Resistance to reinfection with Haemonchus contortus in lactating and barren ewes following a single infection with larvae conditioned for inhibited development.

Ewes were infected in November 1977 with third stage larvae of Haemonchus contortus which were conditioned for inhibited development at +15 degrees C for 35 days. Worm counts in December and May and weekly faecal egg counts revealed that the high burdens of inhibited larvae present in December virtually disappeared during winter, while the faecal egg counts remained low. This happened whether the ewes were pregnant or not. Indications of a slight postpartum rise in faecal egg counts, however, were observed in May in three out of five lactating ewes. After experimental reinfection of seven barren ewes and five lactating ewes in May 1978 a considerable resistance to H. contortus was observed in the barren ewes, while the resistance to reinfection was negligible in the lactating ewes.