Changes in LC50 in an in vitro egg development assay during the patent period of Haemonchus contortus in 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 contortus was 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 micrograms ml-1 thiabendazole, after 40 to 60 days 0.09 and 0.35 micrograms ml-1, while after 77 days 0.07 and 0.24 micrograms ml-1 were recorded.