Worm replacement with susceptible Haemonchus contortus benefits weight gain, reduces anthelmintic treatments and impacts sheep breeds differently.

This study evaluated the effect of partial and total replacement of Haemonchus contortus in sheep breeds. Pregnant ewes of White Dorper (DO), Santa Inês (SI) and Texel (TX) breeds were allocated into three groups: Control (C), Partial Replacement (PR) and Total Replacement (TR). PR and TR ewes received anthelmintics (AH), were artificially infected with H. contortus-susceptible isolate and grazed on resistant-infested or worm-free paddocks, respectively. Control animals were untreated and naturally infected. 106 lambs were born and kept in the paddocks of their respective mothers. Their egg count per gram of feces (FEC) and packed cell volume (PCV) were recorded every 21 days, until 189 days old. Fecal Egg Count Reduction Test revealed AH efficacy of 85 %, 92 % and 97 % in the C, PR and TR groups, respectively, for ewes and 60 %, 74 % and 98 %, respectively, for lambs at day 147. SI animals (p < 0.001) received fewer AH treatments and presented a higher PCV than DO and TX, reaching similar weight gain to the other breeds. Male lambs (p < 0.001) presented a higher FEC, lower PCV and lower frequency of Famacha score 1. Higher age at weaning resulted in a lower FEC (p = 0.0073), higher PCV (p = 0.002), and higher frequencies of Famacha 1 and body condition scores 3 and 4. AH treatment was more efficient after worm replacement, reducing FEC (p < 0.001) and favoring weight gain in the PR and TR groups. Avoiding early weaning, adopting selective AH treatments and using more resistant sheep breeds may delay the reestablishment of resistance after worm replacement.