Is partial worm replacement effective for Haemonchus contortus control in different sheep breeds?

Partial worm replacement (PR), consisting of the elimination of an anthelmintic-resistant parasite population from the host but not from the environment, was evaluated as a tool to improve anthelmintic treatment efficacy in White Dorper (DO), Santa Inês (SI), and Texel (TX) sheep breeds. Artificially infected ewes were kept either in a nematode-free paddock (total replacement - TR) or in a paddock naturally contaminated with resistant helminth larvae (PR) and compared with untreated ewes harboring resistant helminths kept in a naturally contaminated paddock (control - C) over two breeding seasons. In the third breeding season, because of reestablishment of resistance, ewes from the PR and TR groups were reinfected with the susceptible isolate, and their lambs were evaluated every 21 days for fecal egg count (FEC), packed cell volume (PCV), live body weight, FAMACHA© score, and body condition score (BCS). Benzimidazole (F200Y) and levamisole (S168T) resistance-associated polymorphisms were genotyped in Haemonchus contortus larvae. Lambs in the C group had higher FEC and lower PCV than lambs in the other groups, in addition to lower final weight than in the TR group (p < 0.05). DO lambs showed higher FEC, lower PCV, and higher final weight, whereas SI lambs showed higher PCV (p < 0.05). FAMACHA© 3 predominated in the C group and in DO lambs (p < 0.05), which also required significantly more anthelmintic treatments. Reinfection reduced the allele frequencies associated with benzimidazole and levamisole resistance, mainly in the PR group, positively affecting the evaluated parameters. Although reversion of anthelmintic efficacy was transient, strategic reinfection at the end of the dry season may offer an alternative for producers unable to implement TR. When combined with the use of resistant breeds, this approach may help reduce production losses in countries where effective drug classes are unavailable.