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General Information
EIA is a viral disease of members of the horse family. Identified in France in 1843 and first tentatively diagnosed in the United States in 1888, it has commanded a great deal of attention over the years.
 The EIAV is categorized as a retrovirus: it contains the genetic material RNA, which it uses to produce DNA. This DNA is then incorporated into the genetic makeup of infected cells.
EIA is significant historically because it is the first disease of horses proven to be caused by a "filterable virus"--one that can survive a special laboratory filtering procedure and remain infectious (Vallee and Carre 1904). EIA is the first retrovirus-induced disease proven to be transmitted by insects (Stein et al. 1942). And EIAV is the first persistent virus for which "antigenic drift" was defined (Kono 1972). (Antigenic drift is the virus' ability to change its form sufficiently so that it is no longer vulnerable to existing antibodies.) Finally, EIA is the first retrovirus-induced disease for which a diagnostic test was approved (Coggins and Norcross 1972).
More recently, the EIAV has been recognized as a lentivirus, the type that causes slowly progressive, often fatal diseases. It is a close relative to the human immunodeficiency virus (HIV), which cause acquired immunodeficiency syndrome (AIDS). In fact, EIAV was the first virus shown to be related to the HIV through cross-reaction in tests of blood serum (Montagnier et al. 1984). These two lentiviruses share many structural and biochemical features, and EIAV is thought to serve as a useful model for many aspects of HIV research, especially for discovery of common mechanisms of immunologic control (Montelaro and Issel 1990).
An effective test for antibodies specific to EIAV was described in 1970 by Leroy Coggins, D.V.M., and collaborators (Coggins and Patten 1970) and was rapidly adopted by authorities around the world. The agar-gel immunodiffusion (AGID) or Coggins test was shown to correlate with horse inoculation test results for EIAV and, therefore, could be used to identify EIAV carriers (Coggins et al. 1972). Because only members of the horse family were shown to be infected, programs based on serologic testing were designed and adopted to help control the spread of EIAV (Campbell 1971). Since 1972, more than 15 million blood samples have been collected from horses in the United States and tested for antibodies against EIAV (Cordes, pers. commun.), partially in response to State, Federal, and/or international regulations concerning EIA.
 In the United States, the percentage of samples with positive results has decreased dramatically from over 3 percent to less than 0.2 percent since testing was initiated in 1972 (Cordes, pers. commun.). Since 1978, 92 percent of the test-positive samples have originated from horses located in what is referred to as the "hot zone" (fig. 1). The risk of becoming infected with EIAV is greatest in this region, in part because environmental conditions are ideal for the insect vectors that transmit the virus, and, presumably, because a significant number of untested reservoirs of EIAV exist. The threat of exposure to EIAV-infected horses will continue to exist wherever horses congregate.
Although this document is not intended to be an exhaustive review, it is designed to introduce the database on EIAV, the infection and the disease it causes, and programs designed to control its spread. We hope that this document will augment the information published previously by USDA staff (Hourrigan and Knowles 1974), Equus magazine (Equine Health Publications 1977), the American Quarter Horse Association (Tashjian 1985), and the American Association of Equine Practitioners and Bayer Corporation (1995), and serve as a catalyst for additional progress in the international control of this infectious disease.
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