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Further studies of the effect of selection with levamisole on a benzimidazole resistant population of Ostertagia spp. of sheep

Waller, P.J., Dobson, R.J. and Donald, A.D. (1983) Further studies of the effect of selection with levamisole on a benzimidazole resistant population of Ostertagia spp. of sheep. International Journal for Parasitology, 13 (5). pp. 463-468.

Link to Published Version: http://dx.doi.org/10.1016/S0020-7519(83)80009-5
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Abstract

Fresh isolations of Ostertagia spp. from sheep in a continuing field experiment confirmed a substantially lower level of benzimidazole resistance in the progeny of worms which had survived repeated doses of levamisole than in the progeny of those not exposed to any anthelmintic for 7 months. These resistance levels had remained unchanged for 4 months. Five generations of laboratory selection with levamisole had no effect on the level of benzimidazole resistance in a highly resistant line of the parasite isolated from the same field experiment. The effect of levamisole treatment in the field experiment was probably not due to a negative correlation between benzimidazole and levamisole resistance in the parasitic stages, but could be explained by the population dynamics of the parasite. It is postulated that levamisole treatment resulted in the replacement of a highly benzimidazole resistant parasitic population by one reflecting a lower frequency of resistant individuals in the infective larval population on pasture at the time of the first levamisole dose. Despite gradually increasing persistent faecal egg counts of Ostertagia spp. in the grazing sheep receiving 4-weekly doses of levamisole, the progeny of these worms, either as fourth stage larvae or as adults, were not resistant to levamisole.

Publication Type: Journal Article
Publisher: Elsevier BV
Copyright: © 1983 Published by Elsevier Ltd.
URI: http://researchrepository.murdoch.edu.au/id/eprint/8070
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