­tive trypanocide against it is quina-pyramine.

T. evansi trypanosomiasis is ex­tremely rare in cattle in Africa, where the disease occurs mainly in camels. It is encountered more frequently, however, both in cattle and in water buffaloes in southeast Asia. The best treatment is quinapyramine.

TRYPANOSOMIASIS IN SMALL RUMINANTS

Sheep and goats are seldom affected by trypanosomiasis and there is little information on treatment. If neces­sary, the treatments indicated forcattle, with diminazene and isometa-midium, can be used.

Figure 5. Geographical distribution of animal trypanosomiasis.

TRYPANOSOMIASIS IN PIGS

T. simiae, which is found mainly in domestic and wild pigs, presents a special problem because of its low vulnerability to the various trypano-cides, requiring the application of considerably higher doses than those used against other trypanosomes. Two treatments appear to be effec­tive: an extremely high dosage of isometamidium (12.5 to 35 mg/kg); or a combination of quinapyramine sulfate (7.5 mg/kg) with diminazene (5 mg/kg).

However, the rapid course of this form of trypanosomiasis usually makes any therapeutic action impos­sible, so that it is necessary to rely on preventive rather than curative treatment.

EQUINE TRYPANOSOMIASIS

T. congolense and T. vivax

Diminazene is not as well tolerated by horses as by cattle. Local reac­tion and fatal poisoning, with kidney or brain lesions, have been reported. Homidium and isometamidium can be used on horses, although both drugs often cause local reactions; doses should therefore be divided so as to inject no more than 10 milli-litres per injection site.

T. brucei and T. evansi

Quinapyramine sulfate is the most effective trypanocide against these two trypanosomes, but this drug is often poorly tolerated and is likely to cause serious local reactions and general disorders. It is therefore ad­visable to administer the dose in two or three parts at six-hour intervals.

CAMEL TRYPANOSOMIASIS

Quinapyramine sulfate is the pre­ferred treatment for T. evansi try­panosomiasis in camels, but suramin sodium is still used in many coun­tries although its cost is markedly higher and cases of drug resistance have been observed. Suramin-resis-tant strains of T. evansi remain sen­sitive to quinapyramine.

Conclusions

Several drugs are now available which are highly effective (except in the case of T. simiae) and easy to use; but for each of these products there are specific instructions which must be observed. Care and expert advice must always be taken before any large-scale treatment is started.

Cattle being injected with a drug to control trypanosomiasis

PART II. CHEMOPROPHYLAXIS AND THE RAISING OF TRYPANOTOLERANT LIVESTOCK

P. FlNELLE

The first article in this series exam-ined the possibilities afforded by direct action on the parasites responsible for animal trypanosomiasis by the use of trypanocidal drugs on diseased animals. This second paper reviews two other methods of control: pro­tecting susceptible animals by the use of preventive drugs, and making use of the natural resistance of certain breeds of cattle to trypanosomiasis.

Chemoprophylaxis

There are three trypanoprophylactic drugs which can be used: quinapyra-mine prophylactic (Antrycide Prosalt), pyrithidium (Prothidium), and isome-P. Finelle is Animal Health Officer, Animal Production and Health Division, fao, Rome. tamidium (Samorin-Trypamidium). Quinapyramine can also be used in complex forms with suramin, but as this formula is not on the market it must be prepared by the user as follows:

The methods of using these trypano­prophylactic drugs are shown in Table 1.

CATTLE TRYPANOSOMIASIS

T. congolense, T. vivax, T. brucei

Quinapyramine (Antrycide Prosalt) was the first trypanoprophylactic drug that was sufficiently active for use in common practice. However, it has fallen into disuse because of the frequent appearance of drug-resistant trypanosome strains. Moreover, its prophylactic action, extending over two to three months, is considerably less than that of more recent products. Isometamidium and pyrithidium af­ford protection ranging from three to six months, depending on the risk. In principle it would be advisable to make a preliminary trial in each case in order to determine the treat­ment rate. In practice, a four-month cycle may generally be adopt­ed — three injections per year. Iso­metamidium is most frequently used, particularly because of its lower cost. As in curative treatment, and especially since higher doses are administered for prevention, it is advisable to give the injection in a muscle where the local reaction is not likely to affect the price of the carcass substantially For large animals it is also advisable to divide the dose so as not to inject more than 15 ml per injection site. If trypanosomes reappear before an­other preventive injection has been given, a curative treatment with diminazene should be administered so as to eliminate theisometamidium-resistant trypanosomes.

Table 1. Use of trypanoprophylactic drugs

¹ IM = intramuscular injection; sc = subcutaneous injection.

² May and Baker Ltd.

³ Specia.

⁴ Boots Pure Drug Co. Ltd.

⁵ Imperial Chemical (Pharmaceutical) Ltd.

T. evansi

In areas where T. evansi is prevalent, quinapyramine prophylactic (An-trycide Prosalt) can be used.

PROPHYLACTIC TREATMENT OF SLAUGHTER CATTLE

In Africa, tsetse-free livestock pro­duction areas are often located far from the large cities; this means that slaughter animals have to travel a long way to market, often through tsetse-infested zones. These journeys, usually made on the hoof, frequently last for several weeks during which the animals may contract trypano-somiasis that is all the more acute because the cattle come from regions free of infection and therefore have no immunity. Moreover, their re­sistance is lowered by travel stress. It is therefore necessary that trypano-prophylactic treatment be adminis­tered before livestock intended for slaughter enter tsetse-infested areas. Because (a) a large number of ani­mals are to be treated at low cost, (b) a comparatively short period of protection is required (about one month), and (c) drug resistance is unlikely since the animals are to be slaughtered, the following drugs may be used:

TRYPANOSOMIASIS IN SMALL RUMINANTS

Chemoprophylactic treatment of try­panosomiasis in small ruminants is rare, but it appears that the measures indicated for cattle can be applied equally to these animals.

Figure 1. Herd of N'dama trypanotolerant cattle in Ivory Coast. These cattle are humpies s, and their coats are light fawn in colour.

TRYPANOSOMIASIS IN PIGS

For the prevention of T. simiae in­fection in pigs, the following can be used:

EQUINE TRYPANOSOMIASIS

T. congolense, T. vivax, T. brucei

Isometamidium and pyrithidium can be used for horses and donkeys under the same conditions as for cattle, although such treatments may cause temporary lameness. It is advisable to administer deep intramuscular in­jections and to divide the dose if a large amount is to be injected.

T. evansi

Quinapyramine (Antrycide Prosalt) is the most effective, but this product causes serious local reactions in horses. The protection period is from three to four months.

TRYPANOSOMIASIS IN CAMELS

Quinapyramine can also be used to prevent T evansi trypanosomiasis in camels.

Drug resistance

The discovery of trypanocidal drugs with preventive action raised high hopes that their use would make it possible to turn subtropical Africa into a flourishing livestock produc­tion area. It must be admitted that most of these hopes have not been realized. Although these drugs do provide protection, which in some conditions may last up to six months, all of them frequently give rise to the formation of drug-resistant try-panosome strains. This drug resis­tance occurs when the'trypanosomes are in contact with a trypanocide administered in a subcurative dose insufficient to ensure the destruction of the parasites. This situation may be due to one or more of the follow­ing factors:

1. the application of insufficient doses, due in particular to underesti­mating the weight of animals;
2. the formation of abscesses fol­lowed by partial rejection of the drug;
3. a cyst-forming reaction which prevents the diffusion of the product;
4. preventive treatments at too long or irregular intervals;
5. halting the application of try-panoprophylactics while the animals are still exposed to the risk of in­fection ;
6. the occasional use of preventive drugs in curative treatments.

Trypanoprophylactic drugs should therefore be used with considerable caution, especially since there is a cross drug resistance between various trypanocides and drug-resistant try-panosome strains which may persist for a long time even after passage through tsetse. In fact, these drugs can be used without danger only on controlled livestock, where it can be certain that the treatment rate and application requirements will be fully observed. These prerequisites sharply limit the possibilities of applying chemoprophylaxis under traditional African livestock production conditions.

Figure 2. Herd of small short-horned humpless cattle in Dahomey. This breed is the second of the trypanotolerant breeds in west Africa, where it is known by different names.

Raising of trypanotolerant livestock

The low susceptibility of some west African cattle breeds to trypano-somiasis has long been known. Early workers observed that such livestock were able to survive and thrive in areas infested with tsetse where other breeds, especially zebu, could not exist.

These trypanotolerant livestock are the small, humpless cattle of west Africa, of which there are two distinct breeds. One is the N'dama, which seems to have originated in the Fouta Djallon massif in Guinea and whose area of distribution covers southern Senegal and Mali, Guinea, north­western Ivory Coast, and northern Ghana. The horns of these ani­mals are lyre-shaped and the tawny coat is characteristic. The other breed, according to the region in which it is found, is called Baoule, Laguna, Samba, Muturu, Dahomey, and west African short-horned cattle. It is found in Ivory Coast, Ghana, Dahomey, Togo, and in the southern regions of Mali, Upper Volta and Nigeria. These cattle are smaller than the N'dama and more powerfully built; their coats are usually black or piebald black, and they have short pointed horns. The areas of distribution of these two breeds and the zebu are often poorly defined, and many crossbreeds can be found.

There is still very little known about trypanotolerance. It seems to depend on two groups of factors: hereditary and acquired characteristics.

HEREDITARY CHARACTERISTICS

Trypanotolerance is a feature of the small, humpless cattle of west Africa. By studying the behaviour of zebu cios=breds it has been shown that the ujscfjtibility of these animals to trypanosomiasis is intermediate between that of pure humpless and zebu breeds and is approximately proportional to the degree of zebu blood.

ACQUIRED CHARACTERISTICS

Humpless cattle raised in tsetse-free areas have no resistance to trypa­nosomiasis and behave like those of other breeds; their serum does not contain antibodies and when they become infected the course of the disease is acute and results in death. Trypanotolerance is therefore in part an acquired immunological phenom­enon. It is also relative and may break down in certain conditions, particularly in the case of too fre­quent infections which may succeed in overcoming the animal's immuno-logical defences. Moreover, all the causes capable of affecting the pro­duction of antibodies can also reduce it or cause it to disappear. These include malnutrition, overwork, in­testinal parasitism and infectious diseases.

The mechanism of trypanotolerance may therefore be explained as follows: the trypanotolerant breeds have a hereditary capacity to produce try-panosome antibodies; but the pro­duction of antibodies is set off by infections contracted while the young animal is still protected by the mother's antibodies. Subsequent production of antibodies is maintain­ed and strengthened by subsequent infections, but it can be reduced and even eliminated by all the factors which exert an unfavourable action on the immunological defences.

Introduction of trypanotolerant live­stock

The west African trypanotolerant livestock which have been imported into the central African countries have enabled a significant develop­ment of livestock production in areas unsuited to the raising of zebu and where there had been no cattle production previously. The number of trypanotolerant livestock in various countries of central Africa can be estimated at about 220 000 head in Zaire, 35 000 in Congo, 15 000 in the Central African Republic, and 5 000 in Gabon.

Two systems for the introduction of trypanotolerant animals can be adopted. In the first, imported breed­ing stock is assembled on ranches, and the increase in the herd, the offspring, is distributed to the farmers. This permits an effective control of the herd and is perfectly suited to N'dama cattle, which respond well to ranching. The short-horned cattle however, appear to settle better in small herds. In the second system, the breeding stock is distributed directly to the farmers, which has the advantage of immediately involv­ing the village people in the opera­tion. A farmer is given several females and a bull, which are to be repaid in cattle later as the increase in his herd makes this possible. These will be used to start new herds. Whichever system is applied, the operation is faced with technical and human problems.

Figure 3. Crossbreeding between N'dama and west African short-horned cattle is frequent. This crossbred Dahomian heifer with short horns and a fawn coat is a good example.

TECHNICAL PROBLEMS

These are primarily the problems con­nected with any importation of ani­mals, in particular the danger of intro­ducing contagious diseases: rinder­pest and contagious bovine pleuro-pneumonia. Since trypanotolerance is related to local strains of trypano-somes, the transferred animals may be susceptible to other strains. It is therefore advisable to ensure strict sanitary inspection during the first few months after importation. If necessary, trypanocidal treatments should be given to help the animals overcome trypanosome infections and enable them to adapt their production of antibodies to new strains against which they have no immunity.

HUMAN PROBLEMS

Trypanotolerant livestock are usually distributed in areas where cattle husbandry is a completely new ac­tivity. The operation therefore re­quires considerable organization and resources, at least for the first few years, its success depends on the training of the new stock-raisers. Under prose i': conditions, the in­troduction of" y anotolerant cattle is one of the most Hxtive methods for developing livestock production in countries where tr/panosomiasis is prevalent. It is costly, requiring considerable personnel, and is slow to start, but these drawbacks are more than offset by the results, which are permanent, whereas the methods considered previously, chemotherapy and chemoprophylaxis, must be repeated constantly.

A fourth method, vector control, can also be employed, and will be the subject of the third article in this series on African animal trypano-somiasis.

PART III. CONTROL OF VECTORS

P. FlNELLE

Deforestation and bush clear­ing — an indirect method of tsetse control.

Tsetse flies are the chief vectors of African trypanosomes, and also serve as intermediate hosts in which the parasites multiply actively. Various methods may be used to control these vectors: indirect meth­ods, that attempt to alter the environ­ment so as to make it an unsuitable habitat for tsetse flies, and direct methods — chemical and biological — aimed at destroying the insects or at eliminating their ability to breed.

INDIRECT METHODS

Deforestation

The microclimate that is established by plant cover provides the most suitable combination of temperature and humidity for the tsetse fly because it limits variations in climate to a minimum. The fly concentrates in certain types of vegetation, which vary for the different tsetse species. When this vegetation is cleared, changes occur in the microclimate that may cause the species concerned to disappear. Use of this selective de­forestation method therefore requires a very precise knowledge of the biology of the species concerned in the prevailing conditions.

Destruction of the vegetation can be done manually, by felling the trees, or by ringbarking in the case of plant species for which this technique is effective. Mechanical means can be employed with quicker results, but these can only be used in flat country. The use of arboricides has not proved very practical as they are expensive and slow-acting products that do not work well with all plant species.

Regardless of the technique used, the selective destruction of vegetation presents two major drawbacks: it is generally a very expensive oper­ation and it increases soil erosion, which is liable to cause sterility in the cleared land. This method is now seldom employed, except to establish deforested barriers to prevent areas cleared by insecticide spraying from being reinvaded.

Elimination of wild animals

As well as being trypanosome car­riers, wild animals are an important source of food for the tsetse fly. Studies of these hosts have shown that the tsetse fly obtains much of its nourishment from a small number of wild animal species, which differ for each species of fly. A control method has therefore been developed with the aim of eliminating the pre­ferred hosts. This method has been employed to a considerable extent in some countries of east and south­ern Africa with noteworthy results, but at the cost of the massive de­struction of big game.

However, the elimination of wild animals, even if restricted only to host species, is not easy to accomplish, especially when it involves destroying small animals like the warthog, the bush pig and the small antelope which are the favourite hosts of many testse species. It has also been observed that the tsetse is not rigidly dependent on specific animal species, and that when the preferred hosts disappear it can feed on other species.

Because of these difficulties, and the increasing concern for wildlife pro­tection, the control of tsetse fly by the selective elimination of wild ani­mals is not to be recommended at present.

DIRECT METHODS

Insecticides

The treatment of tsetse-infested zones with insecticides is currently the most common method of eradication. Insecticides may be applied from the ground or from the air.

APPLICATION FROM THE GROUND

The method consists in applying a persistent insecticide where it has the most chance of coming into con­tact with tsetse flies, that is, on their most frequent resting places. The insecticide is applied in the dry sea­son, not only to prevent it from being washed off by rain but also because the severe conditions prevail­ing in this season force the flies to concentrate on certain types of plant which provide a more favourable microclimate. Only one treatment is applied; the insecticide must there­fore have sufficiently long persis­tence, exceeding the maximum pupa­tion period, to act on the newly hatched insects coming from pupae deposited before the treatment.

The insecticides used are usually chlo­rinated hydrocarbons, chiefly DDT and dieldrin, both of which persist for several months on vegetation. DDT is more frequently used in regions with a Sudanian climate and a long dry season, while dieldrin is preferred in more humid regions with a Gui-nean climate. DDT is applied in the form of wettable powders or emul-sifiable concentrates, diluted to ob­tain a final concentration of be­tween 2 and 5 percent according to the needs of the region. Dieldrin is used in concentrations varying between 1.8 and 2 percent, obtained from an emulsifiable concentrate. Insecticides can be applied with pressurized sprayers, or motorized or high-capacity vehicle-mounted sprayers. The choice of equipment is essentially a matter of convenience in use, according to the local vegeta­tion and the physical features and extent of the area to be treated.

Ground spraying with portable prepressurized sprayers.

The preferred resting sites of the tsetse fly vary with the different spe­cies, the season and local ecological conditions. It is therefore essential to have accurate knowledge of the biology of the species in the region to determine the types of vegetation to be treated. In Nigeria, for exam­ple, in the Sudanian regions infested by Glossina morsitans submorsitans and G. tachinoides, a 2 percent DDT solution is applied exclusively on the supports where the tsetse flies rest during the hottest hours at the end of the dry season. In the case of G. morsitans, these supports are shady tree trunks with a diameter of over 20 centimetres from ground level to a height of about 1.5 metres. For G. tachinoides it is necessary to treat all tree trunks, visible roots overhanging the banks of streams and woody vegetation near water, all up to a height of about 1 metre. When forest galleries are relatively narrow and well separated from both banks of streams, only 5-metre strips are treated along each bank in the case of G. tachinoides and 10-metre strips for G. morsitans. If the forest is broader and the banks are not clear, treatment is effected in strips, about 20 metres wide, along the outside edge of the forest and inside the forest galleries in the direction of the stream, at intervals of about 100 metres. In flood plains, where the forest is divided into thick­ets, only the edges of the thickets and narrow parallel strips inside them, about 20 metres apart, are sprayed. Operations of this kind in northern Nigeria have given highly satisfactory results, and the zones where tsetse flies have reappeared and require further treatment do not exceed 1 percent of the total area treated.

The technique of selective spraying with persistent insecticides using ground equipment has been and con­tinues to be widely and successfully employed in several countries against various species of tsetse fly. It has been most extensively applied in northern Nigeria, where it has led to the clearing of some 125 000 square kilometres, and where the programme is continuing at the rate of about 12 500 square kilometres a year. It is an attractive method because of its effectiveness, its rela­tively low cost, and because it results in reduced environmental contami­nation. It should be stressed, how­ever, that it requires thorough prior studies of tsetse fly ecology to deter­mine the conditions for using in­secticide, and large, well-equipped and well-trained spraying teams, as well as a dense network of roads and tracks. The method is of real value only in regions where the habitat of the tsetse fly is relatively restricted, at least for part of the year. These various conditions, which cannot often be met, have led to the adoption of aerial spraying, which gives quick results and requires limited personnel.

AERIAL SPRAYING

The first attempts to control tsetse fly by the aerial spraying of insecticide were made in 1948 in Tanzania, and the first large-scale operation was carried out shortly afterwards in South Africa, leading to the elimina­tion, although at a very high cost, of G. pallidipes in the Zululand region. Research, particularly in Tanzania, has led to improvements in this tech­nique and to appreciable reductions in its cost. The .insecticide can be applied either as an aerosol without residual action, distributed over the whole infested zone, or as a deposit, with a persistent effect, on the pre­ferred resting sites only.

Ground spraying with portable motorized sprayers.

Aerial spraying.

Aerosol

The aerosol method has been used in various countries, including Rwanda, Kenya and Tanzania, but has been applied most widely in Zambia.

The insecticide is sprayed in the form of fine droplets with a diameter be­tween 10 and 60 \im, which because of their lightness remain suspended in the air and can penetrate through the vegetation and reach the adult tsetse flies. However, the insecticide is not deposited on the vegetation and has no persistent action, so the treatment must be repeated to reach the tsetse flies which had been in the pupal stage at the time of the first operation.

DDT, BHC, dieldrin, endosulfan, iso-benzan, fenthion and pyrethrum have been used. Light single-engined air­craft are generally employed for these operations, fitted either with heat generators working off the engine exhaus| pipe or with rotary sprayers. In Zambia, heavier two-engined air­craft, which allow more rapid operation, are used successfully. Re­cent advances in aerial spraying tech­niques have enabled considerable reductions to be made in rates of ap­plication, with highly concentrated insecticides; in Zambia, endosulfan is sprayed at the rate of 30 grams per hectare. Operations are carried out during the dry season, at a time when many trees lose their leaves, and at hours when the meteorological conditions are favourable, at dawn and just before sunset. The air­craft are guided either by ground teams using radio or markers, or by the aircraft's own navigating equip­ment.

Aerial sprayings are repeated four or five times, at three-week intervals. The conclusion to be drawn from the various, operations for controlling tsetse flies by the aerial spraying of nonpersistent insecticides is that total elimination is possible provided the treated area is suitably isplated from other contaminated regions. In Zam­bia aerial spraying has led to the eradication of G. morsitans from nearly 15 000 square kilometres at a lower cost than spraying with ground equipment. However, the method suffers from several disadvantages: it requires total coverage of the tsetse-infested zone and all that this implies from the point of view of environmental pollution; it requires several applications staggered over about 100 days; the insecticide, which is dispersed in the form of fine drop­lets, is often carried away by the wind to areas outside those to be treated. Because of these disad­vantages the application of long-lasting insecticides is sometimes pre­ferred.

Persistent insecticides

Various tests involving the aerial spraying of persistent insecticides have been made, using an inverted-emulsion insecticide or by selective spraying from helicopters.

Inverted emulsions. In these tests, performed in Kenya under a who/ fao project, dieldrin was used as a water-in-oil emulsion instead of the normal oil-in-water formula. With these inverted emulsions, in which the oil phase is the continuous one, evaporation is limited and the drop­lets are coarser and less sensitive to atmospheric conditions, so that they reach the target zone with greater accuracy. The vegetation of the region chosen was composed of very dense thicket infested by G. pal­lidipes. Applications were made from aircraft and helicopters, but the latter proved to be more expensive. The results of these preliminary tests proved that the technique was ef­fective in the conditions prevailing in the region, and that its cost was competitive with that of the other methods. The tests should be con­tinued and extended to other species and other types of vegetation.

Selective spraying. The principle un­derlying the selective spraying of persistent insecticides by helicopter is similar to that of selective spraying using ground equipment: the aim is to treat only the vegetation used as a refuge by the tsetse fly, when weather conditions for the insect are most severe. This method has been suc­cessfully employed in northern Nige­ria in regions infested by G. morsitans submorsitans. A helicopter flying at a low speed (about 40 kilometres an hour) 1 or 2 metres above the treetops sprays (in the dry season) 10 percent dieldrin in the form of droplets with a diameter varying between 90 and 200 (xm, over a width of about 20 metres, at the rate of 1.5 kg of active product per hectare. This technique has turned out to be effective, but expensive if the price is related to the area treated. How­ever, if account is taken of the fact that only 10 percent of the total area is effectively treated, the cost of this method calculated in terms of the cleared area is comparable to that of unselective spraying by aircraft.

PROBLEMS OF CHEMICAL CONTROL

Whatever the technique used, chem­ical control of the tsetse fly raises several general problems.

Isolation of the cleared region

It is obviously necessary to prevent the cleared region from being reinvad-ed by insects from neighbouring con­taminated areas. This can be achieved either by establishing barriers which have been deforested or treated with persistent insecticides and which are sufficiently broad to prevent cross­ing by the tsetse fly, or simply by re-treating the edge of the already treated zone the following year. It is also necessary to check the move­ments of vehicles, livestock and pos­sibly of game, all of which can transport tsetse flies over great dis­tances.

Pollution

Chemical control of the tsetse fly certainly raises the pollution level of the area concerned. The insec­ticides used, usually chlorinated hydrocarbons, are toxic to other insects, including useful species such as bees or predatory insects. They are also very toxic to fish. Their toxicity as far as birds and mammals are concerned is still not very clear, but it is known that they can ac­cumulate in fats.

It must be noted, however, that the levels and quantities of insecticides used for tsetse fly control are much lower than those for controlling crop parasites, and hence account for only a very small part of the general pollution caused by pesticides. Nevertheless, it is highly desirable that new insecticides should be tested with the aim of finding a product with a more selective action and with fewer effects on the environment than the organochlorinated com­pounds currently used. Biological control methods may also provide a solution to this problem.

Biological control methods

The biological control of tsetse flies is as yet only in the experimental stage. Two methods, which have been proved against other insects, may be considered: the use of organisms pathogenic to tsetse flies, and genetic control.

PATHOGENIC ORGANISMS

Little is known about the pathology of tsetse flies. While there is in­formation concerning insect parasites which prey on tsetse pupae and which in nature certainly play a part in limiting the number of flies, so far there has been no success in breed­ing these parasites in the labora­tory.

Likewise, no work has been done on organisms that are pathogenic to tsetse flies, an approach which has been so promising in the control of other insect species. These matters should receive the very close attention of research laboratories.

GENETIC CONTROL

Research on the genetic control of the tsetse fly has made great strides since it became possible to breed these insects in the laboratory on a large scale. It is now possible to consider methods involving induced sterilization or the transmission of lethal genes. The underlying prin­ciple is that as the female tsetse fly generally copulates only once at the beginning of its life, it will produce no progeny when inseminated by a male whose spermatozoids have un­dergone chromosomal modifications that render them incapable of fertiliz­ing the egg. By releasing a sufficient number of sterile males in a region so that they have a greater chance of mating with the females than the existing normal males, a reduction and eventually an extinction of the population through reduced numbers of progeny can be achieved.

Contact with various chemical prod­ucts can cause sterility in males, and the gamma irradiation of adult males has the same effect. Laboratory studies have shown that although the longevity of the males is reduced, they live long enough to copulate several times and retain their ability to mate. The advantages of this system are obvious. The species is used to destroy itself, without dis­turbing the natural biological equi­librium. The males seek out the females and can track them down in places inaccessible to man.

The application of this method re­quires a large number of males, which is now possible through recent improvements in laboratory breed­ing techniques. One important prob­lem which remains to be solved concerns the behaviour of the arti­ficially bred insects when they are released in a natural environment. First experiments suggest that after some days of adaptation the sterilized males tend to behave like normal insects, although their longevity is significantly curtailed. However, the practical and economic feasibility of this method can be fully established only after pilot tests have been carried out on the most important tsetse fly species.

The main factor governing feasibility is the size of the natural tsetse fly population. It will certainly be ef­fective to release sterile males after the population has been reduced by treatment with a nonpersistent in­secticide. From this point of view, the release of sterile males may be regarded as a supplement to chemical control, opening the way to the complete elimination of the tsetse fly after a brief and nonpolluting insecticide treatment.

Conclusions

The methods of controlling animal trypanosomiasis are numerous and varied; each possesses advantages and disadvantages and these must be assessed in the light of local data and the end results that are sought.

In the fourth and final article in this series, the economic problems raised by animal trypanosomiasis and its control will be considered, in order to assess the relative costs and benefits of the various methods.

PART IV. ECONOMIC PROBLEMS

P. FlNELLE

In the previous three articles the methods that can be used to control African animal trypanosomiasis and its principal vector, the tsetse fly, have been examined. However, con­trol of the disease is not exclusively a technical problem because, as the Joint fao/who Expert Committee on African Trypanosomiasis has stated: "The problems caused by the disease should be viewed against the wider background of the general social and economic needs of the countries concerned."

Trypanosomiasis control is expensive. Before large-scale operations are undertaken, an economic analysis is necessary for the region in question to show the extent of the socio-economic losses due to the disease, to determine the priority of trypano­somiasis control in development plan­ning, and to furnish the data needed to estimate the economics of pos­sible control methods by a com­parison of their cost with the results that can be expected of them.

Socioeconomic consequences

It may therefore be useful to assess the socioeconomic consequences of African animal trypanosomiasis, and the cost of the different control methods.

INFLUENCE ON LIVESTOCK DISTRIBUTION IN AFRICA

The number of cattle in Africa is assessed at around 160 million head, but distribution is very uneven among the various regions. Two phyto-climatic zones are virtually unsuitable for cattle raising, owing to lack of pasture: the desert or semidesert zone roughly corresponding to the regions where annual precipitation is less than 300 millimetres, covering about 12 million square kilometres, and the zone of dense rain forest, estimated to cover 3 million square kilometres. It can therefore be as­sumed that of the 30 million square kilometres of the African continent only half carry pastures suitable for livestock raising. These are the areas covered by wooded steppe or savanna. Yet even in these 15 million square kilometres, with their obvious suit­ability for grazing, livestock produc­tion is very unevenly distributed. While the number of cattle exceeds 20 per square kilometre in the most favoured places, there are areas which are completely devoid of livestock despite the presence of good-quality grazing and plentiful water supplies. This anomaly is partly due to animal trypanosomiasis, which occurs in tropical Africa over about 10 million square kilometres. As the infected areas include the 3 million square kilometres covered by equatorial forest, the land suitable for grazing in which livestock raising is limited by animal trypanosomiasis can there­fore be estimated at 7 million square kilometres.

The consequences of animal trypano­somiasis vary in gravity from place to place. Broadly speaking, there are:

In every case, however, trypano­somiasis leads to considerable under-exploitation of natural resources, and to a lower level of animal produc­tion than could be achieved if the disease were eliminated.

The influence of the tsetse fly on animal production is nowhere more clearly illustrated than in Tanzania, where the geographical pattern of cattle distribution is almost exactly the opposite of that of tsetse distribution.

ANALYSIS OF SOCIOECONOMIC CONSEQUENCES

The socioeconomic importance of African animal trypanosomiasis is extremely difficult to assess, as the data available are fragmentary and frequently very approximate. In the Present state of knowledge it is possible only to enumerate the various consequences of trypanosomiasis, in the hope that this list can serve as a base for assessments at the local level. Two sets of consequences, direct and indirect, can be identified:

1. The direct consequences, represent­ed by the economic losses due to the disease and to the various expen­ditures incurred in controlling it.

1. mortality;
2. disease, which manifests itself in emaciation, retarded growth, abortion, temporary sterility and various organic lesions;
3. the cost of detection and treat­ment of infected animals (veterinary service personnel, trypanocidal drugs, equipment, operating expenses);
4. the cost of preventive operations (chemoprophylaxis, tsetse fly control, development of trypanotolerant live­stock);
5. the cost of research on animal trypanosomiasis control.

2. The indirect consequences of ani­mal trypanosomiasis affect:

1. human health, as the shortage of meat and milk causes protein deficiencies which are particularly harmful to children;
2. agriculture, because the lack of draught animals and manure reduces agricultural output;
3. livestock production: (i) trypa­nosomiasis limits the possibilities of introducing improved breeds, which are highly sensitive to this disease, thus preventing the upgrading of local livestock by crossing with im­ported sires; (ii) the presence of try­panosomiasis causes livestock to be concentrated in limited grazing areas, which results in their overuse and deterioration; (Hi) seasonal variations in the incidence of trypanosomiasis prevent some pastures from being grazed throughout the year and com­pel herdsmen to practise transhu-mance, which holds them back from integration in the national com­munity;
4. the economy: the deficit in animal production compels countries where trypanosomiasis is rife to resort to imports of meat and dairy prod­ucts, a practice harmful to their balance of trade.

TENTATIVE ESTIMATE OF DEFICIENCY TO BE MADE UP

While it is impossible to make even a rough assessment of the various socioeconomic consequences of tiy-panosomiasis, one can nevertheless try to estimate the meat production of the regions concerned, if the disease were controlled, on the basis of the following criteria:

Although very approximate, this esti­mate shows how animal trypanoso­miasis control could contribute to the development of animal production at a time when demand for animal protein, especially beef, is constantly growing and when projections indi­cate a serious shortfall in the years to come.

Cost of controlling African animal trypanosomiasis

Estimating the cost of controlling trypanosomiasis is no easier than the other estimates of the cost of the disease, as some expenses cannot be accurately quantified. The fig­ures available are not always com­parable because they are calculated according to criteria that vary with each country. Therefore, the data given here aim only at supplying an order of magnitude, which will allow an assessment and comparison of the average costs of the possible control techniques.

CHEMOTHERAPY

Approximate cost of a curative dose for a 300-kg bovine animal:

Diminazene (3.5 mg/kg) 15 cents Isometamidium (0.5 mg/kg) 19 cents

The cost of application is difficult to calculate, as it must include a pro­portion of the costs of the veterinary service and of its budget (peisonnel, equipment and operation) devoted to trypanosomiasis detection and treatment. However, this can be estimated to be around 50 cents. It may therefore be assumed that the cost of curative treatment for a 300-kg bovine animal varies between 65 and 70 cents.

CHEMOPROPHYLAXIS

Approximate cost of a preventive dose for a 300-kg bovine animal:

Isometamidium (1 mg/kg) 38 cents Cost of the opetation 50 cents

As preventive treatments must be repeated on average every four months, the annual cost of chemo-prevention for a 300-kg bovine ani­mal would be about US$2.65.

INTRODUCTION OF TRYPANOTOLERANT LIVESTOCK

An analysis of the cost of importing trypanotolerant livestock was made in 1966¹ following the import into the Central African Republic of 254 animals from Upper Volta and Ivory Coast. The expenditure was broken down into the following percentages:

The average purchase price of an animal was US$40, and its total aver­age imported cost was US$247 (1966).

TSETSE FLY CONTROL

Data on the costs of tsetse fly control are numerous but difficult to compare because they depend on many factors, especially: the evaluation of the area of the cleared zone and its relation to the area actually treated, the ac­counting system used for certain ex­penses (management personnel, equipment amortization), the ex­penses involved in preliminary sur­veys, subsequent surveillance and conservation measures, and the utili­zation of the cleared zone.

Variations in the parity of the different cunencies involved also make com­parisons difficult.

Deforestation

It is impossible to state average costs for deforestation operations because of their great variation. An example is the cost of deforested barriers in northern Nigeria in 1970, which varied between $3 500 and $4 200 per square kilometre.

Ground spraying

Costs vary greatly with the region and the species of tsetse fly. Some recent examples are given in Table 1. Air spraying

Table 2 gives the average cost of the various methods based on recent operations.

These data show how difficult it is to forecast the cost of tsetse control operations. Consequently, it would be hazardous to advise which tech­nique would be the most economic in a given area. Recommendations can be made only after pilot trials have been carried out.

Table 1. Cost of ground spraying

Table 2. Cost of air spraying

Conclusions

This attempt to analyse the economic problems raised by African animal trypanosomiasis shows that accurate data are so limited that it is almost impossible at present to draw up even an approximate report.

Aware of these significant limitations, fao plans to undertake a two-year study which will include a number of local surveys in carefully selected regions. The study will furnish the basic data for an assessment of the socioeconomic importance of try­panosomiasis and the costs of the various methods used to control it, and should draw the attention of interested governments and assistance organizations to the necessity for a very substantial increase in funds for field operations if the disease is to be controlled to an extent that would allow a significant expansion in animal production.

References

FAO. 1969. African trypanosomiasis: report of a joint FAO/WHO Expert Committee. Rome, fao Agricultural Study No. 81.

International Scientific Council for Trypano Research and Control. 1971. Thirteenth meeting. Lagos, OAU/SCTR. Publication No. 105.

Mulligan, H.W. 1970. The African Trypanosomiases. London, George Allen and Unwin Ltd.

Park, P.O., Gledhill, J.A., Alsop, N. & Lee, C.W. 1972. A large-scale scheme for the eradication of Glossina morsi-tans morsitans Westw. in the Western Province of Zambia, by aerial ultra-low volume application of endosulfan. Bull. ent. Res,, 61: 373-384.

^* Animal Health Officer, Animal Produc­tion and Health Division, fao, Rome.

P. Finelle Animal Health Officer, Animal Production and Health Division, fao, Rome.

P. Finelle is Animal Health Officer, Ani�mal Production and Health Division, fao, Rome.

P. Finelle is Animal Health Officer, Ani­mal Production and Health Division, fao, Rome.

¹ Lacrouts, M., Sarniguet, J. & Tyc, J. Le cheptel bovin en République centrafri-caine. Paris, Secretariat d'Etat aux affaires étrangères, 1966.