Piroplasmosis Found in New Mexico Horses

As part of a racetrack screening program, three New Mexico horses have been identified as infected with Theileria equi, a causative agent for equine piroplasmosis. These infections are noteworthy as these horses are not epidemiologically linked to those involved in a larger ongoing investigation centered on horses from a South Texas ranch.

Information on the new cases, and an update on the Texas investigation, was included in a Dec. 24 report issued to the World Organization for Animal Health (Office International des Epizooties, or OIE) by John Clifford, DVM, deputy administrator of the USDA's Animal and Plant Health Inspection Service. See the report.

The positive New Mexico horses did not show any clinical signs of disease. Preliminary results of the investigation indicate that the transmission of the organism might have resulted from management practices (use of shared needles or substances between horses) rather than by a tick vector, the OIE report noted. More than 1,300 New Mexico horses have been tested via the screening program.


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Officials in the United States have screened all imported horses for piroplasmosis for nearly 30 years. The disease was officially eradicated from the United States in 1988. It is spread by some species of ticks, the use of contaminated needles, and possibly through blood-contaminated semen of infected stallions.

Clinical signs of equine piroplasmosis can include a host of nonspecific problems, such as fever or anemia, and some infected horses might appear healthy. Blood tests are needed to diagnosis the disease. The only treatment is a potent type of chemotherapy that can have serious side effects in some horses.

The larger piroplasmosis investigation remains under way, with 357 confirmed positive horses. All of the positive horses have direct links to the index premises in Kleberg County, Texas. The OIE report stated these include horses that currently or previously lived on the index premises, or live on a premises immediately adjacent, or other "dangerous contacts" (a positive foal born to an infected mare was listed as an example of such).

Positive horses have been located in 12 states, with 289 positive horses on the index ranch in Texas, 41 on other premises in Texas, two in Alabama, two in California, five in Florida, one in Georgia, two in Indiana, five in Louisiana, one in Minnesota, two in North Carolina, four in New Jersey, one in Tennessee, one in Utah, and one in Wisconsin. All known positive horses are under quarantine.

More than 1,500 horses have been tested for equine piroplasmosis as part of the epidemiological investigation, including 587 horses exposed to positive horses outside of the index premises. All of these cohorts have tested negative, the report stated.

As a result of the current investigation, Canada and several U.S. states have restricted the importation of horses from Texas. Horse owners and veterinarians shipping horses are urged to check with animal health officials in your state of destination to ensure the animals have met all entry requirements.

More information on equine piroplasmosis from the USDA Animal and Plant Health Inspection Service:

About the Author

Erin Ryder

Erin Ryder is a former news editor of The Horse: Your Guide To Equine Health Care.

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