Foal Pneumonia: Symptoms, Treatment and Prevention

Foal Pneumonia: Symptoms, Treatment and Prevention

Rhodococcus equi (R. equi) pneumonia is often not recognized until it is well advanced and, therefore, is difficult to treat.

dun mare and foal (Credit: Briana Malmquist)

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Note: The American Quarter Horse Foundation has funded research regarding foal pneumonia, including "Entry and Modulation of Signaling Pathways of Macrophages by Rhodococcus equi." The Foundation fills a vital role by being just one of four private institutions funding research. Make a contribution at


By Dr. Thomas R. Lenz 

Rhodococcus equi (R. equi) is a bacterium that causes pneumonia and death in young foals between 3 weeks and 5 months of age. 

R. equi can be cultured from the soil of virtually every horse property, but the disease tends to affect foals on some farms and not others. Infections in adult horses is extremely rare. 

What causes Rhodococcus equi ?

Affected foals either inhale or ingest the bacteria during their first few days of life, but pneumonia develops weeks or months later when the antibodies the foal ingested with the mare’s colostrum start to wear down. The most common clinical disease caused by R. equi is pneumonia. 

Early clinical signs might include a slight increase in respiratory rate and a mild fever. In some foals, the only early sign is a decrease in nursing and lethargy. As the disease progresses, you might see a higher fever, rapid heart rate, increased effort to breath including nostril flaring and an increased abdominal effort. A cough or nasal discharge may or may not occur. 

 The incubation period following the inhalation or ingestion of bacteria varies from nine days to four weeks, and symptoms are caused by the formation of large abscesses in the foal’s lungs. In addition to causing pneumonia, R. equi abdominal lesions are present in about 50 percent of the foals that die. R. equi is also found in the joints of approximately 25-30 percent of clinical cases. All are serious locations for abscesses as they can cause, in addition to pneumonia, colic and joint abscesses that may result in permanent lameness. 

How do you treat foal pneumonia?

R. equi pneumonia is often not recognized until it is well advanced and, therefore, difficult to treat. It has been proven that catching the disease early in its course dramatically improves the odds of curing it. A variety of screening techniques have been used to detect the disease early, including close observation for increased respiratory rates or failure to nurse, monitoring rectal temperatures twice daily, frequent blood samples, and ultrasounding the foal’s chest weekly. 

 Over the past 10-15 years, control of R. equi infections at many farms, where the disease commonly occurs each year, has relied on ultrasounding the foal’s chest and the initiation of antibiotics prior to the development of clinical signs. Additional diagnostic tools your veterinarian may use not only to diagnose R. equi pneumonia but also to differentiate it from other diseases include transtracheal wash where a small needle is passed through the foal’s trachea and a tube inserted to collect fluid samples from the lung. The collected fluid is then cultured and checked for R. equi DNA as other common bacteria may cause pneumonia. Radiographs (X-rays) of the chest can also confirm the disease by locating lung abscesses caused by R. equi

Although it appears an effective vaccine is near licensing, prevention of the disease at farms where R. equi pneumonia is an annual problem has been the best approach. The intravenous administration of commercially available and licensed plasma containing specific antibody against R. equi is recommended as an aid in preventing pneumonia. Therefore, it is a common practice on properties where the disease annually to provide foals a transfusion of at least one liter of Hyperimmune plasma (HIP) no later than the second day of life. On farms with high annual R. equi exposure, it’s recommended that a second dose of HIP be administered at 2-4 weeks of age. Although the administration of HIP is effective in most cases, the foals should still be monitored closely, as some may still develop the disease. 

 Foals with pneumonia caused by R. equi shed a higher number of bacteria in their manure than healthy foals. Therefore, sick foals and their dams should be isolated from healthy foals whenever possible. Although rare, severe pneumonia infections can occur in people with suppressed immune systems so practice good hygiene when handling sick foals. 

For additional information on R. equi infections in foals, go to the AAEP’s guides for infectious diseases at infectious-disease-control/using-guidelines or talk to your local veterinarian. 

About the Source

Thomas R. Lenz, DVM, M.S., Diplomate of the American College of Theriogenologists, is a trustee of the American Horse Council, past chairman of AQHA’s research committee, past president of the American Association of Equine Practitioners and a member of the AQHA Animal Welfare Commission. He is a member of the American Quarter Horse Hall of Fame. 

Funding Equine Research: American Quarter Horse Foundation

The American Quarter Horse Foundation fills a vital role by being just one of four private institutions funding research. Since 1960, more than $11.7 has been awarded to various colleges and Universities to further our understanding of equine management and health and improve the overall welfare of our horse.

Our horses give us their all; whether in the thrill of victorious moments in the show ring or in the solace we find in their companionship. The American Quarter Horse Foundation is committed to giving back to the horses we love by challenging the frontiers of knowledge through research.