A natural outbreak of an infectious catarrh in a colony of Swiss mice is reported. The disease was generally characterized by a peculiar chattering sound during life and by a rhinitis, an otitis media, and a pneumonia at autopsy. The pneumonia was slowly progressive and terminated fatally in a high percentage of cases. The mortality in a group of 75 naturally infected mice was 95 per cent over a period of 11 months.

The disease was readily reproducible in susceptible mice by the nasal instillation of exudate from any locus of infection. It was also transmissible by direct contact. In both naturally and experimentally infected animals there was an incubation period of 10 days or more before symptoms were apparent. Recovery from the disease was not observed.

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