West Nile Virus

By: Katie Jones, CVT

Mosquito Isolated on WhiteWith the snow beginning to melt we have started to anxiously await the warmth of summer. With the warm weather though comes all the other “joys” of summer. One of these is the infamous mosquito. Mosquito do more than pester you out on the trails; they also bring and spread disease to you and your horse. West Nile Virus has become spread through the whole country. The disease was first reported in the United States in 1999 in New York, with the first reported case in Minnesota in 2002. In 2013, the USDA reported 64 confirmed cases of West Nile Virus in 23 states (The Horse).


West Nile Virus is circulated in a complex cycle; which can be seen in several species of lifecyclemosquitos. The migration patterns of wild birds are believed to be the cause of the national spread of the disease. The cycle begins with the mosquito biting infected wild birds which carries the disease in their salivary glands. They then spread the disease to whoever becomes its next blood meal. Humans and horses are incidental hosts; otherwise known as dead end hosts. Once the disease has entered the blood stream the virus multiplies and travels throughout the system. The virus then crosses the blood-brain barrier where it enters into the brain interferes with normal central nervous system function. In newly acquired regions of the disease, humans and birds contract it first, followed by horses. Being bit by an infected mosquito is the only way to contract this virus. It can not be spread from human-human or animal-animal.

Most cases are reported in horses from August to the first frost in fall; however, prevention should start occurring in early summer.

Though the horse may be infected with the disease, they do not always show signs. West Nile Virus commonly will cause inflammation of the central nervous system (CNS). Here are a few common signs a horse has been infected with West Nile Virus:

  • Loss of appetite
  • Depression
  • Fever
  • Weakness or paralyzed hind limbs
  • Impaired vision
  • Ataxia
  • Head pressing
  • Aimless wondering
  • Seizures
  • Inability to swallow
  • Walking in circles
  • Hyperexcitability
  • Coma

If your horse is experiencing any of these commonly seen with encephalitis, the horse should be examined by your local veterinarian. Not all horses that are showing encephalitis signs have West Nile Virus; there are several other diseases with encephalitis symptoms.

At this time there is no specific treatment for West Nile Virus. Treatment for horses is focused at supportive care and anti-inflammatories. Even without a specific treatment for the West Nile Virus, it is important to get diagnosis by a local veterinarian in order to know mosquitos are carrying the virus in the area.

Although there is no specific treatment for West Nile Virus, there is an equine vaccine recommended to prevent the contracting of the disease. Like all vaccines, it will not guarantee one hundred percent protections for your horse.

To reduce risk of developing West Nile Virus in your area, here are a few tips:West_Nile_Logo_Fnl

    • Remove anything that accumulates standing water
    • Remove unused tires from your property
    • Keep horses indoor during peak periods of mosquito activity (dusk to dawn)
    • Clean ruff gutters
    • Turn over wheelbarrows
    • Empty and refill outdoor water troughs daily
    • Stock ornamental pools with fish
    • Decreased the number of birds around your barn and pasture
      > Eliminate roosting areas
    • Use topical preparations containing mosquito repellent

Though West Nile Virus is a serious and detrimental disease, horse can recover. Consider talking to your local veterinarian about ways that you can cut down the mosquito population in your area, as well as adding the West Nile vaccine to your spring preventive care checklist.

For more information call your local health department or check these websites:
Center of Disease Control and Prevention
American Mosquito Control Association
Minnesota Mosquito Control