DURATION OF THE PERSISTENT ACTIVITY OF MOXIDECTIN AGAINST HAEMONCHUS-CONTORTUS IN SHEEP

Weaned lambs were infected with Haemonchus contortus 35, 28, 21, 14 an d 7 days after treatment with moxidectin at 0.2 mg/kg and 35 and 14 da ys after treatment with ivermectin at the same dose rate, Worm counts 14 days after infection showed that moxidectin prevented the establish ment of over 99% of infective larvae for 28 days and reduced the estab lishment rate at 35 days by 96%, relative to ivermectin. There was no difference in the protective efficacy of ivermectin at 14 or 35 days. The persistence of moxidectin is likely to provide advantages in nemat ode control, particularly when used as a strategic early summer treatm ent or as a pre-lambing treatment to ewes. Implications of the persist ent activity of moxidectin for the development of resistance during th e decay phase are discussed.