Contagious Equine Metritis and Equine Viral Arteritis

Herd of Arabian horses on the field returns home

In preparation for breeding season, it is worth having some facts on hand to protect horses against infectious venereal diseases. Two reportable diseases—contagious equine metritis (CEM) and equine viral arteritis (EVA)—can be controlled through knowledge and surveillance.

Contagious Equine Metritis (CEM)

CEM, caused by the bacteria Taylorella equigenitalis, is a foreign animal disease. The bacteria are surface contaminants of the genitalia. A stallion does not experience systemic effects. However, a mare might present with copious vaginal discharge up to a couple of weeks following exposure. She might not conceive on the breeding in which she was exposed. Both mare and stallion can be sources of spread. Foals can become infected during parturition.

United States Outbreaks

Since 2006, there have been six outbreaks of CEM, reported Angela Pelzel-McCluskey, DVM, MS, equine epidemiologist for the USDA-APHIS-Veterinary Services. She recalled one outbreak that was massive and multi-state, lasting two years (2008-2010). More than 1,000 horses were directly exposed in 48 states. Twenty-two stallions, one gelding and five mares were chronically infected from the outbreak. They were a source of infection to other horses.

While an infected stallion does not show systemic disease, and clinical signs in a mare are usually transient, the consequences to breeding stock and the industry are significant. As an example, in the late 1970s and early 1980s, Kentucky experienced an economic disaster due to mares’ failure to conceive or pregnancy loss due to CEM infection during breeding. Mares developed a copious vaginal discharge. Stallions were out of commission for breeding until treated to a negative status, explained Pelzel-McCluskey.

Most mares will eventually clear the infection, said Pelzel-McCluskey, but some will end up with a persistent infection. She also noted that the infection and surface contamination remain, even after gelding.

Infected stallions are contaminated for life unless treated appropriately.

Other Transmission Sources

CEM’s causative bacteria can live on surfaces for several days—longer on wet or moist surfaces. Arnd Bronkhorst Photography

Transmission can occur through sources other than direct breeding. For example, inadequate biosecurity practices can lead to stallion-to-stallion transmission. This occurs from contaminated wash buckets, hands and artificial vaginas (AVs) used for semen collection. At one Kentucky facility, one stallion spread T. equigenitalis (the organism that causes CEM) to seven other stallions of multiple breeds. This happened in just one breeding season due to equipment contamination. These stallions potentially disseminated the bacteria to other mares and stallions.

Transmission of CEM Through Equipment

Because CEM is a bacterial infection, disinfection of equipment is achievable with chlorhexidine, dilute bleach or Roccal-D. To prevent bacterial contamination, use disposable plastic liners in wash buckets. You should also wrap the collection dummy with new plastic wrap for each stallion. You should clean the AV with soap and water and a disinfecting solution. During collection, stallion handlers should wear disposable gloves. Pelzel-McCluskey cautioned against using sponges for cleaning. The organism can live for days on a sponge or a wet surface.

Transmission Through Fresh, Cooled and Frozen Semen

Mares generally become infected through semen from live cover, or from cooled or frozen semen. Even when appropriate antibiotics in common semen extender preparations kill Taylorella, cooled shipped semen can carry the bacteria.

Pelzel-McCluskey noted, “There is either too great an infectious bacterial load in the semen and antibiotics used are overwhelmed, or there has been improper handling of semen extender. For example, the extender needs to stay refrigerated. But, if one is preparing it and an emergency interrupts the procedure and it is left out on the counter, then it loses some efficacy.”

Frozen semen with extender, often used in international shipping, also can retain CEM bacteria. She stressed that it is best not to rely on semen extender antibiotics to kill the organism. Instead, ensure that a stallion tests negative for T. equigenitalis before breeding or collecting.

Shipped semen can be a source of CEM infection. Breeding equipment must be thoroughly cleaned and disinfected. Arnd Bronkhorst Photography

CEM Surveillance 

Pelzel-McCluskey reported that currently there is not enough surveillance for CEM in active breeding stallions. Because of this lack of surveillance throughout the United States, the big outbreak of 2008-2010 was identified only when stallions were directly swabbed for CEM for export of semen to the European Union (EU). Kentucky has its own surveillance program to monitor stallions and breeding mares imported from endemic countries. These horses must be quarantined and tested per Kentucky’s protocol.

Import Testing for CEM

The USDA import requirements have some exceptions from import testing. This specifically applies to racing Thoroughbreds or Thoroughbreds in training, even if the horses come from CEM-affected countries. Because of the large number of Thoroughbreds imported for racing into Kentucky, that state’s program allows racing and in-training, intact males to wait and be tested once they transition from racing into the breeding shed.

Pelzel-McCluskey commented, “To protect their industry, Kentucky created their own state CEM testing requirements to pick up the horses that entered the USA through this loophole.” No other state implements routine CEM testing.

Mares and stallions of breeding age imported from specific CEM-affected countries are required to undergo CEM testing prior to entering the United States. Some people have figured out a workaround for getting horses into the country without a thorough CEM testing protocol.

Mexican Border Loophole

Pelzel-McCluskey said hundreds of stallions and mares arrive at the Mexican border as imports from that country. However, the horses actually originated in Europe. These are mostly Luisitano, Andalusian, warmblood and Friesian horses that fetch a good price in the United States. In fiscal year 2021, 4,828 horses entered the U.S. through the Mexican border. Because Mexico is considered a non-CEM-affected country, the horses do not have to be tested for CEM to enter the United States. It is unclear how many horses cross the border illegally from Mexico each year.

The EU has an identification program. Many horses are microchipped in their country of origin. This way, it is possible to see that they originated from a CEM-affected country. Additionally, Pelzel-McCluskey noted that it is obvious that these are not Mexican-based horses since the horses don’t respond to Spanish. Instead, they respond to German, Dutch, etc.—languages in CEM-affected European countries.

Testing Procedures 

Swabbing of stallions is required for CEM testing. Arnd Bronkhorst Photography

Mexican importation allows importers to avoid an arduous and expensive testing procedure that costs about $5,000-$7,000 for stallions. An imported stallion must stay in an approved CEM quarantine center different from animal import quarantine centers or ports through which horses pass through first.

At the CEM-quarantine center, the stallion has direct swabs of genitalia taken and cultured. Then he is test bred to two prequalified test mares. On Days 3, 6 and 9 post-breeding, veterinarians take direct swabs of the mares’ clitoral fossa and sinuses where Taylorella bacteria like to hide. At 21 days post-breeding, they perform a complement fixation test. At 28 days post-breeding, they obtain a full set of cultures from the mares, including cervical or endometrial swabs in non-pregnant mares. Cultures must sit on the culture plate for seven days, said Pelzel-McCluskey. This all takes time and is expensive. The stallion is quarantined at the center throughout the test period.

Test Breeding of Mares

The United States is the only country requiring test breeding of two mares and samples collected post-breeding over 28 days. According to USDA data published on the 2008-2010 CEM outbreak, using only a single set of direct swabs of genitalia reportedly missed 22% of positive stallions involved in the outbreak, said Pelzel-McCluskey.

Imported mares must also remain at a CEM-quarantine center, with direct swabs taken on Days 1, 4 and 7 for culture and a complement fixation test. To be able to enter the country, the mare must test negative. The expense for mares is significant, albeit less costly than for the stallions because their stay takes about two weeks compared to a full month for the stallions. That is assuming the mare doesn’t test positive. If positive, the mare has to be treated and held for 21 days before being retested. 

CEM Treatment

Treatment involves five consecutive days of chlorhexidine scrub and packing of genitalia with nitrofurazone ointment or silver sulfadiazine cream. Treatment only knocks back bacteria growth if not performed for the recommended time and with the recommended procedure. There is a required 21-day wait time after topical treatment before performing the CEM testing protocol. A horse that has been on oral antibiotics must wait seven days to test. The wait times are to ensure that the organism has ample time to grow back to detectable levels if it was not completely eliminated by treatment.

For an infected stallion, it is important to look at the semen collection log for seven days before and seven days after the time of collection in order to determine whether other stallions might have been exposed. Stallion-to-stallion transmission has been confirmed during outbreaks up to four days before or after collection of an infected stallion. Taylorella lives on surfaces for several days, but wet/moist materials enable greater longevity. It is important to decontaminate everything as carefully as you would do for neonatal intensive care. Infected horses are quarantined, tested, treated and retested. Not all horses will clear the infection with a single treatment. It could take two to three rounds of treatment and retesting, and all these efforts cost money.

CEM in Geldings 

Infected stallions that are subsequently gelded still maintain the lifelong surface contamination of their genitalia. While they are not breeding animals, some geldings mount mares in the pasture, potentially passing on an infection.
Another transmission from geldings occurs with procedures such as sheath cleaning. The sheath cleaning bucket can become contaminated with Taylorella. It is easy to see how the bacteria can spread throughout the barn, even among geldings.

Equine Viral Arteritis (EVA) 

A virus causes EVA. Stallions can shed the virus intermittently in their semen. An active infection also involves nasal shedding.

Because pregnancy loss is associated with EVA, it is important for breeding mares to be vaccinated at least 21 days prior to breeding (live or artificial insemination) to a shedding stallion. Vaccine requires approval by the state veterinarian for use on a mare. The mare must also be quarantined for a couple of weeks post-vaccine because of the risk of her shedding live vaccine virus. EVA vaccination has a good efficacy for protection.

Kentucky has a robust EVA program for stallions, since horse breeding is an important industry for that state.

Testing, Vaccination and Spread 

There is no way to differentiate on serologic testing between a vaccinated and exposed stallion, other than with a vaccine certificate that is part of a stallion’s documentation. If the stallion is seropositive and the certificate is lost, then the stallion’s semen must be tested by PCR or virus isolation to determine whether he is currently shedding the virus.

Vaccination Programs

EVA stallion vaccination programs involve first testing the stallion as seronegative before administering the vaccine. The same applies to a mare—she is tested and if seronegative, she can be vaccinated. It is possible to test nasal swabs for PCR; EVA can be shed from the respiratory tract for weeks to months.

Nasal Shedding

Risk of infection is high due to nasal shedding of EVA. So, regulatory changes are needed, said Pelzel-McCluskey. She receives calls from practitioners who have identified an infected mare that contracted EVA from breeding or through nasal shedding. They ask, “What should we do with the rest of the barn?” She gave an example from New Mexico in 2006: A Quarter Horse breeding operation experienced abortions and foal loss due to EVA. Outside mares came in for breeding, then took the infection home.

Usually, infection enters a facility from a previously imported stallion (Friesians or warmbloods, for example). There is no comprehensive EVA testing or control program in the United States. Therefore, there is no requirement for EVA testing of imported horses. Pelzel-McCluskey reminded owners who are actively breeding their mares that they should find out the EVA status of their selected stallion. A stallion owner should disclose information to mare owners that the stallion is an EVA shedder so they can arrange to vaccinate the mare in advance of breeding.

Take-Home Message about CEM and EVA

USDA-APHIS-Veterinary Services National Epidemiologist for Equine Diseases Angela Pelzel-McCluskey, DVM

It is important to stop propagation and spread of venereal disease, stressed Pelzel-McCluskey. Stallions that continuously shed CEM or EVA can spread disease logarithmically without testing and/or good biosecurity practices. Widespread outbreaks can result in a significant economic impact to the equine industry due to abortions, fetal loss and international trade restrictions. To optimize control of equine venereal diseases going forward, regulatory changes are necessary.

Veterinarians can help owners ask appropriate questions when selecting a stallion for breeding or when breeding a mare to an unknown stallion. While performing duties on the farm, veterinarians have an opportunity to discuss the risk of equine venereal diseases. Sound education and advice can help to mitigate and stop the spread of venereal diseases.

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