EEE, WEE, SLEV, and VEE are viruses carried by wild birds and transmitted by mosquitoes to horses and humans. Symptoms in horses can range from no signs of disease to depression, trouble walking or paralysis, convulsions, or other neurologic signs and death. Horses are dead-end hosts to EEE, WEE, and SLEV, which means that they usually do not spread these viruses to other animals or humans. Horses with a certain type of VEE infection, however, may be able to spread the virus through mosquitos.
People develop these diseases by being bitten by an infected mosquito. Most people have no symptoms, but for those who become sick, symptoms can range from mild flu-like symptoms to arthritis, respiratory distress, gastrointestinal upset, encephalitis, shock, or death. Very young or elderly patients are more likely to develop severe infections.
Human cases of SLEV have decreased from 13 cases in 2008 to 3 cases in 2012. Sporadic cases of EEE were reported in horses in 2012, but no cases of WEE were reported. VEE has not been seen in the United States since 1971.