From a contagious respiratory disease of cats an agent has been transmitted to mice, rabbits, guinea pigs, hamsters, and embryonated eggs. When inoculated intranasally into cats, it produces a disease like that seen in the naturally infected animals. Parenteral injection causes only a mild fever. From cats killed during acute illness, the agent was demonstrated regularly in the discharges from the eyes and nose and in the pneumonic lung but not in other organs. The agent can be demonstrated in the nasal mucosa of cats 1 to 2 months after injection.
Cultures of egg yolk sac containing the agent showed no growth, and cultures of active lung suspensions were usually negative, such positive findings as were obtained being due to respiratory contaminants. Attempts to pass the agent through Berkefeld N filters were generally unsuccessful. Stained preparations of egg yolk sac membranes and of infected mouse or hamster lungs showed typical elementary bodies. Centrifuge experiments showed that the agent and the bodies sedimented at the same rate. Complement fixation experiments using a suspension of partially purified bodies as antigen were negative with control sera and positive with sera from recovered cats. It is concluded that the respiratory disease of cats is due to a virus that forms elementary bodies.