Effects of Benzimidazole Anthelmintics on Nematode Parasites of Sheep

Scott, Elspeth Warren (1988) Effects of Benzimidazole Anthelmintics on Nematode Parasites of Sheep. PhD thesis, University of Glasgow.

Abstract

A benzimidazole resistant strain (HFRO strain) and a benzimidazole susceptible strain (Bearsden strain) of the sheep abomasa] parasite Ostertagia circumcincta were obtained and passaged through parasite naive lambs. Using egg hatch assays to detect resistance day to day changes in the susceptibility of the eggs from adult O. circumcincta of these strains to the effects of the benzimidazoles, thiabendazole and albendazole were noted. Treatment of the lambs which were parasitised with the HFRO strain with fenbendazole at the normal dose rate caused a temporary increase in the resistance level measured by egg hatch assays. However, 12 treatments with fenbendazole over a 14 generation period with the HFRO strain produced a negligible rise in benzimidazole resistance for this strain. A dose titration trial to determine the effectiveness of fenbendazole against adult parasites of the HFRO strain of O. circumcincta confirmed that the parasites were highly resistant to the effects of the benzimidazole anthelmintics. Treatment with increasing doses of fenbendazole used in the dose titration trial failed to increase the level of resistance in the eggs produced by the adult parasites remaining after treatment. The establishment of 'clones' of the HFRO strain of O. circumcincta by implantation of pairs of adult parasites into the abomasa of lambs was investigated. The offspring of these parents showed a reduced level of resistance to the benzimidazoles measured by egg hatch assays but after treatment with fenbendazole selection for resistance occurred rapidly. The DNA from the two strains of O. circumcincta was isolated and compared after sectioning with restriction endonucleases. The banding obtained after gel electrophoresis was similar for the two strains. Studies were carried out on the motility of infective larvae of the HFRO and Bearsden strains of O. circumcincta in the presence of benzimidazole anthelmintics, their metabolites and physostigmine. Prolonged incubation in the presence of benzimidazoles was required to reduce motility. Differences in the motility in the presence of physostigmine between Denzimidazole resistant and benzimidazole susceptible strains of parasites were found. The activity of the benzimidazole metabolites against larval stages and eggs of O. circumcincta was studied and discussed. A possible mode of action of the benzimidazole group of anthelmintics is binding to nematode tubulin causing depletion of microtubular structures with disruption of cellular transport systems. Tubulin was extracted from the pig intestinal nematode Ascarissuum and the sheep nematodes O. circumcincta and Haemonchus contortus The benzimidazoles albendazole and mebendazole were shown to inhibit the binding of colchicine to tubulin extracts from Asuum. Tritiated albendazole, fenbendazole and mebendazole were synthesised and were used to study the binding of these drugs to tubulin extracts from Asuum, O. circumcincta and H. contortus Binding of these benzimidazoles was variable and was influenced by the protein content of the tubulin extract. High concentrations of tritiated benzimidazoles bound to extracts synthesised using low numbers (< 100,000 ) of infective larvae of O. circumcincta and H. contortus. The differential binding of tritiated benzimidazoles to tubulin extracts from nematodes as a test for resistance in these parasites to the effects of benzimidazole anthelmintics was investigated using tritiated fenbendazole, albendazole and mebendazole. Extracts from H. contortus larvae which were resistant to the benzimidazoles bound less mebendazole and albendazole than extracts from benzimidazole susceptible larvae. With tubulin extracts from different strains of O. circumcincta results were more variable and depended on the ligand used in the assays. Displacement of albendazole binding to tubulin extracts from H. contortus by albendazole and its metabolites , albendazole sulphoxide and albendazole sulphone, was found with albendazole proving to be the most potent inhibitor. However, in assays using O. circumcincta tubulin extracts albendazole, albendazole sulphoxide, albendazole sulphone and colchicine failed to inhibit, consistently the binding of tritiated albendazole. An ELISA to measure binding of anti-alpha and anti-beta tubulin antibodies to extracts from O. circumcincta larvae found reduced binding of both antibodies to extracts from first stage and third stage larvae of the benzimidazole resistant strain compared with similar extracts from the benzimidazole susceptible strain. Treatment of the larvae with benzimidazoles prior to the ELISA for tubulin did not alter the antibody binding to extracts from both the resistant and susceptible strains of O. circumcincta.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Veterinary science, Animal sciences, Animal diseases, Parasitology
• Date of Award: 1988
• Depositing User: Enlighten Team
• Unique ID: glathesis:1988-77692
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jan 2020 11:53
• Last Modified: 14 Jan 2020 11:53
• URI: https://theses.gla.ac.uk/id/eprint/77692