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Equine infectious anemia, commonly known by the abbreviation EIA, has been infecting horses in the United States for over 100 years. However, it was not commonly seen as a problem until the late '60s/early '70s when an outbreak of the disease was seen in the increasing thoroughbred racing industry. At this point, scientists and researchers began studying to find ways of controlling the virus.

EIA is a viral disease that is, to this point, untreatable. It operates by destroying blood cells and is related to HIV, which causes AIDS in humans. Also known as “swamp fever,” EIA appears to be most present in humid, wet areas. This is mostly because of the abundance of insect vectors (including deerflies and horseflies) in wet, swampy areas that carry the virus.

The transmission of EIA is most sufficient through these insect vectors. These insects are able to transfer large enough amounts of infected blood to transmit the disease from an infected horse to a healthy one. Also, since insect vectors have painful bites, their meals are often interrupted by their host, in which case the flies will move to another host to finish their meal; thus, the flies have the ability to spread the disease to a number of horses quite quickly. EIA can also be transferred by the use of contaminated syringes, needles, and surgical tools. The virus can also be transferred as easily as using tack on several horses that has been in contact with an abrasion of an infected horse.

EIA is able to produce three different types of infection: acute cases, chronic cases, and inapparent cases. In acute cases, the virus takes over and destroys red blood cells, which causes anemia and inflammation that causes organ damage. Other symptoms include high fever, depression, weight loss, and blood shot eyes. Acute cases often result in death; however, those horses that survive tend to become chronic cases. In chronic cases of EIA, horses remain in a continuous state of having non-life-threatening symptoms or have episodes of acute cases, which are easily brought on by heat and work-related stress. Inapparent cases are horses that are carrying the virus but show none of the symptoms and appear to be healthy, yet they are able to transmit the disease.

Extensive study and research began in the mid-1960s in order to find a way of identifying the virus in both sick and inapparent carriers of EIA. It was not until 1972 that a reliable test, developed by LeRoy Coggins, was invented. The Coggins test uses agar-gel-immunodiffusion methods in order to detect the presence of antibodies to the virus in a blood sample. Because of its reliability, the Coggins test is used as the standard test for all traveling horses.

Horses with EIA have the virus for life. There have been enough cases that have been traced back to inapparent carriers to make those horses a threat to the surrounding horse population. Because of this possible danger, all horses that have tested positive for EIA must be either humanely destroyed or branded with a special number and quarantined for life. The quarantine requires a 200-yard buffer zone between the infected horse and other horses. Special quarantine farms have been created to which positive horses can be sent in order to live the rest of their lives in the company of other horses.

The very strict controls on EIA-positive horses have been enforced for the past twenty-five years. Because of this control, EIA is no longer a serious threat to horses and horse owners in the United States. The numbers of recognized cases have fallen steadily in the past few years. Hopefully, they will continue to do so.