Tropical Rhodococcus equi outbreak? has reported on an outbreak of Rhodococcus equi that has claimed the lives of seven ponies at an equestrian facility on the island on Mayotte (a French protectorate off the coast of Madagascar). Local agriculture officials report that two other horses are also affected, but recovering, and the facility has been quarantined

There are a few strange aspects of this report. Rhodococcus equi is an important cause of respiratory disease in foals, in which it can cause serious abscesses in the lungs. However, it's extremely rare in mature horses, and it seems that the horses affected in Mayotte were adults. It's not impossible, but an outbreak of Rhodococcus in adults would be incredibly surprising, indicating either something that made these horses remarkably susceptible to the bacterium, or the presence of a strain of Rhodococcus more able to cause disease in adults. A more likely explanation is that it's not actually Rhodococcus. There's no mention of what type of disease the horses had or for what other infectious agents tests were done. A Department of Agriculture official stated that the diagnosis was made by blood tests, but blood testing is pretty useless for diagnosis of Rhodococcus. So, I'd consider the diagnosis highly questionable without further information.

Whatever the cause, something that kills seven horses on a farm is remarkable and thorough testing is needed to determine exactly what's happening. In the unlikely event that this was caused by Rhodococcus, more work needs to be done to explain why the outbreak occurred and why it was so severe. If (as is likely) it wasn't Rodococcus, knowing the actual cause is important for controlling further spread and preventing problems in the future.

Fortunately, the remote nature of this location makes it rather unlikely that whatever's happening there will spread to another region soon.

Image: Location of Mayotte (click image for source)

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John Prescott - February 22, 2012 3:57 PM

Interesting report, but it's anecdotal and needs to be fully documented in the peer-reviewed literature. Current serological tests for R. equi have low sensitivity and low specificity and are not recommendned by the ACVIM Consensus Statement for diagnosis of the disease.
People interested in the pathogenesis of rhodococcus equi have always been looking for a pristine desert island type of environment where the following question could be answered. Is Rhodococcus equi pneumonia limited to foals because of an age-related innate immunity, so that it's not seen in adults OR is it not seen in adults because they quickly usually develop immunity as foals? If the latter is the case, then in a pristine environment one might see an outbreak of pneumonia in adults following introduction of the infection. This desert island outbreak might have answered the question, but we need full and proper documentation with well accepted critera, which are NOT serological.

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