Comparison of anthelmintic control programs for sheep in Virginia.

Haemonchus contortus is the most serious parasitic problem encountered by sheep producers in southwestern Virginia. Four anthelmintic control programs for grazing lambs were tested. Group TR received monthly SC injections of ivermectin (200 micrograms/kg of body weight). Group SI received the same dose of ivermectin at 0, 3, 6, 9, and 12 weeks after the start of the grazing season. Group SL was given levamisole (8 mg/kg, PO) 0, 3, 6, 9, and 12 weeks after the start of the grazing season, and group TA received ivermectin (200 micrograms/kg) 0, 8, 16, 20, and 24 weeks after the beginning of grazing. None of the 4 programs provided satisfactory control of parasites as indicated by fecal egg counts and serum pepsinogen concentrations, although group-TR lambs gained significantly more weight than lambs in groups SI and TA. Group-TA lambs developed clinical haemonchosis in early August and required additional treatment at that time. These findings suggest that reliance on anthelmintics alone may not provide the most effective and economic control of parasitic infection.