Vol. 17, No. 18
THE JOHNS HOPKINS MICROBIOLOGY NEWSLETTER
Monday, May 4, 1998
A. Provided by Marguerite-Hawkins, Epidemiology and Disease Control Program, Division of Outbreak Investigation, Maryland Department of Health and Mental Hygiene.
From April 23, 1998 through April 30, 1998, 9 outbreaks have been reported to the Maryland Department of Health and Mental Hygiene as follows:
B. The Johns Hopkins Hospital. Information provided by Dr. Frank Holmes, Dept. of Pathology.
Case Report
A 32 year-old, HIV-positive, white male presented with a history of fever and cough. A chest X-ray revealed the presence of a cavitating lesion in the left upper lobe. Fluorochrome stains done on sputum were negative for tuberculosis. A bronchoalveolar lavage was performed. Bacterial cultures grew Rhodococcus species.
Rhodococcus equi
Originally isolated in 1923, this organism is best known as a cause of pneumonia in horses. The first case of human infection by R. equi was reported in 1967. In the last two decades, there has been a substantial increase in the number of human cases, especially in immunosuppressed patients and particularly those with AIDS. The organism's pathogenicity is related to its ability to survive within macrophages by interfering with phagolysosome fusion. Cell-mediated immunity seems important in controlling the infection.
Lung disease is the most common manifestation of R. equi in humans, usually presenting as fever and cough. Typically, there is a localized upper lobe infiltrate that enlarges and eventually cavitates. It can mimic mycobacterial and nocardial infections. Involvement of extrapulmonary sites might be seen primarily as an abscess or a mass lesion, possibly mimicking a malignancy. A few patients give a history of exposure to horses or other herbivores, but most do not.
Pathology
With its ability to survive in macrophages, R. equi leads to an acute and chronic inflammatory process with a massive collection of histiocytes. In this regard, it is similar to mycobacterial infections in AIDS patients and Whipple's disease. Well-formed granulomas are typically not seen. Within the sheets of macrophages are microabscesses of neutrophils as well as clusters of intracellular and extracellular bacteria. The microabscesses can expand and coalesce, eventually forming large abscess cavities.
One notable association is that of R. equi with malakoplakia. Malakoplakia is best known in relation to chronic urinary tract infections (particularly those due to Escherichia coli) in which distinctive yellowish plaques develop in the bladder mucosa. The plaques are composed of sheets of histiocytes containing Michaelis-Gutman bodies, which represent calcified phagolysosomes. The majority of cases of malakoplakia outside the bladder have occurred in immunosuppressed patients, and one of the most frequently identified organisms is R. equi, especially in cases of pulmonary malakoplakia.
The differential diagnosis includes conditions in which large numbers of histiocytes aggregate. These include mycobacterial infections, nocardia, fungal infections (especially histoplasmosis and cryptococcus), Pneumocystis carinii, Whipple's disease, storage diseases, and histiocytosis X.
Microbiology
R. equi is a gram-positive coccobacillus varying from coccoid to long curved and clubbed forms. It can be cultured in ordinary nonselective media with aerobic incubation at 37C. The organism is nonfermentive, catalase positive, often urease positive, and oxidase negative. It is widespread in the environment, being found particularly in soil and in the intestines of herbivores.
Treatment
R. equi is susceptible to many antibiotic drugs, including erythromycin, rifampin, and ciprofloxacin. It is recommended that a combination of antibiotics be used and that they be given for at least two months because of the risk of relapse with shorter courses. In AIDS patients, indefinite prophylaxis has been recommended because of the high risk of relapse.
References
Colby TV, Kwon KY. Rhodococcus equi - An old bug with a new name; relevance for the anatomic pathologist? Advances in Anatomic Pathology 2:263-69,1995.
Verville TD, Huycke MM, Greenfield RA, et al. Rhodococcus equi infections of humans. Medicine 73:119-32,1994.