Acute responses generally occur 7 days to 30 days after initial exposure. Horses may develop fever, go off feed, or die suddenly. Eventually, the virus destroys red blood cells, causing severe anemia. Equine in the acute stage may test negative for 16 days to 42 days after exposure, until the animal's immune system has had time to produce detectable antibodies. Symptoms will subside, but may reoccur.
Acute episodes can emerge after stress from hard work, hot weather, pregnancy or other disease.
Inapparent infections exhibit no outward symptoms, yet produce positive tests. However, if these horses are stressed, symptoms can appear, and the animal's weakened immune system may not fight off other diseases. These horses pose the greatest danger of spreading EIA at events or gatherings, because no one suspects the animal has a health problem. No cure or vaccine for EIA is known. Infected horses produce antibodies, which will not rid the body of the EIA virus.