The enhanced screening programme of racehorses in New Mexico continues to pick up cases of equine piroplasmosis.
The US Department of Agriculture, in its latest report to the World Organisation of Animal Health (OIE), on January 25, said the screening programme had so far tested more than 3000 horses for the blood-borne parasitic disease. Thirteen horses had returned positive tests.
The department, in its report a month earlier, on December 24, said 1300 horses had been tested, with three positive as of December 15.
The New Mexico Livestock Board euthanized five of the horses. The remaining positive horses are under isolated quarantine.
"Preliminary results of the investigation indicate that the transmission of the organism may have resulted from management practices - use of shared needles or substances between horses) rather than by a tick vector," said Dr John Clifford, deputy administrator for the department's Animal and Plant Health Inspection Service.
Dr Clifford said a comprehensive investigation into all cases was continuing.
The New Mexico cases are unrelated to the outbreak which was first identified in a ranch in Kleberg County, Texas.
As of January 20, equine piroplasmosis has been found in 364 horses directly linked to that Texas property.
The positive horses either currently live, or previously lived, on the ranch, or lived on premises immediately next door, or were dangerous contacts (positive foal born to an infected mare, or temporarily boarded on index premises). The positive horses are currently located in 12 states, with 289 of them still on the index ranch in Texas. The remaining horses are on premises in Texas, Alabama, California, Florida, Indiana, Louisiana, Minnesota, North Carolina, New Jersey, Tennessee, Utah, and Wisconsin.
Georgia is now free of equine piroplasmosis positive horses.
"Over 1600 horses have been tested for equine piroplasmosis as part of the epidemiological investigation," Dr Clifford says.