Protecting Horses Against Equine Infectious Anemia

That annual Coggins test has been part of your routine as a horse owner for so long, you may not realize why it’s important.

Equine Infectious Anemia (EIA), also referred to as “swamp fever,” is a blood-borne infectious viral disease that affects equids around the world. First tentatively diagnosed in the United States in 1888, EIA surged during the 1960s and 1970s, infecting thousands of horses. According to the American Association of Equine Practitioners (AAEP),10,371 cases were identified in U.S. horses in 1975.

Although EIA remains a threat, thanks to widespread testing and surveillance, the number of positive horses in the U.S. today is minimal. The United States Department of Agriculture (USDA), the agency responsible for keeping track of EIA cases in the U.S., reports that out of 1,187,536 EIA tests conducted in 2018, only 51 horses tested positive.

Transmission of EIA

In some equine viral diseases, the disease actually replicates in the insects that transmit it. A good example of this is West Nile Virus, which is transmitted to horses by mosquitoes that have fed on infected birds. EIA is different in that the biting flies that typically spread it don’t circulate the virus in their own bodies, but simply serve to “transport” infected blood from a horse that has EIA to a “clean” horse when feeding between nearby horses.

EIA is most commonly spread by biting flies, such as horseflies and deer flies.

“The reason horseflies are so effective as vector insects is that they can carry large amounts of blood on their mouthparts compared to other biting flies,” says Todd C. Holbrook DVM, Diplomate ACVIM, ACVSMR. A board certified specialist in equine internal medicine, sports medicine and rehabilitation at Oklahoma State University’s College of Veterinary Medicine, he holds the June Jacobs’ endowed chair and is a professor of equine medicine at OSU. Holbrook has served as an international team veterinary consultant with the U.S. Endurance team for the last two decades.

If the horse fly’s feeding pattern is interrupted, say, by the horse switching its tail, then the fly will immediately try to land on another horse close by to continue feeding. If the first horse is infected with EIA, the fly can transfer contaminated blood when it feeds on the next horse.

“It used to be that we knew where pockets of positive horses were based on untested herds and tracing back pockets of positive tests, but we don’t now. The Gulf Coast states have always been EIA ‘hotbeds’ because of the insects, but positive cases have popped up in states that have traditionally been negative,” notes Holbrook. “Nowadays the evidence is mounting that it’s less connected with fly transmission than with iatrogenic (caused by humans) transmission.”

Iatrogenic infection occurs when contaminated blood from an infected horse is introduced to a “clean” horse, by such inappropriate actions as sharing needles/syringes, re-using intravenous tubing, or other equipment, and incorrect handling of multi-dose drug vials, including inserting a used needle into the vial.

Because iatrogenic infection is completely avoidable by following standard precautions, it’s all the more tragic when a horse is infected through human carelessness.

Testing for EIA

Until the Coggins test was developed to confirm the presence of EIA, a horse owner was only aware of the disease if their horse died, or showed clinical signs–something not all horses do.

Today, the Coggins test, also known as Agar gel immunodiffusion (AGID), remains the most widely accepted procedure for diagnosing EIA. Blood samples are submitted by an accredited veterinarian, state or Federal animal health official, and EIA tests are conducted in USDA-approved laboratories.

Since there’s no such thing as “Coggins disease” (although some people have mistakenly referred to EIA this way), you may have wondered why this routine blood test is known as a “Coggins” test. For that, we can thank the late Leroy Coggins, DVM, a 1957 Oklahoma State University graduate, who developed a test for antibodies specific to EIA. Testing was initiated in 1972 and was quickly adopted by animal health authorities around the world.

Each state has its own regulations regarding the movement of horses, whether within the state or across state lines, including to events, sales, etc. Most states require EIA testing every 12 months, although some require it every six months. Your vet can tell you what is required in your state.

While many horse owners comply with state regulations and routinely test their horses, not all horse populations are tested regularly. Today, positive EIA tests in North America tend to occur in groups of horses that were previously untested. For example, a horse/pony/mule/donkey that has lived its entire life at the same farm is offered for sale and thus tested, or a group of wild horses is rounded up by the Bureau of Land Management (BLM) and tested before being put up for adoption, according to standard protocol.

The latter actually happened in 1998 when a group of wild horses in Utah was rounded up by the BLM. In this particular case, 10% of the horses rounded up tested positive for EIA, indicating the virus had likely been circulating in that population for many years.

Clinical Signs of EIA

Classified as a retrovirus, EIA is closely related to the human immunodeficiency virus (HIV), and like HIV, it attacks the immune system. Clinical signs of EIA include:

  • Fever
  • Going off feed
  • Lethargy
  • Pale mucous membranes
  • Laboratory findings of anemia and/or low platelet count
  • Weight loss/loss of condition

EIA is a somewhat sneaky disease as it has three phases of infection:

  • Acute
  • Chronic
  • Subclinical carrier

Survivors of the acute phase can become chronic with off-and-on signs, and remain carriers of the disease. Some horses aren’t observed as ill during the acute and chronic phases, but because the virus remains in their blood, these horses are considered subclinical carriers.

Holbrook explains that when a horse is initially infected with EIA, high fever and other signs of severe illness are common. Weight loss and anemia also develop if the horse lives. Often the horse doesn’t die, but after this acute phase, it may become chronic and experience cyclical bouts of fever and anemia, yet appear normal much of the time. This horse is described as a subclinical carrier, meaning he is infected with EIA, but doesn’t show clinical signs.

“The subclinical carrier is the most common scenario, which is why an owner can’t expect to notice clinical signs,” says Holbrook. “The most important thing horse owners need to know is that you won’t recognize EIA. Horses that have it will usually look normal but can be subclinical carriers and potentially put other horses at risk.”

Positive Tests

So, what happens in those rare situations when a horse tests positive for EIA?

The final answer depends on your state. Most states follow USDA guidelines, which specify that a positive horse is either euthanized or remains in life-long quarantine no less than 200 yards from any other equids. Some states have stricter protocol and require euthanasia for any equid that tests positive for EIA.

“Once a horse has it, he has it. There is no cure and no vaccine,” cautions Holbrook. “EIA is a lifelong infection and euthanasia is the most appropriate management decision.”

Negative Coggins

You already know that you need negative Coggins paperwork to travel with your horse, and enter any show, competition or event. But you may not be thinking about a Coggins test when caught up in the excitement of shopping for a new horse.

“As veterinarians, we recommend a prepurchase exam, but if that’s not something you want to have performed, at least have a Coggins test done,” Holbrook urges.

“If not, you could purchase a horse that looks totally normal, and could have a disease that requires you put him down,” notes Holbrook. “It’s a very cheap test given the consequences of a positive test, which could mean losing your horse and possibly exposing horses who belong to others.”

Here’s a quick lesson on why continued annual testing is important for all horse owners. Let’s say you’re an avid trail rider who routinely haul your horse to ride the public trails in your state. This past weekend, after enjoying a great ride, you tie your horse to the trailer in the parking area at the trail head and join your friends for a picnic lunch there before heading home. Your trailer is parked near other trailers, where multiple horses are also tied.

What you don’t know is that the horse tied to the trailer next to yours was recently purchased, but the new owner neglected to ensure the horse had a negative Coggins, since he looked healthy. The majority of subclinical carriers of EIA look totally normal, but unfortunately, if a horsefly bites that horse and then bites yours, your horse could be infected with the virus.

Protecting Against EIA

Even if you never plan to show or haul your horse anywhere, that annual Coggins test is still necessary. Every horse on the property should be tested every year, including, the pony or donkey you got as a pasture companion, and even the old retiree who isn’t going anywhere.

Wide-spread testing and universal regulation have greatly reduced the spread of EIA and losses from the disease are negligible today. Although numbers have dropped significantly, even one owner being forced to euthanize their beloved partner because of a positive test is one case too many.

Do your part to minimize the risk of EIA:

  • Continue annual Coggins tests for every horse
  • Don’t allow any horse on the property unless you have proof of current negative Coggins
  • Use fly repellentand physical barriers such as fly masks to reduce your horse’s exposure to biting flies
  • Never share needles, syringes, IV sets, multi-dose vials of medication between horses
  • Make sure dental/surgical equipment is thoroughly sterilized between each use

EIA Tip: Protecting your horse against Equine Infectious Anemia (EIA) includes keeping up with the annual Coggins test and using diligent fly control to protect against biting flies that can transmit the disease. Build an effective No Fly Zone that includes the physical barrier of a fly mask, on-horse repellent products, and feed-through fly control.

Did you Know? EIA is a “reportable” disease in the United States, meaning that any horse, pony, donkey, mule, (or zebra!) that tests positive must be reported to the USDA. Depending on the state, a positive equine must either be euthanized or kept permanently isolated in strict life-long quarantine away from other horses.

Courtesy of Farnam’s Stable Talk