Thirteen groups of 4 South African mutton Merinos grazed for 4 weeks with the flock on Kikuyu pastures and were slaughtered for total and differential worm counts at necropsy. Subsequently 12 groups of 8 week tracers grazed on the pastures and were killed for worm counts post mortem. The following were present in most sheep: Teladorsagia (syn. Ostertagia) circumcincta, Trichostrongylus axei, Trichostrongylus colubriformis, Dictyocaulus filaria and Oesophagostomum venulosum. Haemonchus contortus, Nematodirus spathiger and Trichuris skrjabini were less frequently recovered. Optimal conditions for infestation of grazing sheep occurred from June (late autumn)-October (spring) when mean temperatures in any 4 week period were <20 °C and a total of >40 mm of rain fell on 8 or more separate days. When the mean temperatures exceeded 20 °C pastures were safe, sheep acquiring< 1 000 worms in 4 weeks.
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