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Horses vulnerable to disease threat
By Laura Whelan
MISSISSIPPI STATE -- Although Equine Infectious Anemia is not a new threat to horse health, its potentially deadly consequences and lack of a cure make it an especially harmful risk.
"EIA has commanded a great deal of attention over the years," said Dr. Stanley Robertson, Mississippi State University Extension Service veterinarian. "This disease has no vaccine, treatment or cure, and it is often difficult to differentiate it from other fever producing diseases, like anthrax, influenza and equine encephalitis."
Robertson said EIA is most frequently transmitted between horses by large biting insects like horseflies and deerflies. EIA is a retrovirus, producing DNA that becomes part of the genetic makeup of infected cells. As biting flies travel from one horse to another, they transmit the virus through any infected material they picked up from an EIA carrier. The disease can be diagnosed by the Coggins test, which identifies antibodies specific to the EIA virus.
In the acute form of EIA, horses can die within two to three weeks.
"The acute form of the disease is the most damaging and the most difficult to diagnose because the signs appear rapidly, and often an elevated body temperature is the only symptom," Robertson said.
The disease also can appear in chronic form, meaning the horse survives the first acute bout, but may develop recurring clinical disease. Symptoms include high fever, hemorrhages on the mucous membranes, depression, weight loss, swelling of the legs and underbody surfaces, and anemia.
However, Robertson said most horses with EIA show few symptoms.
"The majority of infected horses are unapparent carriers that show no overt clinical abnormalities as a result of infection," he said. "They survive as reservoirs of the infection for extended periods and are considered virus carriers for life."
Veterinarians or horse owners suspecting EIA in an animal should contact state or federal animal health authorities immediately.
Horse owners can take several precautions to reduce the risk of infection. Robertson suggested using disposable syringes and needles, cleaning and sterilizing all instruments thoroughly after each use, and keeping stables sanitary by removing debris and manure promptly. Control insects with the help of a veterinarian and avoid placing horses in favorable insect habitats.
Robertson said it is important not to intermingle infected and healthy animals or breed EIA-positive horses. Isolate all new horses until they have been tested, and owners must comply with state laws requiring certification of negative EIA tests.
Robertson said one horse in Mississippi is known to have tested positive for EIA this year; in 2002, 11 horses tested positive; and in 2001, 13 horses tested positive. Approximately 40,000 horses have been tested for EIA each year in Mississippi for the last three years.
The impact of EIA on horse owners is greater than just dealing with the disease itself; they also must comply with EIA regulations regarding testing and the disposal of positive animals. According to the EIA Law (House Bill 539), all horses in the state where the public participates in equine activities must be accompanied by the original copy of a current negative EIA test. Any horses moving within the state or sold at public or private sales also must have proof of a negative test.
"EIA is a serious problem because there is no vaccine or treatment for the disease. If an animal tests positive on the Coggins test, the owner has two options -- the horse is either quarantined or destroyed," Robertson said.
Since 1978, 92 percent of positive EIA tests in the United States have originated from a region known as the "hot zone" because of the warm environmental conditions that are ideal for insects that transmit EIA. Mississippi is included in the hot zone.
"But overall statistics indicate that the number of tests positive for EIA in the United States has decreased greatly, from 3 percent in 1972 to 0.10 percent today," Robertson said.
It is difficult to produce a vaccine for this deadly disease because the virus can vary its protein makeup, making it difficult to produce an antibody that will fight off every form of the disease, Robertson said.
"For an EIA vaccine to be effective, it would have to protect the horse against all the different types of EIA viruses," he said. "Research is currently under way to develop a vaccine and, hopefully, one day a successful cure will be developed."