Experimental infection of mice with Rhodococcus equi: Differences in virulence between strains

The growth kinetics in outbred mice of clinical and environmental isolates of Rhodococcus equi were followed by serial bacterial enumeration of organ homogenates. Clinical isolates multiplied until Day 4 before being progressively cleared, but could still be recovered from the liver at 3-4 weeks post-infection. Intravenous inoculation of clinical strains was associated with histopathological responses very similar to those elicited by intravenous infection with various facultative intracellular parasites. Whereas lesions in mice and foals at 7-9 days following respiratory infection are those of severe bronchopneumonia with massive consolidation, a week later the patterns of host response have diverged as the murine lesions resolve. The type strain, NCTC 1621 and 4-6 environmental isolates were eliminated without prior multiplication and these strains caused negligible lesions. The two environmental strains which behaved as the clinical strains were recovered from a stud with an R. equi problem. No association of colonial morphology of R. equi with virulence was apparent.