Keeping Tabs on Equine Infectious Anemia

Keeping Tabs on Equine Infectious Anemia

Photo: Stephanie L. Church, Editor-in-Chief

With neither cure nor vaccine in existence, veterinarians and ­researchers must supervise the equine infectious anemia virus ­closely to avoid widespread disease.

Microorganisms have ruled the earth since the dawning of time. Despite our valiant disease-prevention efforts, miraculous discoveries (such as antibiotics and anti-inflammatory drugs), and medical advances (including treatment methods), these wily microscopic monsters continue to evolve as rapidly as we find tools to fight them. The equine infectious anemia virus (EIAV) is a prime example of this continual war between mammal and microbe.

"The equine infectious anemia virus is similar in many ways to the human immunodeficiency virus (HIV), the cause of AIDS in people," says Robert Mealey, DVM, PhD, Dipl. ACVIM, associate professor of immunology and infectious diseases at Washington State University's College of Veterinary Medicine.

For example, both viruses:

  • Are lentiviruses, a viral variety known for persistent infections resulting in slow, progressive disease;
  • Are retroviruses (RNA viruses that replicate as part of the host cell's DNA); and
  • Cause lifelong infections.

Just like HIV in humans, EIA can spread insidiously from one horse to another--developing subtly and gradually enough to be well-established before becoming apparent. And also like HIV, we do not want a retrovirus running rampant in the equine community. Infections are not preventable by vaccination, can go undetected, and are incurable.

However, "unlike HIV, which eventually destroys the immune system resulting in fatal disease, most horses infected with EIAV do not die from the infection," notes Mealey. "Instead, their immune systems are able to keep the virus at low levels, and they live their lives persistently infected yet otherwise normal."

In this article we'll review EIA and how veterinarians and researchers are helping control its spread.

Disease Spread

The EIA retrovirus is primarily spread by biting insects such as horseflies and deer flies. The horse's defensive reaction to the painful bite interrupts a fly's feeding activity, sending it on to a nearby horse to finish feeding. Thus, the fly transfers blood from an uninfected horse to other horses via its mouthparts. The good news is the virus doesn't live very long on the insect's mouthparts, so it can only spread EIAV to horses within very close range.

Other methods of EIAV spread include blood or plasma transfusions; use of contaminated needles, surgical instruments, and teeth floats; transfer from mare to foal in utero; and possibly aerosol transmission.

Unlike other infectious diseases that "simply" multiply, causing abscesses (Rhodococcus equi), or wreak general havoc on certain body parts such as the spinal cord (e.g., Sarcocystis neurona, the causative agent of equine protozoal myeloencephalitis), EIAV adds insult to injury. Once a horse is infected, the virus uses the horse's "cellular machinery" to convert its RNA to DNA, which is then directly incorporated into the infected horse's own DNA.

"The EIAV infects a type of white blood cell called macrophages and persists throughout the life of these cells in tissues like the spleen, liver, and lungs," adds Mealey.

Most horses survive the acute stage of infection, which is usually mild and short-lived, and subsequently have mild to severe bouts of fever, anemia (low platelet counts that can lead to hemorrhage), jaundice (yellowing of the whites of the eyes and gums), weight loss, swelling in the limbs, and weakness.

"Most of the time, the disease can go unnoticed, which is why testing is so important," Mealey explains.

Diagnosing EIA

According to the American Horse Council, the U.S. horse population is approximately 9 million, and according to the USDA's Animal and Plant Health Inspection Service (APHIS), only 1.5 to 2 million horses are tested for EIA each year. This means 75-80% of American horses are untested and the actual number of EIA-positive horses could be much higher than testing shows. Nonetheless, current APHIS statistics indicate the number of EIA reactors (affected horses) in the United States is actually low.

"For 2011, we anticipate the numbers to be higher than those reported in 2010 and 2009, as we identified 40 infected horses in Arkansas out of a single herd of 80 head this year," says Angela Pelzel, DVM, a Western region epidemiologist with the USDA-APHIS. "That single finding alone will trump previous years' numbers, and we still had additional findings in our regular high-risk states that have to be added to that group." (As of press time, Feb. 29, state officials were still submitting their 2011 numbers and a year total was not yet available.)

The Coggins test, an agar gel immunodiffusion test developed by Leroy Coggins in 1970, has been used successfully worldwide to identify affected horses and, thus, help officials control the spread of EIA along with a number of EIA enzyme-linked immunosorbent assay (ELISA) tests. Using these test methods, the percentage of EIA-positive horses in the country has declined from almost 4% in 1972 to less than 0.003% in 2010. It is important to note, however, that these figures do not necessarily reflect the EIA prevalence in the general horse population for reasons beyond those highlighted by the Horse Council's statistics.

"These data are biased by repeated testing of high-quality horses competing in events that require negative test results," explains Mealey. "Testing is only required for horses that are entering exhibitions or competitive events, being moved interstate, changing ownership, being imported, or entering auctions or sales markets."

According to Charles Issel, DVM, PhD, Wright-Markey Chair of Equine Infectious Diseases at the University of Kentucky's Gluck Equine Research Center, EIA surveillance strategy could be modified to permit less frequent testing of equids that aren't at risk for EIA and a heightened focus on finding and testing the remaining untested reservoir population (horses that are infected but haven't been tested/identified).

"Smarter testing also means using the most effective tests possible to find the highest number of EIAV-infected equids," he says. "Today that means adopting a three-tiered testing strategy that most effectively uses the strengths of the available laboratory tests for EIA. These tests include the most accurate test--the (Coggins) test--and the more sensitive ELISA-based tests in combination, by using the ELISA-based tests first (the first tier) and the more specific Coggins to confirm positive ELISA results (the second tier). If test results between the first and second tier are mixed or unclear, a third tier of immunoblot testing to detect antibodies can settle the question of whether the horse has EIA."

Issel adds that Italian officials conducted national surveillance testing for EIA recently using this three-tiered strategy and found significantly more positive equids than would have been found if only Coggins testing were completed.

Although EIA is not considered epidemic (disproportionately prevalent in a particular area) or pandemic (impacting a large geographical area) in the United States, it is not a "back-burner" disease. Recall that HIV was initially equally uncommon, yet it spread rapidly from only a handful of people in 1981 to hundreds of thousands within a matter of years. This is why veterinarians and health officials strongly advocate routine EIA testing.

Controlling EIA

Federal officials have established for individual states a standardized set of best practices or recommendations for EIA control, called the EIA Uniform Methods and Rules (UM&R, available publicly).

"Most states do follow the (prevention and surveillance) recommendations in the UM&R for EIA in general, and all of the states follow the UM&R for dealing with EIA reactor horses," relays Pelzel. Entry requirements for proof of negative EIA testing vary from state to state. "For example, some require a test within 12 months and others within six months or within the calendar year. EIA surveillance requirements in each state (i.e., whether that state requires an EIA test for change of ownership, whether it requires an annual EIA test for all horses as do Louisiana and Arkansas, etc.) also vary."

A Positive Test

What happens to horses diagnosed with EIA? Sadly, the choices are fairly limited: They are either euthanized or placed under lifetime state quarantine where they live in isolation.

"While we do have a small number of EIA-positive horses being maintained under lifetime quarantine in the U.S., most EIA-positive horse owners elect to euthanize an infected horse," Pelzel says. "Given that there is currently no cure for the disease, that it is not often simple or inexpensive to keep a horse under lifetime quarantine, and that the horse must be kept isolated from other horses to prevent transmission, many times euthanasia is the more humane option for both the horse and the owner."

The stigma associated with having an EIA-positive horse also drives owners to elect to euthanize infected horses, she notes.

Vaccination Woes

We've all heard about scientists' attempts to develop an AIDS vaccine, but the endeavor has been largely unfruitful. In December 2010 Canadian researchers made a breakthrough and developed a vaccine worthy of the U.S. Food and Drug Administration's nod of approval, and it is currently being tested in humans.

For horses the only equivalent vaccine existed in China, where an EIA epidemic in the 1970s reportedly resulted in the slaughter of 424,000 equids. From this outbreak scientists developed a live attenuated vaccine, used successfully in millions of horses between 1975 and 1990 until China considered the disease "controlled" and discontinued the vaccine program.

"Because the policy in most countries is EIA surveillance where the goal is to detect and remove positive horses, vaccination has not been widely used as a means of disease control," explains Mealey.

Currently, no vaccines are approved for general clinical use in the States. Regardless, considerable research has been devoted to vaccine development, and different kinds of vaccines (e.g., inactivated, recombinant protein, subunit, and recombinant vector) have had variable results experimentally, and one experimental live attenuated vaccine has shown to be effective.

According to Mealey, "The major hurdle is that the viruses constantly mutate and can escape immune responses. Vaccines that elicit a good immune response to a particular strain of the virus won't protect against all the different mutations of that strain or against other strains, of which there are many.

"Right now it is pretty tough to get federal funding to study EIAV, but work on determining how the horse's immune system controls the virus is ongoing," Mealey adds. "The fact that EIA is not a huge problem right now in the U.S. makes vaccine development a relatively low priority. However, infected horses are present all over the world, and sporadic outbreaks in the U.S., Ireland, and other parts of Europe tell us that this virus still poses a significant risk to equine health and more research is needed."

Because EIAV is very similar to HIV, the results of EIAV immunity studies in the horse might also help researchers find ways to protect people against AIDS.

Take-Home Message

EIA is a real threat to horses the world over. And although the virus is not normally deadly, veterinary officials must monitor it vigilantly to ensure detection of cases remains sporadic.

"A safe and effective vaccine would be good to have in the arsenal," Mealey concludes. "Importantly, given the current control programs that are based on serologic tests, it would be critical to be able to distinguish a horse that tested positive because of vaccination from a horse that was actually infected with EIAV."

Until an effective vaccine is developed, testing is the best way to control EIA. Don't let your horse be one of the estimated 7 million that aren't tested. It is a quick, easy, inexpensive blood test that can be performed at any time to help prevent spread of this disease.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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