Information gathered from the USDA:

Early this March, a fourth stallion in Wisconsin tested positive for contagious equine metritis, or CEM, a treatable reproductive disease of horses. The stallion, a 26-year-old Saddlebred housed in Winnebago County, has been quarantined. Like the three stallions that tested positive previously, this one was exposed to the bacterial infection at an artificial insemination center.

Nationwide, traceback has identified 16 infected horses (including the four Kentucky horses that since have been released from quarantine) and nearly 700 exposed horses across 46 states, according to the U.S. Animal and Plant Health Inspection Service web site.

The state veterinarian's office and the UK Livestock Disease Diagnostic Center first discovered the organism in December 2008 in a 16-year-old Quarter Horse during routine testing prior to shipment of semen to Europe. The state veterinarian's office and USDA's Kentucky office established the protocol for identifying, locating and treating infected and exposed horses. They joined with practicing veterinarians to carry out the protocol.

"While the source of the outbreak is still not known, some have presumed that Nanning, the first CEM-positive stallion we reported in Wisconsin, was the original source. This result makes it clear that he was not," said Wisconsin State Veterinarian Robert Ehlenfeldt, DVM. "This latest infection dates to 2006 and possibly 2005. The first CEM-positive stallion we reported was infected in a different insemination facility, and he was infected in 2007 or later, our investigation shows."

State and federal animal health personnel have conducted the investigation by examining the breeding records and movement history of each infected horse to find other exposed animals. At each step, any exposed animals are quarantined, tested and treated. Owners of exposed animals are contacted by state or federal animal health officials. There is no need for them to have their animals tested if they have not been contacted.

There is no human health risk and no risk to horses in the general population.

CEM is a contagious bacterial infection that passes between mares and stallions during mating. It can also be transmitted on contaminated insemination equipment. Stallions do not suffer any symptoms, but the infection causes inflammation in the mare's uterine lining. This may prevent pregnancy or cause the mare to abort if she becomes pregnant. The disease is treatable with disinfectants and antibiotics.