The effects of anthelmintics on ovine larval nematode parasite migration in vitro.

The anthelmintics ivermectin, levamisole, morantel tartrate and thiabendazole all inhibited, in vitro, the motility of third stage larvae (L3) of Trichostrongylus colubriformis. The bioassay, based on the inhibition of L3 migration from agar gels, yielded sigmoid dose-response curves for ivermectin, levamisole and morantel tartrate, but not thiabendazole. The concentration of levamisole giving 50% inhibition of migration (EC50) was determined for Cooperia curticei, Haemonchus contortus, Nematodirus spathiger, Ostertagia circumcincta, Trichostrongylus axei, T. colubriformis and T. vitrinus. EC50s differed between species but within species the EC50s for ensheathed and exsheathed L3 were similar except for N. spathiger which showed significantly higher EC50 for the ensheathed L3. No difference between EC50s for levamisole-resistant and susceptible strains of T. colubriformis were found. Similarly, morantel-resistant and susceptible strains of T. colubriformis could not be differentiated in this bioassay. The inhibition of L3 motility by known anthelmintic compounds in this bioassay suggests that the bioassay could be used as a screen for potential new anthelmintics.