(ii) LDA on the H. contortus isolate.

The LDA on the H. contortus isolate measures the potency of anthelmintics in inhibiting the development of trichostrongyle nematodes from eggs to infective L3 larvae. Nematode eggs were recovered from fresh fecal matter by using a standard procedure described previously (36). Briefly, eggs were incubated with 30 μg of inactivated E. coli bacteria, and 10 μg of amphotericin B (Fungizone; Squibb) was added per ml of egg suspension to avoid proliferation of fungi during larval development. Tubes were incubated at 23°C for 48 h. By this time, eggs had hatched and developed to the L1 or L2 stage. After 48 h, larvae were supplemented with nutrient medium, Earle's balanced salt solution, yeast extract (1 g of yeast extract/90 ml of saline solution [pH 7]), and anthelmintics and then incubated for 7 days at 23°C. The proportion of developed L3 compared to the total number of larvae (L3 plus undeveloped stages [L1 plus L2]) present under each condition was calculated and expressed as a percentage, with the mean number of developed larvae under control conditions being fixed at 100.