Haemonchus contortus: attempts to immunise lambs with irradiated larvae.

Two doses of irradiated Haemonchus contortus larvae stimulated firm resistance to challenge in adult worm-free sheep but not in two-month-old lambs. Parenteral administration of larval antigens in adjuvant together with the two doses of irradiated larvae did not reduce the susceptibility of the lambs. However four doses of the attenuated larvae stimulated three of six lambs to resist the challenge. Similar concentrations of serum IgG and mucosal IgA and IgG antibodies were detected in the sheep which had been vaccinated with irradiated larvae only, regardless of their immune status. The lambs which had been injected with worm antigens had much higher levels of circulating and mucosal IgG antibodies, but mucosal IgA concentrations were similar to those in the other immunised groups. Histological examination of the abomasal mucosa did not reveal any differences between resistant and susceptible vaccinates.