INTERACTION BETWEEN OSTERTAGIA-CIRCUMCINCTA AND HAEMONCHUS-CONTORTUS INFECTION IN YOUNG LAMBS

Twenty-one-week-old, worm-free, pen-reared lambs were infected with ei ther 6000 O. circumcincta L3 per week, or 3000 H. contortus L3 per wee k, or both (9000 L3 per week). Egg counts were monitored throughout th e experiment, and worm burdens and larval establishment rates of both worm species were estimated after 4, 7, 10 and 13 weeks of infection. After 10-13 weeks of infection with H. contortus only, establishment o f O. circumcincta was lower than in previously uninfected controls, de monstrating that a high level of immunity to H. contortus affords some cross-protection against O. circumcincta. Total H. contortus worm bur dens and egg counts (about 2000 worms and 3000 e.p.g., respectively) i n sheep infected with both worm species were less than half those obse rved in sheep infected with H. contortus alone (about 5000 worms and 1 0,000 e.p.g., respectively), Cross-protection between the two species was observed, but was probably less important than the reduction in H. contortus establishment that was caused by O. circumcincta disrupting abomasal physiology.