The prevalence of anthelmintic resistance in nematode parasites of sheep in southern Latin America: Uruguay.

This survey was conducted on 252 farms randomly distributed over all the sheep raising areas in Uruguay. The study involved farms with more than 600 sheep, which represented 80% of the total sheep population of the country. Three anthelmintic groups were assessed, namely, benzimidazoles, levamisole and avermectins. Overall, the results showed 80% of sheep flocks had benzimidazole resistance, 71% had resistance to levamisole, and 1.2% of flocks showed resistance to avermectins. Approximately 28% of farms had resistance to one anthelmintic group, 64% to two anthelmintic groups, and 1% resistance to all three groups. Only 7.5% of farms had no detectable levels of anthelmintic resistance. More than 80% of farms had Trichostrongylus populations resistant to both benzimidazoles and levamisole. Resistance was recorded in all three anthelmintic groups for Haemonchus and resistance also occurred to benzimidazoles and levamisole in Ostertagia.