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An experimental investigation of the nature and the mechanism of the "self-cure" phenomenon in helminthiasis of sheep has been carried out. As a result of the "self-cure" reaction there was no release of heterologous antibodies into the blood stream of the animal. The administration of an antihistamine drug usually prevented the fall in egg count which is characteristic of the "self-cure" reaction, but did not interfere with the subsequent rise in titre of the specific or homologous antibody in the serum. On 7 of 13 occasions when sheep showed the "self-cure" reaction, the blood histamine level rose to about 0.05 µg per ml. during a period of about 24 hours between the second and fourth day after the administration of larvae. On none of 13 occasions when sheep failed to manifest "self-cure" was there any rise in blood histamine. The difference in behaviour of the two groups was significant statistically. The rise in blood histamine was demonstrated when sheep infested with either H. contortus or Trichostrongylus spp. were given H. contortus larvae and manifested the "self-cure" reaction. It was shown to occur also when "self-cure" occurred in sheep infested with Trichostrongylus spp. following the administration of Trichostrongylus spp. larvae. Increase in blood histamine was not demonstrated when Trichostrongylus spp. larvae were given to seven sheen infested with H. contortus and when "self-cure" did not take place. The increase in blood histamine as not associated with any consistent alteration in the concentration of eosinophils. There was no evidence that the increase in blood histamine was the direct cause of "self-cure". Sheep which manifested "self-cure" showed a strong local reaction to the intradermal inoculation of H. contortus antigen. Continued infestation with mature H. contortus or with mature Trichostrongylus spp. did not sensitize the sheep and sheep which failed to manifest "self-cure" did not exhibit an intradermal reaction. An oedematous change was evident in the mucous membrane of the Abomasum or small intestine, depending upon the site of attachment of the adults, on the day on which a rise in blood histamine occurred after the administration of larvae. The intake of H. contortus larvae produced this change only in the abomasum of a sheep which had been infested with H. contortus and only in the small intestine of a sheep which had been infested with Trichostrongylus spp.

An outbreak of Aujeszky's disease occurred in a flock of 160 ewes which had been shorn and housed adjacent to pigs which were excreting Aujeszky's disease virus. The 29 affected sheep developed either the classical 'mad itch' signs associated with Aujeszky's disease in ruminants or signs of encephalitis. Aujeszky's disease virus was isolated from the central nervous system of the affected ewes. Five farm cats also became ill and died. Histological lesions of viral encephalitis were observed in one of the cats and Aujeszky's disease virus was isolated from its brain.

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