Prophylactic efficacy of moxidectin for periparturient ewes and mid-summer lambs
Taylor, S.M.; Edgar, H.; Kenny, J.
Veterinary Record 133(11): 270-271
1993
ISSN/ISBN: 0042-4900 PMID: 8236651 Document Number: 414772
40 Cheviot cross Suffolk ewes aged 3 to 5 years with positive faecal nematode egg counts, which had lambed approximately 3 weeks previously and were grazing pasture used by sheep were allocated into 4 groups to be treated with moxidectin 0.1% oral drench at 0.2 mg/kg, ivermectin (Oramec) 0.08% oral drench at 0.2 mg/kg or fenbendazole (Panacur) 2.5% oral drench at 5 mg/kg; a fourth group was the untreated control group. During the 8 week observation period geometric mean faecal egg counts (measured weekly) of the control group rose from 273.3 before treatment to 1011.9 on day 35 after treatment. The efficiacy of treatment (calculated using the WAAVP recommended method) was 100% on day 7 after treatment for the moxidectin treated group and fell until day 49 when it was 99.8%. Efficacies of treatment were 99.8% and 99.5% on day 7, 96.6% and 89.5% on day 35 and 64.5% and 36.8% on day 49 for the ivermectin and fenbendazole groups, respectively. In the second experiment 66 lambs, offspring of the ewes in the first experiment, with positive faecal counts were allocated by weight and randomised block to 5 groups and kept on the same pasture as before. During the 5 week experiment the geometric mean faecal egg count of the control group increased from 410.8 to 1671.5 on day 28 after treatment. Respective efficacies for groups receiving moxidectin 0.1% oral drench at 0.2 mg/kg, moxidectin 1% injectable at 0.2 mg/kg, ivermectin (as before) or fenbendazole (as before) on day 7 after treatment were 100%, 100%, 100% and 99.5% and remained of this order till day 21 when efficacies were 99.6%, 100%, 87.7% and 15.4% respectively. On day 35 efficacies had declined to 95.3, 97.3, 31.5 and 0%, respectively. Culture of faecal samples showed the nematode populations were mainly Trichostrongylus, Cooperia and Ostertagia species. Thus although all the anthelmintics were highly effective at day 7 after treatment, the length of their prophylactic potency differed. The prolonged activity of moxidectin may be beneficial but a slowly declining efficiacy may invite selection for resistance to its method of action.