Rhodococcus equi: More Than Just Pneumonia
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Rhodococcus equi is a common pathogen in foals between the ages of 1 and 6 months of age that is most infamous for its ability to cause pneumonia. Classic R. equi infection results in the formation of large abscesses throughout the lungs of young foals (see picture left), which can be especially difficult to treat because the bacteria are able to hide from the body’s immune system by living within white blood cells. However, this organism’s bag of tricks doesn’t end with lung abscesses – it can also travel to other parts of the body and cause all sorts of trouble. These kinds of infections may occur with or without the classic lung infection, and are referred to as extrapulmonary disorders (EPDs).
At the 2008 Forum of the American Association of Equine Practitioners (AAEP), researchers at Texas A&M presented the results of a study they recently completed looking at EPDs in 150 foals with R. equi infection over a 20 year period. Here are some of the highlights:
- 74% of the foals had an EPD associated with their R. equi infection. On average foals had two EPDs each and up to as many as nine EPDs in a single animal.
- Many of the EPDs did not cause separate, detectable clinical signs. These were therefore most often found on necropsy in foals that died.
- The most common EPD was diarrhea, which occurred in 50 foals (33%).
- Immune-mediated polysynovitis (inflammation and swelling of the joints without infection of the joints themselves) was the second most common EPD
- 31 foals (21%) had ulcers and inflammation somewhere in their intestine (ulcerative enterotyphlocolitis), all of which were diagnosed at necropsy
- 25 foals (17%) had abscesses in the abdomen, 71% of which could be detected by ultrasonography
Thirty-nine different EPDs were identified in the group. Other EPDs included uveitis (inflammation of the eye), hepatitis (inflammation of the liver), septic arthritis (joint infection), lymphadenopathy (enlarged lymph nodes), peritonitis (inflammation of the lining of the abdomen) and septicaemia (bloodstream infection).
- Among the foals with EPDs, 43% survived hospitalization, whereas 82% of foals without EPDs survived. However it is very important to remember that many EPDs were only detected at necropsy, therefore they were more often found in foals that died, but it is unknown if they actually occurred more commonly in one group or the other.
- Risk factors for foals developing EPDs included longer time from onset of clinical illness to referral (e.g. foals that were sick at home for longer before being sent to the hospital were more likely to have an EPD), higher heart rate on admission and a higher white blood cell count.
It’s important to remember that even though an infectious pathogen may usually affect an animal in a certain way, “the bugs don’t read the textbooks” (as we often say), and they can cause problems in other ways. That’s just one of the reasons it’s so important to have your veterinarian perform a full physical exam of your animal if it is sick - even if it looks similar to something you’ve seen before - in case the pathogen causing the problem starts to affect other parts of the body, which may require more or different kinds of treatment.
Photo credit: M. Anderson
