A recent cluster of equine infectious anemia (EIA) cases has once again highlighted the importance of proper training of the individuals that work in equine veterinary clinics to help prevent the spread of disease via iatrogenic (relating to illness caused by medical examination or treatment) transmission.

A cluster of EIA-positives that were infected throughout May 2024 and another cluster from mid-June through the end of August 2024 were found to have originated at a Texas equine clinic. All the EIA-infected horses had been hospitalized in the intensive care unit (ICU) at the clinic and had an intravenous (IV) catheter placed and flushed.

As of May 30, 2025, the USDA Animal and Plant Health Inspection Service (USDA-APHIS) has identified 21 horses confirmed positive for EIA in four states—California, Colorado, Oklahoma, and Texas. The affected horses are of varying ages, breeds, and disciplines.

“I don’t believe we have found all of the exposed horses for testing just yet,” said Angela Pelzel-McCluskey, DVM, USDA-APHIS equine epidemiologist. “I think there are more that need to be added to the list.” She added that anyone who is concerned  their horse might have been exposed should contact their veterinarian and have their horse(s) tested.

The incubation period of horses infected with EIA is usually 15 to 45 days, and horses that have been infected with the virus are lifelong carriers. Horses showing clinical signs of the disease are more of a threat to healthy populations because of higher levels of the virus circulating in the blood.

Horses that survive the initial clinical phase of the disease usually become outwardly inapparent carriers and remain a lifelong reservoir for the disease. There is no treatment for the disease and no vaccine to prevent an EIA virus infection. If confirmed positive, horses are usually euthanized to prevent ongoing transmission to other horses. If not euthanized, horses must be permanently quarantined with separation from noninfected horses by at least 200 yards to prevent spread of the disease by biting flies. Infected horses cannot be moved from quarantined premises except by approval of state and federal animal health officials. Clinical signs of EIA might include fever, depression, low platelet count, anemia, red or purple spots on the mucous membranes, edema, muscle weakness, and atrophy, although chronically infected horses often show no obvious clinical signs of the disease.

Veterinarians diagnose EIA by testing antibody levels in the blood. They most commonly use the Coggins test, which is an agar gel immunodiffusion (AGID) test. An ELISA (enzyme-linked immunosorbent assay) test is also available to detect EIA antibodies. Practitioners must collect blood and submit it to certified laboratories for testing.

The USDA-APHIS has reviewed the complete medical history of the 21 horses confirmed EIA positive that were treated at the Texas clinic.

The current investigation indicated at least one or more employees of the clinic were reusing needles or syringes to draw up and flush IV catheters with heparinized saline in the ICU as a routine procedure, which caused repeated instances of blood contamination into the bottles. “The original sources of the EIA virus were likely bush-track or other high-risk horses that were admitted to the clinic ICU just prior to each cluster of cases in May and August,” said Pelzel-McCluskey.

The USDA-APHIS began investigating the cases in September 2024, when a 3-year-old Quarter Horse mare in Wise County, Texas, was confirmed EIA-positive. The horse was used for barrel racing and had no flat-track racing history, and her cohorts were EIA negative. Another Wise County case, an 8-year-old Andalusian mare with no racing or import history and with all negative cohorts, was found EIA-positive the following month. The next two months brought three more cases from the same county. The horses showed no common EIA risk factors and had no contact with each other.

“The only thing we could come up with is that all of the horses were seen by the same veterinary clinic,” Pelzel-McCluskey said. “That for a small county area is not strange, so it was a head scratcher.” She noted the horses home premises were not located adjacent to each other, so it was not a local geographic focus of natural fly-bite transmission.

Through investigation, officials determined the only common factor was that the horses had been hospitalized in either two clusters May 2024 and August 2024; three horses in May and two in August for other reasons and EIA was not diagnosed.

At the request of the USDA-APHIS, the horse owners began sharing medical records of the five infected horses. At the same time, four more cases with connections to the same veterinary clinic were discovered in January, February, and March of this year—two more horses from Wise County, one from Denton County, and one located in Oklahoma.

Pelzel-McCluskey said the next step in the investigation is to compile a more complete list of exposed horses from the clinic’s appointment book to identify horses that had an IV catheter placed at the clinic during the affected time range, while completing regulatory EIA testing of all horses that were potentially exposed. Testing will also be completed on cohorts of any new EIA-positive cases.

About 1.3 million horses per year are tested for EIA. So far in 2025 there have been 44 EIA positives. In 2024 there were 147 positive cases, and 120 of those were in the primary high-risk category of Quarter Horse racehorses that contracted the disease via iatrogenic transmission by unhygienic injection practices of owners and trainers, not veterinarians.

The rate of EIA positives has been on the rise since 2016 when there were only 44 cases recorded, with the majority being natural transmission via fly bites. The iatrogenic-transmission EIA cases now being found in Quarter Horse racehorses frequently have ties to unsanctioned or bush track racing.

“I believe that veterinary clinic owners and employees have become complacent, and people don’t believe that something like this could happen, but it does happen, and it is happening at a very large scale with a lot of negative outcomes,” Pelzel-McCluskey said. “I would like state animal health officials to reach out to remind people that we do have iatrogenic transmission of EIA, and this can occur in clinics if the expected procedures for sterile technique are not being followed by all employees at the clinic.”

By Leslie Barlow and Dr. Angela Pelzel-McCluskey for the Equine Disease Communication Center (EDCC)