A Coggins test is a blood test that checks whether a horse is infected with Equine Infectious Anemia (EIA), a serious viral disease with no cure and no vaccine. If you own, buy, sell, or transport a horse, you’ll almost certainly need a current Coggins test at some point. Most states require a negative result before a horse can cross state lines, attend shows, or change ownership.
The Disease Behind the Test
Equine Infectious Anemia is caused by a lentivirus in the same family as HIV. The virus targets a type of white blood cell called a macrophage and, once inside the horse’s body, stays there for life. There is no way to clear the infection. An infected horse becomes a permanent carrier, capable of spreading the virus to other horses even when it looks perfectly healthy.
EIA spreads primarily through biting flies, especially horseflies and deerflies. When a fly feeds on an infected horse and then moves to a nearby horse, it can transfer virus-laden blood on its mouthparts. The virus can also spread through shared needles, contaminated surgical instruments, or blood transfusions. Transmission from a mare to her foal during pregnancy is possible as well.
Signs of Infection
Within a few weeks of infection, a horse may develop a high fever, lose its appetite, and become severely anemic. Other acute signs include jaundice (a yellow tint to the gums and whites of the eyes), rapid breathing, rapid heart rate, swollen limbs, nosebleeds, and blood-stained feces. In the worst cases, sudden death occurs before any signs are even recognized.
Horses that survive the acute phase often become chronic carriers. Many show no visible symptoms at all, which is exactly what makes routine testing so important. These silent carriers can experience flare-ups triggered by stress or strenuous work, and during those episodes they shed large amounts of virus. Over time, chronically infected horses may gradually lose weight and become emaciated.
How the Test Works
The test is named after Dr. Leroy Coggins, the veterinarian who developed it in the early 1970s. Before his work, there was no reliable way to identify carrier horses, and the disease spread unchecked. By 1975, more than 10,000 horses (about 4 percent of those tested at the time) were found positive.
A veterinarian draws a blood sample and sends it to an approved laboratory, where it’s screened for antibodies against the EIA virus. Two main lab methods are used:
- AGID (agar gel immunodiffusion): The original Coggins test. It requires 24 to 48 hours of incubation and is read by at least two certified analysts. Sensitivity and specificity both approach 100 percent, making it the gold standard and the official confirmatory test in the U.S.
- ELISA: A newer, faster method that can produce results in under an hour. Sensitivity reaches 100 percent with some kits, though specificity can vary. A positive ELISA result is typically confirmed with an AGID test before any regulatory action is taken.
Only a licensed, accredited veterinarian can draw the blood and submit the paperwork. You cannot order or perform this test yourself.
What It Costs
The AGID version of the Coggins test typically runs between $29 and $91, with a median around $46. A quick ELISA test costs more, ranging from $32 to $165, with a median near $75. On top of the test fee, you’ll pay for the farm call or office visit. If you also need a health certificate for travel, expect an additional $24 to $76 for that exam and paperwork. Many horse owners bundle the Coggins test with their horse’s annual wellness visit to save on trip charges.
When You Need One
Federal regulations require horses moving across state lines to be individually identified and accompanied by a Certificate of Veterinary Inspection. Most states require proof of a negative EIA test as part of that certificate, though the specific validity window varies. Some states accept a Coggins test done within the past 12 months; others require one within 6 months or even 30 days. Always check the importing state’s requirements before you travel.
Beyond interstate travel, you’ll commonly need a current negative Coggins for boarding at a new stable, entering competitions or trail rides, selling or buying a horse, and breeding. Many boarding facilities require annual proof as a condition of keeping your horse on the property.
What Happens if a Horse Tests Positive
A positive result triggers a strict regulatory process. The horse must be placed under quarantine within 24 hours while a confirmatory AGID test is run. All horses within 200 yards of the positive animal are also quarantined, since biting flies can easily cover that distance.
Once confirmed positive, the horse is classified as a “reactor.” At that point, the owner faces three options: euthanasia, slaughter at a federally inspected facility, or permanent lifetime quarantine at the premises of origin. The horse can never attend shows, trail rides, or any gathering of horses again. It must remain at least 200 yards from all other horses for the rest of its life, and regulatory officials will periodically inspect the quarantine to verify compliance.
Confirmed reactors must also be permanently marked. A state-assigned code number followed by the letter “A” is applied to the left shoulder or left side of the neck using a hot iron, chemical brand, or freeze mark, with characters at least two inches high. Alternatively, a lip tattoo with characters at least one inch high can be placed on the inside of the upper lip. This marking ensures the horse is identifiable as EIA-positive for life.
Reducing the Risk
Since there is no vaccine or treatment, prevention comes down to controlling exposure. Biting flies are the primary vector, so reducing their habitat makes a real difference. Remove standing water, keep manure managed, and use fly sheets, masks, and repellents during peak fly season. Keeping horses spaced apart (particularly away from untested populations in wooded or swampy areas where horseflies breed) lowers the odds of transmission.
Never share needles, syringes, or any blood-contaminated equipment between horses. Use a fresh needle for every injection, even when vaccinating a group on the same day. Before purchasing or introducing a new horse to your property, require proof of a recent negative Coggins test. That single piece of paper remains the most effective tool for keeping EIA out of your barn.