Many kinds of sexually transmitted diseases can affect horses; protect your stallions and mares from the most common offenders.

The horse's reproductive system is not perfect and often throws up roadblocks when we, as breeders, seek a live, healthy foal during a given year. There are so very many things that can go wrong. Despite the best efforts of scientists and researchers around the world, the overall success rate for mares becoming pregnant and carrying foals to term remains relatively low.

Included in the multitude of "things that can go wrong" are sexually transmitted diseases that raise an ugly head without warning, then stubbornly stick around long enough to frustrate an entire breeding season, or beyond.

Types of Problems

An example is contagious equine metritis (CEM), a disease that at one time wasn't found in the United States, but at this writing it is alive and kicking, with a current outbreak costing horse owners money and causing concern.

Unfortunately CEM isn't the only sexually transmitted disease that can strike. Also included in the lineup are equine viral arteritis (EVA), a disease that wreaked havoc in a segment of the Quarter Horse industry back in 2006; equine coital exanthema, a viral disease caused by equine herpesvirus-3; dourine, which at the moment is confined to South and Central America, the Middle East, and Africa; and bacterial assaults by Pseudomonas aeruginosa and Klebsiella pneumoniae.

These diseases can bungle up a breeding program--causing issues that range from endometritis (inflammation of the uterus) in the mare and infection in the stallion to outright abortion.

Contagious Equine Metritis

The USDA Veterinary Services Laboratory reported in September 2009 that 22 stallions in the United States had tested positive for CEM, along with five mares. Another 962 horses, the laboratory reports, have been exposed to Taylorella equigenitalis, the bacterium that causes CEM. None of the positive horses has yet been identified as the source of the outbreak.

Infection spreads through direct transmission of bacteria from mare to stallion or teaser, or from stallion or teaser to mare, at the time of mating or teasing. It is also transmitted to mares if semen from infected stallions is used in artificial insemination (AI).

According to officials from the Department for Environment, Food and Rural Affairs in the United Kingdom, where the disease first was reported in 1977, indirect infection also can occur through contaminated water, utensils, and instruments; the hands of staff and veterinarians who handle the mare's tail and genital area or the stallion's or teaser's penis; and by stallions'/teasers' genital-to-genital or nose-to-genital contact with mares.

Infected mares often develop endometritis and don't become pregnant. Some infected mares will abort. The disease has little physical effect on the stallion, but he can be a carrier for years and infect mares either through natural cover or AI.

The first cases of CEM in the United States were diagnosed on March 9, 1978, on Thoroughbred farms in Central Kentucky. In April of the following year there was an outbreak in Missouri. The disease was quickly eradicated in both states.

In 2006, according to Donna Gilson of the Wisconsin Department of Agriculture, three Lipizzaner stallions imported from eastern Europe into Wisconsin tested positive for CEM after their arrival, but they had not been used for breeding. (An outbreak was avoided entirely.)

The recent outbreak in was discovered on Dec. 15, 2008, when the state of Kentucky officials confirmed that a 16-year-old Quarter Horse stallion on a Central Kentucky premises had tested positive for CEM. The stallion was undergoing routine testing by the University of Kentucky Livestock Disease Diagnostic Center as a preliminary step to shipping frozen semen to the European Union. The stallion in question had arrived at the breeding facility the previous February from Texas. Further testing revealed three other stallions at the premises were positive for CEM.

The USDA's Animal and Plant Health Inspection Service (APHIS) reported in September that in addition to the initial 27 cases reported (22 stallions and 5 mares), 274 exposed or positive stallions have been found in 31 states and 715 exposed or positive mares have been found in 46 states. When that report was issued, only Hawaii and Rhode Island had not reported positive cases.

The disease is treatable, but the process can be time-consuming. Here is how treatment is described in an APHIS fact sheet: "The mare cannot be successfully treated until the CEM bacteria clear from the uterus, a process that may take several months. The external genitalia of the mare and stallion can be treated with disinfectants and antibiotics. Once daily for five consecutive days, the external genitalia should be gently scrubbed with two percent chlorhexidine in a mild detergent solution and rinsed with a warm saline solution. The external genitalia should then be coated with an antibiotic ointment, such as nitrofurazone."

In September APHIS stated that the majority of positive and exposed stallions and mares involved in the current outbreak had completed testing and treatment, and at that time they were negative for CEM. (For more information on this see page 11.)


Pseudomonas aeruginosa and Klebsiella pneumoniae can cause endometritis in the mare that often appears similar to clinical signs of CEM. These bacteria normally are harbored on the stallion's genitalia and are transmitted through breeding.

Equine Viral Arteritis

The current CEM outbreak demonstrates the speed at which disease can spread. The same can be said for an EVA outbreak that occurred in 2006. It started with one popular Quarter Horse stallion in New Mexico, and before it had run its course EVA had been confirmed in six states and suspected in three more.

Peter Timoney, MVB, PhD, FRCVS, of the Gluck Center, said in an extensive interview at the time that cases of EVA can present with a combination of the following clinical signs: fever, swelling (particularly of the limbs), loss of appetite, depression, swelling of the male external sex organs and the mammary glands in mares, conjunctivitis (inflammation of the outermost eye layer), nasal discharge, skin rash, and abortion in pregnant mares. The abortion rate often is 50% or higher. Adding an insidious element is the fact that the disease can also be spread via the respiratory route (inhalation of infected nasal secretions).

The virus responsible for EVA is equine arteritis virus (EAV). Timoney pointed out that EVA is not a new disease. It likely has existed within the equine population for a very long time. It was first identified as an etiologically separate disease (caused by something entirely different than other diseases) of horses in 1953, following an outbreak at a Standardbred farm in Ohio.

In 1984 there was a major outbreak on Thoroughbred farms in Kentucky, and in 1993 the disease struck at Arlington Park Racetrack in Chicago. A massive vaccination program at Arlington stopped the disease in its tracks.

Between that time and 2006, there were no major outbreaks, although the disease surfaced in various locales and among varying breeds through the years. One small outbreak among a group of Warmbloods was traced to a European stallion that transmitted EVA to mares in the United States through shipped semen. One of the problems involved in controlling EVA is that the virus can survive freezing and, thus, could be present in frozen semen.

Posing a major danger element in the spread of EVA, Timoney says, is the widespread use of transported semen and embryo transfer. Some farms that specialize in embryo transfer have as many as 500 or more mares on the site as embryo recipients. In many cases, Timoney explains, they are confined in what amounts to a feedlot setting with a high number of mares in close proximity. If there should be an outbreak of EVA, he says, it could quickly spread through the entire population via the respiratory route. The problem becomes compounded even more when one realizes that each of the recipient mares ultimately will be sent off to another location and, if infected, she could spread the disease to a new, susceptible population at another location.

In the case of the 2006 outbreak stemming from the New Mexico stallion, semen was shipped to mares in 18 states. Fortunately EVA cropped up in only nine of them. However, mares infected from the semen spread the disease to others in their midst via the respiratory route.

There is some good news in regard to EVA. Timoney explains: "Mares do not become carriers of the virus. A mare will experience the infection; she will shed the virus for a limited period of time by various routes, primarily by the respiratory route; the virus will circulate in her bloodstream for up to 2½ weeks, and then she'll develop a very solid and long-lasting immunity as a result of that infection. The virus will be eliminated from her system, and she will become a serologically positive protected mare, but not one that represents any risk whatsoever in terms of shedding the virus at that point and acting as a source of infection of any susceptible in-contact horses."

Also good news is the fact that a relatively inexpensive vaccine is available to prevent EVA. The only problem during the 2006 outbreak was that Fort Dodge, which manufactures the vaccine, had cut down on production because of lack of demand and, for a time, there wasn't enough to meet the crisis.

Timoney recommends that mares be vaccinated three weeks before breeding and that stallions and colts between 6 and 9 months old be vaccinated to eliminate the risk of becoming carriers.

It's important to note that testing should be completed before vaccinating horses that might be exported. A vaccinated horse will test positive for EVA after vaccination and might be denied entry into many countries.


The best news available about dourine is that it is not present in the United States. Dourine is caused by infection with the protozoal parasite Trypanosoma equiperdum, which most often is passed from stallion to mare via breeding, but also can be passed from mare to stallion. No vaccine is available. According to officials at the Center for Food Security and Public Health at Iowa State University, the infection can result in neurologic signs and emaciation. The mortality rates are high. Dourine has been eliminated in some areas of the world by humanely destroying any horse that tests positive for the disease.

Equine Coital Exanthema

The final sexually transmitted disease to be discussed is the most mild of the lot. Equine coital exanthema is a viral disease caused by equine herpesvirus-3. Affected mares suffer local problems of the perineal area, such as blisters that might break and become inflamed. The stallion can be affected, with lesions showing up on the penis. Normally, veterinarians say, the treatment of choice is sexual rest until the condition clears up.

Veterinarians say this is potentially a dangerous "behavioral" disease because of the pain that it causes the stallion. If a stallion is hesitant to get an erection and breed a mare, this behavior might be enhanced if he is asked to breed a mare while he has penile lesions because he anticipates discomfort. To a lesser extent this happens in the mare.

Take-Home Message

There are many sexually transmitted diseases in horses. Good preventive measures, and vaccination where appropriate, can help prevent problems. If a stallion or mare gets a sexually transmitted disease, it is up to the owner to ensure that horse is isolated to protect other horses.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

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