Changes in lc50in an in vitro egg development assay during the patent period of Haemonchus contortusin sheep

Using an in vitro egg development assay with thiabendazole as anthelmintic the LC50 of eggs produced by two sheep infected with either a susceptible or a resistant strain of Haemonchus contortuswas followed during the patent period from 22 days to 77 days after infection. In both sheep the LC50 followed the same pattern. At the start of the patent period the average values for the susceptible and the resistant strain were, respectively, 0·05 and 0·21 μg ml–1thiabendazole, after 40 to 60 days 0·09 and 0·35 μg ml–1, while after 77 days 0·07 and 0·24 μg ml–1were recorded.