Efficacy of abamectin against ivermectin-resistant strain of Trichostrongylus colubriformis in sheep

The efficacy of two formulations of abamectin, i.e. oral and injectable was determined against ivermectin-resistant strain of T. colubriformis in sheep. Twenty-four lambs were infected with 10,000 third stage larvae of ivermectin-resistant strain of T colubriformis. Twenty-four days post-infection, the lambs were divided randomly into four groups of six animals each according to egg counts. The first group was left untreated and kept as a control. The second group was treated with ivermectin (oral) at 0.2 mg kg(-1) body weight. The third group was treated with oral formulation of abamectin at 0.2 mg kg(-1) body weight. The fourth group was treated with injectable formulation of abamectin at 0.2 mg kg(-1) body weight. Fecal egg count and controlled slaughter tests were employed to determine the efficacy of abamectin (oral and injection) against ivermectin-resistant strain of T. colubriformis in sheep. Reduction in arithmetic mean fecal egg counts achieved by ivermectin (oral), abamectin (oral) and abamectin (injection) was 66, 98 and 76%, respectively 10 days after treatment. Ivermectin (oral), abamectin (oral) and abamectin (injection) reduced arithmetic mean worm burden by 63, 97 and 74%, respectively. The findings demonstrated that abamectin oral formulation was more effective than abamectin injection against ivermectin-resistant strain of T. colubriformis in sheep. (C) 2004 Elsevier B.V. All rights reserved.