Rhodococcus equi is a soil-borne bacterium that is responsible for a variety of disorders seen in foals from approximately one to four months of age.  Pneumonia is the most common condition seen.  Other conditions include abdominal abscessation, diarrhea, uveitis (inflammation of the eyes), polysynovitis (joint inflammation), osteomyelitis (bone infection), and brain abscesses. 

R.equi is a normal inhabitant of soil, and the highest concentrations are found in soil contaminated with herbivore (cattle, horse) manure.  The progression of the disease is very insidious, with exposure likely occurring during the first week of life.  Ingestion of the bacterium provides the most common route of exposure.  This route rarely leads to pneumonia, however may lead to abdominal abscess formation. R.equi can be isolated from the feces of adult horses, and has been isolated from the feces of foals 1-2 weeks of age (perhaps the foal eating the mare’s manure is not as beneficial as we think). Inhalation of the bacterium into the respiratory system leads to the development of pneumonia.

Early detection of the disease is critical for a successful outcome.  On farms where the disease is endemic, and cases are seen annually, screening of foals at a month of age may provide a means of early detection prior to the development of clinical signs.  Useful screening tools include thoracic ultrasound, which may identify a lung abscess even before elevations in other screening methods such as the temperature, and white blood cell count/fibrinogen are noted.  The clinical signs seen are no different from those seen with other causes of pneumonia, and include increased respiratory rate and effort, cough, and nasal discharge.  Other useful diagnostics include chest x-rays and transtracheal washes.  Bacterial culture or Polymerase Chain Reaction (PCR) of the tracheal aspirate may yield a definitive diagnosis.

Treatment requires the use of antibiotics with the ability to penetrate the abscesses.  Conventional therapy includes the use of erythromycin and rifampin.  Erythromycin has been associated with many adverse side effects including diarrhea (both in the foal and mare), very high fevers (hyperthermia) and acute respiratory distress.  Newer generation antimicrobials (Azithromycin and Clarithromycin) with potentially less side effects are now available.  These newer drugs should be used in conjunction with rifampin. In a recent study, Clarithromycin was shown to be more effective than both erythromycin and Azithromycin. 

Unfortunately there is no effective vaccine for R.equi.  Thus far the only proven method for prevention or minimizing occurrence is transfusion of R.equi hyperimmune plasma, which is expensive and not always effective.  Other preventative measures that have been employed on farms with an endemic problem include treating all foals with azithromycin during the first 2 weeks of life.   Studies evaluating the efficacy of this protocol are ongoing.