Immunological studies on equine phycomycosis.

One in vivo and 2 in vitro tests were developed to study immunological aspects of phycomycosis in clinically infected, recovered and normal in-contact horses. Serum from all infected horses gave positive readings in an agar-gel double diffusion test; serum from normal and recovered horses did not react. A complement fixation test detected antibody against Hyphomyces destruens in 82% clinical cases at an average titre of 20. Serum from recovered and in-contact horses reacted sporadically at positive titre. An intradermal hypersensitivity test (Heaf test) was used to detect evidence of cellular immunity to H. destruens. Positive tests were observed in 64% of clinically infected horses, 100% of recovered animals and 31% of normal in-contact horses. Negative tests in the clinically infected group were thought to be due either anergy in chronic cases or no stimulation in very acute cases. It was concluded that many horses showed evidence of past contact with H. destruens and had acquired resistance to infection.