Rhodococcus Infections in the Foal

Ultrasound image of a lung abscess in a three-month-old foal. The foal was diagnosed with R. equi pneumonia. (Photo courtesy of researchgate.net)

By Lauren Lamb, DVM 

Rhodococcus pneumonia is one of the most common causes of pneumonia in foals one to four months of age. Rhodococcus equi (R. equi) is the bacterial that causes an infection in foals. R. equi infections are usually localized to the lungs and cause abscess formation within the lungs. Although rare, R. equi can also cause infection in other body systems such as the eye, bone, joints and abdomen (diarrhea or abscess). Adult horses rarely develop a R. equi, unless there is an underlying severe immunodeficiency present.

R. equi can be found on most horse farms globally. The R. equi can survive in the environment for long periods of time and withstand some of the harshest climates. Just because the bacteria is present in the soil does not automatically result in a foal becoming infected with R. equi. High stocking densities, low soil moisture and low pasture height can also increase the risk of a foal developing a R. Equi infection.

Rhodococcus equi has a relatively simple lifecycle. The bacteria is inhaled from the environment, usually in the form of dust. The airborne bacteria is transported to the lungs, where several abscesses are formed and the pneumonia begins. A secondary route of infection is via ingestion of bacteria, followed by entry of the bacteria into the blood stream. Once the bacteria is in the blood stream it will seed the lungs.

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Foal’s lungs showing small abscesses throughout the lung field. (Photo courtesy of askjpg.org)
Radiographs of a three-month-old foals lungs infected with R. equi. (Photo courtesy of veteriankey.com)