Tally of U.S. Horses Exposed to CEM Hits 820

The ongoing investigation into contagious equine metritis (CEM) now includes more than 820 exposed or positive horses, according to the USDA.

The investigation began in mid-December 2008, when a Quarter Horse stallion on a Kentucky farm tested positive during routine testing for international semen shipment.

According to the USDA's Animal and Plant Health Inspection Service (APHIS), as of May 1 the positive horses included 18 stallions and five mares. In addition to these horses, they had confirmed the locations of 797 horses exposed to Taylorella equigenitalis. The 820 horses were located in 48 states. There were 171 exposed or positive stallions in 27 states and 649 exposed or positive mares in 45 states. Authorities were still actively tracking six exposed mares and four exposed stallions.


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An exposed horse is one that was bred to a positive horse, either naturally or via artificial insemination, or one that is otherwise epidemiologically linked to a positive horse, as determined by animal health officials.

Of the 171 stallions involved, a total of 41 had completed their entire testing and treatment protocol and been determined to be negative for T. equigenitalis. Of the 649 mares, a total of 365 had completed testing and treatment.

APHIS spokesman Jim Barrett said the USDA will be taking a collaborative approach and covering some costs associated with testing these horses, which includes assisting the veterinarian taking samples, mailing test samples, and laboratory testing. The estimated cost of veterinary services for testing ranges by state from $1,500 to $5,000 per animal tested.

Contagious equine metritis is a transmissible, exotic venereal disease in horses. It usually results in infertility in mares and, on rare occasions, can cause mares to spontaneously abort. Infected stallions exhibit no clinical signs but can carry the CEM bacteria for years. CEM is commonly transmitted during sexual intercourse but also might be transmitted indirectly through artificial insemination or contact with contaminated hands or objects. It can be treated with disinfectants and antibiotics.

About the Author

Erin Ryder

Erin Ryder is a former news editor of The Horse: Your Guide To Equine Health Care. She owns a portly gray gelding named Duncan and dabbles in several equestrian disciplines, with an emphasis on dressage.

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