Equine infectious anemia (EIA) is a viral disease in horses caused by a lentivirus, the same group of viruses that causes AIDS (acquired immune deficiency syndrome) in people, and immune-deficiency disease in cows and cats.
Like AIDS, the horse won’t get EIA by ”casual contact” with infected horses. The greatest risk is with exposure to blood when there is a break in the skin. Most horses become infected when a large biting insect feeds on an infected animal, has residual blood on its mouth and bites an uninfected animal within four hours. The association with biting insects is where the disease got the name ”Swamp Fever.” Sharing of needles between horses is another possible route for spreading the infection.
The incubation period for EIA is about 30 days on the average, with a range of 14 to 40 days. Symptoms of the infection range from a short-lived fever to a full-blown acute disease with high fever, petecchial hemorrhages (small red dots) on the mucus membranes, anemia, jaundice, weakness and edema of the belly and/or legs. In some cases, the horse may die during this acute phase.
Survivors of the acute phase will either become inapparent carriers or develop a chronic disease characterized by slow wasting and cycling fevers. They may appear healthy between attacks of acute symptoms but lose condition, have low energy, anemia and often low platelet counts.
Inapparent carriers are just that — horses carrying the virus but showing no symptoms. These horses may go their entire lives without having EIA-related problems but could be pushed into showing symptoms by any significant stress.
What the Virus Does
Although anemia is a symptom, the virus doesn’t directly attack the red blood cells. It prefers a form of white blood cells. Early in the infection, the virus sets up shop in these cells located in the horse’s spleen.
The appearance of symptoms coincides with immune system activation. This virus triggers a reaction called the ”complement cascade.” This is similar to what happens in horses with strangles that develop the complication of Purpura Hemorrhagica. Circulating complexes of viral antigen and the horse’s antibodies activate the complement system. Cells coated by these complexes, including red cells, are targeted for destruction. Blood clotting is activated, producing the hemorrhages on the mucus membranes and the potential for hemorrhage in the GI tract, urinary tract or nosebleeds. The antigen antibody complexes can also precipitate in the kidneys, causing kidney damage.
If the horse survives, the disease symptoms go into remission when the immune system has established sufficient control. By this time, however, the virus is residing in virtually every tissue in the horse’s body, including the brain.
EIA viruses mutate frequently. Every time the mutation causes a sufficient different virus, the immune system has to start all over again. To further complicate things, the mutation may be equally, more or less aggressive, and this in turn determines whether or not the horse will show symptoms on the outside of the battle being waged on the inside.
Apparently Healthy Carriers
Like HIV (human immunodeficiency virus, which eventually causes AIDS), once a horse is infected with this virus they will have the virus in their body for life. The infection can never be cleared.
During the acute phases of EIA, just 1 cc of blood from the horse contains enough virus to infect a million horses. When chronically ill with EIA, this drops to 10,000. Horses that have no symptoms, however, are unlikely to transmit the disease by natural pathways of biting insects. The chance of that occurring may be 1 in 6 million. So why are healthy but Coggins-positive horses required to be euthanized or kept at least 200 yards away from negative horses’
We discuss this question in our interview with Dr. Charles Issel on page 15. (Note: It has been determined that horseflies interrupted when they’re feeding may try to finish their meal on another horse if it’s less than 200 yards away but are more likely to try their luck again with the first horse if no other horses are that close by.)
Basically, blood from these horses still contains virus and can cause disease in another horse, even if the carrier has shown no symptoms for a while. Also, when a healthy carrier is give an immunosuppressive drug, this releases the immune-system control on the infection and the level of circulating virus can rise quickly, making the horse a much more significant danger to other horses via biting fly spread. There are many things in the lifetime of a horse that are known to have a negative impact on the immune system, including long trips, severe injuries, surgery, heavy exercises or the stress of battling other infections.
Some evidence suggests that horses that have been carrying this virus for a prolonged period of time without showing symptoms may be harboring mutations with little-or-no potential to cause disease. In other words, it’s possible that not all apparently healthy carriers pose the same threat to the general population. Unfortunately, there’s currently no way to reliably identify those horses. Even if there was, we don’t know if the virus could still revert to a more dangerous strain if the horse’s immune response was weakened, or could mutate back to a dangerous strain on its own.
For these reasons, the experts still advise that any Coggins-positive horse should be isolated from noninfected horses.
EIA In The United States
According to the 2005 National Animal Health Monitoring System’s survey of horse owners, only 37.6% of the survey participants’ horses were Coggins tested. This makes it difficult to say with certainty how many horses are carrying this virus. However, testing statistics are available at the USDA web site(http://www.aphis.usda.gov/vs/nahps/equine/eia/).
There’s no question that testing and removing positive horses from the general population works — at least for the population of horses being tested. That’s the catch. Horse owners are unlikely to test routinely. They test when they are required to do so — for shows, fairs, racing, shipping, sales or to enter facilities that require a negative test. The same negative horses are often tested year after year, making the statistics potentially look more rosy than they truly are. Despite this, testing and isolation or euthanasia of EIA-positive horses has reduced the risk of EIA, at least for horses moving within populations where testing is done.
Today, when EIA outbreaks occur, it’s usually because horses come in contact with horses from groups that are not routinely tested, such as wild horses mingling with domesticated ones in the western states, or horses moved from farms or camps that don’t test into the general population.
Where Do We Go From Here’
A tremendous amount of money is spent on Coggins testing every year, but compliance is a small price to pay because you are also assured that the horses your horse comes in contact with are tested. The system does have cracks, since a negative test today doesn’t guarantee the horse will remain negative until the next test.
The only sensible way to further lower the risk of horses catching EIA, or even eradicate the threat of EIA, is by more testing — catching the horses that are currently flying under the radar. Laws requiring mandatory testing vary widely from state to state, but aggressive laws lead to steady declines in the number of positives. Measures such as requiring Coggins testing of all horses that go through sales, all horses changin g hands privately, and all horses that leave the home farm for any reason will get us closer to eliminating this disease.