Studies on Haemonchus contortus. XI. The effect of a bovine strain of Trichostrongylus axei in Merinos on natural pastures heavily infested with H. contortus.

Sheep grazed on natural pastures heavily infested with infective larvae of Haemonchus contortus. Sixty-eight weaned Merinos were divided into 6 groups on Day 0 (23 November 1977), and on Day +14 (7 December) 79 Merinos were divided into 7 groups. There were 2 groups of undosed controls and other groups were either dosed with infective larvae of Trichostrongylus axei (bovine strain) only on Days 0 and +14, or in combination with H. contortus, or with subsequent doses of H. contortus, 28 days later. One group (Group 12) was dosed with T. axei and treated with a subcutaneous injection of di-iodonitrophenol (DNP) on Day +14. With the exception of 2 sheep, the sheep of the first 6 groups survived until slaughter in March and April 1978, while many sheep (43) of the latter 7 groups died or were killed in extremis from March-May. T. axei dosed on 23 November (Day 0) protected Group 2 by greater than 50% in greater than 50% of sheep. In the latter 7 groups the best results were achieved when DNP was combined with predosing with T. axei. The poor results were probably due either to delayed predosing with T. axei or a massive challenge in the wettest summer on record.