Canada has confirmed that semen from one of the infected stallions was imported into the country and it has launched an inquiry.
In Kentucky, four stallions - three quarter horses and one paint - have been confirmed infected with the venereal disease.
Each of the four stallions stood the 2008 season at a breeding facility in Woodford County that specialises in stallion collection for artificial insemination.
Three of the four infected stallions remain at the original premises with the fourth at another Woodford County address.
The Kentucky Department of Agriculture says five other stallions at the facility have returned negative initial tests to the bacterium that causes the disease.
Thirteen other stallions had left the facility and relocated out of state after the breeding season. One has been returned to Kentucky and is currently quarantined at a private quarantine facility.
Results of testing are expected soon. The US Department of Agriculture's Federal Veterinary Service is co-ordinating the testing and treatment of the stallions in other states.
Mares had received semen from one of the positive stallions, according to officials. They have been quarantined ending tests for the disease.
"There is no evidence supporting or suggesting the organism has spread to any population outside the defined groups - which are being tested to determine the extent of the outbreak," Kentucky's agriculture department said.
"Kentucky's thoroughbred industry is not in any way associated with or threatened by the disease incident."
North of the US border, the Canadian Food Inspection Agency (CFIA) has launched an investigation after confirming that horse semen was imported into Canada from one of the stallions.
Shipments of frozen semen from one of the Kentucky stallions were sent to Ontario and Alberta in spring of 2008.
CFIA and provincial animal health officials are currently tracing the shipments to identify potentially exposed animals.
To date, potentially infected farms have been identified in Ontario and Alberta. As a precaution, the CFIA has quarantined animals on the farms, and these measures will remain in place until all animals have tested negative for CEM. As investigations in Canada and the US continue, animals on additional farms may be quarantined.
CEM is a highly contagious disease that affects the reproductive tract of horses. The disease can cause temporary infertility in mares. In most cases, CEM can be successfully treated with disinfectants and antibiotics.
CEM is a notifiable disease in Canada. This means that all suspected cases must be reported to the CFIA for immediate investigation by inspectors. There are international trade implications if a country loses its CEM-free status.
Until more information is available from the US, the CFIA is recommending that the equine industry and importers in Canada exercise caution and refrain from importing breeding horses, embryos and semen from the US.
CEM is primarily spread directly during natural breeding, but can also be transmitted during artificial insemination and through contaminated instruments and equipment, such as tail bandages, buckets, sponges and gloves. Therefore, horse owners and veterinarians should maintain strict hygiene when handling breeding mares and stallions to prevent infection.
Infected stallions tend to be the major source of infection, as they can harbour the disease for years without showing any clinical signs. The primary symptoms of infection in mares are short-term infertility and vaginal discharge, but some mares can also carry the disease without clinical signs. Any horse owner or veterinarian who suspects a horse under their care may be infected with CEM should immediately contact their local CFIA District office.
"The CFIA will continue to work with provincial counterparts, affected producers and the equine industry in this response effort. Additional information will be provided as it becomes available."