Anthelmintic resistance and its importance in the control of nematode parasitism of small ruminant livestock in the tropics / subtropics

Livestock raised in the humid tropics are exposed to a wide range of helminth parasites which are responsible for enormous losses in productivity, and often high levels of mortality, particularly in young animals. Control methods rely almost exclusively on the use of anthelmintic drugs. Frequency of use depends on the farmers desire to treat his animals and/or his ability to pay for the drugs. However there is an increasing tendency for farmers to resort to treatment with their greater understanding of the losses caused by parasites and particularly due to the fact that cheap, generic drugs are becoming more readily available. As a result, athelmintic resistance in helminth parasites has developed, but its importance varies between parasite taxa, between parasites of different host species and between different regions/countries in tropics. By far the greatest problem with anthelmintic resistance is associated with the nematode parasites of small ruminants, particularly with Haemnchus contortus, a major pathogen of sheep and goats throughout the humid tropical/sub-tropical regions of the world. The worst situation is generally in the large-scale sheep farming countries within these regions (continents of Africa, South America, and Australia). Generally speaking, resistance in the Asian countries is not as serious, although individual countries have major problems (eg. Malaysia, Fiji). Evidence is clear-cut that anthelmintic resistance is increasing-in prevalence, magnitude and spectrum. Unfortunately there is a tendency to seek a solution when it is too late, when total anthelmintic failure has occurred. However, some adjuncts and/or alternatives to parasite control are available, but none in isolation will solve the problem, nor will they offer immediate respite. The way to sustainable nematode parasite control in the future is to incorporate a range of control options into an Integrated Pest Management (IPM) package for nematode parasites of livestock.