Studies on Haemonchus contortus. II. The effect of abomasal nematodes on subsequent challenge with H. contortus.

Fifty-thousand infective larvae of Trichostrongylus axei dosed to 7-9-month-old Merinos protected them against a subsequent challenge with 50 000 infective larvae of Haemonchus contortus. Protection varied with the period between an initial dosage of T. axei and challenge with H. contortus as follows: After 90 days it was more than 80% effective in more than 80% of sheep; at 119 and 180 days there was still a significant reduction in the numbers of H. contortus recovered (P < 0,05 and P < 0,001, respectively). Prior infestation with either Ostertagia circumcincta or with Haemonchus placei was ineffective against challenge with H. contortus.