Weaned Dorper lambs on natural pasture were predosed with 40 000 infective larvae (L₃) of Trichostrongylus
axei, irradiated (0,3 kGy) L₃ of T. axei or closantel at 10 mg/kg either in September or November 1978 and
were compared with Merino yearlings predosed with 40 000 L₃ of T. axei in November 1977. The following
summer (December- March) only 178,6 mm of rain fell and very few H. contortus were present on pastures.
Artificial challenge with 20 000 L₃ of Haemonchus contortus with the local strain from the University of
Pretoria's experimental farm was given 6--7 months after predosing with T. axei. When compared with the
controls, significant reductions occurred only in Group 11 (T. axei irradiated at 0,3 kGy on Day +63)
(P=0,025); Group 2 (T. axei on Day 0) (P = 0,003) and Group 4 (T. axei and closantel on Day 0) (P = 0,049).
We concluded that predosing with T. axei was unsuccessful in Dorpers and Merinos artificially challenged 6--7
months later with H. contortus.
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