Resistance reversion after Haemonchus contortus replacement in a sheep flock: monitoring of benzimidazole and levamisole resistance-associated polymorphisms and impact on lamb survival

Haemonchus contortus is a highly prevalent gastrointestinal nematode whose control is hindered by anthelmintic resistance. Worm replacement (WR), where a resistant parasite population is substituted for a susceptible one, was performed through an artificial infection of nematode-free ewes with a susceptible H. contortus isolate. The infected ewes were kept in a nematode-free paddock (total replacement—TR) or in a paddock naturally infested with resistant helminths (partial replacement—PR) and compared to untreated ewes kept in a naturally infested paddock (control—C). Lambs born in each group, over two breeding seasons, were evaluated from 63 to 189 days old and subjected to lifesaving anthelmintic treatments (AHt) with the combination of benzimidazole and levamisole when fecal egg counts (FEC) ≥ 10,000 and packed cell volume (PCV) ≤ 24% or when PCV ≤ 22%, and a Kaplan–Meier survival analysis of the AHt requirement was performed. Benzimidazole (F200Y) and levamisole (S168T) resistance-associated polymorphisms were genotyped in H. contortus every six months after WR. In lambs, the FEC mean increased (4647 to 8506) and the PCV mean decreased (30.8% to 27.2%) between the first and second breeding seasons; the AHt mean in C (2.4) was higher than in TR (1.4) and PR (1.6) in the second breeding season; and inferior survival probabilities were observed in C compared to TR throughout the trial and to PR in the second breeding season. A methodology for S168T genotyping using real-time PCR and allelic discrimination with fluorescent hydrolysis probes was developed. Resistance frequencies similar to the C group for the F200Y and S168T polymorphisms, respectively, were reached after 1 and 7 months in PR and after 7 and 13 months in TR. The positive effects of WR on phenotypic health traits in lambs lasted for at least two breeding seasons. However, molecular anthelmintic resistance in H. contortus was reestablished in the first year, reinforcing the role of molecular methods in the early diagnosis and monitoring of resistance. Although the resistance reversion showed a time-limited effect, WR was able to control the detrimental effects of haemonchosis in lambs born in two consecutive breeding seasons until slaughter age.