Potential for misinterpretation of the faecal egg count reduction test for levamisole resistance in gastrointestinal nematodes of sheep

The faecal egg count reduction rest (FECRT) is widely used to assess the presence or absence of levamisole resistance in gastrointestinal nematodes in sheep flocks, However, in a recent study the FECRT appeared to falsely indicate the presence of levamisole resistance. One possible explanation for this result could have been that the anthelmintic failed to completely remove all immature stages of levamisole-susceptible strains, which then developed into egg laying adults and gave misleading results in the FECRT. Artificial infection of lambs with levamisole-susceptible, benzimidazole-resistant strains of Ostertagia circumcincta, Haemonchus contortus and Trichostrongylus colubriformis was followed by treatment with levamisole when the infections were between 2 and 10 days old; Levamisole was 84%, 88% and 98% effective against these immature stages of the three species, respectively. The positive control, ivermectin, was more than 99% effective against all three species. The percentage faecal egg count reduction on samples taken 11 and 20 days post-treatment with levamisole was 80% and 78%, respectively. These results demonstrate that, due to the maturation of immature stages, the FECRT based on faecal egg counts taken 11 or more days after treatment may give results indicative of levamisole resistance despite the absence of levamisole resistant strains of gastrointestinal nematodes.