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Principles of control on Nematodes

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.

1. ?Parasite species present

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.

2. ?Herd structure and grazing management

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.

3. ?Availability and abundance of infective larvae on pasture

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.

4. ?Type of climate

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.

5. ?Genetic resistance

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.

5.1 ?Parasite resistance within breeds

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.

5.2 ?Parasite resistance between breeds

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.

6. ?Control of gastro-intestinal nematodes

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.

6.1? Control in savannah-type climates with one or more distinct dry season(s)

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

6.2 ?Control in arid climates

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.

6.3 ?Control in humid climates

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.

7. ?Control of lungworms

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.

8. ?Control of filarial nematodes

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.

9. ?Control of Toxocara vitulorum

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.

?Note: Source of this article: from

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