Selection of ivermectin-resistant Trichostrongylus colubriformis in lambs

摘要: A mixed population of Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis was passed for four generations through lambs. In successive passages on Day 6 (D6), lambs were dosed subcutaneously with ivermectin at 100, 200, 250 225 micrograms kg-1 body weight, respectively. No larvae H. contortus recovered after the first passage following third drug exposure only T. present. Larvae selected strain developed to stage, sheathed in vitro concentrations that inhibited development from original colubriformis. definitive control test then run 45 lambs, compare this strain. The treatments 150 subcutaneous (SC) D6, 200 SC intraruminal (IR) D6 D21; all compared non-treated controls, or as applicable. IR greater than 99% effective both strains, although treatment 85 48% strain, D21 58% against There significant (P less 0.001) differences between strains fecal egg output worm counts. immature partial resistance.