CAR bacillus is a gram-negative, filamentous rod of uncertain classification. Recent studies suggest that CAR bacillus isolates of rat and rabbit origins may be distinct strains and suggest that, in mice, isolates of rat origin may be more virulent than those of rabbit origin.
Transmission is probably via contact exposure to the respiratory system.
However, primary infection of rats with CAR bacillus has been recently reported. Lesions of CAR bacillus infection are similar to those of MRM, and the reader is referred to that section for a full description. In addition, CAR bacillus infection produces severe bronchiolectasis, pulmonary abscesses, and atelectasis of entire lung lobes. These lesions are due mainly to accumulation of pus in the airways. Large numbers of CAR bacilli can be observed between cilia on respiratory epithelial surfaces and cause the ciliated border to appear dense.