Slight resistance to the residual effect of closantel in a field strain of Haemonchus contortus which showed an increased resistance after one selection in the laboratory.

On the strength of a complaint of suspected resistance to closantel, a field strain of Haemonchus contortus was isolated from sheep on a farm near Pretoria and tested for resistance to the remedy, both without exposure to closantel in the laboratory (designated the "unselected" substrain of the parasite) and after a single selection with closantel (called the "selected" substrain). For comparative purpose a few sheep were treated with rafoxanide to ascertain whether the efficacy of this drug was unchanged. While the unselected substrain appeared to show only slight increased resistance to the residual effect of closantel drenched at a dosage rate of 5 mg/kg, there was a dramatic increase in resistance after one selection. The residual efficacy (determined by the modified NPM test of Groeneveld & Reinecke, 1969, as outlined by Reinecke, 1973), 13-14 days after treatment with closantel was reduced from the registered claim of the remedy in South Africa of greater than 80% effective in greater than 80% of the treated flock to "ineffective", or less than 50% effective in less than 50% of the treated flock. Furthermore, there also appeared to be a slight increase in the resistance of this substrain to rafoxanide, as its efficacy was reduced from 99-100% to about 92%. This sharp reduction in efficacy after a single selection with closantel seemed to indicate that the slight increase in resistance of the unselected substrain was probably due not to possible variation in the repeatability of the trail method but to repeated treatment with closantel on the farm.