The American and Canadian horse breeding industries are on alert after a break-out of contagious equine metritus (CEM) on a Kentucky stud farm.

Four stallions at the DeGraff Stables/Liberty Farm Reproductive Centre in Woodford have tested positive for the infection that can cause temporary infertility in mares and spontaneous abortions.

The last known cases of CEM in the US were in 2006 and the country is considered free of the disease.

The infection was spotted on 10 December when semen from one of the stallions — Quarter horse Potential Investment — was being routinely tested prior to export to Europe.

Another three stallions standing at the same premises — Hot Lopin Sensation, Indian Artifacts and Repeated in Red — have subsequently tested positive.

Semen from one of the infected stallions was sent to Ontario and Alberta in Canada in spring 2008.

The Canadian Food Inspection Agency is trying to trace those horses that may have been infected and have recommended that Canadian breeders halt imports of breeding stock, semen and embryos from the US.