Streptococcus zooepidemicus in Horses: Symptoms & Treatment

Streptococcus equi subspecies zooepidemicus, usually shortened to Strep zoo, is one of the most common bacteria involved in disease in horses. It lives quietly on the skin and mucous membranes of most healthy horses and only causes problems when the animal's defences are lowered. When it does, it is remarkably versatile: it drives secondary respiratory infections, wound and skin infections, infections in newborn foals, and it is the single most common bacterial cause of infertility in broodmares.

This guide explains what Strep zoo is, how it differs from the bacterium that causes strangles, the signs it produces, whether it is contagious, and how it is treated, including why one hidden form of the infection is so difficult to detect.

What is Streptococcus zooepidemicus?

Strep zoo is a beta-haemolytic streptococcus (Lancefield group C). It is an opportunistic pathogen: a normal resident of the equine upper respiratory tract, skin and lower genital tract that causes no harm in a healthy horse. It becomes a problem when something else weakens the horse first, a viral infection, the stress of transport, a foaling injury or a wound, giving the bacteria a chance to multiply and invade.

Strep zoo vs Strep equi: not the same disease

This is the most common point of confusion. There are two closely related subspecies:

• Streptococcus equi subsp. equi causes strangles, a specific, highly contagious disease with abscesses of the lymph nodes. It is an obligate pathogen.
• Streptococcus equi subsp. zooepidemicus (Strep zoo) is the opportunist described here. It does not cause strangles, is far less contagious, and produces a range of secondary infections rather than one specific disease.

Symptoms of Streptococcus zooepidemicus in horses

Because Strep zoo can infect several body systems, the signs depend on where it takes hold.

Respiratory infection

Strep zoo is the most frequent bacterial complication of viral airway infection and of long-distance transport (sometimes called shipping fever). Signs include fever, nasal discharge, cough and, in serious cases, pneumonia or pleuropneumonia. It is usually a secondary invader after a virus or stress has damaged the airway lining.

Uterine infection in mares

In broodmares, Strep zoo is the number one bacterial cause of endometritis. In a German study of 28,887 endometrial samples, beta-haemolytic streptococci accounted for 79.7% of all positive cultures (Köhne et al., 2024). Here the problem is different: the infection is often subclinical, with no obvious signs. The mare cycles normally and looks healthy, but repeatedly fails to conceive or loses the pregnancy early. This hidden form is covered below.

Wounds, skin and foals

Strep zoo also infects wounds and surgical sites, and in newborn foals it can cause joint infections (joint ill) and blood infection (septicaemia). A note for owners: Strep zoo is occasionally zoonotic, so basic hygiene around sick horses is sensible.

Is Streptococcus zooepidemicus contagious?

Yes, but far less than strangles. It can spread between horses through direct contact and respiratory secretions, and it is already present in most horse populations. Because it is an opportunist rather than an obligate pathogen, it usually only causes disease when the receiving horse is already stressed or immune-compromised. It does not sweep through a yard the way strangles does. Sensible biosecurity around a sick horse is still worthwhile.

Treatment of Streptococcus zooepidemicus

Strep zoo is usually highly sensitive to penicillin, which remains the first-choice antibiotic, guided where possible by culture and sensitivity. The same German study found 99.5% of beta-haemolytic streptococci were penicillin-sensitive, with multidrug resistance in only 3.1% of isolates (Köhne et al., 2024). For an active infection, a wound or a respiratory case, treatment is generally straightforward.

There is one important catch. Antibiotics only work on bacteria that are actively growing. And in one setting, the mare's uterus, Strep zoo has a way of avoiding exactly that.

The hidden problem in broodmares: dormant Strep zoo

In chronically infected mares, Strep zoo retreats deep into the uterine lining and switches into a dormant, low-metabolic state, protected within a biofilm. In this state the bacteria stop dividing, with two consequences that make the classic problem mare so frustrating:

• Standard swabs come back clean. Culture depends on bacteria growing on a plate; dormant bacteria do not grow, so the swab is falsely negative even though the infection is present and preventing conception.
• Antibiotics do not clear it. A dormant, non-dividing bacterium is not killed by penicillin. The mare may seem to respond, then fail to conceive again the next season.

This is why a mare can have a clean culture, a normal examination and a fertile stallion, and still not get in foal season after season.

The way through is to reactivate the dormant bacteria first, so they become detectable and treatable. This is what bActivate does: a veterinarian instils the growth medium into the uterus, and within 24 to 48 hours previously invisible Strep zoo becomes visible to a standard culture. The veterinarian can then identify it and treat it with targeted antibiotics. In independent clinical evaluations this approach achieved an 83% pregnancy rate in 64 problem mares at Hagyard Equine Medical Institute and an 89% pregnancy rate at Kildangan Stud (Godolphin). The underlying mechanism was published in Veterinary Microbiology (Petersen et al., 2015).

bActivate is a diagnostic aid, not a treatment in itself, and it applies only to this dormant uterine form, not to respiratory or wound infections, which respond to standard antibiotic therapy.

Frequently asked questions

Is Streptococcus zooepidemicus the same as strangles?
No. Strangles is caused by Streptococcus equi subspecies equi. Strep zoo is a different, opportunistic subspecies that causes secondary infections and is much less contagious.

What antibiotic treats Strep zoo in horses?
Penicillin is usually the first choice, ideally guided by culture and sensitivity. It works well on active infections, but not on dormant uterine bacteria that are not growing.

Why does my mare have a clean swab but still won't get in foal?
Because dormant Strep zoo deep in the uterine lining does not grow on a standard culture, so the swab reads negative while the infection is still present. Reactivating the bacteria before culture reveals it.

Can Streptococcus zooepidemicus infect people?
Rarely. Human cases are uncommon and usually linked to unpasteurised dairy or close contact with infected animals.

References

• Köhne M, et al. Frequency of potentially pathogenic bacterial and fungal isolates among 28,887 endometrial samples from mares. Journal of Equine Veterinary Science 2024;133:105008. Link
• Petersen MR, et al. Activation of persistent Streptococcus equi subspecies zooepidemicus in mares with subclinical endometritis. Veterinary Microbiology 2015;179(1-2):119-125. PMID 26123371
• Nielsen JM. Endometritis in the mare: a diagnostic study comparing cultures from swab and biopsy. Theriogenology 2005;64(3):510-518. PMID 15978661

Further reading

• The Horse: What's Plaguing Your Problem Mare?
• EquiManagement: Biofilms and the Equine Uterus

If you manage a mare that keeps failing to conceive despite clean swabs, a hidden Strep zoo infection is a likely explanation. Read more about how bActivate detects it or review the clinical evidence.

Written by the Bojesen & Petersen Biotech ApS team. Medical oversight: Prof. Anders Miki Bojesen DVM PhD (University of Copenhagen) and Dr. Morten Rønn Petersen DVM PhD Dipl. ACT.