What Happened to an EIA-Free America?

Humans, not insects, are now the major source of equine infectious anemia (EIA) infections.

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Humans, not insects, are now the major source of EIA infections

Most EIA Cases occur via iatrogenic transmission in Quarter Horses participating in unsanctioned racing. | Getty images

Many people likely believe all horses in the U.S. have Coggins tests performed routinely and, thanks to stringent Coggins testing rules, the country is therefore free from equine infectious anemia (EIA). In reality, the U.S., like the rest of the world, has many cases of EIA each and every year. While the current prevalence of EIA in the American equine population remains very low, around 0.004%, this serious disease still poses a risk not only to our resident horses but also those visiting from other countries.

Where We Are With EIA

While we would like to be free from EIA, here are some statistics from the USDA’s Animal and Plant Health Inspection Service (APHIS) showing just how far from it we truly are:

  • In 2022, 96 horses were confirmed EIA-positive in 16 states.
  • In 2021, about 1.4 million tests were conducted, identifying 103 positive cases.
  • In 2020, about 1.3 million tests were conducted, identifying 29 positive cases.

As we can see from Figure 1, APHIS data show these case numbers are fairly consistent dating back to the early 2000s.

If these numbers surprise you, it might be a relief to know we’re in a much better position now than 50 years ago. A report published by the American Association of Equine Practitioners showed EIA cases peaked in the 1960s and ’70s, with 10,371 horses diagnosed with EIA in 1975 alone.

How EIA Virus Causes Anemia

To briefly review, the retrovirus that causes EIA consists of a single strand of genetic material (RNA). Once that virus enters the horse’s bloodstream, it infects white blood cells called macrophages as well as endothelial cells lining the walls of blood vessels throughout the body. The virus integrates its own genetic material into the horse’s DNA inside these cells, inciting an immune response that involves both arms of the immune system—humoral (concerning production of antibodies) and cellular (attacking invading antigens). This zealous response results in an immune-mediated destruction of red blood cells, causing anemia. The immune system’s excessive activity can also damage several other tissues.

Clinical signs of disease might occur at the time of initial infection, or the original infection might be so mild it goes unnoticed. Such signs include fever, depression, and petechial hemorrhages (small red or purple spots) on the mucous membranes inside the mouth, for example. After recovering from acute infection, horses remain infected for life. During the subacute/chronic infectious stages, horses can suffer intermittent bouts of fever, depression, petechial hemorrhage, weight loss, anemia, enlarged lymph nodes (lymphadenopathy), and icterus, or jaundice. Some horses become inapparent carriers, never showing any outward clinical signs.

A Change in Mode of Transmission

Traditionally, EIA was viewed as a vector-borne disease, similar to the encephalitides (Eastern, Western, and Venezuelan encephalitis viruses) and West Nile virus spread by biting insects.

In the case of EIA, the virus can be transmitted mechanically on the mouthparts of biting insects from one horse to another. The most common insects known to transmit EIA virus are horseflies and deerflies, but stable flies can also be responsible for viral spread. Flies can’t travel very far, and the virus does not persist on their mouthparts very long, which is why an EIA-positive horse can be kept in quarantine as long as they remain 200 yards (600 feet, 183 meters) from uninfected horses.

“Previous to 2016, the majority of the cases every year were in untested/undertested populations with natural fly bite transmission,” says Angela Pelzel-McCluskey, DVM, MS, equine epidemiologist with APHIS Veterinary Services, in Fort Collins, Colorado. “In the past few years, since 2017, the epidemiology has shifted. Now, the majority of our cases every year are in Quarter Horse racehorses with iatrogenic transmission as the cause of spread. Most of these have connections to unsanctioned racing.”

Iatrogenic spread means horses became infected when owners/trainers reuse needles, syringes, and IV sets; through contamination of multidose drug vials and using illegally imported blood and plasma products from other countries; and by direct transfusion of blood between horses for the purpose of increasing athletic performance (blood doping).

In the 96 cases of EIA identified in 2022, 84 of those were diagnosed in current or former Quarter Horse racehorses infected via iatrogenic transmission.

Texas is no stranger to EIA, frequently identified as a hot spot for disease, having the most cases per year. In a disease alert to veterinarians, the Texas Animal Health Commission (TAHC) reports, “Cases of this blood-borne disease have seen a rise in Texas in recent years due in large part to humans. People play a large role in EIA transmission by using contaminated medical equipment, such as needles, syringes, IV sets, and medications. When equipment is contaminated with blood from an infected horse it can spread the disease to healthy, unexposed horses when used again.”

Attempting to minimize/eliminate EIA, “equines must meet TAHC EIA testing requirements when changing ownership, participating in an assembly, or stabled within the state of Texas,” the statement continues. “These requirements are in place to help protect the health of the animals and the animal industry from disease. When these requirements are ignored, there is a chance animal health could be at risk.”

Humans Are Spreading EIA

Pelzel-McCluskey says unsanctioned horse racing, commonly called bush track racing, is a sport deemed illegal in states with an established sanctioned horse racing authority. It usually involves highly organized match racing of Quarter Horses at locations other than sanctioned racetracks, with no established rules for horse or rider safety.

“Bush track races have no veterinary oversight, track surface standards, welfare standards, or medications monitoring employed,” says Pelzel-McCluskey. “Use of electronic shock devices, excessive whipping, racing of horses while lame, catastrophic breakdowns, and administration of narcotics to horses immediately before the race are all practices routinely documented at bush track venues.”

So far, Pelzel-McCluskey says, state and federal animal health officials have located 107 bush tracks in 28 states.

“More likely exist, and the use of social media in recent years to communicate and advertise races has caused significant expansion of the activity nationwide,” she adds.

“Whether at bush tracks, stables, or rescue centers, when contaminated equipment is used among multiple horses, the disease is likely to spread,” notes the TAHC.

Traceback of infected Quarter Horses often reveals clusters of cases, and further probing often leads to identification of involvement in bush tracking. Many Quarter Horses diagnosed with EIA are tested by someone not participating in bush tracking who acquired a horse for barrel racing or some other purpose. To compete, the new owner of that Quarter Horse would have to have it EIA-tested, and that’s when officials find positives, explains Pelzel-McCluskey.

Because bush track participants rarely use veterinarians or seek Coggins tests, new cases of EIA in this population are often only found by accident. Further investigation of these cases is also hampered by owners’ and trainers’ unwillingness to share information about other participants or disclose where horses were acquired or stabled for traceback, she says. Additionally, many of the horses were once involved in sanctioned racing and have lost their registered identity, so complete traceback might not be possible.

What Can Be Done?


“Obtaining yearly negative EIA Coggins tests and opting for regular, voluntary testing is important in maintaining an EIA-free herd,” says the TAHC. Specifically:

  • Have your horse tested annually, and obtain the results. Retain proof of the negative Coggins test in the horse’s records.
  • Ensure all the horses in a boarding ­facility/environment are tested annually.
  • When purchasing a horse, require proof of a recent negative Coggins test at the time of sale or have testing performed.
  • Only participate in events requiring proof of a negative Coggins test for all horses.

Defer to Your Veterinarian

The TAHC encourages equine owners to seek guidance from a private practice veterinarian before performing any injectable treatments.

“Never using blood-contaminated equipment is key to protecting all equine from EIA,” the TAHC notes.

Fly Control

Mucking stalls regularly, disposing of manure away from horse stabling areas, and using fly sprays or natural predators to minimize fly presence will abrogate the spread of EIA virus via insects. Even though most cases are now caused by humans, don’t ignore vector spread.


As with any infectious disease, biosecurity protocols will help control disease spread and outbreaks. Separate horses with fevers, reduced feed intake, and/or lethargy from your other horses, and contact your veterinarian immediately.

“EIA is a regulated disease in all states and veterinarians are required to report suspect or positive EIA cases to state and federal animal health officials,” says Pelzel-McCluskey.

Upon confirming an EIA-positive case, state animal health officials will:

  • Quarantine the affected premises.
  • Isolate the positive case.
  • Identify and test all exposed horses.
  • Conduct an epidemiological investigation to identify the source of infection.

Exposed horses will not be released from quarantine until they are retested negative at least 60 days after isolation or the positive horse is removed from the premises. EIA-positive horses must either be euthanized or maintained in permanent quarantine at least 200 yards from other equids to prevent continued transmission

EIA-Free Status Roadblocked by Bush Tracking

The APHIS data are clear: Very few EIA cases are currently diagnosed in horses outside of bush tracking. For that small handful of cases infected via natural transmission, our sources say the universally accepted precautions described herein for insect control will continue to keep the prevalence of EIA at a low level in most equine populations.

As for the uptick in EIA cases in bush tracking that are spilling over into the general horse population, Pelzel-McCluskey says the only solution is to make this type of racing illegal—a feat far easier said than done.

“Bush tracking is expanding without any hinderance, and social media is spurring it on,” she says. “This is a fan-based ‘sport’ now, rather than a group of guys getting together to match-race their horses in a back pasture. These are sophisticated, highly organized events that often involve other criminal activity. These events are state-of-the-art with starting gates, drone videos, photo finishes.

“Making unsanctioned racing illegal at a federal level would allow the highest level of law enforcement to become involved in the situation,” she continues. “Bush tracking goes well beyond veterinarians, APHIS, the USDA and involves more than just equine health, safety, and welfare.”


Written by:

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she’s worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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