Prevention and control of parasitism in livestock
- Part 2 -

Kalyan Sarma *

Safe Pasture

The generation of safe pastures generally relied on the prevention or reduction of contamination during periods of peak egg shedding and then allowing the larvae to die off during periods when the pasture was not grazed. It is assumed that spelling a pasture, allowing it to remain ungrazed for a period of time, will reduce the number of infective larvae on pasture.

Resting a pasture can reduce the number of infective larvae. Twice weekly removal of excessive faeces provided superior nematode strongyle cyathostomin and ascarid control, as well as a 100% increase in grazing area, and freedom from drug related problems. The routine management systems on these farms include removal of faeces once, or twice, every 2 months.

This practice results in the reduction of nematode egg counts to less than 300 eggs per gram (EPG). The life cycle of parasites is interrupted when faeces are removed, or collected to use for fuel, building material and for composting.

Strategic Anthelmintic Treatment

A strategic treatment aims to remove a worm burden. The treatment of young animals at weaning, in conjunction with a move to a spelt pasture. Anthelmintic treatment should be given before extreme climatic conditions of either very high or very low temperatures or intense droughty.

Hot dry conditions over summer result in very low larval availability so that anthelmintic treatment at this time is followed by very low rates of re-infection. Larvae of H. contortus cannot survive extreme cold such as occurs in a northern continental winter, so treatment of housed sheep or goats in winter should be very effective in controlling this species.

For some parasite species, such as H. contortus and T. colubriformis, resistance to new infection can occur before resident infections are expelled. An anthelmintic treatment at this time to remove the resident worms result in an extended period of low egg production because of acquired immunity to new infection. The best example of a strategic anthelmintic treatment is the single treatment at 10 days of age with pyrantel to control Toxocaravitulorum in calves within a few days of birth via the colostrums.

Biological control

Biological control on pasture includes the use of predatory fungi to kill a variety of nematode species and substantially reduce the intensity of infection. Challenges to fungal control have been a requirement for the daily administering of fungi to the host and achieving the required fungal density inside the dung.

However, a nematode-killing fungus, Duddingtonia flagrans, recently discovered in New Zealand, was shown to have a trapping efficiency rate of 78% and activity for up to 90 days on pasture, providing a viable alternative to reduce animal mortality from nematode infections. Control of the snail intermediate hosts by foraging flocks of ducks has shown to be of practical value in the control of Fasciolagigantica infections of ruminants raised in the rice-producing areas of South-East Asia.

The benefit is shown to be two-fold. Not only do the ducks seek snails as a food source, but the free-living stages of Echinostoma Revolutum, which is a common trematode parasite of ducks, out-compete F. gigantica in utilising snails as intermediate hosts.

Another finding of great practical importance in fluke control of buffalo and cattle in paddy farming areas is that encysted metacercaria is mainly confined to the bottom third of rice plants.

An extension programme, advising farmers to feed only the top two-thirds of freshly cut rice stalks to their animals, encouraging the use of ducks, avoiding the grazing of harvested rice fields close to cattle pens and using only a single anthelmintic treatment with triclabendazole in the dry season, has been successfully promoted in West Java for several years. Separating hosts from their faeces is the simplest, cheapest and most effective form of biological control of parasitic diseases.

A variety of birds rely heavily on coprophagous invertebrates as a food source and in seeking these, they tear bovine dung pats apart thus destroying the environmental buffering capacity of these large faecal masses. It is their invertebrate prey, notably dung beetles, scarabs and to lesser extent earthworms, however, that is capable of rapid and often complete dung removal and thus is indirectly responsible for significant reductions in the number of free-living stages of parasites (Waller and Faedo, 1996).

Nutritional management

The supplement is also a major strategy to control parasites. The plane of nutrition is an important determinant of the response by animals to parasitism by affecting the development and establishment of parasites and also influencing the magnitude of their pathogenic effects. Many reports in the literature attest to the synergistic relationship between helminth infection and malnutrition.

Well-nourished animals are generally more resistant to the effects of parasite infection. Therefore, nutritional supplementation may reduce the requirement for chemotherapeutic control. Feed supplementation, particularly with high quality protein, is often necessary to maintain a better immune response to internal parasites than animals whose nutritional status is compromised low protein diets are responsible for causing infection because they produce less IgA (immunoglobulin).

High protein diets have been shown to improve the pregnant ewe's immune response to parasites after lambing. Lambs receiving protein supplementation have reduced faecal egg counts. Additional dietary protein, selenium, as well as minerals may each play a role in countering infections presumably through mechanisms such as enhancing host immunity or maintaining digestive tract integrity.

Parasite-inhibiting plants have also been tested for their ability to reduce egg shedding and pasture seeding density. Copper wire particle (COWP) capsules were developed to overcome the copper deficiency in ruminants grazing on mineral deficient and marginal grazing lands. The copper particles pass to the abomasum where they lodge in the mucosal folds and release ionic copper over an extended period.

An additional important benefit of ultra-low-dose copper therapy is on reducing certain parasite infections in grazing livestock. Research has shown that 2-5 g COWP capsules administered orally to sheep resulted in a high-level anthelmintic effect against H. contortus, as well as extended protection (approximately 3 months) against incoming infection of this parasite.

Genetics of host resistance

One alternative approach to controlling parasitic nematode infections is to use the natural diversity of the host genome to reduce parasite transmission. In cows, studies have shown that the number of nematode eggs/gram (EPG) in faeces was influenced by host genetics with an estimated heritability of 0.30.

A small percentage of the herd was responsible for the majority of parasite transmission, a distribution strongly suggesting that genetic management could reduce overall parasite transmission. Breeding to obtain livestock that is genetically resistant to nematode infection is the ultimate in sustainable parasite control.

Good examples of genetic resistance can be found across the wide spectrum of animal parasitic disease entities of the tropics; such as the resistance of Bos indicus cattle to the cattle tick, Boophilusmicroplus, trypanotolerance of the N'dama and West African shorthorn cattle, nematode (specifically H. contortus) resistance in the East African Maasai, Florida ad Louisiana Native, Barbados Blackbelly and the St. Croix breeds of sheep, and trematode resistance in Javanese thin-tailed sheep. Cattle breeds of Ongole/Nellore, Sahiwal, Ponwar hill cattle and buffaloes and resistance to tick infestation.

Chemotherapy

The use of anthelmintic is still the mainstay for nematode control. The successes have been cyclical and directly related to the timely introduction of new drugs as resistance to older drugs has surfaced. Effective drug utilization dates back to the 1960s with the development of benzimidazoles (BZ), followed by the imidothiazoles-tetrahydropyrimidines in the 1970s, and the production of macrocyclic lactones (avermectins and milbemycin) in the 1980s.

Avermectins and milbemycin emerged as compounds having a high efficacy against ectoparasites and effective in simultaneously killing nematode worms in the host. As result invermectin, doramectin and milbemycin became well accepted for the treatment of parasitic invasions in livestock.

During the last 35 years, the pharmaceutical industry has produced a succession of highly effective, broad-spectrum anthelmintic and veterinarians and livestock producers have come to expect that worm control is easy, either by drenching or injecting cattle, sheep and goats with these products.

This has made helminth control easy but has not fostered conservative use of the products. Control of ectoparasites with acaricides may be directed against the free-living stages in the environment or the parasitic stages on the host. Acaricides can be used by dipping, washes, spraying, pour-on, spot-on or by injections.

Insecticide ear tags are commercially available in some countries for the control of ear ticks. The following are the strategies for the use of chemical anthelmintic.

1. Regular treatments at intervals at or near the length of the pre-patent period of the parasite.
2. Animals are treated therapeutically, whenever production losses and /or uncontrolled disease is considered to be significant
3. Treat all animals in the herd or flock.
4. Treat only those animals that are perceived to need treatment use of a narrow-spectrum drug, closantel, in combination with a minimum number of treatments with broad-spectrum anthelmintic. Closantel is particularly effective against Haemonchus and has a persistent effect for 2 to 3 months.
5. Proper drenching of the drug.
6. Reduce feed before drenching: Restricting access to fee for 24 hours before drenching slows the flow of gut contents containing the drench from the rumen. Reduced feed intake prolongs drench uptake, extending the effective duration of the killing effect

To be continued...