A 65-year-old white man was evaluated for swelling of his left arm. He was receiving prednisone and azathioprine for a heart transplantation that took place 4 years previously. His physical examination showed a scar from a recently resected squamous cell carcinoma with sutures still present on the dorsum of his left hand and a 10 x 7 x 3 cm indurated, tender, erythematous swelling of the left upper arm. An abscess that extended deep beneath the bicep muscle was incised and drained. Cultures grew the bacterium Rhodococcus equi. Although he denied exposure to animals, he was an avid gardener and used horse manure as fertilizer.

The natural habitat of R. equi is soil, particularly soil associated with herbivore manure. Persons exposed to horses, livestock farmers, and gardeners have a higher risk for infection. Inhalation is the major route of inoculation, and infections in humans primarily involve the lungs. Skin and soft-tissue abscesses have been reported only in humans with disseminated disease, although primary skin infection with R. equi do occur in animals [1]. However, after reviewing the bibliographies of relevant articles identified through a MEDLINE search, we found that an R. equi-like bacterium was isolated from a wound in an immunocompetent child, but the isolate showed unusual biochemical and growth characteristics [2]. Another report only gives circumstantial evidence for R. equi as the cause of a subcutaneous foot lesion in a patient with the acquired immunodeficiency syndrome (AIDS) [3].

In our patient, R. equi was isolated in pure culture from the subcutaneous abscess. The patient worked with horse manure, which has been shown to contain high numbers of R. equi organisms [4]. It is logical to speculate that the recent surgical resection of a carcinoma distal to the site of the lesion provided the portal of entry for the organism. We believe that this is the first report of a primary soft-tissue infection caused by R. equi in humans due to direct inoculation rather than to hematogenous dissemination.

Since we first submitted this case report, we have become aware of two cases of R. equi causing a wound infection [5]. One was in a 9-year-old boy, the other in a 4-year-old child who sustained a nail puncture injury that resulted in an infected wound and knee joint. Together with our patient, these represent the first three described patients with primary skin and soft-tissue infections in humans caused by R. equi.