The virus has a life cycle that mainly involves wild birds. Remember it was a veterinarian who discovered West Nile Virus in crows in New York City back in 1999. Mosquitoes pick up the virus when they feed on an infected bird and then transfer the virus to horses, or humans, when they bite them. These viruses cannot live outside of a host and do not survive in the environment for long and the virus does not exist in high enough levels in the horse’s blood stream to transmit to other mosquitoes, horses or humans. Therefore isolation of infected cases is not required.
EEE, WEE and WNV all act very similarly in the horse. The virus replicates in lymph nodes and infects the neurons in the brain. Clinical signs can appear anywhere from 2-3 days to as long as 3 weeks after infection. Signs may include: fever, depression, loss of appetite or neurologic signs (incoordination, staggering, hyperexitable, head pressing, leaning on walls, low head carriage, drooping lips/tongue or recumbency). Some horses may have intense puritis, or itchiness. Many horses infected with West Nile Virus will display very fine fasciculations of the muzzle that are often only noticed by seeing their whiskers quiver. The course of the disease is between 2 to 14 days and fatality rates range from 33% with WNV, to 50% with WEE and up to a staggering 90% with EEE. The only treatment is supportive care (IV fluids/nutrition, anti-inflammatories and corticosteroids). Horses that are lucky enough to recover may have residual neurologic deficits.
So with the early onset of the warm weather, and hatching of billions upon billions of mosquito eggs, I would strongly recommend moving up your horse’s vaccination schedule to cover them against these devastating diseases as soon as you can. The bugs aren’t going to wait to bite your horse just because you’re not due for your spring shots till April!
And don't forget about Fido and Fluffy! While dogs and cats rarely get these viral encephalidities, mosquitoes carry heartworm disease and the warm weather will bring out other undesirable critters like fleas and ticks which carry a whole set of diseases of their own like tape worms and Lyme Disease. So, if you take a break from your flea/tick/heartworm prevention in the winter, now is the time to start back up again!!