Anthelmintic resistance

Abstract

Since the first reports of resistance to the broad spectrum anthelmintics were made some three decades ago, this phenomenon has changed from being considered merely as a parasitological curiosity to a state of industry crisis in certain livestock sectors. This extreme situation exists with the small ruminant industry of the tropical/sub-tropical region of southern Latin America where resistance to the entire broad spectrum anthelmintic arsenal now occurs. In contrast, the cattle industry does not appear to be threatened--or so it seems. Although field reports of resistance have been made to the range of broad spectrum anthelmintics in nematode parasites of cattle, it appears that the evolution of resistance in cattle parasites is not as dramatic as for sheep worms. However, one cannot remain confident that this state of affairs will remain static. Concern is shared amongst parasitologists that we have not looked closely enough. In regions of the world where internal parasites are considered a problem in cattle and drenching occurs frequently, no widespread surveys have been carried out. It appears that because of the very high costs and risks associated with taking a new active drug down the development track to marketing, that the pharmaceutical industry has, in general, turned away from this activity. By implication, the international small ruminant industry is too small for these companies to make the necessary investment. This begs two questions: What is the fate of the sheep (and goat) industries in those parts of the world where resistance is rampant and immediate ameliorative parasite control options are required? What will be the response if significant resistance is found in cattle parasites? There is a body of opinion which suggests that if resistance becomes an issue in the control of cattle parasites then the pharmaceutical industry will find it commercially attractive to re-enter the anthelmintic discovery and development business. This is based on the simple commercial fact that by far the greatest anthelmintic sales are associated with the cattle industry. However, this market is specific and sectoral, with by far the greatest sales in North America and Western Europe, where the prevalence of resistance is likely to be low and remain so, more-or-less indefinitely. So the chances of the above scenario occurring must be considered low. Remarkable developments have recently occurred in non-chemotherapeutic parasite control options, for example worm vaccines, host selection and biological control. Also, there seems to be greater acceptance of various grazing management practices designed to reduce the frequency of anthelmintic treatment. However, they collectively cannot be expected to offer immediate salvation to farmers now faced with chemotherapeutic failure to control nematode parasites in their flocks. The future for these farmers must be considered bleak, because compounded with these problems are the poor commodity prices for sheep and goat meat and fibre, resulting in relentless reductions in f...

Anthelmintic resistance in nematodes of horses

“…The likelihood that such resistance will develop at some future time is suggested by the high prevalence of AM-resistant Haemonchus contortus in sheep and goats throughout the world [91,100,104], and the increasingly common reports of AM-resistant parasites of cattle. Avermectin-resistant Cooperia spp.…”

The FAMACHA system for managing haemonchosis in sheep and goats by clinically identifying individual animals for treatment

“…It appears unlikely, due mainly to the very high costs involved [21,24,37], that development of new, unrelated anthelmintics will offer a sustainable solution to the problem. Therefore other strategies which do not rely heavily on the chemical treatment of entire flocks or herds continue to receive special attention.…”

Recovery of Trichostrongylus colubriformis infective larvae from three grass species contaminated in the autumn

“…The anthelmintic resistance of most species involved in parasitic infections is a major problem for sheep breeding not only in Brazil (AMARANTE et al, 1992;ECHEVARRIA et al, 1996) but also in any other place where this activity is relevant (WALLER, 1997;BARTLEY et al, 2004BARTLEY et al, , 2006. In the past two decades, this problem has led to further studies once it is a great threat to small ruminant production.…”