Chlamydia psittaci induced pneumonia in a horse.

An agent lethal to embryonated chicken eggs was isolated from lung tissues of a quarter horse mare with a fatal respiratory disease. The lesions induced in embryonated chicken eggs, the tinctoral properties, the ultrastructural morphology, the resistance of the organism to sodium sulfadiazine, and the presence of a chlamydial complement fixing antigen, identify this isolate as a member of the family Chlamydiaceae and suggest the agent to be Chlamydia psittaci. Two Shetland ponies experimentally infected with the isolated agent developed subclinical infection as demonstrated by an increase in complement fixing antibody titers. Post mortem examination of one pony revealed microscopic evidence of a generalized chlamydial infection. Lesions of interstitial pneumonia and focal hepatic necrosis were observed, and Chlamydia psittaci subsequently was reisolated from the lung tissues of the pony.