EIA, or equine infectious anemia, is a viral blood disease that infects horses, donkeys, and mules for life. There is no cure and no vaccine. Once a horse is infected, the virus permanently inserts itself into the animal’s DNA, making the horse a carrier forever, even if it never shows symptoms again. In 2024, the USDA recorded 147 positive horses across 63 premises in the United States out of nearly 1.3 million tests performed.
What Causes EIA
The virus behind EIA belongs to the same family as HIV in humans: it’s a retrovirus, specifically a lentivirus. Like other retroviruses, it works by converting its RNA into DNA and then weaving that DNA directly into the horse’s own genome. This is why infection is permanent. The virus can’t be “cleared” because it becomes part of the horse’s genetic material in certain white blood cells called monocytes and macrophages.
The disease itself isn’t caused by the virus destroying cells directly. Instead, most of the damage comes from the horse’s own immune system reacting to the infection. Inflammatory signaling molecules and the body’s adaptive immune response drive the fever, anemia, and other symptoms. The amount of virus circulating in the blood fluctuates over time, and clinical flare-ups tend to coincide with periods when viral levels spike.
How EIA Spreads
EIA spreads through blood-to-blood contact, and there are two main routes: biting insects and contaminated equipment.
Horseflies and deer flies are the classic natural vectors. These large biting flies feed by tearing skin and lapping blood. When a fly bites an infected horse, gets interrupted, and lands on a nearby horse to finish feeding, it can mechanically transfer virus-contaminated blood on its mouthparts. This is purely mechanical transmission, not biological. The virus doesn’t replicate inside the fly.
The second route has become increasingly significant. Since 2017, the USDA has identified a growing number of EIA cases linked to human-caused (iatrogenic) transmission, particularly among current or former Quarter Horse racehorses connected to unsanctioned “bushtrack” racing. The practices that spread the virus this way include:
- Reusing needles, syringes, or IV equipment between horses
- Contaminating multi-dose vials by drawing from the same bottle with used needles
- Blood doping, where blood is transfused between horses to boost athletic performance
- Importing unlicensed blood or plasma products from other countries
All of these practices are preventable. A fresh needle and syringe for every horse, every time, eliminates the most common iatrogenic risk.
Stages of the Disease
Acute Stage
The first episode after infection typically lasts one to three days. The horse develops a fever, becomes lethargic, and its platelet count drops. Many horses show only mild signs at this point, or no signs at all, which is part of what makes EIA so difficult to catch early.
Chronic Stage
Horses that develop more serious disease enter a chronic phase that can stretch over days to months. Recurring fever and worsening anemia are the hallmarks. The horse may lose appetite, lose weight, and develop jaundice (yellowing of the gums and whites of the eyes). Swelling in the legs, chest, and abdomen is common. Some horses develop nosebleeds, red or purple spots on their gums, blood in their feces, rapid breathing, and a fast heart rate. Sudden death can occur during this stage.
Horses that survive the acute and chronic phases may still experience flare-ups later in life, often triggered by stress, hard work, or other illness. These recurrences range from mild weight loss and failure to thrive to full-blown fever and edema.
Inapparent Carrier Stage
Most infected horses eventually settle into an inapparent carrier state where they look and act completely healthy. They show no clinical signs. But they still carry the virus in their blood and can serve as a source of infection for other horses, particularly through biting flies in close quarters. This is the most dangerous stage from a herd management perspective, because these horses are silent reservoirs.
How EIA Is Diagnosed
The standard diagnostic tool is the Coggins test, formally known as the agar gel immunodiffusion (AGID) test. A veterinarian draws blood and sends it to a lab, where the sample is checked for antibodies against the virus. The Coggins test has been the gold standard since the 1970s. Faster screening tests also exist, but a positive result on a screening test is typically confirmed with the Coggins method.
One important limitation: it takes time after infection for a horse to produce enough antibodies to register on any test. A horse in the very early days of infection can test negative while actively carrying the virus. This is why repeat testing and careful attention to potential exposure matter.
Testing Requirements for Travel and Sales
Most states require a negative EIA test (a current Coggins) and a Certificate of Veterinary Inspection before a horse can cross state lines. The specific rules, including how recent the test must be, vary by state. Horse shows, racetracks, breeding farms, and auction houses commonly require proof of a negative Coggins before allowing a horse on the grounds. Many states also require testing at change of ownership.
These requirements are the backbone of EIA surveillance in the United States. The nearly 1.3 million tests performed in 2024 reflect how deeply Coggins testing is woven into routine equine management.
What Happens to a Positive Horse
A horse that tests positive faces a grim set of options. Because there is no treatment that eliminates the virus, regulators generally require one of two outcomes: euthanasia or lifetime quarantine under strict conditions.
If the owner chooses quarantine, the positive horse must be kept at least 200 yards from all other horses. All exposed horses within 200 yards of where the positive horse was kept are also placed under quarantine and must be tested. The quarantine distance is designed to exceed the typical flight range of horseflies, reducing the chance of fly-borne transmission to neighboring animals. The positive horse can never attend shows, sales, or any public gathering, and it can never be transported without regulatory approval.
Prevention
With no vaccine available, prevention relies entirely on testing and biosecurity. The most effective measures are straightforward:
- Annual Coggins testing for every horse, and additional testing before travel or sales
- Never reusing needles, syringes, or IV sets between horses
- Using only licensed blood products, administered by a veterinarian using tested donor horses
- Fly control around barns and pastures, including removing standing water, using fly sprays, and keeping horses stabled during peak biting-fly hours
- Isolating new arrivals until a current negative Coggins is confirmed
Iatrogenic transmission is, as the USDA puts it, completely preventable. The natural fly-borne route is harder to eliminate, but reducing fly populations and keeping horses away from known positive animals dramatically lowers risk. For most horse owners, the annual Coggins test is the single most important tool: it identifies carriers before they can silently spread the virus through a barn.