Almost 1,000 horses may have had disease
posted January 18, 2010
Contagious equine metritis reemerged in late 2008 after being considered eradicated for years. In the process, the highly transmissible venereal disease affected potentially 1,000 horses in all but two states.
It also created concerns about how CEM is transmitted, how to protect against its spread, and how to properly test for it. The Department of Agriculture is continuing to learn more about the outbreak and attempting to prevent any further occurrences.
The subject was discussed at length during a tabletop session Dec. 6, 2009, at the American Association of Equine Practitioners Annual Convention. Four experts reflected on the epizootic and fielded questions.
Tracing their steps
Dr. Ellen M. Buck, veterinary medical officer with the USDA Animal and Plant Health Inspection Service and an equine import specialist, said the department's definition for horses confirmed to have CEM is "any horse that has had the isolation (of the bacteria) and identification of Taylorella equigenitalis confirmed by the National Veterinary Services Laboratories."
An exposed horse is "one that was bred, naturally or by AI, to a stallion determined to be positive or one that is otherwise epidemiologically linked to a positive horse."
Fifteen laboratories have been approved by APHIS to perform cultures for T equigenitalis.
As of the end of January, 27 horses have been found to be positive for CEM. Of the 22 stallions confirmed, nine were from Wisconsin, four from Kentucky, three from Illinois, three from Indiana, and one each from Georgia, Iowa, and Texas. CEM was also confirmed in five mares.
An additional 964 exposed horses have been identified and located. The total 991 positive or exposed horses have been located in 48 states—Hawaii and Rhode Island were the lone exceptions.
The 276 stallions are in 31 states and the 715 mares are in 46 states. About 92 percent, or 912 horses, of the total are considered negative, meaning they've been tested and were not found to have CEM.
The positive stallions are of 11 breeds and have a median age of 11 years.
Indirect transmission of T equigenitalis to stallions happened at five facilities in three states, according to USDA-APHIS. Direct and indirect transmission to mares happened once during live-cover breeding, four times during AI with fresh semen from three stallions, and once during AI with fresh and cryopreserved semen.
Infection in mares was detected anywhere from 19 days to 11 months after presumed exposure. Four mares had vaginal discharge and one had a normal pregnancy.
Treatment regimens and results varied. Some practitioners used two topical treatments plus oral antimicrobial administration, according to Dr. Buck. Others used just a single topical treatment.
The beginning of the end
To date, none of the positive horses has been definitively acknowledged as the source of the outbreak. Investigators believe a stallion from Denmark imported in late 2000 is considered the most likely source. The horse was an unusual breed, Dr. Buck said, and not widely bred.
"We're confident there aren't multiple source introductions, but one common source," Dr. Buck said.
The USDA-APHIS is working on identifying the scale of the incident, the exact source of the infection, and the duration of infection, as it attempts to return the United States to CEM-free status.
Dr. Buck estimates the investigation will conclude by May, which is when the exposed pregnant mares are expected to foal.
The agency would then like to proceed with voluntary collection of samples from 3,000 stallions nationwide. The number of breeding stallions in each state from which samples would be collected would be roughly proportional to each state's equine population, she said. APHIS would pay only for shipping and testing of samples, not veterinary care or treatment if a horse tests positive.
"The survey is not designed to set a testing standard or show that these horses were CEM-free," Dr. Buck said.
APHIS will also continue to test for the disease in stallions and mares imported from countries that are CEM-affected.
Where it all began
CEM was first reported in England and Ireland in May 1977. Other countries that have been affected by CEM in some way since then include Japan, Australia, most European countries, Iran, Turkey, Brazil, and the United States.
The appearance of CEM internationally is important because of the ease with which it can be spread among countries through the shipment of mares and stallions, said Dr. Peter J. Timoney, a faculty member in the Department of Veterinary Science at the University of Kentucky. He also is a member of the USDA's National Animal Health Surveillance System Steering Committee.
In fact, a sizable increase in the international movement of horses has occurred in the past 30 years as the number of stallions being used for dual-hemisphere breeding has increased. So it follows that the disease's potential to cause widespread short-term infertility in mares and the frequency of occurrence of the carrier state in stallions and mares are causes for concern.
Dr. Timoney said this could cause substantial economic losses in previously unexposed breeding populations just as the major disease outbreaks in Great Britain in 1977 and Kentucky in 1978 did. Today, however, CEM is one of the most internationally regulated equine diseases.