Equine Infectious Anemia (EIA), a blood-borne virus that attacks the immune system of horses, mules and donkeys, is sometimes fatal, but not always. Once a horse is infected, however, it becomes a lifelong carrier of the virus.
Related to the human immunodeficiency virus (HIV), which causes AIDS in humans, the EIA virus reproduces in the body’s white blood cells – those responsible for defending the body against infection. Parts of the virus attach to red blood cells, which are then attacked by the immune system. Anemia eventually results – a condition in which the body does not have enough healthy red blood cells to provide adequate levels of oxygen to body tissues.
The disease was first identified in France in the mid-1800s and diagnosed in the United States in 1888, where it took on the still often-used label of Swamp Fever because it was mistakenly believed the virus only occurred in the wet, humid regions of the southern United States. EIA is now found worldwide, including Canada where its presence is felt mainly in the western provinces.
Disease Stages
The EIA virus is transmitted primarily by biting insects such as horse and deer flies. Horses become infected when the insect’s feeding is interrupted – by a swishing tail or other movement, perhaps initiated by the pain of the bite or sting. The fly moves on to another horse with the virus still on the mouthpart. The virus does not live long on the mouthpart, and flies don’t travel long distances, therefore, EIA is more likely to be spread among horses within close proximity. “The fly isn’t going to bite one horse and then go a mile down the road and bite another,” said Dr. Wendy Wilkins, disease services veterinarian with the Saskatchewan Ministry of Agriculture.
Less commonly, the EIA virus can be passed by re-used needles and syringes and other blood-contaminated equipment. “Back in the ‘70s and ‘80s, when we were using one needle for 20 horses, people were largely responsible for spreading it. But people don’t do that anymore. Now it’s the flies,” said Wilkins.
The incubation period generally lasts from two to four weeks following infection. There are three general stages to the disease – acute, chronic and inapparent.
In the acute stage, the virus vigorously multiplies in the animal’s bloodstream. Horses can die within a couple of weeks of exposure, but most survive. The horse runs a sudden high fever, has decreased appetite and depression as well as a loss of condition that can lead to weakness, incoordination and an inability to stand. Other symptoms include jaundice, the appearance of tiny red spots on the mucous membranes, and swelling in the legs, under the chest and other areas.
At this point, EIA can be difficult to diagnose because it can take anywhere from seven days to more than a month for antibodies (immune system proteins that target and attack bacteria and viruses) to be detected, rendering diagnostic tests ineffective.
Symptoms in the chronic stage vary from horse to horse, but high concentrations of the virus are still in the blood. The animal can alternate between periods of illness and health.
Typically, by eight months to a year after infection, horses become inapparent carriers that do not show clinical signs, and the concentration of the virus in their body is lower than horses with clinical or acute signs. The level of virus can increase when the body is under stress or the immune system is strained, as it begins to replicate and symptoms appear.
Estimates say that 90 per cent of infected horses, however, are inapparent carriers with no discernible symptoms. And, in some cases, horses are infected, but never show symptoms. This is where the inherent danger of EIA exists – carriers are able to move within the horse population and are a source of infection for other horses. There is really no way of determining how many horses are affected or where, except through testing protocols.
Testing for EIA
Most horse owners have heard of the Coggins test, which identifies EIA antibodies in a horse’s blood. Proof of a negative Coggins test is often required before horses are allowed on a property or show facility, prior to a horse’s purchase or when travelling across borders.
Prior to the test’s invention by Dr. Leroy Coggins of Cornell State University in 1970, EIA was diagnosed solely by clinical signs. Development of the Coggins led to EIA control programs across North America, including Canada where, in 1971, EIA was designated a reportable disease under the Canadian Health of Animals Act.
Under Canada’s current EIA control program, owners voluntarily pay to have their horses tested. All suspected cases must be reported to the Canadian Food Inspection Agency (CFIA), which has legislated control and reporting measures for the disease, and will respond by implementing control measures.
The CFIA restricts the movement of equines on farms where there are suspected EIA-infected horses. Quarantines are implemented and all equines that have been in contact with the suspect horse within the previous 30 days are tested. Test-positive horses are retested and those with clinical signs are ordered destroyed. Horses that are confirmed to be infected, whether they show clinical signs or not, are ordered to be euthanized or put in permanent quarantine in a fly-proof facility. They are permanently identified with a brand or tattoo. Owners receive up to $2,000 in compensation for horses destroyed.
While the Coggins remains the leading diagnostic test, c-ELISA (competitive enzyme-linked immunosorbent assay) tests are becoming more prevalent. They can offer results in an hour, rather than the Coggins’ 24-hour turnaround. The CFIA has used c-ELISA tests for surveillance testing since 2006.
Canada’s Control Program
In 2010, there were 23 positive tests out of more than 52,000 animals tested: 14 in BC, seven in Alberta and two in Quebec. But, in 2011, Canada saw a spike in in equine infectious anemia cases with a total of 179. The outbreak hit Saskatchewan particularly hard with more than 90 horses ordered to be destroyed. From 2005 to 2010, the province had only four cases, all of which were detected in 2009. And, to kick-off this year, by the end of January, equine infectious anemia had been found in three northern Saskatchewan premises (although these were facilities that had already been confirmed to be infected in 2011). “In my opinion it’s a combination of two factors,” said Wilkins. “From what I understand, the last two years in a row, we’ve had perfect weather conditions for the reproduction of the biting flies, which primarily spread the disease.”
An entomologist (a zoologist who specializes in insects) explained to Wilkins that horse and deer flies prefer conditions such as those found in the areas along the “at-risk” swampy forest ridge areas of the western provinces’ northern regions – where the majority of horses tested positive. “It took two years and now we have a population explosion of these insects,” she noted.
Wilkins also attributes EIA’s spread to the “lack of testing and the complacency of Saskatchewan horse owners.” The province had fewer than 1,200 voluntary tests done in 2010 out of a population of more than 100,000 horses. “It’s been steadily decreasing since then and, in the last three years in particular it’s reached its lowest point,” she said. “I’m hop-ing we’ve reversed this trend now.” Wilkins is referring to publicity and promotion under-taken by the CFIA, the Saskatchewan Horse Federation and the Ministry of Agriculture to make owners aware of the disease and encour-age them to have their horses tested.
In fact, the increased numbers nationwide has also prompted the CFIA in conjunction with Equine Canada’s Health and Welfare Committee to undertake a review of the coun-try’s EIA control program. “Equine Canada strongly recommends that all horse owners have their horses tested regularly for EIA,” stated an Equine Canada bulletin issued in September 2011. “Horses should be tested whenever there is potential for them to be cohabitating and intermingling with other horses. This includes, but is not limited to, boarding and breeding stables, race tracks, horse shows and other events where horses come together for competitive or recreational purposes.”
Both agencies have solicited feedback from the public and industry stakeholders on the proposed changes and a review will be re-leased soon.
Prevention
There is no real treatment for equine infectious anemia. Establishing biosecurity plans for equine facili-ties will, at least, help minimize risk. “First and foremost have your horses tested,” said Wilkins. “And that means all your horses. If you have a dozen horses, but you only show one or two, it doesn’t make sense just to test the ones that are leaving the farm. It’s really important to test the ones that are on the farm too.” Test horses yearly, more often in high-risk zones.
Wilkins also suggests that when an owner takes horses to shows or events, they should ensure that the other horses are EIA-free as well. “Nowhere up to this point in Saskatchewan did any event organizers require EIA testing. I really encourage horse owners to encourage their organizations to make that a requirement, so they know that every horse is safe at that event.”
Consider all horses EIA carriers until they test negative. Isolate all new horses to a farm for at least 45 days, checking their temperature daily to check for fever and keeping an eye out for other signs of illness.
Wilkins also recommends reducing the fly population by keeping facilities clean, draining pastures and eliminating swampy, fly-breeding areas.
A point Wilkins wants to stress to horse owners is that, although they should be diligent and aware of EIA, “We don’t want people to panic. It’s not a fast-moving disease. And certainly no reason to be cancelling horse events this spring and summer.”
HIV Vaccination Connection
The development of an Equine Infectious Anemia (EIA) vaccine could depend on ongoing research to arrest the human immunodeficiency virus (HIV).
Both EIA and HIV are lentiviruses, those characterized by a long incubation period and a persistent, progressive decline, potentially leading to death. In fact, when HIV was discovered in 1983, it was classified as a lentivirus in large part because of its similarity to the EIA virus.
A report entitled Equine Infectious Anemia Virus (EIAV): what has HIV’s country cousin got to tell us?, published in the July/August 2004 edition of the journal, Veterinary Research, states that EIA virus studies over the past five decades have produced “interesting results” on the control and nature of lentiviruses. “These studies are of interest in the context of HIV and efforts to develop a vaccine.”
As one 2009 study published by the University of Pittsburg’s Center for Vaccine Research, in the journal Retrovirology notes, “Given EIAV’s role as an animal model for HIV vaccine studies, the associated costs of equine testing and the general issue of equine health, the development of an effective EIAV vaccine holds a multifaceted significance.”
In part, what makes the equine infectious anemia virus so interesting to HIV researchers is that it is much faster developing and more highly mutatable than other lentiviruses. It is very good at evading detection by the immune system.
More than 90 per cent of infected horses will progress from a chronic disease state to inapparent carrier status (horses with the disease that don’t show clinical signs). When under stress or the immune system is under duress, these horses can begin to develop symptoms as the virus starts to grow again, adapt and become more prevalent within the body. Many researchers consider this one of the keys to lentivirus vaccine development.
The virus-specific immune responses in inapparent carriers “appears to offer the enduring broadly protective immunity against virus exposure that has been the elusive goal of AIDS vaccine research for the past 30 years,” said a 2011 paper out of the Center for Vaccine Research at the University of Pittsburg, called Equine Infectious Anemia Virus Infection and Immunity: Lessons for AIDS Vaccine Development.
In 1983, Chinese scientists from the Harbin Veterinary Research Institute announced it had developed a vaccine, in response to an outbreak of the disease that had begun in the 1960s and reportedly the country was able to eradicate equine infectious anemia through mandatory vaccination. There are no recent statistics available to support this, however. Researchers at Harbin Veterinary Research Institute are apparently continuing to develop a vaccine, mainly as it relates to HIV, however.
Research facilities around the world, including Washington State University College of Veterinary Medicine, University of Pittsburgh and Maxwell H. Gluck Equine Research Center at the University of Kentucky, continue to work on developing EIA vaccines for use in North America. Were the vaccine used in China available here, it wouldn’t work on North American strains of the virus because EIA is highly mutable.
But, there are standing difficulties in finding a vaccine. Being live-attenuated vaccine, or a weakened form of the organism that causes the disease, horses that are given the vaccine will test positive for the disease for the rest of their lives. How, therefore, would positive results caused by a vaccine be distinguished from naturally occurring equine infectious anemia? Is there a possibility that the attenuated vaccine would become virulent based on the virus’s ability to mutate and replicate?
Seemingly, there are many questions to be answered and much research to be done before EIA, HIV and other lentiviruses can be successfully vaccinated against.
