American scientists think they may have identified the cause of the outbreak of equine venereal disease contagious equine metritis (CEM) in America — a Friesian stallion imported in 2004.

Testing for CEM is now being undertaken on horses in 39 states in America.

The investigation involves 291 mares and 43 stallions that are believed to have been exposed to the Taylorella equigenitalis bacterium and another nine stallions have tested positive for CEM.

One of the stallions with CEM is Friesian stallion Nanning 374, who was imported to the States from Holland.

Nanning has been linked to one of the first batch of stallions to test positive for CEM on 10 December 2008 at the DeGraff Stables/Liberty Farm Reproductive Centre in Woodford, Kentucky.

A spokesman for the office of Kentucky State Veterinarian Robert Stout said: “This evidence strongly supports a conclusion that the source of introduction of the bacteria to Kentucky has been identified.”

But he added that they could not categorically state that the Friesian was the starting point of the infection.