Shetland ponyThe bacterium Streptococcus equi subsp. zooepidemicus (often simply called Streptococcus zooepidemicus) is one that I think I understand less and less, including the risks this predominantly equine-associated bug poses to humans. It’s a well-known and common bacterium in horses, both healthy and sick, and can also be found in dogs and cats (where it can cause serious disease). Despite what has to be very common and sometimes abundant exposure in equine personnel in particular, human disease is rare. Yet, disease can occur. It seems like it’s being reported more often in the medical literature, but whether that’s because human infections are becoming more common or just more commonly identified and written about is a big question.

A recent case report entitled “Zoonotic necrotizing myositis caused by Streptococcus equi subsp. zooepidemicus in a farmer” (Kittang et al, BMC Infect Diseases 2017) once again shows the potential for nasty human infection.

The affected person was an otherwise healthy 73-year-old farmer who owned two healthy Shetland ponies. He was admitted to hospital with pain in his groin. His condition deteriorated and a CT scan identified suspected necrotizing myositis (flesh-eating disease). Emergency surgery was performed to remove the infected tissue in this case, as this type of bacterial infection is so severe and rapidly progressive that the best chance of survival is for the infected tissue to be removed ASAP – antibiotics alone are unlikely to to be effective. Culture of the affected tissue yielded S. zooepidemicus. Thirty days, two surgeries and lots of very intensive care later, the patient was discharged from the hospital. Rapid diagnosis, initial administration of antibiotics that were effective against the bacterium and most importantly, rapid surgery, were probably the most important factors in this successful outcome.

The link to horses is through the nature of the bacterium and not any specific investigation. Since S. zooepidemicus is a horse-associated bacterium and the man had contact with ponies, the potential link is obvious. However, the bacterium can also be found in other animal species, so it’s still only a presumptive link. As is typically (and unfortunately) the case, swabs of the ponies were not taken to see if the same bacterial strain could be identified.

How the man was infected isn’t clear. He reportedly had some minor abrasions and blisters on his fingers at a time when he was handling the ponies. So, it’s possible the bacterium from respiratory secretions of a pony got into the wounds and spread to the muscles in his thigh. That’s a pretty oddball occurrence for minor skin wounds and healthy horses. Considering the number of horse personnel that have skin wounds at any time, this highlights the potential but presumably exceptionally low overall risk. The man’s age probably increased his risk of infection, but other reasons why he (and not the large number of other people similarly exposed to horses on a daily basis) got sick are not obvious.

Does this change anything?

  • Not really. We’ve known that S. zooepidemicus can cause disease in people for a long time. This type of infection is different but not overly surprising. It’s another reminder of the ever-present potential of exposure to zoonotic pathogens from healthy animals.

What should be done?

  • A little hygiene goes a long way. Keeping wounds covered and washing potentially exposed sites are probably the best preventive measures when working around apparently healthy animals. That’s particularly true for people at increased risk of disease, such as people over 65 year of age, young children, pregnant women and people who have a compromised immune system.

So, don’t run screaming next time you get horse slobber on your hands, but don’t forget to wash your hands.