The ongoing investigation into contagious equine metritis (CEM) now includes more than 820 exposed or positive horses, according to the USDA.

The investigation began in mid-December 2008, when a Quarter Horse stallion on a Kentucky farm tested positive during routine testing for international semen shipment.

According to the USDA’s Animal and Plant Health Inspection Service (APHIS), as of May 1 the positive horses included 18 stallions and five mares. In addition to these horses, they had confirmed the locations of 797 horses exposed to Taylorella equigenitalis. The 820 horses were located in 48 states. There were 171 exposed or positive stallions in 27 states and 649 exposed or positive mares in 45 states. Authorities were still actively tracking six exposed mares and four exposed stallions