Bug of the Month: Streptococcus zooepidemicus
Streptococcus zooepidemicus (technically Streptococcus equi subspecies zooepidemicus), commonly called Strep zoo, is an important bacterium in equine medicine. It is one of the most common bacteria isolated from infections in horses. Like other streptococci, S. zooepidemicus is a Gram positive coccus, meaning that it stains purple with Gram stain and has a ball shape (coccus). Streptococci tend to stick together in chains (see picture right) which are often described as "string of pearls."
Streptococcus zooepidemicus can be found in the upper respiratory tract of healthy horses. In most instances, it lives there without causing any problems. Strep zoo is considered an opportunistic pathogen - a bacterium that can live in harmony with its host, but can also cause disease in certain situations. Strep zoo can cause a variety of infections, including most commonly pneumonia, lung abscesses and guttural pouch infections. More severe conditions, like bloodstream infections, joint infections and meningitis, can also occur, but are uncommon and most often affect neonatal foals.
Diagnosis of disease caused by S. zooepidemicus requires culture of appropriate samples collected from the horse. Care should be taken when interpreting cultures of samples collected from the nasal passages or upper respiratory tract, since S. zooepidemicus is often present there even in normal animals, so finding it does not necessarily mean it is causing disease.
Unlike some bacteria that cause infections in horses, S. zooepidemicus tends to be susceptible to numerous antibiotics, including penicillin and trimethoprim-sulfa. However, since some strains can be resistant to various drugs, culturing the bacterium and testing its susceptibility to antibiotics in the lab is still required to determine the optimum treatment, .
Strep zoo is of minimal risk to people. Human infections caused by this bacterium have been reported but they are considered very rare.
