A disease syndrome referred to as infectious catarrh, encountered under natural conditions of exposure in a rat colony, was transmitted to selected animals and maintained for 5 years by nasal instillation or contact.

During this period 37 passages were made in 156 rats, the rates of pneumonia, otitis media, and rhinitis being 28, 63, and 87 per cent, respectively.

After the 12th passage, Brucella bronchiseptica and Actinobacillus muris (B. actinoides var. muris), which were originally present, were no longer cultivable from infected rats.

By reason of the maintenance of infectious catarrh in the absence of the latter and also because of its non-invasiveness on nasal instillation, it is now believed that Actinobacillus muris is of no direct etiological significance.

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