by Sally Colby

Horse owners who board, compete in shows, participate in group trail rides or moved a horse from one state to another are familiar with an important piece of paper known as the Coggins test — proof that the horse has tested negative for equine infectious anemia, or EIA.

Dr. Angela Pelzel-McCluskey, veterinarian and equine epidemiologist for the USDA/APHIS veterinary services, said the U.S. has had an EIA control program in place since the 1970s, which is when Dr. Leroy Coggins developed the test to identify horses with EIA. “We’ve gained a lot of ground against the disease,” said Pelzel-McCluskey, “but most people in the equine industry don’t know much about the disease or why they’re testing their horses for it.”

There are several acceptable methods for the Coggins test. One is the agar gel immunodiffusion test (AGID) which involves placing blood on clear gel to elicit a reaction to an antigen for the EIA virus. The other test is the enzyme-linked immunosorbant assay, (ELISA) which is faster and easier. Both are licensed and approved for use in the U.S.

The reason for testing for EIA is simple: there’s no cure and the horse is infective for life. EIA is a blood-borne retrovirus, in the same class as HIV that causes AIDs. Many horse owners believe mosquitoes spread the disease, but Pelzel-McCluskey says mosquitoes are not capable of transmitting the EIA virus. “Vectors that transmit EIA are certain biting flies,” she said. “Horse flies, stable flies and deer flies are the most common transmitters. They have spongy mouthparts that hold the virus and blood so it can be transmitted from horse to horse very quickly. They also have a very strong and painful bite. In response to the painful bite, the horse tries to swat the fly or move. The fly then goes to another nearby horse and transmits the infection via blood on its mouthparts.” If one positive horse is introduced to a group of horses, the disease spreads, sometimes over many years, and a large proportion of the herd can become infected by natural fly transmission.

Pelzel-McCluskey said a horse that survives the initial stage of infection is infective for life and can be a reservoir for infection for other horses. Owners’ options are limited, and the choice is often euthanasia. The alternative is permanent quarantine of the infected horse, at least 200 yards away from any non-infected horses, along with vector control to manage biting flies. Pelzel-McCluskey said the 200-yard rule is the result of research to calculate how far flying vectors can travel with the virus on mouthparts with the virus still remaining transmissible. State and federal animal health authorities check the premises of EIA positive horses for compliance.

The clinical signs of EIA are non-specific and can vary widely among horses. Clinical signs in acute cases may include fever, anemia, jaundice, depression, lymph swelling, generalized weight loss, weakness and decreased appetite; which are also signs for many other diseases. Some horses will develop severe anemia and die suddenly.

The incubation period for EIA — the time between exposure and signs of illness or a positive test — is highly variable and ranges from one week to 45 or more days after exposure. Exposed horses may not test positive immediately because the test is for antibodies generated by the horse’s immune system after it has ‘seen’ the virus for a certain period of time. For negative horses exposed to a positive horse, there’s a 60-day wait to maximize the incubation period and minimize the risk of missing a true positive.

Today, thanks to testing, the actual incidence of EIA is extremely low. Pelzel-McCluskey said in the 1980s, about 4,000 cases of EIA were identified in the U.S. annually. Testing and management has reduced that figure to fewer than 100 cases every year. “Currently, our U.S. prevalence is down to 0.004 percent of the population,” said Pelzel-McCluskey. “Last year we had only 46 cases, and we tested one and one half million horses.”

However, recent reports of EIA infection in racing Quarter Horses in 13 states are cause for concern. Pelzel-McCluskey said in 2018, at least 46 EIA-positive horses were identified; 33 of which were Quarter Horse race horses. These horses likely acquired EIA through iatrogenic transmission, which is the transmission of a disease in which humans are involved, usually through contaminated equipment.

“A person is unknowingly transmitting the disease between horses using equipment that’s contaminated with blood from a positive horse,” said Pelzel-McCluskey. “That equipment is reused needles and syringes, reused intravenous lines, or double dipping into a multi-dose drug vial that’s contaminated from blood from the horse.”

For the remaining positive horses in 2018, the mode of infection was undetermined or they were untested, or they came from undertested populations in which a whole herd is not tested and not protected from biting flies.

Pelzel-McCluskey said EIA is also spread in the unsanctioned Quarter Horse racing industry, also known as bush tracking, when blood is transfused from one horse to another to gain a competitive advantage. The sanctioned race industry is helping to clean up the problem by identifying cases of EIA and requiring horses to have a negative Coggins test prior to arriving at the track. In addition, some bush track racing participants have learned they may be purchasing infected horses and are having horses tested.

The EIA virus has been isolated from the semen of infected stallions, and Pelzel-McCluskey said the breeding of EIA positive horses in the U.S. is strictly prohibited. The virus can also be transmitted from mare to foal in utero.

Pelzel-McCluskey says most testing is state-mandated, with states requiring a Coggins test prior to entry. Some states have their own regulations for change of ownership or for assembly points such as a sale, show or other event. Individual events usually require a negative Coggins test within a certain time period, usually one year.

Most states require a negative Coggins within the past month for state-to-state movement. “That’s an arbitrary thing,” said Pelzel-McCluskey, adding that some states have a six-month requirement. “It was to reduce the burden on the equine industry so they wouldn’t test so frequently yet try to get some sort of recent test before the horse enters that state. We’re trying to walk the fine line between ‘does the horse have recent exposure; does it have recent evidence that it tests negative? We don’t want to wait several years before the horse is retested to prove it really is free of the disease.”

If horses don’t travel, aren’t exposed to other horses and have a history of negative EIA status, are they at risk of contracting EIA? “You still might have neighbors who may not properly test or manage their horses,” said Pelzel-McCluskey. “Work with your veterinarian to determine whether you can back off on (testing) based on actual risk.”