Decreased ivermectin and moxidectin sensitivity in Haemonchus contortus selected with moxidectin over 14 generations

Ivermectin resistance in the nematode Haemonchus contortus has been reporte d in many parts of the world and many ivermectin resistant isolates have be en found to have reduced sensitivity to moxidectin. However, it is unclear whether parasites that are selected with moxidectin would demonstrate reduc ed sensitivity to ivermectin. In this study, the effects of moxidectin and ivermectin on an unselected strain and a strain of H. contortus derived fro m the unselected strain but selected over 14 generations with moxidectin, w ere compared in jirds. The recovery of adult worms and fourth stage (L-4) l arvae following treatment were compared between strains and anthelmintics. Moxidectin-selected H. contortus showed reduced sensitivity to ivermectin a s well as to moxidectin. Doses of 0.1 mg/kg of moxidectin and 0.4 mg/kg of ivermectin were necessary to obtain an efficacy of 95% or above against the moxidectin-selected strain of H. contortus compared with 0.025 mg/kg for m oxidectin and 0.1 mg/kg for ivermectin required for a similar efficacy in t he unselected strain. (C) 1999 Elsevier Science B.V. All rights reserved.