Rescuers speculated that the man rode his horse too close to the edge of the wooden bridge that had low railings and fell over the side with the horse. Horse Dies of Eastern Equine Encephalitis in Virginia State agricultural officials in Maryland were urging horse owners there to vaccinate their horses for Eastern Equine Encephalitis the Baltimore Sun reported in its September 3 edition. An unvaccinated horse died of the disease on Virginia's Eastern Shore. The disease is mosquito-borne. Because of the particularly wet summer, there has been an abundance of mosquitoes to carry the virus.
People can also catch the disease, although that is rare.
When humans do contract the disease, it is usually fatal.