Monitoring the second generation of lambs after Haemonchus contortus replacement in ewes: effects of climate, sheep breed, and integrated control strategies in the tropics.

Anthelmintic resistance in Haemonchus contortus compromises parasite control and productivity in tropical sheep production systems. This study monitored the second generation of lambs following the H. contortus replacement. A multidrug-resistant H. contortus population was replaced by a susceptible isolate in periparturient ewes of the White Dorper (DO), Santa Inês (SI), and Texel (TX) breeds. They were kept in a nematode-free paddock (TR, n = 60) or in a paddock naturally infested with resistant helminths (PR, n = 60), and compared with untreated ewes harbouring resistant helminths from natural infection and kept in a naturally infested paddock (C, n = 60). The 121 lambs born in the second breeding season were phenotyped every 21 days based on faecal egg count (FEC), packed cell volume (PCV), weight, FAMACHA© score, and body condition score. In the second birth season, the efficacy of anthelmintics was reduced to 40%, 86% and 89% for the C, PR, and TR groups, respectively, compared with 60%, 74%, and 98%, respectively, in the first birth season (n = 106). SI and TX lambs had lower (p = 0.008) FEC than DO. The PR and TR groups had higher weight gain (p < 0.001), higher PCV (p = 0.009), and twice the anthelmintic efficacy compared with C. Humidity affected (p = 0.016) the number of infective larvae recovered per kg of dry pasture matterL3/kg DM in pasture. These findings demonstrate that the beneficial effects of worm replacement in ewes on anthelmintic efficacy and animal performance were temporary, persisting into the second generation of lambs, but diminishing over time. The sustainability of this strategy depends on climatic conditions, sheep breed resistance, and integration with broader parasite control programs.