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

Abstract

The serological reactions to infestation with H. contortus were studied in penned sheep. For critical studies, sheep born and reared in a worm-free environment were used. In infestation with H. contortus, the antibody response was stimulated by the administration of third-stage infective larvae and as the infestation developed the antibody response declined. Established infestation with mature H. contortus did not stimulate the formation of antibodies. The sera of sheep dying from infestation with H. contortus were negative to the complement fixation test. Ground, mature H. contortus given to sheep by mouth or injected into the rumen did not elicit any antibody response. Infective larvae of H. contortus, killed by heat and given to sheep by month, did not stimulate an antibody response. When heat-killed larvae mere injected into the rumen, a rise in titre at times took place, but it was usually transient. When a dose of infective larvae of H. contortus was superimposed upon an existing infestation of H. contortus or Trichostrongylus spp. either the existing infestation was eliminated or the egg output of the parasites was suppressed. This phenomenon was accompanied by a rise in serum titre. Elimination of the infestation of H. contortus or Trichostrongylus spp. was not usually followed by resistance to the development of the superimposed larvae. The injection into the rumen of heat-killed larvae of H. contortus did not have the same effect as the oral administration of live larvae. Efforts to protect sheep against infestation with either H. contortus or Trichostrongylus spp. by means of vaccines prepared from ground, infective larvae were unsuccessful. Evidence was obtained that the artificial stimulation of antibodies, by means of intravenous injections of formalinized, ground larvae, influenced the course of existing infestations with H. contortus.