Resistance to Haemonchus contortus in sheep given repeated daily doses of 10 000 infective larvae

Abstract

Two experiments on the effects on sheep of prolonged repeated (5 days in every week) infection with 10 000 infective larvae of Haemonchus contortus are reported. In the first experiment 7 sheep were infected for 6 months. Regular observations were made of haematocrit and faecal egg count. Two sheep required anthelmintic treatment once, 2 sheep required 3 treatments, the remaining 3 sheep required no anthelmintic support. At the end of the 6-month period all 7 sheep had acquired a high level of resistance. In the second experiment, this acquisition of resistance while subject to a weekly intake of 5 × 10 000 L 3 was studied in 7 sheep which were fitted with abomasal cannulae. These sheep were infected repeatedly for 8 to 22 weeks. The acquired resistance was not associated with an abnormally high gastric pH. Large numbers of globule leucocytes were present in the fundic mucosa of all 14 sheep at the conclusion of the experiments.