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

South America has a large population of small ruminants. Currents estimates are approximately 100 million sheep and 23 million goats. A large percentage of these flocks are raised in the humid tropics/sub-tropics. Nematode parasitism is singly the most important disease of these animals and typically farmers resort to frequent anthelmintic treatment in attempts to control this problem. Because of this reliance on drugs, price consideration is an important determinant in a farmer's choice of anthelmintic. In some circumstances, this has led to unethical practices of drug adulteration and substitution, or the offering to farmers of cheap, sub-standard generic products. The prevalence of anthelmintic resistance has not been investigated in any widespread sense in South America, although some of the first reports of resistance were made in southern Brazil almost 30 years ago. The following series of papers outline surveys conducted in the humid topics/sub-tropics of southern Latin America to assess the significance of resistance to the broad-spectrum anthelmintic groups in nematode parasites of sheep flocks.

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

Sixty-five sheep farms in the northern provinces of Buenos Aires, Entre Rios, Corrientes, Cordoba and Sante Fe were used in this survey on anthelmintic resistance. Anthelmintic groups tested were the benzimidazoles, levamisole, the combination levamisole + benzimidazole product and the avermectins. The overall level of resistance was 46% of properties, with resistance to the individual drug groups being 40%, 22%, 11% and 6%, respectively. On a large proportion of farms the resistance recorded was to two, or more, anthelmintic groups. Haemonchus contortus, Ostertagia spp. and Trichostrongylus spp. were the most abundant parasite species recorded. Resistance was greatest in the province of Corrientes where the frequency of treatment is generally high due to the endemic nature of H. contortus. Management practices were also important with resistance greatest on farms where frequent drenching is carried out and on farms carrying only sheep.

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

This survey was conducted in the southern Brazilian state of Rio Grande do Sul and involved 182 farms located in 26 counties. In addition to the three major broad-spectrum anthelmintic groups (viz. benzimidazole, levamisole and ivermectin) the combination benzimidazole and levamisole and the H. contortus specific anthelmintic, closantel, were tested by the faecal egg count reduction method for the prevalence of anthelmintic resistance. Resistance was found to be 90%, 84%, 13%, 73% and 20%, respectively. This is a crisis situation. Immediate, drastic action needs to be implemented, otherwise the sheep industry in this region (approx. 10 million head) will soon face a lack of any effective anthelmintics with the inevitable consequences of major restructuring or abandonment.

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

This survey to detect anthelmintic resistance in nematode parasites of sheep was conducted on 11 farms in the Occidental and 26 farms in the Oriental Regions of Paraguay using the faecal egg count reduction test (FECRT). The anthelmintic groups tested were the benzimidazoles, levamisole and avermectins (both oral and injectable). Overall the levels of resistance were 73%, 68%, 73% and 47%, respectively. Levels of resistance were similar for all three important nematode genera, viz. Haemonchus contortus, Ostertagia and Trichostrongylus. This survey clearly indicates that a large, and ever increasing, proportion of sheep farmers are rapidly approaching the time when they will have exhausted all chemotherapeutic options to control parasites. Unless they face having to abandon their sheep farming operations, radical changes will need to be implemented with urgency.

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.