Clinical Presentation

19-year-old woman with AIDS, employed as an equestrian trainer, with a CD4 count of 140 cells/μL presenting with fever, cough, increasing fatigue, and chest pain

Radiologic Findings

Background

Rhodococcus equi primarily causes zoonotic infections in grazing animals, namely horses and foals. Although R. equi rarely infects immunocompetent humans, it is emerging as an important pathogen in immunocompromised persons, especially those with AIDS and a CD4 count <200 cells/mm³. Human infection results from inhalation of contaminated aerosols.

Etiology

R. equi is an aerobic, Gram-positive or Gram-variable, weakly acid-fast coccobacillus. It is found worldwide in the soil and in feces of some animals, especially horses. Thirty percent of infected persons have a history of animal exposure, particularly to horses.

Clinical Findings

Eighty to 90% of patients with R. equi infection are immunocompromised. Approximately 50–60% of these patients are HIV-positive, 15–20% have lymphoma, leukemia, or other underlying malignancies, and 10% are transplant recipients. Symptoms in immunocompetent patients are similar to those in immunocompromised patients. Necrotizing pneumonia is the most common pulmonary manifestation of infection. Affected patients have fever and cough (>80%). Fatigue, chest pain, dyspnea, hemoptysis, and weight loss may also occur. Extrapulmonary manifestations of R. equi infection include abscesses (e.g., soft tissues, brain, retroperitoneum), meningitis, pericarditis, endophthalmitis, lymphadenitis, osteomyelitis, septic arthritis, etc. R. equi infections occur in all age groups, with a mean age of 34–38 years. Risk factors for infection in immunocompetent patients include chronic renal insufficiency, alcoholism, and diabetes. Approximately 50% of R. equi infections in immunocompetent patients are due to trauma. Although the diagnosis of R. equi