7.1 Principles of control: Nematodes
7.2 Principles of control: Trematodes
7.3 Principles of control: Cestodes
7.4 Anthelmintics
7.5 Anthelmintic resistance
7.1.1 Parasite species present
7.1.2 Herd structure and grazing management
7.1.3 Availability and abundance of infective larvae on pasture
7.1.4 Type of climate
7.1.5 Genetic resistance
7.1.6 Control of gastro-intestinal nematodes
7.1.7 Control of lungworms
7.1.8 Control of filarial nematodes
7.1.9 Control of Toxocara vitulorum
The principle of a parasite control strategy is to keep the challenge to young livestock by the pathogenic trichostrongyle parasites at a minimum rate. This is achieved in the following ways.
(a) Controlling the density of livestock (stocking rate). Overstocking forces the animals to graze closer to faecal material and closer to the ground, and may result in the consumption of a higher number of infective larvae.(b) Periodic deworming.
(c) Strategic deworming when conditions are most favourable for larval development on the pasture.
(d) Separating age groups in the more intensive production systems.
(e) Reducing the effects of gastro-intestinal parasites by assuring an adequate plane of nutrition. Control programmers should reduce the effect of parasites to sub-economic levels.
(f) Using grazing management to minimize the uptake of infective larvae and to create safe pastures.
The development of such programmes requires a thorough knowledge of the types of parasites present (including their biology and epidemiology), herd structure and grazing management, parasite seasonal availability and survival and the weather conditions in particular areas.
Most infections are mixed infections and involve several species of gastrointestinal parasites. The pathogenicity is usually high when Haemonchus, Mecistocirrus, Trichostrongylus and Oesophagostomum are present. The presence of Haemonchus, especially in sheep, requires immediate control measures to prevent severe weight losses and mortality. Ostertagia may be a severe problem in certain areas, especially at higher altitude. According to the season, the ingested infective larvae of Haemonchus, Trichostrongylus and Ostertagia may become inhibited in the mucosa of the gastro-intestinal tract. This usually happens prior to the dry season. Development continues at the beginning of the following rainy season. Cooperia and Trichuris ovis are in most cases relatively non-pathogenic. Faecal egg counts should always be obtained on a herd or flock basis.
If possible data should be acquired on the number of animals, the broad age structure of the herd/flock and the time of calving/lambing in relation to the rainy season(s). Furthermore information should be obtained on the grazing management practiced as the control measures may differ for animals grazing communal lands, confined animals (zero grazing), tethered animals, animals in crop rotation systems and herds/flocks under transhumant/nomadic systems.
If herbage larval counts or tracer animal studies have been performed at regular intervals and grass samples picked at the same time of the day on each sampling occasion, a pattern of seasonal availability of (L3) can be established for any particular pasture. Similarly, trends in relative abundance of (L3) by season and by location can be established. On the basis of these results, the timing and frequency of anthelmintic treatments can be proposed.
Strategic control programmes are based on the seasonality of development and survival of (L3) on the pasture. This is strongly dependent upon climate. Climatic data from each study site or from local meteorological stations are therefore required.
7.1.5.1 Parasite resistance within breeds
7.1.5.2 Parasite resistance between breeds
With the widespread development of resistance to anthelmintics and the high cost of developing new drugs, the interest in exploiting and developing animals that are genetically resistant to helminth parasites has increased considerably. Studies have shown that resistance is heritable in cattle, sheep and goats but this approach to the control of helminths is currently being exploited principally in sheep. Two strategies can be followed in the development of genetically resistant sheep; one is the selection for resistant individuals within breeds; the other is the exploitation of sheep breeds which appear to have increased helminth resistance.
It has been shown that within any one breed, lambs of certain rams or ewes have increased resistance to helminth parasites and some selection for resistance is feasible. Based on the results from breeding programmes for resistance against helminths, it has been concluded that faecal worm egg counts are a reasonable comparative indicator on which to base selection of resistant animals with heritabilities similar to those for production. This allows a simple and practical method of-breeding for resistance.
A number of studies have shown that some sheep breeds are more resistant to helminth infections, specifically Haemonchus contortus, than other breeds. The Red Maasai, the Barbados Blackbelly and the St. Croix are examples of breeds which have demonstrated resistance traits.
7.1.6.1 Control in savannah-type climates with one or more distinct dry season(s)
7.1.6.2 Control in arid climates
7.1.6.3 Control in humid climates
The ideal approach is an integration of:
· adjusting stocking rate
· optimum use of safe pastures
· strategic use of anthelmintics
· use of resistant breeds or genotypes
Overstocking is a major problem in large parts of the world particularly in Africa outside the tsetse-infested areas. In addition to contributing to pasture degradation and soil erosion in certain marginal areas, it also forces the animal to graze closer to faecal material which inevitably results in the uptake of higher number of infective larvae. Reducing the stocking rate can significantly reduce the parasite burden of grazing livestock.
Improving grazing management and introducing the safe pasture concept can reduce the use of anthelmintics, minimizing the risk of developing anthelmintic resistance. Ungrazed pastures are parasitologically safe at the end of a prolonged period of dry weather (10 weeks or more). Other types of safe pasture are those used for hay/silage production and those previously grazed by other species. In some countries safe pastures are created by letting cattle graze pasture first, and following with sheep/goats. Grazing different species of livestock together may reduce the overall parasite burden of the species in question but this will not usually be sufficient for efficient parasite control. Fields of harvested cereal crops are also safe. If safe pastures are available, treat young stock with an anthelmintic at the onset of the rains and place them on the safe pastures entirely separated from the older animals.
Based on the seasonality of development and survival of (L3) on the pasture, the timing of strategic anthelmintic use can be determined and integrated into control programmes
With a number of anthelmintics available in several formulations (ready-to-use drench, paste, powder, mineral premix, urea/molasses/anthelmintic blocks, pour on, injectable, slow release and pulse release devices), it is recommended that the ideal formulation is selected by considering availability, price, parasite species to be treated, type of livestock, livestock numbers and type of management.
In many cases a separation of young newly weaned animals from the older animals is not possible and it is important that all animals in the herd/flock are treated. An effort should be made to convince livestock owners using communal grazing areas to include their animals in the programme. It is recommended to treat animals at the beginning of the dry season in order to eliminate the parasite burden, enabling them to better cope with the nutritional stress during the dry season. A treatment prior to the rainy season using a larvicidal drug will prevent the "rains rise" and contamination of pastures at a time when conditions are becoming favourable for egg and larval development.
Older animals may experience an increase in their parasite burdens at the beginning of the rainy season. This "rains rise" is due to further development of previously (end of last rainy season) inhibited larvae in the gastro-intestinal mucosa. If not treated, this "rains rise" results in a heavy contamination of the pasture with eggs, and subsequently (L3) which when ingested by young, susceptible animals may result in severe clinical disease.
More than one treatment at the beginning of the rainy season may be necessary in the following cases:
· all animals in the herd/flock (communal grazing) were not treated and contamination of pasture continues resulting in rapid reinfection of animals· drugs not effective against arrested larvae were used
· the pasture was not parasitologically safe after the dry season
The second treatment if necessary should be given three weeks after the pre-rains treatment. Two successive treatments three weeks apart should prevent livestock from acquiring parasite burdens.
|
NOTE: In sheep, the interval between treatments should be 2 weeks to prevent haemonchosis |
Infection with gastro-intestinal parasites in arid regions is often limited to local areas with surface water or irrigation. Infection over a wider area may occur during the brief intermittent periods of rainfall. Haemonchosis is the main hazard at such times. With the infrequent and/or low level of infection which may occur under these climatic conditions, animals are usually highly susceptible, and disease is often severe. The timing and frequency of anthelmintic treatments under such climatic conditions will vary greatly from place to place. As such, they should be based on results of prior epidemiological studies described earlier.
Humid climates are permanently favourable for the development of infective larvae. In these climates it is important to ascertain levels of parasitism and the epidemiology of the species present in order to determine satisfactorily the frequency and timing of strategic anthelmintic dosing.
The adjustment of stocking rate is very important as a controlling factor in humid climates. If a high stocking rate is maintained, regularly repeated treatments with anthelmintics may be essential. However, regularly repeated treatments throughout the year may not be economically feasible, and a strategic anthelmintic regimen should be devised based on optimal cost/benefit assessments.
The introduction of control programmes for gastro-intestinal nematodes using grazing management and strategic use of modern anthelmintics will often result in partial control of lungworms as well (Dictyocaulus species, Protostrongylus). This effect is enhanced by the use of anthelmintics with a prolonged period of activity (such as ivermectin) or slow-release and pulse-release devices. Several drugs are available in these formulations, including most of the benzimidazoles, immidazothiazoles and ivermectin and these are effective against both adults and larvae.
The pathogenic effect of Mullerius capillaris is usually limited to small sub-pleural nodules and the infection only rarely requires specific treatment. The benzimidazoles are efficient but at 3-5 times the normal dose.
Irradiated (L3) vaccines have been developed for the control of D. viviparus in cattle and D. filaria in sheep and goats. The attenuated larvae are administered orally in 2 doses 4 weeks apart. The animals should preferably be confined during the treatment and for 2 weeks after to allow time for an adequate resistance to develop. The vaccines produce a strong immunity which is maintained if animals are continuously exposed to reinfection.
The principle of control of the filarial nematodes is based on either reducing the number of microfilaria, eliminating the adults, or both.
The most common treatment of Stephanofilaria infections is local application of organophosphate compounds. Broad-spectrum benzimidazoles and ivermectin have been reported to be efficacious.
Onchocerciasis treatment is usually directed towards the microfilaria; diethylcarbamazine and ivermectin both have high activity against these.
Animals infected with Parafilaria can be treated with nitroxynil or high doses of fenbendazole or levamisole, given daily for four days. This will eliminate the adult parasites and reduce the lesions significantly.
The treatment of epidemic cerbrospinal nematodiasis caused by migrating larvae of Setaria species is very difficult. Diethylcarbamazine and levimisole have been used but the results are not conclusive. Early treatment is crucial. Treatment with ivermectin has been shown to be efficacious against adult Setaria.
Newborn calves are infected with T. vitulorum larvae (L3) through the colostrum and milk. The majority of larvae are excreted during the first 10 days post partum. In some parts of the world calves may become infected ingesting infective eggs from the environment. If calves survive the infection they develop a strong immunity against the parasite and the adult parasites are eliminated from the intestine. The principle of control is to prevent the parasites from having an effect on the calves and from contaminating the environment with eggs. Both objectives can be met by treating calves between 10 16 days of age, eliminating the immature parasites before they can harm the calves and start egg production.
Farmers should be advised to treat all calves between 10-16 days of age, repeating the procedure as more calves reach the age for treatment. With large herds this may require a regular weekly treatment programme.
Several efficient drugs are available including piperazine, pyrantel/morantel, levamisole and fenbendazole. The treatment can be administered either as a drench or as a paste. The efficacy of treating cows in an attempt to eliminate somatic migrating larvae of T. vitulorum has not been adequately demonstrated, and is not considered justifiable.
7.2.1 Fasciola hepatica and Fasciola gigantica
7.2.2 Control of paramphistomes
7.2.3 Control of schistosomes
Effective control of most trematode infections is based on strategically applied chemotherapy. Improvements in current farm management can reduce the chances of infections by limiting the contact between intermediate and final hosts. Furthermore, direct action may be taken to reduce or eliminate intermediate host populations. The use of one or more of these measures in an integrated strategy should be based on sound economic assessments of the diseases and the relative merits of control options. Some animal husbandry systems such as zero-grazing (cut and carry) and tethering of animals may minimize the risk of trematode diseases.
7.2.1.1 Strategic chemotherapy of ruminants
7.2.1.2 Chemical control of snails
7.2.1.3 Biological methods of snail control
7.2.1.4 Managemental methods of snail control
Efficient control of fascioliasis requires a well planned and executed, integrated control programme designed for each farm, area, country or region. The available strategies which can be used individually or in combination are:
· Strategic application of anthelminthics, eliminating the parasites from the host at the most appropriate time for effective prevention of pasture contamination.· Reduction in the number of intermediate host snails by chemical or biological control.
· Reduction in the number of snails by drainage, fencing and other management practices.
· Reduction in the risk of infection by planned grazing management.
Seasonal strategic application of effective anthelmintics specific for trematodes, as well timed prophylactic and curative treatments, play an important role in the control of liver fluke infections. Strategic treatments have been developed for several regions of the world based on meteorological data. However, it is advisable to supplement meteorological data with sound epidemiological information in order to improve the timing, and thereby the efficiency, of treatments. The basic principles of strategic anthelminthic application (treatment/prophylaxis) are:
(a) Prophylactic treatment of ruminants towards the end of a period of ecologically reduced activity of the parasites and the intermediate hosts.One treatment is therefore recommended towards the end of a period when larval development in the fluke eggs or in the snails has been retarded, and when the reproductive rate of snails is low or their activity is impaired (such as during a prolonged dry season, or extreme cold). At that time, a prophylactic effect can be achieved by reducing the pasture contamination of eggs before favourable climatic conditions for larval development and snail activity resume.
(b) Curative treatment about one to two months after the expected peak infection of the hosts.
A curative effect can be achieved by one treatment to remove the residual fluke burden acquired from metacercariae which had survived on the herbage.
(c) Additional treatment in highly contaminated areas where seasonal variations do not significantly affect the life cycle of the flukes.
These additional treatments may be required occasionally, when the seasonal climatic conditions are favourable for parasite and snail development, or in areas where high metacercariae intake often occurs as a result of restricted grazing of wet areas during dry seasons.
The most important prerequisite for efficient chemotherapy and chemoprophylaxis is a prior knowledge of the epidemiology of the disease based mainly on meteorological data and seasonal surveys in hosts.
The economics of chemotherapy should be evaluated for each farm, area and country, including assessments of the availability of anthelminthics, their price and the economics of the livestock production system in which they are to be used. More treatments are necessary if the drugs available (or selected on the basis of cost) are those that are only effective against mature flukes. Efficient control programmes can be developed based on less frequent treatments with drugs effective against early immature and immature flukes. However, the price of these drugs is considerably higher than those effective only against older flukes and their use may therefore be restricted to the more intensive livestock production systems.
If animals are grazing communal areas, it is important to achieve a synchronized reduction in pasture contamination of eggs, if possible. Ideally all animals in the area should receive treatment within a short period of time.
The use of molluscicides for the control of snail intermediate hosts is a potential tool for the control of fluke infections. Before considering chemical control of snails it should be noted that:
· many habitats are topographically unsuitable for the use of molluscicides and it is often very difficult to apply them effectively.· they are toxic to the environment
· cooperation between neighbouring properties is required for effective cover
· regular (at least yearly) application is required because rapid repopulation of snails may occur
· they are not species-specific and may destroy edible snails highly valued as food in some communities.
· they are expensive
Reports from several parts of the world indicate that a number of plants have molluscicidal properties. Planting of these trees and shrubs along streams and irrigation channels can reduce the number of snails in a population. The efficacy of this method for control of flukes has not yet been assessed.
The introduction of large numbers of ducks into rice fields after harvest has been used to reduce the snail population. The ducks eat the snails and the fluke species specific to the ducks compete with the fluke species of ruminants in the infection of snails. It is reported that snails infected with duck flukes will not become infected with flukes of livestock.
The introduction of edible snail species unsuitable as intermediate hosts into the habitat of the host snails may prevent the flukes from completing their life cycle.
The important management methods of controlling fluke infections are:
(a) To prevent snail habitats from developing by regular clearing of drainage channels in vegetation which provides suitable sites for snail development. Good drainage and the building of dams at appropriate sites in marshy and low lying areas may reduce the snail problem.(b) To keep livestock away from pastures contaminated with metacercariae. This may only be possible when the number of animals involved is small.
(c) Establish proper watering facilities to prevent animals from drinking from lakes, ponds and streams.
Most of the principles described for the control of liver flukes also apply to the control of intestinal and ruminant flukes.
Outbreaks of clinical paramphistomiasis caused by the immature flukes in calves, sheep and goats often pass undiagnosed, and the importance of this disease may be considerably underestimated.
Control of schistosomes is based on control of the snail intermediate host and treatment of infected animals and humans.
The proposed control strategies for the control of snails outlined in the section on F. hepatica and F. gigantica are also applicable to the control of the intermediate host snails of schistosomes. The managemental methods recommended in section 7.2.1.4, points a and c, may also contribute to the control of schistosomiasis.
7.3.1 Ruminants as final hosts
7.3.2 Ruminants as intermediate hosts
The principles of control of tapeworms are to prevent livestock from becoming infected, and when infection has occurred, to limit the pathogenic effect. The strategies available to achieve these differ according to whether the ruminants act as the final host for the tapeworm or the intermediate host for the larval stages.
As infections with Moniezia, Thysaniezia and Avitellina species are considered virtually non-pathogenic, specific treatment for these parasites are not recommended. Furthermore, young animals which are not treated develop an immunity to reinfection, whereas regular treatment may interfere with the development of immunity and result in repeated re-infection. In the case of severe infections, it is advisable to modify the treatment and control strategies selected for nematodes to include the use of a broad spectrum anthelminthic that controls both tapeworms and roundworms. A number of the benzimidazoles, such as albendazole, cambendazole and mebendazole, effectively control tapeworms.
Control may also be directed at the intermediate host, the oribatid mites. It has been shown that the habitat of the mites can be destroyed by ploughing and where this is feasible, newly sown pastures can be kept free of tapeworms for several years.
The apparent lack of pathogenic effects and the absence of diagnostic techniques render treatment of hepatic tapeworms unjustified.
7.3.2.1 Cysticercosis
7.3.2.2 Coenurosis
7.3.2.3 Hydatidosis/echinococcosis
7.3.2.4 Regional/national hydatidosis control programmes
With the availability of sensitive and specific serological tests, it is now possible to diagnose cysticercosis in living ruminants. However, at present it is not considered feasible to treat animals due to the high cost and the possible public health significance of dead, calcified cysts in meat and organs. Drugs which have shown efficacy against larval cestodes include praziquantel, mebendazole and albendazole. Control of cysticercosis and the adult tapeworms is therefore based on the prophylaxis of the infections.
To prevent infection of cattle, sheep and goats, all final hosts, namely man (Taenia saginata) and dogs (T. ovis, T. hydatigena) should be treated according to need. In addition humans should be educated in the use of high standards of personal hygiene and latrines to prevent the spread of T. saginata eggs.
Standardized methods of meat inspection should be implemented to detect infections at slaughter, and infected meat and organs should be condemned, treated (by cooking until grey) frozen to prevent infected meat reaching humans and dogs.
Considerable progress has been made in the development of vaccines against cestode larvae in ruminants, and a commercial vaccine is now available in Australia against Cysticercus ovis. It is anticipated that vaccines against other larval cestodes may become available in the future.
Clinical diagnosis of coenurosis is difficult and clinical signs can be confused with other disorders of the central nervous system, such as hypocalcaemia, listeriosis, cerebral abscess, tumours and Oestrus ovis infection (in sheep).
No specific treatment is available for this infection and slaughter of the animal is usually recommended.
Prevention of the disease includes the treatment of dogs with taenicidal drugs and the education of farmers and butchers so that offal and condemned material are not fed to dogs after slaughtering a parasitized animal.
Immunodiagnostic methods, radiology and ultrasound scanning are used routinely in diagnosing hydatidosis in humans, but these methods are rarely if ever applied in veterinary medicine.
Surgery to remove cysts is still the only effective treatment in human cases. Various drugs, such as mebendazole and praziquantel, are presently being tested; it appears that high doses are required, administered over a long period of time. Animals are not treated for hydatidosis.
Control of hydatidosis and Echinococcus granulosus is therefore based on prophylaxis of the infection in man, livestock and dogs.
· Public education programmes should convey the message that dogs infected with E. granulosus present a danger to the human population and their livestock. Farmers and other dog owners must be aware that uncooked viscera containing hydatid cysts should not be given to dogs, that dogs should be dewormed regularly, and that the handling of infected dogs may increase the risk of becoming infected.· Several anthelminthics are available for the treatment of dogs but a number of these cause segments to disintegrate, allowing the eggs to remain viable and thus spread after being passed. If possible, faeces passed by dogs after treatment should be buried or burned. However, experience gained during several control programmes has shown that these eggs are of minor importance in the epidemiology of hydatidosis. An alternative treatment is to use the purgative drug, arecoline hydrobromide; this is administered following 12 hours of fasting. The parasites are expelled during the subsequent 6 hours, during which time the animal should be confined. The faeces should be destroyed.
· All abattoirs, slaughterhouses and slaughter slabs should rigorously prevent dogs from gaining access to the premises; all offal and condemned material containing hydatid cysts should be destroyed.
Several control programmes in different parts of the world have reduced transmission to the stage where infection of humans is rare. The best known are on the islands of New Zealand, Tasmania, Cyprus and the Falkland Islands. A successful programme has also been undertaken in two regions of Chile. These control programmes have all been based on effective cooperation between veterinarians, technicians and dog owners and have had a strong educational component to each programme.
The objective of control is to reduce the transmission of echinococcosis/hydatidosis from animals to humans. The objective of eradication is to eliminate the transmission between animal hosts. It is very important to assess the potential for eradication/control and it is advisable to exercise extreme caution before decisions are made regarding the preference for eradication over control. Among other factors the following should be considered:
(a) Socioeconomic importance (prevalence, severity of disability, risk of mortality).(b) Epidemiological features of the infection
(c) Annual losses due to infections in livestock
(c) Availability of adequate operational and financial resources
(d) Feasibility of control
Based on evaluations of effective control programmes the following four phases can be distinguished:
(a) Planning. The planning phase includes (1) appointment of the appropriate control authority supported by the necessary legislation. The majority of successful programmes has been implemented by the Ministry of Agriculture or its equivalent. (2) Collection of data from which a benefit-cost analysis can be made and appropriate control strategies identified. Data should be collected on the size of rural dog populations, the incidence of hydatidosis in humans by age group, the reinfection rate of rural dogs and the number of veterinarians and technicians needed to treat and test every 10,000 rural dogs. (3) Development of a computer-based surveillance programme from which progress in control can be determined and cost-effective modifications can be made. (4) Selection and training of staff. (5) Securing of appropriate funding for the programme before entering the next phase.(b) Attack. The attack phase is labour-intensive and therefore very costly. It involves the use of arecoline surveillance and/or 6-weekly dog dosing. The duration of this phase depends on the strategy in use but according to experiences gained takes at least 10 years. To secure success it is important that funding is identified for the whole period before the programme is started. This phase requires constant monitoring to determine when a transfer can safely be made to the next phase.
(c) Consolidation. The consolidation phase transfers activities from nondiscriminatory dog dosing to quarantine of infected farms (or high risk farms). This transfer is often accompanied by the introduction of penalties for keeping infected dogs.
(d) Maintenance of eradication. During this phase all special activities cease and the normal resources of the meat inspection services of the Ministry of Agriculture are used to prevent reintroduction.
7.4.1 Characteristics and selection of anthelmintics
7.4.2 Administration of anthelmintics
7.4.3 Testing of anthelmintics
7.4.4 Summary of anthelmintics for the treatment of gastro-intestinal
An anthelmintic is a compound which destroys or removes helminths from the gastro-intestinal tract and other tissues and organs they may occupy in their hosts.
Currently a good selection of safe anthelmintics is available, some with broad spectrum activity and others with activity against specific helminth infections. Many modern anthelmintics are effective against both adults and larval stages and an increasing number are efficacious against arrested or dormant larvae.
Due to their cost and their tendency to delay or interfere with natural host immunity mechanisms, anthelmintics may not be the most desirable method of managing helminth problems. However, in many circumstances the sensible use of anthelmintic drugs is likely to be the only available method of controlling helminth parasites. They should not be used indiscriminately.
The ideal anthelmintic has the following properties:
(a) A broad spectrum activity against adult and larval helminth parasites.A number of factors influence the efficacy of an anthelmintic drug. Animals often harbour several different species of helminths, which may not have the same sensitivity to a given anthelmintic. In addition, there is usually a difference in sensitivity between adults and larval stages, with immature stages being less sensitive than the adult parasites.
Very few if any of the anthelmintics are completely effective at the recommended doses under field conditions. Some anthelmintics may be very effective in sheep but not in cattle, or vice versa.
(b) A rapid metabolism in the body and short-lived presence at low levels in the milk and/or tissues.
Animals should not be slaughtered for human consumption and milk not distributed to consumers until the drug residues have reached acceptably low levels. The withdrawal period of the drug should be considered before its use.
(c) A low toxicity in the target species. The ratio of the therapeutic dose to the maximum tolerated dose should be as large as possible.
It is desirable that an anthelmintic has a safety margin of at least six-fold.
(d) No unpleasant side-effects to the animal or to the operator.
Drugs may cause vomiting, or pain at the injection site. Some drugs irritate the skin of humans.
(e) Suitable for practical and economical integration into various management systems.
The selected drug(s) should be competitively priced and ready to use in a simple way. They should be stable and not decompose on exposure to normal ranges of temperature, light and humidity, and have a long shelf life.
7.4.2.1 Dosing by mouth
7.4.2.2 Dosing by injection
7.4.2.3 Dosing by external application
It is important to first identify the nature of the parasitic problem in order to select the appropriate drug to treat the infection. The optimal time and mode of administration of the drug should then be considered.
|
WARNING: Many formulations of anthelmintics are easily adulterated and it is strongly recommended that only registered drugs from authorized sources be purchased. |
A wide variety of formulations and preparations have been developed to provide methods of dosing animals, which are convenient for a wide range of species and circumstances.
The majority of anthelmintics are given by mouth as liquid preparations, pastes, boluses and tablets.
Liquid preparations are usually sold ready to use. Several devices such as syringes, bottles and drenching guns can be used for delivering the dose. Drenching guns are generally preferable and a wide variety, including single dose, multi-dose and automatic types, are available. It is important to keep the drenching equipment clean after use. The dose to be delivered should be checked before-and several times during-dosing to ensure that the correct dose is given to all animals. A graduated cylinder should be included in the field equipment for calibration purposes. It may be necessary to fit a short piece of rubber tubing on the end of the dosing nozzle to protect the mouth and pharynx of dosed animals.
Pastes are relatively easy to administer if a proper dispenser is available. If that is not the case, care should be taken to ensure the animal receives a full dose.
Boluses and tablets can be placed deep in the mouth of the animal by using a dosing gun or a pair of long-handled forceps, both of which can be manufactured locally. Bolus and tablet formulations have the advantage that if the dose is rejected, it is usually the total dose and a replacement can then be administered.
Prolonged protection of grazing livestock can be achieved by incorporating anthelmintics into medicated salt-molasses blocks and prepared mineral mixes, but animals do not always consume the amount required for an efficient treatment. Controlled-release preparations, such as slow release boluses allow the effective delivery of anthelmintics over several months.
A number of anthelmintics are available for injection. The size of needles should be appropriate for the formulation and the site of injection. In order to avoid local reactions (such as abscess formation at the injection site) the highest possible hygienic standards should be maintained.
Several dewormers are now available in a formulation for external application, termed "pour-on" preparations. The active ingredient of the drug is absorbed through the skin reaching its target via the circulatory system. This application form, which is particularly convenient for animals kept under range conditions, has the advantage that only minimum restraint of animals is needed, as the dose is applied to their back while passing through a crush or standing at a feeding trough.
Anthelmintics marketed by international pharmaceutical companies are usually well tested for efficiency and toxicity and can be safely applied according to the manufacturer's instructions. In cases where the origin of the drug is unknown or when it has been obtained from a source without an established reputation, it may be advisable to test the efficiency of the drug before using it in large scale control programmes. A rapid and cheap method of assessing the efficacy of an anthelmintic is to determine the effect on worm egg counts before and after treatment.
Nematodes, Lungworms, Tapeworms and Flukes
Table 7.1 ANTHELMINTICS AND THEIR APPLICATION
|
Generic name |
Route of administration* |
Dose rate (mg/kg) |
Spectrum of activity** | |
|
Benzimidazoles | ||||
|
|
Albendazole |
O |
5-7.5 |
GI, L, T |
|
|
Cambendazole |
O |
20-25 |
GI, L, T |
|
|
Febantel |
O |
5-10 |
GI, L |
|
|
Fenbendazole |
O |
5-7.5 |
GI, L, T |
|
|
Mebendazole |
O |
12.5 |
GI, L, T |
|
|
Oxfendazole |
O/IR |
4.5-5 |
GI, L, T |
|
|
Oxibendazole |
O |
10-15 |
GI |
|
|
Parbendazole |
O |
20-30 |
GI |
|
|
Thiabendazole |
O |
44-110 |
GI |
|
|
Thiophanate |
O |
50-80 |
GI, L |
|
Immidazothiazoles | ||||
|
|
Tetramisole |
O |
15 |
GI, L |
|
|
Levamisole hydrochloride |
O/S0/SC |
7.5 |
GI, L |
|
|
Levamisole phosphate |
O/SC |
8-9 |
GI, L |
|
Organophosphates |
|
|
| |
|
|
Coumaphos |
O/F |
8-15 |
GI |
|
|
Haloxon |
O |
40-50 |
GI |
|
|
Naphtalophos |
O |
30 |
GI, T |
|
|
Trichlorfon |
IM/SC |
10-15 |
GI |
|
Tetrahydropyrimidines | ||||
|
|
Morantel |
O |
10 |
GI |
|
|
Pyrantel tartrate |
O |
25 |
GI |
|
Miscellaneous |
|
|
| |
|
|
Ivermectin |
D/SC/SO |
200 mcg/kg |
GI, L |
*O: Oral
SC. Subcutaneous
SO: Spot-on
IM: Intramuscular
IR: Intraruminal
F: Feed
**GI: Gastro-intestinal nematodes
L: Lungworms
T: Tapeworms
Table 7.2 ADDITIONAL INFORMATION ON ANTHELMINTICS FORMULATIONS
|
Generic name |
Preparation |
|
Albendazole |
Liquid suspension |
|
Cambendazole |
Ready-to-use suspension, paste |
|
Fenbendazole |
Ready-to-use suspension, granules, tablets, mineral pre-mix, licking blocks |
|
Mebendazole |
Ready-to-use suspension, granules, paste, mineral pre-mix |
|
Oxfendazole |
Ready-to-use suspension, tablets |
|
Thiabendazole |
Ready-to-use suspension, powder, granules, tablets, paste |
|
Tetramisole |
Suspensions, granules, tablets, injectable |
|
Levamisole |
Pour-on preparation |
|
Morantel |
Ready-to-use suspension, tablets, granules, paste |
|
Pyrantel |
|
Table 7.3 ANTHELMINTICS FOR THE TREATMENT OF INHIBITED LARVAE
|
Cattle |
Sheep |
|
Albendazole |
Albendazole |
|
Febantel |
Febantel |
|
Fenbendazole |
Fenbendazole |
|
Oxfendazole |
Oxfendazole |
|
Thiophanate |
Levamisole |
|
Ivermectin |
Ivermectin |
Table 7.4 ANTHELMINTICS AND FORMULATIONS FOR STRATEGIC PROPHYLACTIC PROGRAMMES
|
Generic name |
Formulation |
Animal |
|
Albendazole |
Slow release bolus, active 90-120 days |
Cattle |
|
Albendazole |
Pulse release bolus |
Cattle |
|
Oxfendazole |
Pulse release bolus |
Cattle |
|
Morantel tartrate |
Slow release bolus active 60 + days |
Cattle |
Table 7.5 ANTHELMINTICS FOR THE TREATMENT OF LIVER FLUKES
|
Generic name
|
Route of Administration
|
Dose rate (mg/kg) |
Minimum age of fluke in weeks efficiency ³ 90% |
||
|
Sheep |
Cattle |
Sheep |
Cattle |
||
|
Hexachlorophene* |
O |
15 |
20 |
12 |
>20 |
|
Hexachloroethane |
O |
250-300 |
300 |
12 |
12 |
|
Tribromsalan |
O |
20 |
20 |
12 |
>12 |
|
Bithionol* |
O |
75 |
30 |
>12 |
>12 |
|
Hexachloroparaxylene |
O |
150 |
130 |
12 |
12 |
|
Bromophenophos |
O |
16 |
12 |
12 |
>12 |
|
Clioxanide* |
O |
20 |
NR |
12 |
NR |
|
Oxyclozanide* |
O |
15 |
13-16 |
12 |
>14 |
|
Niclofolan*
|
O |
4 |
3 |
12 |
>12 |
|
SC |
NR |
0.8 |
NR |
<12 |
|
|
Nitoxynil |
SC |
10 |
10 |
8 |
10 |
|
Brotianide* |
O |
5.6 |
NR |
12 |
NR |
|
Rafoxanide* |
O |
7,5 |
7.5 |
6 |
12 |
|
SC |
NR |
3 |
NR |
12 |
|
|
Closantel |
O |
7.5-10 |
NR |
8-6 |
NR |
|
SC |
NR |
3 |
NR |
>12 |
|
|
Diamphenetide |
O |
80-120 |
100 |
1 day-6 weeks |
1 day-7 weeks |
|
Albendazole |
O |
4.75 |
10 |
>12 |
>12 |
|
Triclabendazole |
O |
10 |
12 |
1 |
1 |
|
Clorsulon
|
O |
- |
7 |
- |
8 |
|
SC |
- |
2 |
- |
>12 |
|
O = Oral
SC = Subcutaneous
NR = Not Recommended
* = Also effective against paramphistomes (see Table 7.6)
Table 7.6 ANTHELMINTICS FOR THE TREATMENT OF PARAMPHISTOMES
|
Generic name
|
Route of Administration
|
Dose rate (mg/kg) |
Efficiency (%) |
|||
|
Sheep
|
Cattle
|
Immature in small intestine + abom. |
Mature in rumen |
|||
|
Sheep |
Cattle |
Sheep/Cattle |
||||
|
Clioxanide* |
O |
20-40 |
NR |
|
|
|
|
Niclosamide
|
O |
50-100 |
50-100 |
95 |
0-96 |
0 |
|
|
|
NR |
160 |
NR |
92 |
|
|
|
|
|
|
|
|
|
|
Niclofolan* |
O |
6 |
6 |
76-95 |
- |
42 |
|
Oxyclozanide*
|
O |
15 |
15 |
85-100 |
61-96 |
73-100 |
|
|
NR |
18 |
NR |
99-100 |
100 |
|
|
|
|
(2 doses) |
|
|
|
|
|
Rafoxanide* |
O |
75-100 |
25-50 |
99-100 |
9-100 |
63-98 |
|
Bithionol S03* |
O |
40 |
40 |
- |
- |
97-100 |
|
Resorantel |
O |
65 |
65 |
80-90 |
62-99 |
85-100 |
O = Oral
NR = Not Recommended
* = Also effective against liver flukes (see Table 7.5)
7.5.1 Detection of resistance
7.5.2 Testing for anthelmintic resistance
7.5.3 Preventing the development of anthelmintic resistance
Increased productivity in ruminants through the control of helminth parasites will to a large extent depend upon the availability of low cost, effective anthelmintics. It is therefore of great concern that the regular use of anthelmintics has lead to the selection of drug-resistant helminths. This has become a serious problem in many countries, and resistance of parasites to one or more anthelmintics is now widespread, particularly in sheep. The loss of inexpensive drugs against helminths due to problems of anthelmintic resistance may leave many countries without a means of controlling helminth parasitism.
Anthelmintic resistance is often first suspected in cases of apparent anthelmintic failure, but it should be kept in mind that several other factors can be responsible for the lack of efficiency of a drug. These include:
(a) Underdosing. Most farmers usually estimate (guess) the weights of their animals and many surveys have shown that such estimates are often considerably below the actual weight. In addition farmers often use the "average weight" to establish the dose. This automatically results in underdosing. This can be further compounded if a manufacture recommend dosages for a broad weight range.(b) Rapid reinfection. If animals are grazed on heavily contaminated pastures, reinfection occurs immediately and this may give the impression of drug failure. This is particularly relevant where Haemonchus contortus is dominant, as it develops rapidly and is very pathogenic.
(c) Inefficiency against arrested or dormant larvae. Arrested larvae which are unaffected by the anthelmintic being used may continue development immediately after treatment.
(d) Drug resistant parasites are present. Frequent regular treatments using the same anthelmintic given at low dosages over a prolonged period of time, will predispose to the development of drug resistance.
A simple method to test for the presence of drug-resistant nematodes which can be applied in the field is as follows.
(a) Collect faecal samples from a representative group of animals for faecal egg counts prior to treatment. There should be 10-15 animals in the group.(b) The animals should be carefully weighed and dosed according to weight with the drug under suspicion.
(c) A control group of the same size should be identified which is not treated, and these should be sampled.
(d) All animals should be sampled again 7-10 days later for faecal egg counts.
A reduction in egg counts of less than 85% strongly suggests that resistant nematodes are present.
It is then necessary to determine to which drugs the parasites are susceptible and immediately change to an efficient drug. Other tests are also available, such as the egg hatch test, but these require laboratory equipment and are more sophisticated. To confirm that resistance is present, experimental infections in lambs can be established with subsequent treatment, slaughter and worm counts, but these are expensive and time consuming. Thus in many cases the egg count reduction test will be the only feasible test available.
A fuller account of methods for detecting anthelmintic resistance recommended by the World Association for the Advancement of Veterinary Parasitology can be found in Coles et al., Veterinary Parasitology (1992) 4, 35-44.
There is an urgent need for the development and adoption of strategies to prevent the spread of anthelmintic resistance, particularly in nematodes of sheep, and prevent it from becoming a problem in cattle. The following practical measures can be taken to delay the occurrence.
(a) Use the correct dose. It is very important that farmers use a scale or a measure tape to establish the weights of animals in each age group (lambs, weaners, young ewes, older ewes, rams). The dose for each group is then established for the heaviest animal in each group; the use of an "average weight" should be discouraged as this results in underdosing of the heavy animals. This rule also applies to cattle.(b) Maintain drenching equipment. A common cause for incorrect dosing is faulty dosing equipment. It is very important that equipment is tested for accuracy before the start of dosing. Some drenching guns have a low compatibility with certain preparations, and it is recommended that the appropriate equipment is used. Thorough cleaning of equipment after use is important, as some preparations destroy the piston if not removed. In general, drenching guns with a plastic or plexiglass cylinder are recommended.
(c) Reduce dosing frequency. Studies have shown that the risk of anthelmintic resistance increases with increased dosing frequency. It is therefore important to establish the epidemiology of the helminth infections and introduce strategic deworming programmes based on a few well-timed treatments given when it is most advantageous.
(d) Establish treatment and quarantine for all animals introduced to the farm. The source of anthelmintic resistant nematodes can be from purchased animals, such as introduced breeding stock. If nothing is known about the previous deworming history of animals, it is recommended that they be treated prior to introduction with a double dose of levamisole and a modern benzimidazole (fenbendazole, albendazole). It is advisable to keep the newly introduced animals isolated for 72 hours after arrival and treatment.
(e) Alternate anthelmintics. Present information recommends continued use of an anthelmintic for at least a whole season (one year for many tropical and sub-tropical countries) provided it is effective. When changing the anthelmintic, a drug from a different class (see Table 7.1) should be selected.
In addition farmers should consider establishing grazing management practices which reduce the parasite burden and subsequently the need for treatment.
