title: The death rate of Ostertagia circumcincta and Trichostrongylus colubriformis in lactating ewes: implications for anthelmintic resistance.
abstract: Lactating adult Romney ewes were infected, 4 weeks post-lambing, with benzimidazole (bz) resistant strains of Ostertagia circumcincta and Trichostrongylus colubriformis. Commencing 4 weeks after the initial infection the ewes were subjected to challenge 3 times weekly with 5000 L3 of bz-susceptible strains of both parasite species. At weekly intervals over the following 6 weeks, groups of ewes were drenched with a bz anthelmintic (oxfendazole) to remove bz-susceptible parasites and slaughtered to determine adult worm burdens of the bz-resistant parasites. The O. circumcincta infection declined exponentially with a mean daily death rate of 10.6% day-1 and no worms were recovered after 4 weeks or more of challenge. The T. colubriformis infection did not decline 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%. The implications of these death rates on selection for drug resistance following ewe drenching during the post-partum period are discussed with selection pressure likely to be greater for T. colubriformis than for O. circumcincta.
authors.affiliation: AgResearch, Grasslands, Palmerston North, New Zealand. leathwickd@agresearch.cri.nz
authors.forename: D M
authors.initials: DM
authors.lastname: Leathwick
authors.forename: C M
authors.initials: CM
authors.lastname: Miller
authors.forename: A
authors.initials: A
authors.lastname: Vlassoff
authors.forename: I A
authors.initials: IA
authors.lastname: Sutherland
mesh_terms: D000818:Animals; D000871:Anthelmintics / Q000494:pharmacology*; D001562:Benzimidazoles / Q000494:pharmacology*; D004351:Drug Resistance; D005243:Feces / Q000469:parasitology; D005260:Female; D007774:Lactation; D010028:Ostertagia / Q000187:drug effects; D010029:Ostertagiasis / Q000188:drug therapy* / Q000276:immunology / Q000662:veterinary; D010270:Parasite Egg Count; D012756:Sheep; D012757:Sheep Diseases / Q000188:drug therapy* / Q000469:parasitology; D014253:Trichostrongylosis / Q000188:drug therapy* / Q000276:immunology / Q000662:veterinary; D014254:Trichostrongylus / Q000187:drug effects
chemical_list: D000871:Anthelmintics; D001562:Benzimidazoles; C011030:oxfendazole