Inhibited development of trichostrongylids of sheep in northern Nigeria.

Worm counts from Yankassa ewes, lambs and tracer lambs grazing unimproved pasture at Zaria were obtained each month during the rainy season of 1976. In June and July up to 14 per cent of the Haemonchus contortus populations consisted of early fourth stage larvae (L4). In August this increased to more than 55 per cent and in October to more than 95 per cent. From June to August most of the abundant late L4 contained rodlike crystals. More than 95 per cent of the Haemonchus contortus populations of two lambs killed two weeks after infection with infective larvae, cultured for seven days at 25 degrees to 30 degrees C in the dark, were early L4. Inhibited 3rd stage Trichostrongylus spp did not exceed 26 per cent of the worm populations.