Rhodococcus equi pneumonia in foals
Rhodococcus equi is an important bacterial pathogen of horses being associated with pyogranulomatous bronchopneumonia of foals, with about 50% of these foals developing concurrent ulcerative colitis and in some foals intestinal lesions occur alone. It is less commonly associated with clinical disease in other animal species (cats, cattle, sheep, goats, pigs and llamas) and is emerging as an important pathogen in immunocompromised humans (HIV, hematopeitic malignancies, chemotherapy patients, transplant recipients).
In addition to being a common soil inhabitant R.equi is frequently part of the alimentary flora of mammals and birds. There are both virulent and avirulent forms of the bacterium with about 88% of isolates from foals being the highly virulent variant (VapA). Nearly all isolates from pigs are the intermediate virulent VapB strains, while many humans develop clinical disease with the avirulent strains, due to immunosuppression.
Pathogenesis of foal pneumonia is related to aerosol infection with bacteria being taken up by macrophages. These intracellular bacteria survive within macrophages by interfering with phagosome-lysosome fusion and inhibiting oxidative bacteriocidal functions of neutrophils. The primary manifestation of disease is pyogranulomatous bronchopneumonia and abcessation in the absence of pleuritis. Repeated swallowing of respiratory exudate by foals is probably an important source of infection for the development of concurrent intestinal lesions.