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.
The R. equi bacteria replicates in the lungs, is coughed up and then swallowed by the foal. Replication of the bacteria continues as the bacteria travels thru the gastrointestinal tract. Large amounts of R. equi are passed in the feces and regain access to the soil. No evidence currently supports that the R. equi bacteria can be spread from foal to foal directly via aerosolized bacteria being passed in the foal’s breath.
Progression of the R. equi pneumonia is slow. Common clinical signs seen in foals with R. equi pneumonia include fever, cough, depression, labored breathing, tracheal rattle and lethargy. Clinical signs may not be apparent until the disease process has progressed to a life threatening state. Farm history, clinical signs, ultrasound examination and radiographs of the lungs, culture of the airway and blood work can all be used to diagnosis R. equi in foals. Not all cases infected with R. equi will need to have all of these diagnostic test performed.
Learn more about Rhodococcus infections in the March issue!