Suitable experimental groups of calves for controlled anthelmintic tests were created by repeatedly dosing susceptible worm-free animals orally with infective larvae of Haemonchus placei, Ostertagia ostertagi, Oesophagostomum radiatum and Cooperia spp. (C. pectinata plus C. punctata) and giving a single percutaneous dose of Bunostomum phlebotomum. Calves were infested in such a way that at treatment the worms were either present as third stage larvae, fourth stage larvae or fifth and adult stages. Enough calves were infested to enable the data to be interpreted by the non-parametric method. Optimal results were achieved by testing compounds against a specific stage of development. A combined test was evolved where two groups of 11 calves were treated when the worms were at different stages of development but only a single group of 9 control calves was used. For more accurate worm counts delaying the slaughter of calves for 3 to 4 weeks after administering the final dose of infective larvae is advocated. Nylon grit gauze with 500 micron apertures allows worms to migrate more easily into the filtrate of the ingesta than nylon mesh with 225 micron apertures in which they tend to become trapped.
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