Management of Rhodococcus equi pneumonia in foals

Imogen Johns

Department of Clinical Sciences and Services, Royal Veterinary College, North Mymms, UK

Abstract: Rhodococcus equi, a gram-positive facultative intracellular bacterial pathogen, is the most important cause of pneumonia in foals aged 3 weeks to 5 months. The disease occurs worldwide, resulting in significant morbidity and mortality on endemically affected farms. Foals appear to become infected early in life, but clinical signs are typically delayed until 1–3 months of age because of the insidious nature of the disease. Although pneumonia is the most common clinical manifestation, up to 74% of foals may concurrently have extrapulmonary disorders, including both extrapulmonary infections (abdominal abscessation, colitis, osteomyelitis) and immune-mediated disorders (nonseptic synovitis, uveitis). Diagnosis is based on the combination of clinical signs and abnormalities on hematologic screening and thoracic imaging in an appropriately aged foal and is confirmed by bacteriologic culture of the organism. Management of R. equi infections, in particular on farms with endemic disease, combines appropriate treatment of affected foals with preventative measures targeted at preventing infection and identifying foals before the development of severe disease. The combination of rifampin and a macrolide antimicrobial is recommended for treatment, as the combination is synergistic, reaches high intracellular concentrations, and should minimize the development of antimicrobial resistance. The prognosis for survival for foals with R. equi pneumonia is good, especially in foals mildly or subclinically affected, as is the prognosis for future athletic performance. Screening for early identification before the development of clinical signs has been advocated on endemically affected farms, although the most appropriate method, the timing of screening, and the selection of foals requiring treatment have yet to be determined. Recent evidence suggests that a high proportion of foals identified via screening methods such as ultrasonographic evaluation of the thorax can recover without treatment, questioning the “trigger” required for treatment of identified cases.

Keywords: extrapulmonary disorder, infection, rifampin, antimicrobial, immunity, inhalation, virulence