Rhodococcus equi

Background

Facultative intracellular, non-spore-forming, Gram-positive, weakly acid-fast coccobacillus
Within the family Nocardiaceae and order Actinomycetes
Obligate aerobic and facultatively intracellular
Found in dry and dusty soil
Makes red pigment, hence the name

Infects domesticated animals: Horses (and in foals it causes pneumonia), Goats, Pigs, Sheep, and Cattle
Most commonly infects people with cell-mediated immunodeficiency, particularly HIV, with or without a notable infectious exposure

Necrotizing pneumonia is usual presentation, as well as nodules, cavitation, pleural effusion, and lung abscess
- Typically subacute onset with fever, cough, and fatigue, as well as pleuritic chest pain
Specifically in immunocompromised patients, it can cause a cavitary lung disease and is on the differential with mycobacteria and nocardiosis
Extrapulmonary disease can occur with or without pulmonary involvement
- In conjunction with other infections, can have abscesses in liver, spleen, thyroid, kidney, psoas, bone, prostate, intraabdominal cavity, and paraspinous tissue
Extrapulmonary disease without pulmonary involvement has three main presentations:
- Localized infection following traumatic inoculation, causing wound infection, traumatic septic arthritis, or endophthalmitis
- Isolated bacteremia with fever, typically recently after chemotherapy causing neutropenia
- Gastrointestional inoculation followed by lymphatic dissemination, causing peritonitis, pelvic masses, and mesenteric adenitis
- Others include otitis media with mastoiditis, colonic polyp infection, and osteomyelitis