Microbiology, epidemiology, and pathogenesis of Rhodococcus equi infections
- Camille N Kotton, MD
Camille N Kotton, MD
- Associate Professor
- Harvard Medical School
Rhodococcus equi has been recognized as an animal pathogen since its original isolation as Corynebacterium equi in 1923 from foals with pneumonia. Foals less than six months old are uniquely susceptible to the development of a chronic suppurative bronchopneumonia that can lead to lung and bronchial lymph node abscesses, sometimes called "rattles" . Ulcerative colitis and mesenteric adenitis may also occur, usually associated with pulmonary infection. In swine, R. equi is commonly isolated from the lymph nodes of animals with submaxillary adenitis. The organism can rarely cause pulmonary and extrapulmonary abscesses, wound infections, and lymphadenitis in other animals, primarily rooting and grazing mammals. However, when hosts other than foals and swine develop R. equi infections, they are frequently immunocompromised .
The first case of human infection was reported in 1967 ; only 12 additional cases were reported in the next 15 years . While still rare, R. equi has been isolated increasingly, especially as an opportunistic pathogen. Most human infections have been associated with immune system dysfunction, and a dramatic increase occurred in the setting of human immunodeficiency virus (HIV) infection [5-14]. In addition, improved laboratory identification of infections in both immunocompromised and normal humans has led to increasing recognition of R. equi as a pathogen.
The microbiology, pathogenesis, and epidemiology of R. equi infections will be reviewed here. The clinical manifestations, diagnosis, treatment, and prevention of these infections are discussed separately. (See "Clinical features; diagnosis; therapy; and prevention of Rhodococcus equi infections".)
R. equi was originally named C. equi in 1923 based upon its morphologic appearance. Subsequently, the cell wall composition and biochemical reactions were found to be more closely related to Nocardia and mycobacteria than corynebacteria; in 1980, the organism was reclassified in the genus Rhodococcus. (See "Microbiology, epidemiology, and pathogenesis of nocardiosis" and "Natural history, microbiology, and pathogenesis of tuberculosis".)
Part of the phylogenetic group of nocardioform actinomycetes, Rhodococcus is characterized in part by rod-to-coccus growth cycle variation and the presence of tuberculostearic acid and cell wall mycolic acids . Little is known about the pathogenic potential of rhodococci other than R. equi. Some have been isolated from granulomatous lesions in humans , but reports of disease are rare.
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