Equine infectious anemia (EIA) is a rare disease but one that a lot of time, effort and money are put into avoiding. Most people know about this disease by way of the most common test (formerly) used to diagnose it: the Coggin’s test (see image).
Equine infectious anemia is caused by a virus of the same name ( equine infectiious anemia virus, EIAV). EIAV is a lentivirus which, like all other lentiviruses, causes persistent infection. Unlike most other lentiviruses that cause slow, gradual preogression of disease, EIAV infection usually causes a sudden onset of disease (acute phase) followed by recurrent disease. After the initial (acute) phase, horses can appear normal, which means they can be a silent reservoir of the disease. That’s why routine testing for EIAV is required in many circumstances: to detect silent carriers so they cannot continue to transmit infection.
Signs of acute EIA vary, but usually include fever, lethargy and decreased appetite. Anemia (decreased red blood cell count) and thrombocytopenia (decreased platelet count) can be detected. Anemia is more common and pronounced with recurrent infections. Intermittent illness often develops after the first acute episode. Affected horses may experience short (3-5 day) periods of fever, lethargy and decreased appetite. The severity of the anemia often correlates with the severity and frequency of these disease episodes. In some horses, these episodes are very common, long and severe, and these horses often have severe weight loss and anemia. However, most infected horses stop developing obvious signs of disease after a year and seem perfectly healthy. This may be good for the one horse, but it’s bad for other horses in the area to which the virus may be transmitted.
EIAV is a bloodborne virus that can be transmitted by blood-feeding insects, especially tabanid flies (horseflies, deerflies). Stable flies can also transmit EIAV but do so less effectively. Contaminated medical supplies such as reused needles and syringes can also transmit EIAV.
Fortunately, EIA is now rare in most regions and positive tests are quite uncommon. Routine testing for EIA is usually required for shows, sales, transportation and other situations where horses are mixed, in order to detect and remove carriers. Unfortunately, identification of a horse as a carrier is not good for the horse or owner (or other horses in the area) – EIA is not treatable, and horses that have positive tests ("reactors") are quarantined, as are all other horses that are housed within 200 yards. Horses living close to reactors are tested (usually 30 and 60 days after removal of the reactor) and only released from quarantine after getting negative test results at least 60 days after the last reactor was removed. Reactors are usually euthanized. If prompt euthanasia is not chosen, reactors are usually prominently branded or tattooed. They must be kept under quarantine for the rest of their lives, and at least 200 yards from other horses. (200 yards is used because the flies that transmit the disease don’t usually travel that far.)
The best way to reduce the risk of EIA is ensuring that all horses are regularly tested. New horses coming onto a farm must be tested BEFORE arrival or after arrival but while in quarantine at least 200 yards away from other horses. Needles and other items that might be contaminated with blood should never be re-used, both due to the EIA risk and to avoid other potential problems.
Overall, EIA is a very rare disease, but the severe implications of a positive test mean that we need to be vigilant.
Image from http://www.aht.org.uk/science_eia.html