This is a Gram (+) coccobacillus. It is a facultative intracellular organism that causes a persistent infections in macrophages. The name comes from the ability of the organism to form a red colored pigment.

Rhodococcus equi is a well-known cause of pneumonia and enteritis in young (2-6 month-old) foals and can be part of the fecal flora of horses, cows and pigs.

Rhodococcus equi has recently been identified as a growing problem among immunocompromised human patients, with infection leading in 84% of cases to chronic, progressive, granulomatous pneumonia. In contrast, infections are quite rare in immunocompetent people and typically less severe. The mortality rate in immunocompetent patients is 11%, compared to 50-55% in HIV patients.

• This organism can clearly be acquired from environmental sources (soil), but in the most exhaustive survey of cases in HIV patients to date, 7 of 24 people had regular contact with horses.

References:

Arlotti, M. et al. 1996. Rhodococcus equi infection in HIV-positive subjects: a retrospective analysis of 24 cases. Scand. J. Infect. Dis. 28:463-467.

Kedlaya, I., M.B. Ing and S.S. Wong. 2001. Rhodococcus equi infections in immunocompetent hosts: case report and review. Clin. Infect. Dis. 32:e39-e42.

Linder, R. 1997. Rhodococcus equi and Arcanobacterium haemolyticum: two "coryneform" bacteria increasingly recognized as agents of human infection. Emerg. Infect. Dis. 3:145-153.