The experimental establishment of populations of Nematodirus battus and Trichostrongylus colubriformis, nematode parasites of the intestine of sheep, in laboratory rabbits : dose dependent features of the establishment and use of such populations in the study of resistance to anthelmintics

A study was made of Nematodirus battus and Trichostrongylus colubriformis infections in New Zealand White rabbits. Experimental studies with N. battus were not continued because of the unsuitability of the parasite in the host. The distribution and development of T. colubriformis in the intestinal environment of N.Z.W. rabbits was studied at various dose levels. The clinical symptoms and pathology of infected animals was studied. The nature of the changes in gut histology and the architecture of villi was established at two sites in the small intestine by examining paraffin sections and studying the surface architecture of villi. The structure of epithelial cells was examined with the electron microscope and the cellular changes were correlated with observations made on enzyme activity using histochemical techniques. The general picture during infection by large doses of T. colubriformis was that of the intestinal malabsorption syndrome. The development of challenge infections given on days 12 and 20 of a primary infection was studied on day 8 post challenge infection. The dose level at which no marked clinical, pathological or immunological changes occurred was used as the dose for the anthelmintic selection study. Rabbits were dosed with a rat strain T. colubriformis and the anthelmintic thiabendazole was used as a selection pressure at 45 mg/body weight against the adult parasite population. After three generations of selection pressure the anthelmintic selected and non-anthelmintic selected povalations were compared in an anthelmintic titration. Following this experiment it was recognised that an anthelmintic resistant population was present. The criterion of resistance was established by studying uterine e-9 counts and the cell development stages in the two populations in anthelmintic treated and untreated rabbits. It was also noted that the response of sheep and rabbit strain T. colubriformis to the anthelmintic was different, the latter was less responsive to the drug. The role of the host in the modification of the parasite phenotype was considered.