Two of the five horses have been euthanized because of their condition. The three other animals are under observation and quarantine.
The virus is not transmissible to people.
EHV-1 can affect a horse's reproductive, respiratory and nervous systems and can lead to death. The highly contagious disease also can spread rapidly among horses through the air, nose-to-nose contact, contaminated equipment, clothing, and human hands.
"Don't let your horses touch other horses, especially nose to nose," said state veterinarian Bruce King. "Isolate horses that return to the farm from a show or event."
Horse owners also are advised to avoid unnecessary contact with possibly infected horses, and to quickly report symptoms to their veterinarian.
Statewide, no other horses have shown signs of EHV-1, said King.
Symptoms include fever, decreased coordination, nasal discharge, urine dribbling, loss of tail tone, hind limb weakness, leaning against a wall or fence to maintain balance, lethargy and the inability to rise. While there is no cure, the symptoms of the disease may be treatable.