Sexually Transmitted Diseases

You're preparing your mare for the upcoming breeding season. The stud owner insists on a bacterial culture of your mare's uterus prior to live breeding to reduce the chances of her passing any sexually transmitted diseases (STDs) she might have to the stallion. But what about the stallion? The mare has a higher risk of contracting an STD from the stallion than vice versa. With some STDs, the stallion is merely a carrier and exhibits no clinical signs of disease. It's not until the mare becomes infected that the culprit stallion is identified as the source of venereal disease. Some STDs can even be transmitted in cooled fresh or frozen semen.

When a mare contracts an STD, trouble can follow in the form of a lost breeding season, lost pregnancy, or a sick (and sometimes lost) neonatal foal. The worst-case scenario is when STDs spread through a breeding farm, shutting down the farm for the entire season. Or worse yet, the disease is spread to an entire equine breeding community, resulting in tremendous economic losses!

Fortunately, the number of sexually transmitted diseases is low, and owners are becoming wiser about what venereal diseases are out there and how to reduce the risk of their transmission. A.C. Asbury, DVM, Dipl. ACT, Professor Emeritus of the University of Florida and consultant to the Grayson-Jockey Club Research Foundation; and Beverly J. Purswell, DVM, PhD, Dipl. ACT, a professor of theriogenology in the Department of Large Animal Clinical Sciences at Virginia-Maryland Regional College of Veterinary Medicine, discuss the STDs you should be aware of.

Contagious Equine Metritis (CEM)

"Contagious equine metritis is a highly contagious bacterial infection caused by the organism Taylorella equigenitalis," says Purswell. "It hit the United States in the late '70s from Europe. The stallion is a carrier only; it does not affect him, but he passes the bacteria to the mare. In turn, while the mare is infected, she can infect another stallion. When it arrived here in the late '70s, they had to shut down the breeding season in Kentucky, which was extremely costly to the Thoroughbred industry."

CEM causes an acute endometritis (inflammation of the inner lining of the uterus), a process that interrupts fertility, Asbury says. "The organism can establish itself in the clitoral fossa (the folded lining near the clitoris in the mare), which then becomes a reservoir for future spread by natural service. In the stallion, it can colonize on the external genitalia, especially the urethral diverticulum (the normal sac-like outpouching just above the urethral opening), a source of further contamination of mares. CEM can also be transmitted by contaminated instruments or the hands of the personnel preparing the mare for breeding. As a carrier, the stallion displays no clinical signs of the disease. In mares, CEM can be characterized by mucous discharge from the vulva, but many are asymptomatically infected (don't show visible clinical signs).

"Diagnosis is made by recovery of the bacteria, which are very difficult to grow, requiring special media and a low oxygen/ high carbon dioxide environment," Asbury states. Antibodies are detectable in infected mares 10-20 days after exposure to the bacteria.

Serological diagnosis usually is based on finding the antibody to the organism usually by the complement fixation test.

Acute infection can be self-limiting, with the majority of the inflammation subsiding in a week or two. Asbury says, "Some sexual rest is imperative, and the prognosis for fertility is favorable. Residual bacterial colonies in the clitoral region are sources of further infection and must be identified. Those areas should be scrubbed with chlorhexidine solution and packed with chlorhexidine ointment repeatedly until the organism is eliminated."

Managing stallions to reduce the organism on their genitalia requires a similar approach--utilizing thorough cleaning with chlorhexidine solution and packing with nitrofurazone ointment.

"The penis must be extended for these treatments," Asbury states, "and great care taken to cleanse all the folds of the prepuce and the urethral fossa. The standard program is five consecutive days of treatment. Cultures of these areas should be negative before returning stallions to service."

There is no viable immunization program available, but the prognosis for recovery is excellent.

Asbury recommends that state or federal veterinarians be contacted to determine the current regulatory aspects of the disease. Most areas will require treatment with subsequent isolation until cultures are negative.

The United States Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) requires that stallions and mares over the age of 731 days coming into the U.S. for permanent import (i.e., the horse will remain in the U.S. instead of coming in for an event then returning to its country of origin) from CEM-affected countries and regions must undergo testing and quarantine. Affected areas include Japan and many of the European Union countries.

Says Andrea Morgan, DVM, of the National Center for Import/Export, Veterinary Services, APHIS, "Once these horses come through one of our animal import centers, they go through routine tests for certain infectious diseases, a three-day quarantine, and observations for ill health. After that, they're transported in a sealed van to an approved CEM quarantine facility where they undergo additional testing and observation." Morgan says the quarantine period can last up to 60 days.

Some exceptions apply to competition horses, including Thoroughbreds, coming from the United Kingdom, Germany, Ireland, and France. "We allow them to be imported for competition purposes and permanent quarantine, provided they have three negative test results on swabs for CEM in the country of origin," Morgan says. "We also recently made exceptions for purebred horses from Spain, where we required certain certification statements that the horse had never been on a premise that's exclusively a breeding premise, had never been bred, etc. These horses also need to be cultured negative from three separate sets of swabs."

Import regulations on CEM are covered in the Code of Federal Regulations, part 9, section 93.301. They can be seen at the USDA/APHIS web site (www.aphis.usda. gov), keyword search "93.301."

Pseudomonas aeruginosa and Klebsiella pneumoniae

Some strains of Pseudomonas and Klebsiella bacteria have caused multiple, specific incidents of endometritis after breeding to stallions harboring the organisms. These venereal infections are usually passed from the stallion to the mare. They often result from overzealous cleaning of the stallion's genitalia.

"The problem with the stallion is that someone probably has done too good a job of trying to clean his penis prior to breeding by using disinfectants on the penis instead of just washing him with water or mild soap," Purswell explains. "This problem started in the late '70s because of CEM; we knew the stallion was just a carrier, just
on his penis and fossa, so we took care of this by disinfecting the penis."

However, disinfecting the penis removes normal, non-harmful bacterial flora that should be on the penis and fossa, and thus allows proliferation of more pathogenic strains. Repeated scrubbing can also traumatize the skin, exaggerating the problem.

Because the bacteria colonize on the stallion's penis in the urethral fossa (also the clitoral area of the mare), perpetuation of the transmission of bacteria during breeding is enhanced.

"In fact, even artificial insemination may be responsible for the endometritis occurring when the external parts of the vulva or clitoral area (if these bacteria are present) contaminate the gloved hand that passes the insemination pipette," Asbury states.

These bacteria don't affect the stallion unless they ascend up into his internal organs. Once the internal organs are infected, the infection is very resistant to antibiotics, and the stallion might become a lifetime shedder of the organism in his semen.

In most cases, it's not until the mare develops endometritis and a uterine culture identifies the presence of Pseudomonas aeruginosa or Klebsiella pneumoniae (or even both) that the stallion is recognized as a carrier. The endometritis is treated with antibiotics specific to the bacterium.

"Prognosis is good," says Purswell. "It usually takes daily treatments of the uterus for three to five days, but in the process you lose a cycle or two on the mare for that breeding season."

Proper preparation of the stallion is the single most important factor in managing bacterial transmission at breeding. Asbury says that, "Current management recommendations include a thorough wash at the beginning of the season, or after long periods of inactivity, but limiting the routine pre-breeding cleanup to rinsing with clean, warm water and blotting dry with soft paper towels or cotton." Repeated use of antibacterial soaps will disrupt the normal bacterial population. He notes that culturing the stallion's genitalia and semen is not totally satisfactory, as cultures don't usually differentiate strains of the organisms that are not potentially sexually transmitted.

Equine Viral Arteritis

Asbury describes equine viral arteritis (EVA) as a highly contagious viral disease that causes fever, respiratory problems, eye inflammation, dependant edema (swelling of the legs or underbelly), and abortion.

"It spreads from horse to horse by direct contact, mainly contaminated respiratory secretions, and by breeding when stallions shed the virus in their semen, which they often do for long periods of time," he says. "Further concerns in recent years are for fresh or frozen semen being imported for use in domestic mares. Few regulations exist overseas to insure that stallions are free of the virus. Mares bred with infected semen will become antibody positive, even without developing clinical signs of disease."

Neither treating fresh semen nor freezing destroys the virus, allowing transmission by natural and artificial insemination.

"The epidemiology of EVA is puzzling," Asbury says. "The first major incidence of the problem was noted in the 1950s, when it was initially recognized. The incidence was limited by area (mainly Ohio) and by breed (Standardbred). Control was implemented by isolation techniques, and a modified live virus vaccine was developed. Then, other than a few isolated occurrences, the next outbreak occurred in Kentucky in 1984, involving mainly Thoroughbred horses. Since that outbreak, individual cases and small groups of horses have been affected sporadically throughout the country. Today, because of the higher percentage of Standardbred stallions which are carriers, a very high percentage of Standardbreds carry antibodies to the virus compared to other breeds."

Infected horses might show few or no clinical signs. "Occasionally these low levels of the disease erupt into more acute situations, causing the signs described above and concurrent abortions among the pregnant mares," Asbury says.

Abortion can occur in sick or feverish mares, or in mares which show no clinical signs of EVA. However, mares which have a fever and clinical signs can carry healthy foals to term. Both mares and males may develop edema of the abdomen, lower legs and, in stallions, the testicles. Following infection, a high percentage of stallions become shedders of the virus, many for prolonged periods of time."

Purswell adds that the mare's fertility is rarely affected and fatalities are rare. "Usually, the only deaths we see are in the neonatal foal within a few days or weeks of being born," she says. "They can develop a viral pneumonia and die from that. Very few strains actually cause death."

Although there is no treatment, just symptomatic management of the clinical signs, vaccination can prevent EVA. Asbury cautions, though, that vaccinating the stallion can cause problems when horses are moved between a few countries, although a majority of the trade partners with the United States accept vaccinated horses.

Equine Coital Exanthema

Equine coital exanthema (ECE) is a viral disease caused by equine herpesvirus 3; there is no relationship between equine herpesvirus 3 and herpesviruses 1 and 4 that can cause abortion, respiratory, and neurologic diseases. Fairly common, mild, and observed throughout the world, ECE spreads through breeding.

Affected mares suffer local problems of the perineal area, essentially of the vulva, Asbury says. "Initially there are small fluid-filled vesicles (blisters), which break and become inflamed. As a result, a raw, ulcerated condition involves most of the perineal area, and for a period of a few weeks the discomfort to the mare will prevent her from standing for the stallion.

"Lesions on the stallion are confined to the glans penis, and also are characterized by small vesicles, or blister-like lesions," he continues. "As long as these are obvious, and for a period of three weeks thereafter, the possibility of infecting mares exists. Therefore, for both mares and stallions, a three-week sexual rest is advisable."

Diagnosis is made by clinical signs. Usually, treatment is unnecessary, with the herpesvirus lesions healing spontaneously, says Purswell.

"ECE is not transmitted for the lifetime of the horse," she says. "You just sexually rest them while they have the active lesions." However, Asbury recommends treatment with mild, protective ointments to control secondary infection.

After the vesicles heal, the affected areas usually become permanently depigmented (loss of color). "This is very characteristic of ECE," Purswell says. "You will see little white spots all over the penis and around the vulva."


Dourine is a protozoal disease of horses and asses that is transmitted venereally. "The last known or suspected cases in the U.S. occurred in the mid-1950s in southern Arizona, along the Mexican border. Most of those cases were suspicious due to positive serology, but were found to be negative for lesions," Asbury reports. "Since then, dourine has been confined to South and Central America, the Middle East, and Africa."

The causal agent is the protozoa Trypanosoma equiperdum, the only protozoa to produce venereal disease in the horse. "Infected mares show swelling of external genitalia, vaginal discharge, and eventually plaques (circular, raised areas) on the skin," says Asbury. "Stallions develop edema of the penis and scrotum, plus urethral discharges. It causes serious illness and death in a small percentage of infected animals. Anything suggestive of this disorder should be reported to regulatory authorities immediately."

In general, sexually transmitted diseases do not pose great problems for either the small or large breeding operation, as long as preventive pre-breeding exams are regularly practiced and overzealous cleaning of the stallion's penis is not.

"Today, owners are pretty educated on safe breeding," Purswell states. "In general, they are much more knowledgeable." That knowledge directly translates into healthier horses and reduced financial loss." 


About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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