The effect of oxfendazole terminated infections with Haemonchus contortus on the development of immunity in sheep

The relative contribution of third (L3), fourth (L4) or adult stages of Hae monchus contortus to the development of immunity was evaluated in three gro ups of sheep subjected to infections terminated by oxfendazole treatments a t the L3, LA or adult stage. A control group did not receive immunising inf ections. All the groups were challenged with 5000 L3, to evaluate the prote ction provided by the different protocols. All sheep were necropsied at the end of the experiment to count the abomasal worm burdens. A marked reducti on in egg counts after challenge infection was only observed in sheep in wh ich the infection was terminated in the adult stage (Group 4). A significan t reduction in worm burden was also observed in Group 4. The immunising inf ections and/or the challenge infection resulted in moderately elevated IgG antibody levels against L3, LA and adult somatic antigens in all the groups . In contrast, a strong IgG response against H. contortus excretory/secreto ry (ES) antigens was observed in the groups in which the immunising infecti on was terminated in the LA and the adult stage. An elevated lymphocyte pro liferation response against Haemonchus ES antigens was found only in the gr oup that had their immunising infection terminated at the adult stage. The combined data suggest that exposure to and elicited immunological responses to ES antigens are important for the development of immunity against H. co ntortus. (C)2000 Elsevier Science B.V. All rights reserved.