Development of benzimidazole resistance in a Haemonchus contortus strain in the Netherlands following fenbendazole treatment of ewes at parturition.

In contrast to observations in former years treatment of ewes after parturition with 5 to 7.5 mg/kg fenbendazole was not fully effective in removing Haemonchus contortus. Comparison of the H contortus populations in the lambs of these ewes and those of untreated ewes later in the year revealed that the treatment resulted in fenbendazole resistance and also in side resistance to other benzimidazoles. Though the faecal egg counts of the treated ewes were low, the prophylactic effect of the treatment on the population build up of H contortus in the lambs was very limited.