Rhodococcus equi: Genetic Variability and its Clinical Implications

If there are any two words that can cause a foaling operation's manager to shudder, they are probably "Rhodococcus equi," since the bacterium is considered the most common cause of severe pneumonia in foals. According to Noah D. Cohen, VMD, MPH, PhD, of Texas A&M University, there's often a push to identify the farm or region where the foals became infected on a premise with cases. In his presentation at the 2003 American Association of Equine Practitioners' Convention, Cohen revealed that with the use of a DNA fingerprinting method, he and other researchers (from Japan, Argentina, and Ireland) discovered that it was not possible to clearly differentiate isolates from different countries, different regions within Texas, or even on the same farm. Therefore, it is nearly impossible to identify where R. equi came from based on the bacterium itself.

R. equi pneumonia can have a devastating impact because prevalence and case fatality rates are often high, and treatment can be prolonged, expensive, and not very successful. The disease also might diminish future performance in the affected animals. Studies have shown that many farms reputed to be affected by the disease can suffer a loss of clients.

Cohen and others wrote in their paper summing up the research, "Understanding epidemiologic characteristics of an organism is essential for developing methods to control and prevent disease caused by infection with that organism," adding that the "mechanisms of transmissions and spread of R. equi are poorly understood."

He and other researchers set out to compare R. equi isolates from affected foals and their environments in Japan, Argentina, Ireland, and the United States, using a specific type of DNA fingerprinting called pulsed-field gel electrophoresis (PFGE). They assessed each strain's genetic similarity with respect to its country of origin and virulence status (whether it causes disease or not). Additionally, they assessed the same on nine farms located across four geographic regions within Texas.

The researchers found that R. equi pneumonia occurs endemically on some farms, as expected. The variability of isolates was surprising. "We rarely found isolates from the same farm that were identical," said Cohen. When the researchers compared isolates from the same farm, there was only one instance when isolates were considered indistinguishable, one circumstance when isolates were considered closely related (obtained from soil samples collected from different areas of the farm on the same year), and only three isolates that were considered possibly related.

Researchers found considerable diversity among isolates from various continents, and within each country. One might have expected isolates with similar PFGE patterns sprinkled across a particular country, distinguishing its strains from other countries, but that was not the case. "The apparent genetic heterogeneity (variability) among isolates from various continents could be attributable to spread resulting from international movement of horses," the researchers suggested, "but there are other explanations for this genetic diversity."

Cohen said that it is clinically important to note:

  • outbreaks at farms are not necessarily explained by having one particular strain on that farm;
  • Researchers didn't see evidence of specific R. equi endemic strains during outbreaks on farms;
  • You can't incriminate a given farm for passing R. equi to another farm; and
  • You need to consider the host and environmental factors in control of R. equi.

"There is considerable genetic variability among isolates of R. equi," said Cohen in his take-home message, adding that it's only rarely that DNA PFGE can help provide a strong genotypic link among isolates on the basis of source, time point, or location.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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