Venereal diseases are those that can be sexually transmitted. "Vener" is the Latin term for sexual intercourse and, hence, the origin of the term "venereal." The outcome of sexually transmitted diseases in horses can vary depending on the gender of the animal exposed to infection and the causal agent. Infections can give rise to infertility due to endometritis (inflammation within the uterus), the death of a young embryo after conception, the death of a more mature fetus (abortion), or the birth of a sick foal. Some of the infections involved can cause overt systemic illness in both mares and stallions, while others lay quiet in the mare and the stallion, creating a "carrier" state. The response to infection with some of these disease agents can be either "clinical" (readily apparent) or "subclinical" (not readily apparent).

Sexually transmitted diseases include bacterial, viral, or protozoal infections. The true sexually transmitted diseases in horses are not nearly as common as infections of other systems. Some of the diseases that will be discussed have not been reported in the United States for significant periods of time, but are endemic (persistent in the population) in other countries. This is important, as it can greatly affect the importation/exportation of both horses and semen. (There is a good review of venereal diseases of horses by Marcelo Couto, DVM, PhD, and John Hughes, DVM, Professor, from the Department of Reproduction, School of Veterinary Medicine, University of California, Davis, in the textbook Equine Reproduction.)

Bacterial Infections

The more common species of bacteria that can be sexually transmitted are: Pseudomonas aeruginosa, Klebsiella pneumoniae, Streptococcus zooepidemicus, and possibly Escherichia coli. (Taylorella equigenitalis is a true bacterial venereal disease and will be discussed later in this article.) All of these bacteria are very common in the environment of the horse. Pseudomonas is a common inhabitant of the soil, as well as often being a component of the normal flora of the skin, mucous membranes, and intestine of healthy animals. Klebsiella is a component of the normal flora of the intestine, as well as living in the general environment. It can commonly be isolated out of wood shavings.

Streptococcus zooepidemicus (Strep zoo) is perhaps the most common cause of acute and chronic infections in the horse. Strep zoo is a common cause of infections in wounds and abscesses, endometritis, and chronic pneumonia. As most already know, Escherichia coli (E. coli) is commonly found in manure or feces and is therefore ubiquitous in the environment.

Because these bacteria occur commonly in the environment of the horse, they also might be spread by (and perhaps commonly are) non-venereal means. Contaminated breeding equipment can be a potential source of infection.

Streptococcus zooepidemicus and E. coli are part of the normal flora (normal population of bacteria) of the external genitalia of the horse. This provides the opportunity for these bacteria to be transmitted at the time of breeding. Whether infection with either organism results in disease, however, appears to be greatly dependent on the individual strain of the bacterium and the response of an individual horse's immune system.

Some strains of the same bacterium are better at causing disease than others. So, if there is any decrease in local or systemic immunity (see "The Immune System" in The Horse of August 1996), the animal will be more susceptible to the clinical effects of infection.

Streptococcus zooepidemicus and E. coli are more commonly identified with endometritis than Pseudomonas aeruginosa or Klebsiella. One important factor that predisposes the individual mare to endometritis is poor vaginal conformation. The same poor vaginal conformation that results in windsucking creates a predisposition to endometritis. If the vulva tips forward, such that there is greater chance of fecal contamination when the horse defecates, or if there is something abnormal about the vulva that causes a defective seal, then there is a greater likelihood of endometritis.

Some of these mares can also pool urine (see "Urine Pooling" in The Horse of April 1996). Urine pooling is a condition where the conformation of the vagina allows a small pool of urine to remain on the floor of the vagina after urination. This condition can also predispose the mare to chronic endometritis.

Of the four bacteria that have been listed, sound scientific evidence is only available on the significance of Pseudomonas aeruginosa and Klebsiella pneumoniae as venereal pathogens of the horse.

All four of these bacteria can be found in small numbers as part of the normal flora of the stallion's external genitalia. The normal flora of a stallion's external genitalia is complex, and the area harbors numerous bacteria on the skin surface and within the smegma that compete for space with potentially pathogenic bacteria.

This fact plays an important role in the prevention of Pseudomonas and Klebsiella infections. Both of these bacteria have been observed to cause disease following the pre-breeding practice of vigorously scrubbing the penis with strong antiseptic cleansers. This was especially noted after this cleansing practice was adopted following the last outbreak of contagious equine metritis (CEM) in the United States.

Vigorous scrubbing and use of antiseptic cleansers significantly interfere with the normal bacterial flora and "selects" for a population of bacteria that can tolerate these conditions. Pseudomonas aeruginosa is known for its ability to resist antiseptics and disinfectants. In fact, it can occasionally be isolated from containers of alcohol and iodine swabs.

As the numbers of bacteria colonizing the penis of the stallion increase, so do the numbers that are introduced into the mare during natural cover. If the particular bacterial strain involved is highly pathogenic (enhanced ability to cause disease), the mare's defense mechanisms will be overwhelmed and disease might result. It has been suggested that the stallion's penis be washed with pure water, or if antiseptics are used, that the washing technique be a gentle one.

Typically, only mares develop signs of sexually transmitted bacterial diseases. The signs are representative of non-specific inflammation of the reproductive tract. Infertility might be the only clinical sign, or there might actually be a discharge of pus from the vagina. More detailed veterinary examination might reveal hyperemia (redness) of the cervix and/or vagina. The stallion will rarely show any outward signs of disease, and the only clue that a particular stallion is causing a problem might be the occurrence of similar clinical signs or infertility in a high percentage of the mares covered by that stallion.

Many mares with a normal uterine immune defense mechanism will clear these infections shortly after being infected, but some might require additional treatment. If a stallion is suspected of carrying a particular infection and is confirmed infected, the usual treatment is sexual rest. Further treatment might be necessary depending on the individual case, but many of these bacterial infections are reported to clear up following several weeks of sexual rest.

Contagious Equine Metritis

A bacterial venereal disease of considerable significance is contagious equine me-
tritis (CEM). This disease, caused by the bacterium Taylorella equigenitalis, was first reported following an outbreak of venereal disease in the Newmarket area of England in 1977 and in Ireland that same year. The disease has subsequently been reported in additional countries (see sidebar) and the United States. An outbreak affecting several Kentucky breeding farms occurred in 1978; this resulted from two Thoroughbred carrier stallions which were imported from France in the early fall of 1977.

Contagious equine metritis is transmitted primarily via breeding, but mechanical transfer from a carrier or infected animal can occur. The clinical signs in mares affected with CEM develop 8-10 days after being covered or inseminated with infective semen and consist of a copious, grayish vulvar discharge. Most mares are reported to recover spontaneously, with the microorganism persisting in the reproductive tract of some for weeks, months, or years after recovery--these mares become asymptomatic carriers. Stallions do not develop any clinical signs, and many become asymptomatic carriers.

The diagnosis of CEM is based primarily on culturing the organism from the genitourinary tract of infected horses. In mares, swabs are taken from all accessible sites of infection--the endometrium (lining of the uterus), cervix, and clitoral fossa and sinuses. In stallions, samples are generally taken with the penis erect by swabbing the urethral fossa, urethra, preputial folds, and surface of the penis.

Contagious equine metritis has only been reported once in the United States since the 1978 outbreak (in one Thoroughbred stallion and two mares in 1982). This is a tribute to the efforts of the USDA, state regulatory officials, and equine practitioners with respect to protecting our equine industry. The import regulations, which have been stringent, recently have been revised. The Federal Register on Oct. 7, 1996, contained a detailed account of the import regulations specific to CEM. The number of states approved by the USDA to receive and test/quarantine imported horses is limited to: Alabama, California, Colorado, Florida, Kentucky, Louisiana, Maryland, Montana, New Hampshire, New Jersey, New York, North Carolina, Ohio, South Carolina, Tennessee, Texas, Virginia, and Wisconsin. All states mentioned are approved to receive both mares and stallions with the exception of Florida, which can only receive stallions.

For mares or stallions requiring quarantine and testing, the protocol is demanding. Mares must arrive at a U.S. Port of Entry with a USDA import permit, a veterinary health certificate, and a negative CEM culture taken within 30 days of import. From there the animal is quarantined at a USDA Animal Import Center generally for three days, after which it is moved to a post-entry CEM state quarantine facility. While at the state CEM quarantine, the mare will have swabs taken from the clitoral fossa and sinuses to be cultured for Taylorella equigenitalis; these cultures are taken on Days 1, 4, and 7 of quarantine. After the last culture specimen is obtained, the clitoral fossa and sinuses are washed and treated locally with an approved antimicrobial for five consecutive days. At the end of the treatment period, the mare is released if she has had three negative cultures for the CEM organism. If any of the cultures are positive for Taylorella equigenitalis, the mare is quarantined for an additional 21 days and the entire testing procedure is repeated.

Things are more complicated in the case of stallions. A stallion enters the post-entry CEM state quarantine facility after the same initial import procedure as the mare. On Day 1 in the quarantine facility, culture swabs for bacteriological testing are obtained from the urethral fossa, terminal urethra, external surface of the penis, and prepuce. If the first set of samples are negative, the stallion must then breed two test mares. These test mares are provided by the quarantine facility and have to have had three negative cultures for Taylorella equigenitalis as described for an imported mare. After breeding two test mares, the stallion is washed and treated with an approved antimicrobial locally for five consecutive days. The test mares are cultured on Days 3, 6, and 9, and a special blood test is performed on Day 15. If all tests are negative, the stallion is released. If the stallion turns up positive on the initial set of cultures, or if any of the cultures are positive on the test mares, the stallion is quarantined for an additional 21 days and two additional test mares will need to be test bred as before.

Equine Viral Arteritis

Equine viral arteritis (EVA) is a contagious viral disease of members of the horse family that can cause fever, respiratory illness, ocular inflammation, edema (swelling) especially of the limbs, and the risk of abortion. The first account of the disease was in 1953 following an outbreak of respiratory illness and abortion in horses on a Standardbred breeding farm in Ohio. In 1984, an epidemic of EVA occurred on a large number of Thoroughbred breeding farms in Central Kentucky. Sporadic occurrences of the disease have been reported in Arizona, California, Canada, Colorado, Indiana, Kentucky, New York, Ohio, and Pennsylvania since the 1984 Kentucky outbreak.

The clinical signs of the disease can vary between individual horses. There is evidence that a substantial number of horses exposed to the virus develop an immune response, but never develop any obvious clinical signs. The clinical signs in affected horses can range from just a mild fever and limb edema to severe depression and illness. Limb swelling or edema (collection of fluid in the tissue under the skin) is commonly associated with this disease. The infection causes an inflammation of the blood vessels, which in turn causes them to leak protein and fluid into the tissue under the skin. The virus can be transmitted via aerosolized infective respiratory secretions.

Venereal transmission also can occur associated with the presence of virus in the semen of carrier stallions. It has been shown that 20-50% of the stallions which have been infected and recovered from equine viral arteritis can shed virus in the semen for years after the original infection.

Abortions due to equine arteritis virus generally occur late in the acute phase or early in the convalescent phase of the infection. Outbreaks have occurred in which up to 70% of pregnant mares have aborted. Abortion can occur as early as 60 days gestation. Mares which recover from the disease do not shed the virus or become carriers, as is the case in some stallions.

A diagnosis of EVA can be made by isolating the virus from swabs taken from the respiratory tract or from semen, urine, and/or an aborted fetus. Virus isolation is confirmative of a diagnosis of EVA. A negative virus isolation finding does not necessarily rule out the possibility of infection, as virus isolation can be difficult. A blood test can measure the horse's response to the virus. Typically, acute and convalescent blood samples taken two weeks apart are collected, and if a significant rise in antibody levels to equine arteritis virus occurs, this is confirmatory evidence of EVA.

Prevention of a disease is always better than having to deal with the disease. It has been shown experimentally that equine arteritis virus can be detected in the nasal secretions for one to three weeks after infection. Based on this, a quarantine period of at least three weeks should be implemented for new equine arrivals on farms before these animals have contact with the rest of the equine population on the premises. New stallions should be "screened" for presence of the carrier state.

A safe and effective vaccine against EVA is available. It should be remembered that once administered, vaccinated horses will develop antibodies to equine arteritis virus. This might affect the exportation of horses to a small number of countries that have restrictions on the importation of serologically positive horses. It is important to have a blood test performed prior to vaccination to document the pre-vaccination negative status of the animal. Your state veterinarian ( can provide more specific information regarding this.

Kentucky and New York have had control programs for EVA since 1985. Among the requirements in Kentucky is that "the owner/agent of mares booking to known shedding stallions must be notified in writing by the owner/agent of the stallion and a copy must be sent to the chief livestock sanitary official. Shedder stallions are allowed to mate only seropositive (blood test positive) mares prior to vaccination or exposure or mares that have been vaccinated against EVA at least 21 days before mating. The serologic (blood) testing of positive mares must have been conducted on or after Nov. 1 of the previous calendar year. All shedding stallions must be housed, handled, and bred in a facility isolated from non-shedding stallions."

More information regarding Kentucky's control program and the one in New York can be obtained from the state veterinarian of the Commonwealth of Kentucky.

About the Author

Michael Ball, DVM

Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.

Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at or by calling 800/582-5604.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More

Free Newsletters

Sign up for the latest in:

From our partners