Disease and Epidemiology

The simple growth requirements of R. equi are provided by nutrients in herbivore manure and high environmental temperatures. Sandy acid soils in paddocks continually used for breeding mares are likely sites for infection of foals, which inhale the organism with blowing dust. The resulting disease is a pyogranulomatous bronchopneu-monia (Figure 12-1), with cranioventral abscesses a reflection of inhalation of organisms. Swallowing of infected sputum gives rise to ulcerative colitis and mesenteric lymphadenitis (Figure 12-2), with diarrhea. Hematogenous dissemination may result in osteomyelitis in long bones or vertebrae. Affected foals are typically about 6 weeks of age, but age at onset ranges from 1 to 6 months.

FIGURE 12-1 Multifocal pyogranulomatous pneumonia in an Arabian foal.

(Courtesy M. Kevin Keel.)

FIGURE 12-2 Chronic suppurative colitis in a 3-month-old Thoroughbred foal.

(Courtesy M. Kevin Keel.)

Rhodococcus equi infection in pigs causes cervical lymphadenitis, and the resulting granulomatous lesions resemble mycobacterial infection. As such, they may result in condemnation of parts or carcasses at slaughter. Isolation of R. equi from other animal species is rare, but when infection occurs it usually takes the form of pyogranulomatous pneumonia, hepatic abscesses, or lymphadenitis.

Infection by R. equi is frequently isolated from immunosuppressed humans, particularly from cases of pyogranulomatous pneumonia in patients with acquired immunodeficiency syndrome (AIDS). If isolates from patients with AIDS have the virulence plasmid, it is usually of the porcine type, although a small proportion possess the foal type. Thus R. equi probably has little zoonotic significance.

Other types of R. equi infection are equine mastitis and wound infections, bovine ulcerative lymphangitis, lymphadenitis and abscesses in cats, caprine and ovine pneumonia and internal abscesses, canine pyogranulomatous hepatitis and osteomyelitis, septicemia in alligators and crocodiles, and lung abscesses in seals.