THE DEATH RATE OF OSTERTAGIA-CIRCUMCINTA AND TRICHOSTRONGYLUS-COLUBRIFORMIS IN LACTATING EWES - IMPLICATIONS FOR ANTHELMINTIC RESISTANCE

Lactating adult Romney elves were infected, 4 weeks post-lambing, with benzimidazole (bz) resistant strains of Ostertagia circumcincta and T richostrongylus colubriformis. Commencing 4 weeks after the initial in fection the ewes were subjected to challenge 3 times weekly with 5000 L3 of bz-susceptible strains of both parasite species. At weekly inter vals over the following 6 weeks, groups of ewes were drenched with a b z anthelmintic (oxfendazole) to remove bz-susceptible parasites and sl aughtered to determine adult worm burdens of the bz-resistant parasite s. The O. circumcincta infection declined exponentially with a mean da ily death rate of 10.6% day(-1) and no worms were recovered after 4 we eks or more of challenge. The T. colubriformis infection did not decli ne significantly over the 6 weeks of continuous challenge, indicating that the death rate could not be distinguished from zero. The upper 95 % confidence limit for the death rate of T. colubriformis was 4.9%. Th e implications of these death rates on selection for drug resistance f ollowing ewe drenching during the post-partum period are discussed wit h selection pressure likely to be greater for T. colubriformis than fo r O. circumcincta. (C) 1997 Australian Society for Parasitology, Publi shed by Elsevier Science Ltd.