Studies on resistance of sheep to infestation with Haemonchus contortus and Trichostrongylus spp. and on the immunological reactions of sheep exposed to infestation. III. The antibody response to infestation with Trichostrongylus spp.

In infestation of sheep with Trichostrongylus spp. , both the intake of larvae and infestation with adult worms stimulated the production of antibodies. The character of the antibody response in infestation with Trichostrongylus spp. thus differed from that caused by Haemonchus contortus. Sera of sheep dying from infestation with Trichostrongylus spp. were negative to the complement fixation test. It was found that older sheep responded serologically earlier and more vigorously to infestation with Trichostrongylus spp. than did young sheep. Subsequent doses of larvae of Trichostrongylus spp. elicited a more rapid and grester serological response, even in young sheep, than an initial dose of larvae. Previous infestation with H. contortus did not result in resistance to Trichostrongylus spp. , whereas previous infestation with Trichostrongylus spp. heightened the resistance of sheep to subsequent infestation with the same species. It is concluded that the relative resistance of older sheep to Trichostrongylus spp. is due, a t least in part, to an earlier immunological response which is strengthened by subsequent doses of larvae. When infective larvae of Trichostrongylus spp. were superimposed upon an existing infestation of the same species, the egg count declined ;apidly and the development of the superimposed larvae resisted Doses of infective larvae of Trichostrongylus spp. had no apparent effect on the egg counts of sheep infested with H. contortus .