EIA: What's the Big Deal?

Find out where veterinarians are seeing an uptick in equine infectious anemia cases

Each year, like clockwork, our veterinarians take blood samples from our horses during spring health exams. We sign the forms, and off the tubes go to the lab for the Coggins test. We do this year in and year out, with no positives and no questions asked. So why do we keep up with this annual horse care ritual? 

The Coggins test checks for antibodies against the equine infectious anemia virus (EIAV). This virus is significant because much like the human immunodeficiency virus (HIV), its lentiviral cousin, there is no vaccine and no cure. A horse diagnosed positive for equine infectious anemia (EIA) dies, is euthanized, or gets placed under extremely strict quarantine conditions (at least 200 yards away from other equids) for the rest of his life.

How Do Horses Get EIA?

The EIA virus passes from one horse to another via blood. Biting flies, such as horseflies, deerflies, and stable flies, can transmit it after feeding on an infected horse. 

“Unlike flaviviruses such as West Nile virus or Zika virus, EIAV does not replicate in insect tissues, and so the infective dose is limited by the amount of blood that can be carried on the fly’s large mouthparts,” says Frank Cook, PhD, a professor at the University of Kentucky, in Lexington. “EIAV survives for less than four hours on fly mouthparts.

If something interrupts a horsefly’s meal, then the probability of it biting another host is inversely proportional to its distance from the next target. “The further away a potential new host is, the more likely the fly is to return to its original victim,” he says. “It was discovered that if a new horse host was tethered at distances of 50 yards or more, it was virtually (although not entirely) immune from attack, as the fly would almost invariably return to the original host.”

What the Virus Does

EIAV attacks the horse’s immune system. Clinical signs include muscle weakness, progressive loss of condition, and poor stamina. An affected horse might also develop a fever, depression, and anemia.

The disease has three phases of ­infection—acute, chronic, and inapparent carrier—yet not all affected horses develop all phases. Many acutely infected horses die; survivors suffer from the disease’s chronic impacts. Others mount an immune response and persist as carriers. 

Most infected horses develop some level of anemia—red blood cell or hemoglobin deficiency—while still looking fit and fine and able to perform as intended, says Angela Pelzel-McCluskey, DVM, MS, equine epidemiologist of Surveillance, Preparedness, and Response Services at the USDA-Animal and Plant Health Inspection Service’s Veterinary Services. Those that develop an acute crisis experience severe hemolytic anemia (red blood cell destruction) and are likely to die. Most infections, however, seem to progress to a relative state of remission, yet these horses still provide a reservoir for viral infection either by way of insect vectors and/or iatrogenic sources (people and medical treatment). What’s more, the underlying anemia might reduce a horse’s resistance to contracting another disease, such as equine piroplasmosis.

Researchers believe that EIA survivors mount a sufficient immune response to control the extent of the disease in their bodies, says Cook.

“Studies inducing suppression of the immune system with corticosteroid administration resulted in increased blood-associated virus levels and in some cases recrudescence (reappearance of clinical signs) of disease,” he says. “Therefore, it is thought that stress of an inapparent carrier and/or age-related degeneration in the immune system causes that individual to be an active source of infection to others.”

EIA: What's the Big Deal?

Is EIA on the Rise?

Equine infectious anemia is an insidious yet controllable disease. Due to close surveillance (those Coggins tests), the number of cases confirmed in the United States declined over the past few decades.

“When testing first began in the early 1970s with the development of the Coggins test, approximately 4% of all equids tested were positive for antibodies to EIAV,” says Cook. “Removal of these test-positive subjects from the population by the beginning of the 1980s resulted in only around 0.5% test-positives; today the incidence is less than 0.01%.”

In 2001 testing identified 534 EIA-positive horses, whereas in 2015 only 69 horses of nearly 1.5 million tested were confirmed positive. Recent observations, however, show the number of cases on the rise again.

Pelzel-McCluskey describes a disconcerting development in the United States: “Between 2009 and 2013, there were an average of 40-plus cases per year,” she says. “It was felt that there was a steady-state prevalence at our given level of surveillance of 1.5-2 million horses tested each year. But, in the last few years, there has been an uptick in positive cases. The most disturbing trend is that this is due to an increased number of iatrogenic cases, particularly in Quarter Horse racehorses and unsanctioned bush track racing.”

Prior to 2013 USDA-accredited veterinarians did not recognize iatrogenic EIA cases as contributing significantly to the total case number. But by 2013 Pelzel-­Mccluskey noted that about 25% of annual EIAV infections were caused by iatrogenic transmission. In 2014 alone, 54% of EIAV positives were traced back to a potentially preventable iatrogenic origin.

Iatrogenic infection occurs through contaminated blood or blood products, shared needles, or blood-contaminated hands, dental equipment, or other instruments contacting another horse.

“Residual blood volume in a used syringe and needle can be 5,000- to 10,000-fold greater than that carried by even the largest horsefly, and the virus is viable for at least four days,” Cook says. “Horsefly transmission pales to almost insignificance compared to human-­related spread and perpetuation of EIAV. If it was not for iatrogenic transmission, EIAV would be of little significance today.”

Katie Flynn, BVMS, equine staff veterinarian for the California Department of Food and Agriculture, reports that from 2012 through 2015, testing confirmed 39 racing Quarter Horses as positive for EIAV in California alone. “Epidemiologic investigations indicate that the majority of the positive horses participated in Quarter Horse racing and had potential exposure to high-risk practices, such as sharing of needles and other medical equipment or the use of contaminated blood products,” she says.

Sources of iatrogenic infection that currently plague Quarter Horse racing and bush track racing horses include:

  • Reuse of needles and syringes on different horses.
  • Contamination of multidose drug vials by inserting a used needle and/or syringe that deposits blood (with virus) into the bottle. “Infected blood contained within the hub of a used syringe or needle contaminates the drug vial, resulting in disease spread with subsequent drug administrations to additional horses,” says Flynn.
  • Reuse of intravenous (IV) tubing—­invariably when using tubing to connect to an IV catheter, there is blowback of blood into the tube that cannot be cleaned out with water or even antiseptic solutions.
  • Improper cleaning and sterilization of lip tattoo equipment.
  • Blood doping, particularly of bush track horses—this technique takes blood from one horse and gives it to another via IV transfusion.
  • Possible use of unlicensed blood or plasma products imported illegally from other countries, including Mexico and South America.

Further, there’s another disease on the rise that can be transmitted through these methods: equine piroplasmosis. “A critical point is that these blood-borne infections are totally preventable transmissions,” says Pelzel-McCluskey.

EIA - What's the Big Deal?

She suggests veterinarians caring for these high-risk groups of horses always test for blood-borne infections such as EIAV and equine piroplasmosis. Knowing that these animals are at high risk of infection provides veterinarians an opportunity to educate owners about potential sources of iatrogenic infection, she says.

A 2006 outbreak among 29 horses in Ireland illustrates what risks a single iatrogenic EIA infection can pose. “The 2006 Irish outbreak was initially caused by transfusion of a contaminated unlicensed horse plasma product probably from Eastern Europe, and there was also at least one transmission resulting from the reuse of a catheter by a veterinarian,” Cook says.

This outbreak in Ireland was unusual, he says, because several cases might have been caused by aerosol transmission subsequent to power-washing at facilities where EIAV-positive horses were ­identified.

“The sick mare at the Irish clinic was not initially suspected of being infected with EIAV,” says Cook. “However, she experienced one common clinical sign of acute EIA—hemorrhage due to severe thrombocytopenia (loss of platelets necessary to clot blood). Several liters of her blood contaminated the floor, with as much as 1 million infectious EIAV particles per mL of blood. Cleanup of the blood spill with a pressure hose might have aerosolized millions of infectious EIAV particles, each small enough to enter the lung and deliver viruses to lung macrophages (white blood cells) to cause infection. Only those horses present at the clinic at the time of the mare’s hemorrhage became infected. Those that left before or came after were not.”

There’s No Vaccine

Vaccines stimulate naturally occurring protective immune responses against microbial pathogens. “Unfortunately,” says Cook, “Lentiviruses such as EIAV do not stimulate natural, fully protective immunity. In part this is because they have a high mutation rate, which is difficult for a vaccine to duplicate. This is why billions of dollars and many different vaccine approaches against lentiviruses, including HIV and EIAV, have mostly been ­unsuccessful.”

Because of the difficulty in protecting against EIAV with immunization, the only successful approach is through surveillance and testing.

All-Important Testing Policies

EIAV infection is limited to equids so, theoretically, it is possible to eliminate the virus from countries where the incidence is low, such as the United States. However, the key factor is compliance, making this a political issue rather than a solely veterinary one, says Cook.

In regard to EIA testing in the United States, Cook describes two equid ­populations: “Those that compete and show or move across state lines are tested very frequently, whereas those that live out their lives on the same farm may not be tested at all. It is usually within this second group that new cases are discovered, for example, if an owner dies or decides to offer a horse, donkey, or mule for sale, and EIA testing is required or pursued. Therefore, the discovery of new EIA cases in North America is often the result of a chance event.”

Horses that travel out of state and to clinics and shows where a negative Coggins test is required for entry get tested for EIAV repeatedly. Such a “clean” population continues to be tested every year and, so, remains clean. The main danger of EIAV exposure in the frequently tested population of traveling, competing, showing, and breeding horses is if they mingle with animals from the nontested group.

For instance, a clean horse could encounter an untested Quarter Horse that participated in unsanctioned or illegal races—­particularly if he’s already moved on to a second career. Because these horses tend to be natural athletes, many go on to jumping, eventing, or barrel racing careers, for example.

“This can result in an infected horse serving as a reservoir for horses in other disciplines that might otherwise be ‘clean,’ ” says Pelzel-McCluskey.

“Racing Quarter Horses exposed to EIA at a young age that go undiagnosed pose a risk to the equine population,” adds Flynn. “Horse owners should be aware of this potential risk and consult their veterinarian to determine appropriate testing prior to purchase, particularly of a former racing Quarter Horse.”

Each state oversees regulations on horse movement into that state; most adhere to requirements for EIAV testing every 12 months and, in some cases, every six months, says Pelzel-McCluskey. There has been some discussion about codifying the 12-month requirement into a federal rule, but for now there is no established national program of EIA surveillance.

Another concern, says Pelzel-­McCluskey, is that our southern neighbor, Mexico, has no control program or testing for EIAV. With that in mind, she says, we probably won’t be able to ever give up on routine EIAV testing in the United States, due to the high risk of infection sources from south of the border.

She says long-established untested herds can lead to large clusters of positive EIAV cases. And for some of these herds, it’s not due to owner noncompliance. For example, in 1998 the BLM rounded up a group of wild horses in Utah to reduce herd numbers. Testing revealed that 10% were infected with EIAV. Cook says EIA had likely been circulating in the Utah wild horse population for decades and was only discovered during the roundup.

This could pose a threat to the domestic horse population via fly transmission.

Other potential natural reservoirs exist in the United States. Tribal lands out West, for instance, have untested herds, as do some religious groups, such as the Amish in New York and Pennsylvania, says Pelzel-McCluskey. Additionally, large domestic horse herds in the western United States and elsewhere might end up being dispersed following their owners’ death or retirement. As these horses go to sales, testing might identify positive cases.

Take-Home Message

The responsibility is on each of us as horse owners to continue routinely testing our horses for EIAV to control disease spread. The recent increase in positives each year is attributable to iatrogenic sources, yet transmission is preventable through education and testing.

Flynn says EIA prevention relies on obtaining a negative EIA test prior to a horse’s entry into a herd and not sharing blood-contaminated equipment such as needles, syringes, IV sets, multidose vials, or dental or surgical equipment.

Anytime you have a horse evaluated for purchase, travel, or health, be sure to have him tested for EIAV.