Four groups of 6-7-month-old Merino lambs were each dosed with 40 000 infective larvae of Trichostrongylus
axei on 2 November 1976 and subsequently exposed to challenge with Haemonchus contortus on natural
grazing at the University of Pretoria's Experimental Farm in the eastern suburbs of Pretoria. One of these groups
and one group of controls were killed every 8 weeks from the end of December 1976-June 1977. Predosing with
T. axei was >50% effective against 5th stage and adult H. contortus in >50% of sheep for 164 days (Class C),
improving to >60% in >60% of sheep (Class B) 220 days after dosing T. axei. The numbers of retarded 4th
stage larvae (L₄) of H. contortus in the undosed controls as well as in the sheep predosed with T. axei rose from a
low level in summer (December) to a peak in late Autumn (June).
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