This phase may last several days and is the stage during which the horse is most likely to transmit the disease to nearby horses. The second phase is characterized by weight loss, recurring fevers and general weakness. Anemia is likely to be present, and mares can abort during this stage. If horses survive the first two stages, they enter the final or chronic stage, where they often appear normal. An owner may report that a horse is a poor keeper, and the animal may be mildly anemic.
Infected mares can transmit the disease to their foals.
A horse infected with the EIA virus will be a carrier of the disease for life. Although horses in the chronic stage appear normal, they become ill again if subjected to stress, such as shipping or severe weather. Carriers pose a health threat to the equine community. Since the early 1960's, several outbreaks of EIA have occurred at either race tracks or large breeding farms, resulting in the deaths of many horses. The Coggins test, developed in the 1960's by Dr. Leroy Coggins, is based on the detection of antibodies in the horse's blood.