Consequences of anthelmintic resistance on liveweight gain of lambs on commercial sheep farms

AIM: To assess the consequences of anthelmintic resistance on commercial sh eep farms on liveweight (LW) gains and susceptibility to diarrhoea of growi ng lambs. METHODS: Five farms with a history of resistance to benzimidazole drenches were selected. On each farm, 150 ewe lambs were randomly allocated to 1 of 3 treatment groups (n=50). Group 1 was treated 5 times at 28-day intervals with oxfendazole (to which nematode resistance had been demonstrated); Grou p 2 was treated 5 times at 28-day intervals with levamisole or levamisole oxfendazole (to which nematode resistance had not been demonstrated) and; Group 3 was treated with a combination of slow-release albendazole capsules and oral moxidectin (intended to totally suppress nematode infection). Dif ferentiated faecal egg counts (FECs), FEC reduction tests (FECRT), LW gain and degree of faecal soiling around the breech (dag scores) were determined and compared between groups. RESULTS: pre-treatment FECs in Group 1 were similar to or slightly lower th an those of Group 2. Efficacy of oxfendazole in Group I varied between farm s and over time within farms, the reduction in FECs after treatment varying from 0% to >95%. The genera identified as resistant on each farm also vari ed over time. Lambs from Group 3 had lower FECs, lower dag scores and highe r growth rates than lambs from Group 2, which in rum had higher LW gains th an lambs from Group 1. The overall LW gains during the 5 months of the stud y were 7.03 +/- 0.27 kg for Group 1, 8.33 +/- 0.27 kg for Group 2, and 9.91 +/- 0.31 kg for Group 3 (mean + SEM; means differed significantly from eac h other, p<0.01). CONCLUSIONS: The decrease in growth rates associated with the continued use of an incompletely effective anthelmintic were surprisingly small in situa tions where all lambs grazed together.