EIA Outbreak Kills 21 Horses in Southwest US
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A multistate outbreak of equine infectious anemia, likely spread through iatrogenic transmission, has resulted in the deaths of 21 horses in the past year.
June 13, 2025, American Veterinary Medical Association News:
A multistate outbreak of equine infectious anemia (EIA) has resulted in the deaths of nearly two dozen horses.
A U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) investigation confirmed that 21 horses tested positive for EIA—a potentially fatal blood-borne infectious viral disease that produces a persistent infection among equids—as of late May across California, Colorado, Oklahoma, and Texas.
The cases largely occurred in two clusters: one during May 2024 and another from mid-June through August 2024, according to a recent announcement from the Equine Disease Communication Center (EDCC).
Dr. Angela Pelzel-McCluskey, APHIS national equine epidemiologist, said in a May 8 National Equine Industry Stakeholder Call that the ages range from 2- to 14-years-old and that the breeds affected have included Quarter Horses as well as Andalusian, Paint, and Rocky Mountain horses. Few had typical risk factors, such as international travel, and those affected represent different disciplines.
A joint investigation began this past March among APHIS, state veterinary medical boards, and state agriculture departments to identify potentially exposed horses for testing.
“I don’t believe we’ve found all the exposed horses for testing just yet and I think there’s more that need to be added to the list,” Dr. Pelzel-McCluskey said.
She added that the disease likely spread through iatrogenic transmission—unintentional exposure through contaminated medical treatment or equipment—and recommended that anyone who is concerned that their horse might have been exposed should contact their veterinarian and have their horse tested.
Clinical signs for EIA, also known as swamp fever or Coggins disease, include fever, low platelet count, anemia, and muscle weakness.
In a May 30 letter to American Association of Equine Practitioners (AAEP) members, Dr. Tracy Turner, 2025 AAEP president, wrote: “This situation once again highlights the critical importance of strict biosecurity protocols and proper training of individuals who work in equine veterinary clinics to help prevent the spread of disease via iatrogenic transmission.”
During 2023, APHIS’s Veterinary Services confirmed 61 EIA-positive horses on 45 premises and 96 positives on 45 premises in 2022.
“Although the current prevalence of EIA in the U.S. equine population remains very low, at an estimated national prevalence of around 0.004 percent, the epidemiology of EIA-positive cases has shifted in recent years,” APHIS information states. “Most EIA cases were previously found to occur from natural transmission by biting fly vectors in untested and undertested populations. Increasing cases of iatrogenic transmission (disease resulting from medical activity) are now being identified.”
For current information about equine infectious anemia, the Equine Disease Communication Center provides background information and alerts for new EIA cases. The American Association of Equine Practitioners also has disease guidelines and an owner factsheet.
