S. zooepidemicus is not usually contagious between horses, but a 2010 outbreak in Iceland infected 77,000 horses there. | Photo: iStock

Streptococcus equi subspecies zooepidemicus is a Gram-positive bacterium that can cause opportunistic infections in many animal species, including horses, cattle, swine, sheep, goats, chickens, cats, and dogs. The bacteria cause disease when the normal mechanisms with which the body protects itself break down. S. zooepidemicus is actually part of the normal bacterial flora in and on horses’ bodies. It’s responsible for a wide variety of diseases and issues in horses, including pneumonia, abortions, and upper respiratory, wound, testicular, ­and neonatal infections.

S. zooepidemicus vs. S. equi

S. zooepidemicus is related to S. equi subspecies equi, the causative agent of strangles. Both organisms appear as long chains of cocci (spherical bacteria) under a microscope. Strangles differs from the diseases caused by S. zooepidemicus because S. equi is highly contagious from horse to horse, is typically horse-specific, is always considered pathogenic (causing signs of disease), and is usually limited to an infection of the upper respiratory tract. 

Conditions Caused by S. ­zooepidemicus

Pneumonia

Veterinarians consider S. zooepidemicus the most important pathogen associated with pneumonia in horses of all ages. A viral infection, intense training, or prolonged transportation first compromises the horse’s defense mechanisms, enabling S. ­zooepidemicus—a normal inhabitant of the equine throat and tonsils—to take advantage and establish an infection in the lungs. Concurrent infections with other bacteria often cause further ­complications