INTRODUCTION:Rhodococcus equi has increasingly been diagnosed as an opportunistic pathogen. A dramatic increase in incidence has occurred since the recognition of HIV. R equi pulmonary infections most commonly present as a cavitary lung mass. Some of which have been associated with pulmonary malakoplakia.
CASE PRESENTATION:A 39-year-old male with AIDS presented with right sided pleuritic chest pain, productive cough, fever, generalized weakness and hemoptysis. His past medical history was significant for AIDS and hemophilia A. He used to live on a farm, and had close contact with animals including horses and pigs. He had a 40 pack year history of smoking. A chest radiograph (CXR) showed right upper lung infiltrate and a prominent right hilum. Bronchoalveolar lavage (BAL) cultures grew R equi. Highly active anti retroviral therapy (HAART) and vancomycin were started.8 months later, the patient was readmitted for fever, right-sided chest pain and 10 lbs weight loss over 2 months. CXR and CT scan showed a cavitating mass in the right upper lobe (figure 1). Transbronchial biopsy showed few gram positive coccobacilli, with characteristic feature of R equi. Histopathologic exam showed sheets of plump epithelioid histiocytes with abundant eosinophilic cytoplasm, characteristic of von Hansemann’s histiocytes (figure 2 green arrow). Some intra- and extracellular round, dense or target-like calcific Michaelis-Gutmann bodies were also identified (figure 2 red arrow). With these typical features for pulmonary malakoplakia secondary to R equi, the patient was treated with vancomycin and rifabutin with significant clinical and almost complete resolution of the lung lesions.
DISCUSSIONS:Rhodococci are aerobic, Gram-positive, nonmotile, opportunistic coccobacilli which are causal agent of bronchopneumonia, primarily in horses, pigs, and a wide range of farm animals. In humans, Rhodococci are opportunistic pathogens that infect immunocompromised patients including AIDS. This organism is easy to grow, and is sometimes discarded as a “contaminant”. The primary infection site is the lung. Sub acute pneumonia with cavitation occurs, with cough, fever and hemoptysis. In an AIDS patient with cavitary lesions in the lung, in the right clinical setting, R equi infection should be included in the differential diagnosis. Malakoplakia is an uncommon inflammatory disorder which is characterized by aggregation of PAS positive histiocytes often in a mass-like fashion. There is a unique association between AIDS and rhodococci: in all of the few reported cases of pulmonary malakoplakia, R equi was the only isolated organism. The pathogenesis of Malakoplakia is thought to be an acquired defect in macrophage lysosomal function (deficiency of 3′, 5′-guanidine monophosphate dehydrogenase) secondary to immunosuppression. Affected macrophages are called “Von Hansemann” cells, which are large macrophages with foamy, eosinophilic cytoplasm with secondary lysosomes and phagosomes containing partially digested organisms.(figure 2 green arrow). Fusion and calcification of these lysosomes result in the formation of Michaelis-Gutmann bodies, which are intracytoplasmic calcospherite bodies with a central hydroxyapatite core. (figure 2 red arrow).
CONCLUSION:R equi associated Malakoplakia is a treatable inflammatory disorder. Although resembled lung cancer in our patient, recognition and treatment of R equi infection will result in a favorable outcome, as demonstrated. The microbiology laboratory should be alerted about the suspicion of Rhodococcus, so that profuse growth of a coccobacillus is not considered a contaminant and discarded. A detailed exposure history to farm animals and awareness of the condition will help in appropriate recognition and management of this condition.
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