Streptococcus zooepidemicus Infections in Horses

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


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. ­zooepidemicusa 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.


S. zooepidemicus is a common cause of placental infection and inflammation that can lead to early embryonic death, abortion, premature birth, and in utero growth restriction. Mares might show few clinical signs, such as vaginal discharge and premature udder development. They often have a normal temperature and bloodwork and a good appetite. Veterinarians can make a presumptive diagnosis by viewing the placental layers via rectal ultrasound. Bacteria ascending from the urogenital tract mucosa through a relaxed cervix most likely cause the infection. In addition to treating with systemic antibiotics, such as trimethoprim sulfamethoxazole, that target the placenta tissues well, veterinarians often support mares with progestin hormones to maintain the pregnancy to term.

Skin infections

Because S. zooepidemicus exists as normal flora on horses’ skin, it can invade tissues damaged from trauma or maceration (softening/breaking down) due to wet ­environments.


In stallions S. zooepidemicus is often responsible for infection and inflammation of the epididymis (the structure that collects and stores sperm). It could occur independent of other conditions but commonly appears subsequent to infections of the secondary sex glands (vesicular glands, prostate, ampullae, and bulbourethral glands) within the male reproductive tract. The source of the bacteria can be ascending infection from the urogenital passages, as well as venereal contact or blood vessels.

Upper respiratory infections

In the past decades we’ve seen a few published reports of upper respiratory outbreaks caused by S. zooepidemicus. Although it’s usually not considered contagious between equids, 77,000 horses in Iceland were infected in a 2010 outbreak. Researchers documented S. zooepidemicus as the cause of a strangleslike disease in 2013 in Sweden, as well as rhinitis and pneumonia in horses on a Pacific island off Australia in the late 1990s. S. equi typically causes the classic signs of strangles unless proven otherwise, but now we must also consider S. zooepidemicus. Veterinarians can use culture and polymerase chain reaction (PCR, a DNA-based test) to distinguish between the organisms.   


S. zooepidemicus is traditionally susceptible to drugs such as penicillin, but veterinarians should base drug choices on culture and sensitivity results and antibiotics’ ability to penetrate affected tissues.

Zoonotic Potential

In the past 15 years scientists have increasingly recognized S. zooepidemicus as a potential zoonotic disease, meaning it could spread between animals and humans. Early reports of human outbreaks of S. zooepidemicus have been associated with consuming unpasteurized cheese, but more recently disease has been traced to horses. In a 2013 study researchers established proof of S. zooepidemicus transmission from horses to humans. Incidents have been fairly rare, and we don’t yet know enough to identify which horse with a S. zooepidemicus infection or even normal mucosal flora could be a source. Horse handlers should always practice good hand hygiene and might reconsider kissing their horses’ noses.