Prevention and control of parasitism in livestock
Contd from previous issue
However, the subclinical effects in cattle whose outward appearance disguises the adverse economic impact is still a problem of great magnitude. The goals of control are as follows:
a. Prevent heavy exposure in susceptible hosts
b. Reduce overall levels of pasture contamination,
c. Minimize the effects of parasite burdens
d. Encourage the development of immunity in the animals
Endoparasitic infection is probably one of the most economic and production losses in livestock worldwide. It decreases feed intake, utilization of feed, body weight gain, milk production and reproductive performance. It is also important concerning the development of a resistant strain of gastrointestinal parasites to broad-spectrum anthelmintic. Pasture management combined with nutritional supplementation with concentrates and /or forage is the most anti-parasite strategy in organic livestock farming.
Grazing management in worm control programmes based on epidemiological knowledge is simply to provide clean pasture on which stock may safely graze. There are various forms of grazing management usually practised to control worm burden in various places. Grazing management is one of the ways of reducing the frequency of anthelmintic treatment. Grazing management strategies may be of preventive, evasive or diluting.
1. Alternation of host species: Alternate or integrated grazing has been used to control gastrointestinal parasites in ruminants. It is common practice to graze calves followed by older cattle, taking advantage of increased resistance in these older immunocompetent animals. Small ruminants and cattle, small ruminants and horses, or horses and cattle are the most logical candidates for alternate grazing strategies. A recent study of mixed (and alternate) grazing with nose-ringed sows and heifers, showed promising results in controlling Ostertagia infections in the cattle whereas the little effect on the nematode infections of sows was noted.
2. Rotational grazing: This is a grazing management technique involving the intensive subdivision of a pasture in which each constituent paddock is grazed for a short time and then spelt for a relatively much longer time. In a simple example, the total area for grazing subdivided into 15 paddocks, each of which could be grazed for 1 week and spelt for 14 weeks. The grazing time in such a system is thus 1 week, and the rotation length is 15 weeks.
Ticks were eradicated from a costal island in Queensland by removing all known hosts for 5 months and larval survival was almost zero after about 16 weeks. One modelling study for B. micro plus in Australis indicated that a single annual spelling period in summer of between eight and 12 weeks would substantially reduce tick populations.
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 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.
(To be contd)