This disease can also cause chronic weight loss. There is also a significant decrease in the horse's performance level. EIA can be passed from horse to horse by mosquitoes and biting flies, especially horse flies. Needles contaminated with infected blood are also a source of infection. Once infected, the horse may show signs of the disease one week to one month later.
Some infected horses show no signs and become chronic carriers of the disease.
Carrier horses that become stressed will often show signs of the disease. Since EIA is caused by a virus, there is no drug that will directly treat it. About the only thing veterinarians and owners can do is give supportive therapy to bring down the fever and keep the horse comfortable. Most infected horses are euthanized because they will shed the virus during periods of high fever. If the owner chooses to keep an infected horse, the animal must be kept in strict isolation, including insect-proof screening to avoid transmission to other horses.