Equine Infectious Anemia (EIA), sometimes referred to as “swamp fever,” is caused by a lentivirus, a viral family that is similar in structure to the one responsible for HIV in humans. But don’t worry: the equine and human viruses are species-specific, meaning horses can’t catch EIA from humans, and vice versa. However, this disease is not one to take lightly. With a current outbreak and growing cases over the last couple of years, EIA is on the rise, and horse owners need to be prepared.
What is EIA?
This disease targets a horse’s immune system, and it isn’t passed through casual horse-to-horse contact. Instead, it typically spreads when blood-sucking insects, like large biting flies, transfer infected blood from one horse to another.
People, however, can unknowingly contribute to the spread. Reusing blood-contaminated needles, dental tools, or other instruments between horses can transmit the virus. And since the flies responsible for spreading EIA don’t travel far on their own, infected horses moved to new areas are the primary way the disease reaches different regions.
A Changing Landscape
Unfortunately, there’s currently no cure or vaccine for EIA, making prevention and testing absolutely essential. In fact, the disease landscape seems to be changing rather rapidly which is concerning for those in the epidemiology field, such as Angela Pelzel-McCluskey, the National Epidemiologist for Equine Diseases at USDA-APHIS-Veterinary Services.
“EIA in the US has changed epidemiologically, meaning how the disease is spreading, and what types of horses are at higher risk for contracting. The disease is different now than it was maybe 10–15 years ago so that’s pretty that’s pretty concerning. I would say we don’t usually have a very old disease change genealogy right in front of our face.”
Pelzel-McCluskey notes that the changing landscape of EIA is largely attributed to human behavior.
“We’ve been testing for, and essentially euthanizing or permanently quarantining, horses with the disease since the 1970s. We used to have many, many cases in the U.S., but in the last 20 years or so, we’ve significantly reduced the prevalence. Our current national prevalence of EIA in all horses is now modeled to be around 0.004% — that’s really tiny, just a small fraction of the population expected to be infected.”
How Has Equine Infectious Anemia Changed?
A typical story when it came to finding EIA positive horses, even just 10 years ago, went something like this, says Pelzel-McCluskey:
“Grandpa had this whole herd of horses out back. He owned some property, and the horses were just out there grazing for years. They looked fine, but he never did anything with them. Then Grandpa passed away, and the kids or grandkids, who don’t really do horses, decided to package up the herd and send them to the sale barn. And when we tested for EIA at the sale barn, we’d immediately find a whole group of EIA-positive horses. That was the usual story.”
Although rates of the disease had decreased, there’s been a steady uptick of cases in the last few years according to Pelzel-McCluskey. “Unfortunately, our case counts in recent years have increased due to changes in the disease’s epidemiology. Historically, this disease was spread by natural fly-borne transmission between horses, and we would find clusters of positives in herds that were untested.”
And what is the root cause of many of these transmissions according the this epidemiologist? Incorrect handling of needles and syringes. “In recent years we have had iatrogenic transmission so this is human caused transmission, usually using medical equipment; needles, syringes, IV that are used between horses. Not hygienically, correct use of needles or syringes and not veterinarians usually doing this, but owners and trainers handling their own needles and syringes.”
Read the USDA report: 2023 Summary of Equine Infectious Anemia Cases in the United States
What Are the Symptoms of EIA?
The virus hides within macrophages (a type of white blood cell), allowing it to spread quietly. The symptoms of EIA infection are only apparent when the virus is active. Typically, a fever is the first sign, but this can be commonly missed. Then, the next symptom is commonly anemia. This causes weakness, depression, poor oxygen delivery, and possible organ damage.
As the condition worsens, blood clotting issues arise, immune function declines, and internal organs can begin to fail. The horse may develop swelling in the abdomen and legs and gradually lose weight. Unfortunately, standard treatments for these type of symptoms, like dewormers and antibiotics are ineffective, and once symptoms appear, the disease typically ends in euthanasia or death.
Once infected, a horse carries the EIA virus for life and becomes a potential source of infection. Some horses never show symptoms, but those that do may experience:
- Ongoing weight loss
- Muscle weakness
- Reduced stamina
- Fever
- Lethargy
- Anemia
An infected horse may be symptom-free for a long time, until some stress (such as another infection, shipping, hard exercise, etc.) weakens his immune system and the virus becomes activated. There is no cure or vaccine for EIA. Horses that test positive must either be euthanized, die from the disease, or live out their lives in strict quarantine—no less than 200 yards from healthy equines.
The Coggins Test
Diagnosing EIA starts with a Coggins test, which is a blood test designed to detect antibodies to the virus. If a horse tests positive, additional, more sensitive testing is used to confirm the diagnosis. Developed in 1970 by Dr. Leroy Coggins, the test has been a game-changer, significantly reducing EIA cases over the decades.
Strict protocols are in place when it comes to EIA. Horses that test positive are typically either euthanized or placed in lifelong quarantine. Because EIA is incurable and poses an ongoing threat to healthy horses, states mandate regular Coggins testing, especially for horses being transported, shown, or sold. Identifying and isolating infected horses remains critical to controlling the disease’s spread. A negative Coggins test is required by most U.S. states for horses traveling across state lines.
Travel papers aren’t meant to be a nuisance. They’re a line of defense against the spread of equine diseases, and a way to track outbreaks.
Chris/adobe.stock.com
Travel papers might seem like a nuisance, but having a current Coggins test and Certificate of Veterinary Inspection aren’t just ways to minimize the spread of disease. It’s a way to trace infected animals to prevent further infection.
Horse&Rider’s contributing veterinarian, Barb Crabbe, DVM notes, “We’ve seen that testing and inspection requirements play a role in eradicating diseases, as illustrated by: (1) the Coggins test’s role in controlling EIA; (2) controlling disease outbreaks through veterinary inspections that aid with traceability; and (3) preventing the introduction and spread of new diseases, such as piroplasmosis, through testing.”
Getting health papers might seem like an unnecessary step, but according to Crabbe, they’re worth it, “All of these requirements may seem like a nuisance, but they really do play an important role in protecting not only your own horse’s health but also to help prevent and control spread of disease to horses everywhere.”
One needle, one syringe for every horse. Don’t reuse syringes, and if blood gets in the syringes, don’t reuse the needles.”
-Angela Pelzel-McCluskey
Tips For Protecting Your Horse from EIA
When it comes to protecting your horse or your herd from EIA, remember, that prevention is the only route. There is no cure or vaccine for the disease. According to Pelzel-McCluskey, proper handling of medical equipment is one of the keys for reducing outbreaks.
“One needle, one syringe for every horse. Don’t reuse syringes, and if blood gets in the syringes, don’t reuse the needles. We have people that vaccinate horses, one to the other, to the other, using the same needle and syringe, and guess what can happen? They can transmit blood-borne disease, not just EIA, but any blood-borne disease, like anaplasmosis… not good.”
Sharing needles and syringes between horses can open your horse up to not just EIA, but other blood-borne equine diseases. charlymorlock/adobe.stock.com
Another tried and true tip for keeping your horses safe from EIA is to keep a current Coggins test, and to always test horses before you buy them and bring them to your property. Even if the horse looks completely fine, remember that EIA can cause a horse to be asymptomatic. And the disease is endemic in places like Mexico where testing isn’t common, so keeping current tests pulled on your own herd, and testing those you bring onto your property is a non-negotiable for safety says Pelzel-McCluskey.
“Whenever and wherever you get a horse, you should always be testing for EIA…because you just don’t know.”
Alongside these protocols, try and reduce the populations of biting flies around your property. This includes proper manure management and removal, taking care of stagnant water, and regularly disposing of trash that can draw in flies.
Resources for Horse Owners:
USDA – Animal and Plant Health Inspection Service: EIA Archives
Learn About Other Equine Diseases from USDA – APHIS
Stay informed about equine disease outbreaks with EDCC Health Watch text alerts: Sign Up Now
Sources:
Angela Pelzel-McCluskey, the National Epidemiologist for Equine Diseases at USDA-APHIS-Veterinary Services
Barb Crabbe, DVM, Horse&Rider contributing veterinarian
Eleanor M. Kellon, VMD, a Staff Veterinarian for Uckele Health and Nutrition, Inc.
