Streptococcus Between Species

Q. Can Streptococcus equi zooepidemicus be transferred from horses to people? Someone recently told me it can, and now I’m concerned. If humans can catch it from horses, what are the symptoms in people?

A. Streptococcus zooepidemicus is a bacterium that is commonly found in healthy and sick horses. While it is a fairly common cause of respiratory disease in horses (and, uncommonly, other types of infections), it can also be found in the mouth, nose, and throat of healthy horses. This bacterium is different than Streptococcus pyogenes (a Group A Streptococcus), which causes “strep throat” in humans, and Streptococcus pneumoniae, an important cause of respiratory infections and meningitis in people.

Streptococcus zooepidemicus can cause disease in people, but it is very rare. It can range from mild infection to serious eye infections or meningitis. Horses are not the only sources, as human infections have been linked to other species such as dogs and guinea pigs.

Putting the risk into context is important. When one considers how often S. zooepidemicus is present in horses (sick or healthy) and how often people have contact with those horses, the small number of reported infections in people suggests that this bacterium is poorly adapted to cause disease in humans.

While the risk seems to be very low, it is not zero. Disease can occur and rarely can be serious. Striking the balance between being proactive and paranoid can be a challenge, particularly when there is little information about why infections occur in some people. Common sense is the best guide in the absence of clear information. Much of this involves basic hygiene practices (e.g., hand-washing) and limiting contact with sick horses. Wearing eye protection and a mask when working closely with horses with respiratory disease is a reasonable consideration to reduce the risk of exposure, but is not widely done. People with compromised immune systems should be particularly vigilant and consider limiting contact with infected horses, wearing eye protection and a mask if they must work around a horse with respiratory tract disease, and being particularly diligent with hand-washing and other basic hygiene practices at all times.