Studies on Haemonchus contortus. III. Titration of Trichostrongylus axei and expulsion of H. contortus

Abstract

Groups of Merino weaners were dosed with infective larvae of Trichostrongylus axei in numbers ranging from 20 000 - 50 000 and challenged 3 months later with 50 000 infective larvae of Haemonchus contortus. When half the dose of infective larvae of T. axei was given on Day 0 and the balance on Day + 14, efficacy against H. contortus was >60% in >60% of sheep (P < 0,1). A single dose of 40 000 or 50 000 infective larvae of T. axei was > 80% effective against H. contortus in > 80% of sheep (P<0,01). Two doses of 20 000 infective larvae of T. axei followed by a challenge with H. contortus 31-33 days after the initial dose caused a reduction of > 50% in > 50% of sheep (P < 0, 1). This rose to > 60% in > 60% of sheep if the doses of 25 000 infective larvae of T. axei were followed by a challenge with H. contortus 45 days after the initial dose of T. axei. Most of the challenge doses of infective larvae of H. contortus were rejected within 3 days. Surviving worms were retarded in the 4th stage and only a few developed to the 5th or adult stage.