Studies on resistance of sheep to infestation with Haemonchus contortus and Trichostrongylus spp. and on the immunological reactions of sheep exposed to infestation. IV. The antibody response to natural infestation in grazing sheep and the 'self-cure' phenomenon

Weekly tests, over a period of eighteen months, of the sera of a flock of grazing sheep showed a relationship between the serum titres and the faecal egg counts. This relationship was particularly obvious during the summer months. When egg counts fell, the titres rose, and as the titres dropped, the egg counts usually rose again. This relationship between egg counts and serum titres was more striking in individual sheep than in the whole flock because all the sheep did not behave similarly at the same time. The antibody levels rose in individual sheep irrespective of whether the sheep were heavily or lightly infested at the time. There was no evidence that a permanent resistance to Haemonchus contortus was acquired under field conditions. Seven periods of "self-cure" occurred in the grazing flock during the period of observation. On each occasion, the majority of sheep which showed a drop in egg count also showed a rise in serum titre. This behaviour was strikingly similar to the result when infective larvae of H. contortus were superimposed upon an existing infestation of H. contortus or Trichostrongylus spp. in penned sheep. 'Self-cure' was produced in the field by giving naturally infested sheep large doses of infective larvae of H. contortus by mouth. A statistically significant fall in the faecal egg count of H. contortus and Trichostrongylus spp., but not of Oesophagostomum columbianum was produced by this means. The fact that 'self-cure' takes place in naturally grazing flocks during the summer months after rain indicated that the intake of large doses of infective larvae of H. contortus was the exciting cause of the phenomenon.