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Questions and Answers: Contagious Equine Metritis Fact Sheet (USDA/APHIS/VS)  

Contagious equine metritis is considered a foreign animal or transboundary disease making it reportable in the U.S.; positive samples must be reported by laboratories to the state veterinarian and USDA: APHIS: VS.  In Canada, positive samples are reported to the Canadian Food Inspection Agency (CFIA).  There is no evidence that T. equigenitalis is transmissible to humans. 

Background: T. equigenitalis is the causal agent of contagious equine metritis (CEM).  It is a non-systemic, nonlife threatening venereally transmissible disease of equids that can cause short-term infertility in mares and, very rarely, abortion.  This bacterium is a fastidious, microaerophilic, Gram-negative coccobacillus.  This organism was first recognized in the United States when it was found in Thoroughbred stallions and mares in Kentucky in 1978. 

A taxonomically closely related bacterium, T. asinigenitalis,was first discovered in a Mammoth donkey jack in California in 1997.  Primarily an inhabitant of the distal reproductive tract of donkey jacks, this organism has not yet been confirmed as a cause of CEM in horses or donkeys.  T. asinigenitalis is rarely isolated in the U.S. and is not known to cause clinical disease. There is currently no regulatory action taken when T. asinigenitalis is isolated and its main significance is in confirming diagnostically that the Taylorella organism isolated is not T. equigenitalis

Multiple recent incursions of CEM caused by T. equigenitalis in the U.S. have occurred since 2006.  A significant CEM outbreak occurred in 2008-2010 in which over 1,000 exposed horses in 48 states were required to be tested and 23 infected stallions and 5 infected mares were ultimately identified and treated.  The source of that outbreak was determined to be a stallion imported to the U.S. from a CEM-affected country in 2000. 


Transmission:  Although venereal transmission of T. equigenitalis by live cover has historically been the most emphasized route of infection, extensive stallion-to-stallion transmission occurred in the 2008-2010 outbreak through contaminated fomites at multiple semen collection facilities.  Additionally, transmission to mares has occurred via fresh, cool-transported semen in which semen extender containing antibiotics was used. During the investigation of an incursion of CEM into the U.S in 2013, live T. equigenitalis was cultured from the frozen semen of an infected stallion in that incident.  These recent findings have highlighted that the risk of CEMO transmission is not limited to venereal transmission by live cover and that veterinarians need to be alert to continued incursions of this foreign animal disease (FAD). Although uncommon, CEMO transmission from mare to foal either in utero or during parturition has also been documented.  

Clinical Signs: The stallion is an asymptomatic carrier of the organism in which the latter exists as a commensal bacterium on the external genitalia.  Infection in the mare is confined to the reproductive tract.  Mares bred to a carrier stallion by natural cover or by AI with contaminated semen can develop a mucopurulent vaginal discharge that typically lasts 2-3 weeks. Infection is characterized by a degree of inflammation (often marked) of the oviducts, endometrium, cervix and vagina.  The organism can be recovered from a variable percentage of mares for about three months after primary infection in mares with or without clinical signs. The clitoris (clitoral sinuses and fossa) is the most important site of persistence of the organism.  A small number of mares become intrauterine carriers of T. equigenitalis.  Salpingitis may occur that can persist for a longer period than endometritis, cervicitis or vaginitis.  A mare bred with contaminated semen may not conceive on the first or second estrus after exposure.  Subsequent breeding(s) will often result in establishment of a normal pregnancy. 

Although stated in literature that abortion may occur, T. equigenitalis is very rarely a cause of abortion (or permanent infertility).  Mares that become infected may also display shortened estrous cycles with an early return to estrus.  Most mares eventually clear the infection, and do not become long-term carriers.  However, some mares can remain persistently infected for several months or years, and can be a source of the organism for infecting stallions. 

Testing Recommendations: Somerecommend that all stallions being bred or collected at a semen collection facility have a minimum of 1 set of four direct swab cultures (described below) that are culture negative for T. equigenitalis prior to the start of the breeding season.  The test results for the prophylactic annual testing, with identification of the stallion and date of sampling, should be provided by the veterinarian who collected and submitted the swabs to a USDA approved VSL CEM laboratory for testing.  This would mean that stallions that are permanent residents or are sent for breeding management only to the primary facility (not bred at home) are tested once a year for T. equigenitalis prior to the beginning of each breeding season.  

Diagnostic Testing: Prophylactic, annual testing of stallions prior to the breeding season has been proposed, but has not been mandated, by USDA: APHIS: VS as a way of reducing the risk of the spread of CEM.  Mares that develop genital discharges or that ‘short-cycle’ after breeding to an untested or imported stallion should be considered for evaluation for T. equigenitalisinfection.

It is acknowledged that screening domestic (non-imported) stallions for CEM by testing a single set of direct swab cultures is not equivalent to the USDA testing protocol mandated on all post-entry male and female equines above 731 days of age (i.e., being imported into the U.S. from CEM-affected countries).  Specifically, during the 2008-2010 U.S. outbreak, it was documented that out of the 23 infected stallions ultimately found, 18 stallions were identified positive on the first set of direct swab cultures, 1 stallion on the second set, and 1 stallion on the third set of cultures.  Three (3) of the 23 infected stallions were not positive on direct swab cultures and were found positive only after breeding susceptible test mares by live cover. However, one single annual testing (point-in-time sampling) could aid in screening the stallion (or mare) resident population in the U.S. and Canada.  Such testing could provide greater reassurance of the breeding population’s freedom from T. equigenitalis and could help in persuading international trading partners to reduce the level of testing required of horses imported from the U.S. and Canada. 

Media and shipping of samples:  Taylorella equigenitalis can be detected by directly swabbing specific sites in the mare and the stallion, placing each swab in a separate tube of Amies transport medium with charcoal, and sending the swab set to a USDA approved laboratory for testing for CEM.  Samples must be shipped on ice packs to arrive cool at the laboratory and be inoculated onto appropriate media within 48 hours of collection.  Therefore, samples should either be hand delivered or sent by overnight courier Monday through Thursday to the approved laboratory.  The collection of these samples should be performed either by a regulatory veterinarian, or an accredited veterinarian who has been instructed how to collect, handle and transport such samples by a state or USDA: APHIS: VS veterinarian.  These are similar to requirements of the Canadian Food Inspection Agency (CFIA). 

See here for a current list of CEM approved laboratories.    

Collection of direct swab samples from a stallion for prophylactic annual testing:  It is recommended that swabs are obtained, wearing disposable gloves, from four sites on the unwashed external genitalia of the stallion: the shaft of the penis and prepuce; dorsal diverticulum of the fossa glandis (urethral sinus); the fossa glandis; and the distal urethra.  

Collection of direct swab samples from a mare:  Direct swab samples should be obtained, while wearing disposable gloves, from at least two sites, the clitoral fossa and clitoral sinuses. A standard-sized swab should be used to collect samples from the clitoral fossa, while mini-tipped swabs small enough to enter the clitoral sinuses should be used on those sites.  One mini-tipped swab can be used to sample multiple clitoral sinuses.  Collecting more than one set of swab samples at 72 hour intervals (as required for mares imported from CEM affected countries) can increase the chances of detecting the presence of T. equigenitalis in mares.  A third sample site, a deep cervical or endometrial swab, can also be collected.  Washing of the vulva in preparation for a deep cervical or endometrial swab collection should be performed after all clitoral samples have been collected.  Additionally, unlike stallions, mares produce a short-lived systemic antibody response to T. equigenitalis infection, so a serum sample should also be collected on clinical or recently exposed mares for complement fixation testing (CFT) for T. equigenitalis.  

Where there are strong grounds for suspecting the carrier status of a particular stallion (or mare), this must be reported to the USDA: APHIS: VS veterinarian and state veterinarian.  All testing procedures, under their supervision, may then be required according to previously established guidelines. 

Any regulatory work should be reviewed in the Code of Federal Regulations (CFR) prior to initiation of any sample collection to ensure that they are in keeping with the most up to date requirements.

Specific Control Measures – Semen Collection: The following recommendations are for stallions used for semen collection for any reason: 

  • Swab samples should be taken prior to the onset of a stallion’s breeding season
    A minimum of one set of direct swab samples (procured from each of the four designated sites of the stallion as described above) should be negative for Taylorella equigenitalis on bacteriological culture prior to semen collection (results can take up to 7-10 days).  Samples should be transported to a USDA approved laboratory as previously described.Where possible, stallions should not be subjected to semen collection for insemination of mares or natural cover while awaiting results of laboratory testing for CEM.
  • Teaser stallions
    Teaser stallions should be cultured for Taylorella equigenitalis prior to the beginning of each breeding season.  Teaser stallions pose a potential risk of disease transmission due to their frequent exposure to multiple mares on a premises; thus, their health status should be monitored regularly and they should receive the same preventative care such as vaccinations as the other horses at the facility. 
  • Teaser mares—suggested recommendations are indicated due to the risk of inadvertent infection and transmission of disease
    Use of a live mare for semen collection (jump mare) should be avoided if the stallion is trained to a phantom.  Jump mares and teaser mares should be tested for CEM prior to the beginning of each breeding season (culture swabs from the clitoral fossa, clitoral sinuses and cervix/uterus). If semen is collected using a jump mare rather than a phantom, attempts should be made to divert the penis into the AV quickly to avoid penile contact with the hindquarters of the mare.

Specific Control Measures - Natural Cover: 

  • Annual set of  CEM swabs prior to that stallion’s breeding season- Direct swab samples (from each of the four designated sites as described above) should be negative for T. equigenitalis on bacteriological culture prior to breeding the stallion.  Where possible, semen collection for insemination of mares or breeding by natural cover should be postponed until the results of laboratory testing for CEM are known. 
  • Teaser stallions. The contact surface of any shields used to prevent the stallion from breeding the mare should be laundered or cleaned of all visual debris and sprayed with 2% chlorhexidine solution prior to reuse.  Teaser stallions should be tested for CEM annually. 
  • Sampling of dismount semen.  Personnel obtaining dismount samples should wear disposable gloves and use a semen receptacle that can be appropriately disposed of after use. 
  • Consider pre-breeding CEM testing of high-risk mares as described above. 
  • Cleaning perineal area of mares.  Personnel should use a disposable tail wrap and gloves, which are changed between mares.  The vulvar area is to be cleaned with water from a hand-held spray nozzle, or with cotton and water from a bucket with a disposable liner.  A new liner is used for each mare.  The hose nozzle should not touch the hand that is used to wash the mare, and the nozzle should be disinfected daily. 
  • Breeding rolls.  These should be covered with a clean shoulder-length disposable sleeve that is changed between uses and the breeding roll surface cleaned and disinfected.