Twenty-eight American states and 2 in Canada are now testing horses for the infection contagious equine metritis (CEM) following outbreaks at stud farms in Kentucky and Indiana.

Four stallions at the DeGraff Stables/Liberty Farm Reproductive Centre in Woodford, Kentucky and another 3 who stood at that stud in 2008 but are now in Indiana, have tested positive for the infection by the Taylorella equigenitalis bacterium that can cause temporary infertility in mares and spontaneous abortions.

Semen from the Quarter horse stallions at the facility was used widely across the US, leading to the exposure to infection of 18 further stallions and 115 mares.

A spokesman for the United States Department of Agriculture said: “At least 250 additional horses are actively being traced, in at least 27 states. Only 12 [out of 50] states have not been involved in the CEM investigation process so far.

“The CEM positive horses are in quarantine and are being treated with disinfectants and antibiotics.”

It is not thought that the infected semen was exported to the UK.

The infection was first spotted on 10 December when semen from 1 of the stallions at DeGraff Stables — Quarter horse Potential Investment — was being routinely tested prior to export.

Another 3 stallions standing at the same premises — Hot Lopin Sensation, Indian Artifacts and Repeated in Red — subsequently tested positive.

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

The last known cases of CEM in the US were in 2006 when 3 infected Lipizzaner stallions imported to Wisconsin from eastern Europe tested positive in quarantine.