Keeping tabs on Equine Infectious Anemia
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.”
Veterinarians and researchers are helping control spread of the disease.
The EIA retrovirus is primarily spread by biting insects such as horseflies and deer flies. The horse’s defensive reaction to the bite interrupts a fly’s feeding activity, sending it on to a nearby horse to finish feeding.
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.
“Most of the time, the disease can go unnoticed, which is why testing is so important,” Mealey explains.
Current APHIS statistics indicate the number of EIA reactors (affected horses) in the United States is low.
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 percent in 1972 to less than 0.003 percent in 2010. It is important to note, however, that these figures do not necessarily reflect the EIA prevalence in the general horse population.
“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.”
“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.
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.
An EIA epidemic in China 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.
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 seven 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. F
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As a routine management matter, the Teddy Roosevelt National Park plans to remove a few horses from its herd.