The resistance of a field strain of Haemonchus contortus to five benzimidazole anthelmintics in current use.

Using artificially infected sheep, the response of a laboratory strain of Haemonchus contortus, which had not been subjected to anthelmintics for three years, was compared with that exhibited by a parbendazole resistant field strain following treatment with each of seven anthelmintics, the laboratory strain proved fully susceptable to thiabendazole, parbendazole, cambendazole, mebendazole, fenbandazole, levamisole and haloxon, in contrast to the field strain which showed a moderate to marked resistance to all five benzimidazoles, but was fully susceptible to levamisole and haloxon.