Thiabendazole resistance in field populations of Haemonchus contortus.

Six populations of H. contortus were selected for a study of thiabendazole resistance from a collection of 40 populations made during a survey of the efficiency of thiabendazole on the Northern Tablelands of New South Wales. Based on survey results, 3 of these populations were considered susceptible and the remaining 3 were considered resistant. However, when these populations were compared with a known susceptible strain on the ability of their eggs to hatch in a solution containing thiabendazole and 0.1% NaCl all 6 had significantly greater LC50's. The resistance ratios of the LC50 for each of the 6 populations to that of the known susceptible strain were 5.1, 4.3, 3.2, 3.1, 2.3, and 1.6 respectively. Following dosing of their host with 44 mg/kg thiabendazole the resistance ratios of the survivors increased to 5.4, 5.1, 4.7, 4.4, 3.1 and 2.4. Eggs produced by the F1 generation of the worms surviving 44 mg/kg thiabendazole did not revert back to the lower LC50's of the unselected parents. Rather the LC50 remained at a level near that of a known highly resistant strain of H. contortus.