Protection of Merino lambs against Haemonchus contortus by trickle infection of neonates.

In two trials, lambs were trickle infected with 400 or 1000 infective larvae of Haemonchus contortus (HcL(3)), twice weekly for 4 weeks from the day of birth. Following anthelminthic treatment at 6 weeks (Trial 1) or 7 weeks of age (Trial 2), lambs were challenged 1 week later with a trickle infection totalling 5000 (Trial 1) or 10000 HcL(3) (Trial 2). In both cases, significant protection (P<0.05) ranging from 42 to 79% was achieved against egg and worm counts. Serum antibody responses as well as abomasal lymph node cell proliferation and production of interferon-gamma or interleukin (IL)-5 did not differ significantly between immunised and control lambs. The results are consistent with earlier findings that neonatal lambs can generate protective immunity against Trichostrongylus colubriformis, but the underlying mechanism(s) remain to be determined.