West Nile Virus is transmitted by mosquitoes. Horses are considered “dead-end hosts” meaning, they are not capable of spreading the disease to other animals. A horse must be bitten by an infected mosquito to get the disease. The incubation period is 6-10 days.

Clinical Signs

• Nonspecific (lethargy, inappetance, fever, colic)
• Muscle tremors
• Incoordination, falling down
• Muscle twitching (especially in the face)
• Lameness (especially hind limb)
• Paralysis, unable to get-up
• Seizures

There is no specific treatment, only supportive care can be performed. Usually this consists of fluids and antiinflammatories (banamine, deaxamethasone, DMSO), depending on severity. There are a few new products on the market designed to enhance the immune response against the virus, but they are very expensive and their success has not been determined yet.

The prognosis for recovery is incredibly variable, some horses will contract and clear the disease with a minimum of signs, may owners not even aware that their horses were affected. Some horses will go-down and eventually die (or be euthanised). Generally speaking, horses that are already down at the initiation of treatment have a much worse prognosis than those who are still standing.

PREVENTION IS THE KEY! Vaccination has proven to be very successful in preventing the disease and lessening the severity in those who do get the disease. Initial vaccination should begin in the early spring and booster will need to be given in 3-6 weeks. Annual boosters are then required (some parts of the country recommend boostering every six months). Foals born to vaccinated mares should receive their first vaccination at 4-6 months of age and be boostered two more times. Foals born to unvaccinated mares should be vaccinated between 2-3 months of age, and also be boostered two more times.

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