Evidence is presented that infectious catarrh of the rat is caused by Gram-negative coccobacilliform bodies which resemble the agents of fowl coryza and mouse catarrh.

These bodies were demonstrable microscopically in at least 90 per cent of the exudates from the nasal passages and middle ears. The characteristic syndrome was reproduced in selected rats by the nasal instillation of pure tissue culture growths and was maintained for 10 successive passages by the injection of exudate. The experimentally produced disease was also communicable by direct contact.

The biological characteristics of the specific bodies are discussed in relation to the organisms of the pleuropneumonia group.

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