Mycoplasma felis as a cause of pleuritis in horses

M. felis was the only organism recovered from the thoracic cavity of a horse with pleuritis. Large numbers of mildly degenerative neutrophils were in the pleural fluid. The horse developed a serological response to M. felis and recovered during hospitalization. Experimentally, a pony was inoculated in the thoracic cavity with a pure culture of the M. felis isolate suspended in the pony's serum. A control pony was inoculated with serum only. Within 48 hours, the infected pony developed fever, increased respiratory rate, pleural effusion, and signs of pain. A highly cellular exudate with nondegenerative neutrophils and large numbers of M. felis was recovered from the thoracic cavity. The control pony remained normal. The infected pony developed an antibody response to M. felis. The control pony did not. Fourteen days after inoculation, both ponies were killed. Pleural inflammation was present in the infected pony, but not present in the control pony.