Murine respiratory mycoplasmosis in LEW and F344 rats: strain differences in lesion severity.

Pathogen-free weaning rats of the LEW and F344 strains were caged together for two months to eliminate microbial and environmental differences, and then infected intranasally with 10-fold dilutions of viable Mycoplasma pulmonis. At necropsy 28 days post-inoculation, F344 rats had no gross lung lesions, even those given the maximum dose of 1.4 X 10(9) colony-forming units of M. pulmonis. LEW rats often had extensive gross lesions with a gross-pneumonia-dose50 of 1.1 X 10(7) colony-forming units/rat. Histological examination of the respiratory tract (nasal passages, larynges, tracheae, and lungs) and tympanic cavities showed both qualitative and quantitative differences in lesions between the two strains, particularly in the lungs. Hyperplasia of bronchus-associated lymphoid tissue occurred in both strains, but was more extensive in LEW rats. Atelectasis, alveolar consolidation (due primarily to mononuclear inflammatory cells), and suppurative bronchitis and bronchiolitis were seen only in LEW rats. Infiltrates of lymphoid cells into the lungs distal to bronchi and around blood vessels also were seen primarily in LEW rats. These differences between the two rat strains provide excellent model systems with which to dissect the role of cell responses in the pathogenesis of a naturally occurring chronic lung disease.