Equine Infectious Anemia: New Year, New Cases

Veterinarians euthanized two Quarter Horses in mid-January after they tested positive for equine infectious anemia (EIA), according to officials from the Texas Animal Health Commission. At the time of diagnosis, the horses were kept at the same facility in Ector County, West Texas. Meanwhile, mandatory reporting has revealed that seven horses in San Bernardino County, California, have confirmed positive tests.

Appropriate biosecurity and quarantine measures have been implemented by state officials, and local veterinarians and horse owners continue to monitor horses that might have been exposed.

The disease, which is caused by a virus, is colloquially known as swamp fever because of its original prevalence in Gulf Coast states, where environmental conditions are favorable for transmission by blood-feeding insects, especially horseflies and deerflies. In addition to insect vectors, transmission occurs through the use of improperly sterilized needles, scalpels and other surgical instruments, dental equipment, or any other blood-contaminated tools.

The USDA reported 29 positive cases of EIA in 2020. Those cases occurred in six states: Texas (17 cases), Iowa (6), Georgia (3), California (1), Colorado (1), and New Mexico (1). More than 1.3 million tests were performed in that year. According to a summary released by the USDA, 23 of the 29 cases were found in Quarter Horse racehorses suspected or confirmed to be infected through unhygienic practices by trainers and owners, including reusing needles, syringes, or IV sets; administering blood transfusions from untested donors; and using illegal blood products from other countries.^* These horses ran in both sanctioned and unsanctioned races.

In the United States, two types of EIA tests are authorized for use: agar gel immunodiffusion (AGID) test, known as the Coggins test by most horsemen, and the enzyme-linked immunosorbent assay (ELISA) test. In 2020, the tests were used with near equal frequency. A negative EIA test is required for most horse shows, racing events, rodeos, exhibitions, and interstate travel. Interstate movement of horses that have tested positive for EIA is prohibited under federal law unless the horse is being hauled back to the farm of origin, a slaughterhouse, or a diagnostic or research laboratory.

Acutely infected horses may show signs of fever, lethargy, lack of appetite, and atypical bloodwork, namely thrombocytopenia or anemia. Most horses, however, show few clinical signs, and diagnosis may be difficult. Animals remain infected for life. While euthanasia of infected horses is typically not mandated by state enforcement agencies, lifelong quarantine and permanent identification are typical restrictions.

^*USDA. 2021. 2020 Equine infectious anemia cases in the United States.