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Rhodococcus equi Infection in FoalsBy Dr. Bryan Waldridge · June 29, 2011
Rhodococcus equi is a bacterium that causes mainly pneumonia in foals from one to six months of age. The bacterium produces abscesses in the lungs, and the abdominal lymph nodes are often also affected. Occasionally, R. equi also infects bones and joints, and it is not unusual for foals affected with R. equi pneumonia to have effusion (excessive fluid) in their hocks and knees. This effusion does not cause lameness and is believed to be an immune reaction involving antibodies produced against R. equi.
Foals with severe R. equi infections can develop uveitis, an inflammatory reaction involving the iris (pigmented structure surrounding the pupil inside the eye), and these foals may have white blood cells and fibrin in the front chamber of the eye. There have also been some case reports of abortion in mares due to R. equi.
Diagnosis of R. equi infections can be made clinically when there are abscesses in the lung and the foal exhibits the expected clinical signs. However, several other bacteria, including Streptococcus zooepidemicus (a very common pathogen), can cause the same clinical appearance. Lung abscesses can be visualized using ultrasound or radiography. Ultrasound can demonstrate fluid in the chest and abscesses in the periphery of the lungs. Radiographs are more useful to find abscesses deeper in the lungs. Definitive diagnosis of R. equi infection involves culture of the bacterium from the infected site. A PCR test is also available that detects DNA from disease-causing R. equi, and this test can be ran from the same samples submitted for culture.
It is believed that foals that become sick from R. equi have a different immune response than foals that do not become sick. Affected foals appear to produce less γ-interferon (an important immune-modulating cytokine) and mount an antibody response against R. equi rather than a cell-mediated immune response. This is important because R. equi resides inside cells and cell-mediated immunity is required to clear infections.
Reported survival rates for foals with R. equi infections are good, from 69-80%. Foals with antibiotic-resistant R. equi infections have only a 25% survival. Effective antibiotics against R. equi infections include erythromycin, azithromycin, clarithromycin, and rifampin. Foals that survive R. equi infections are less likely to start a race (54% starters) compared to unaffected foals, but their racing performance was not different than the rest of the population.
