“He’s got the horse AIDS” – Has your horse been tested?

By Portia DeLoache DVM


Have you ever tried to register for a local horse show but were denied admission because the copy of your horse’s Coggins “expired” the week prior?  Then, when you call your veterinarian to explain you need a new Coggins ASAP they say “No problem, but unfortunately the price will be double due to being a rush sample.”  Begrudgingly, you agree, the results come back and you’re off to the show.  This is an all too familiar story for horse owners simply because they don’t fully understand many factors associated with Coggins testing and why the regulations are so important.  Most horse people have heard of a “Coggins Test” and many know a negative result is required and only valid for 12 months.  But fewer people know the actual disease this test detects or why testing is so important that the government requires it for interstate travel.  With a recent confirmed EIA positive case so close to home (most recent in Hall County Georgia), you may want to double check your own horses Coggins and testing frequency. 

A “Coggins” is a blood test used to detect Equine Infectious Anemia (EIA) which is a potentially fatal disease caused by a virus.  This virus is specific to equids, meaning only horses, donkeys, mules, and zebras can be infected.  There is no treatment or cure for infected horses.  Initial symptoms are often non-specific and while some chronically infected horses are obviously sick, most become inapparent carriers.  This means they look like normal healthy horses but they carry the virus in low levels in their blood.  These horses are thought to be reservoirs for disease and are the reason routine testing is important as there is no approved vaccine in the US.

   The virus that causes EIA is in the “lentivirus” family.  Human Immunodeficiency virus (HIV) another, better known lentivirus causes Acquired Immune Deficiency Sydrome (AIDS) in humans.  These viruses are very host specific meaning EIA only infects equids and HIV only infects humans.  Viruses in this family attack immune system cells weakening the host’s ability to fight off infection.  EIA in horses causes some immunosuppression but more often causes decreased platelet production which leads to poor clotting which can lead to small hemorrhages and eventually anemia. 


Infected horses may develop severe debilitating disease or look completely normal.  Acutely infected (less than 1 month after exposure) horses will have fever, lethargy and poor appetite.  Once the horse’s immune system responds to the virus the initial signs are not evident.  Most infected horses are inapparent carriers meaning outwardly they look normal but they carry small amounts of the virus in their blood stream.  A few infected horses will have recurrent episodes of illness.  These horses are more obviously sick and often have a higher level of virus in their blood.  Overtime, infected horses may lose weight, have edema (swelling) under their bellies, appear disoriented, and become severely anemic.  End stage infection can even result in visible hemorrhage such as splotchy bruising or nose bleeds and can be fatal.

The EIA virus is spread through blood contamination.  The most common route of infection is when large biting flies that take a blood meal from an infected horse and then bite a non-infected horse.  The fly mouth parts are directly contaminated by blood and can transfer the infection from horse to horse.  As if you needed one more reason to hate them, horseflies are the most common culprit because their painful bite often means their feeding is interrupted and they are more likely to feed from multiple hosts in a short time period.  Interestingly, the virus does not require the fly as part of its life cycle (such as with EEE or WNV and mosquitoes).  Accidental transfer is also possible through inanimate objects as well, such as tattooing equipment, dental equipment, or needles that is not properly disinfected between horses.

There is NO cure for EIA.  Humane euthanasia is most commonly recommended in confirmed positive cases.  Treatment is often not recommended or attempted due to the risk of spreading the disease and the difficulty in providing lifelong quarantine.  There is no approved vaccine in the US.  Because there is no approved vaccine and no cure, the best way to control the spread of disease is to test.


Regular testing is the recommended course for disease control.  Two blood tests are available for testing for EIA in the US.  Both require a blood sample be pulled from the horse by an accredited veterinarian and submitted to an approved lab.  Agar gel immunodiffusion (AGID) is the more commonly used test and takes a minimum of 24 hours to run.  Enzyme-linked immunosorbent assays (ELISA) is faster and often used for rush Coggins testing.  EIA is reportable meaning, if a positive result is obtained the state veterinarian is notified and subsequent recheck testing is performed to confirm the diagnosis.  If a case is confirmed positive humane euthanasia is often recommended as this disease is potentially fatal and highly contagious to other horses.

How do you know when to test?  Short answer:  at least once a year.  All horses over 6 months of age are required to have a negative Coggins test within the last 12 months to cross state lines.  Foals younger than 6 months must be travelling with their dam that must have a negative Coggins or have their own negative test results.  Many show grounds and sale barns also require proof of a negative Coggins test to enter the premises.  We also highly recommend obtaining proof of a negative Coggins test prior to buying a horse especially if you are moving him to a location with other horses.  Please note that a negative test technically means your horse was negative at the time that blood sample was pulled; it in no way prevents infection if exposed after testing.  So just because your horse’s Coggins is only 3 months old does not guarantee he is still negative.  Due to the low prevalence of positive horses, annual testing is usually recommended but in some situations, more frequent testing may be required.

What do recent positive cases mean for you?  If all of your horses are regularly tested (at least every 12 months) and not showing any clinic signs then continue regular testing.  If your equid has not been tested annually, we do recommend testing and annual rechecks.  Some horse owners have companion donkeys or retired horses that have not been tested in several years because “they never leave the farm.”  Unfortunately, because the disease can be spread through biting insects, it is possible for them to be exposed without ever leaving home.  Even if your property is not near other equids making the flies unlikely to carry disease, there is still risk of infection from blood contaminated equipment.  This is the reason veterinarians routinely clean and disinfect all equipment used on their equid patients to prevent transfer of disease.  Having proof of negative Coggins on hand is also helpful in an emergency situation, such as evacuation, when a horse needs to travel on short notice. 


2018 EIA cases by state

Picture from USDA APHIS

For more information and to stay current on recent cases, you can check the Equine Disease Communication Center and USDA APHIS websites for Outbreak Alerts and additional recommendations on testing or monitoring.  If you’re concerned about horses you can also contact your regular veterinarian.

At least once a year, test your horses (even the old ones), test your donkeys, and test your zebras!