Infection by Rhodococcus equi in a patient with AIDS: histological appearance mimicking Whipple's disease and Mycobacterium avium-intracellulare infection.

Rhodococcus equi pneumonia with systemic dissemination is being reported increasingly in immunocompromised patients. This is the first case report of disseminated R equi infection with biopsy documented involvement of the large intestine. The patient was a 46 year old male with AIDS who was diagnosed with cavitating pneumonia involving the left lower lobe. R equi was isolated in culture from the blood and lung biopsies. Subsequently, the patient developed anaemia, diarrhoea, and occult blood in the stool. Colonoscopy revealed several colonic polyps. Histological examination of the colon biopsies showed extensive submucosal histiocytic infiltration with numerous Gram positive coccobacilli and PAS positive material in the histiocytes. Electron microscopy showed variably shaped intrahistiocytic organisms which were morphologically consistent with R equi in the specimen. Disseminated R equi infection may involve the lower gastrointestinal tract and produce inflammatory polyps with foamy macrophages which histologically resemble those seen in Whipple's disease and Mycobacterium avium-intracellulare infection.