Foal Pneumonia: Effects on Future Racing Performance

One of the most common illnesses in foals under the age of six months is pneumonia caused by Rhodococcus equi, a pathogen found in soil. Foals are usually exposed to the infection by inhalation of dust particles contaminated with the bacteria.

The disease may not be identified in its earliest stages when the foal shows few signs. Handlers may notice a slight increase in respiratory rate when newly infected foals are excited or stressed. A mild fever is one of the first signs, and this is followed by increasingly labored breathing, loss of appetite, lethargy, and coughing. Nasal discharge may or may not be seen. Ultrasound can show lung abscesses as the pneumonia develops. In some foals, the infection also causes abdominal abscesses, joint swelling, and inflammation of eye tissues. In rare cases, abscesses also develop in the kidneys, liver, sinuses, guttural pouches, or brain. Diagnosis is by bacteriologic culture.

Though some foals die from R. equi infections, there is evidence that 80 to 90% of infected foals recover without antibiotic therapy. These numbers include foals that show lung abscesses when examined by ultrasound but do not develop other signs of disease. No preventive vaccine has been developed.

Use of antibiotics that are effective against R. equi can cause diarrhea in foals, and caretakers should be alert for signs of dehydration. Foals treated with a combination of erythromycin and rifampin have a survival rate of better than 85%.

In one study that followed foals that survived pneumonia after this treatment, more than half entered race training and started at least once compared to 65% of horses in a control group. Racing results were similar between horses in the two groups, suggesting that R. equi infection does not adversely affect horses that train and race.