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WEST NILE VIRUS


Anne Dick, DVM, MVetSc, Diplomate American College of Veterinary Surgeons

WHAT IS WEST NILE VIRUS?

West Nile Virus (WNV) can cause meningitis and/or encephalitis resulting in coma or death in both humans and horses.  WNV is commonly found in Eastern Europe, Africa, West Asia, and the Middle East.  Its appearance in North America is recent, beginning in 1999. West Nile virus is currently considered endemic in parts of the United States and will continue to spread throughout the US and Canada. The disease is believed to be spread by infected migratory birds flying to breeding and wintering grounds.

HOW IS IT TRANSMITTED?

Mosquitoes transmit WNV after a blood meal from infected birds (primarily Blue Jays and Crows). Mosquitoes transport infected blood to a human or animal host when blood-feeding. Horses (and humans) are considered dead-end hosts meaning that they cannot pass the virus from one horse to another.  It is not yet known if infected mosquitoes or birds can pass the virus on to their young.

WHAT ARE THE SYMPTOMS OR SIGNS OF WNV?

WNV infection in horses may be unapparent or can produce a wide range of neurological disease. The incubation period is approximately 5-15 days.  Clinical signs reported in horses include: hindlimb and forelimb incoordination, recumbency, difficulty rising, hyperresponsiveness to sound and touch, muscle tremors over the muzzle and triceps region, drooping lower lip, stupor, falling to knees, blindness, and seizures.  Fever has been reported in less then 25% of equine cases.

HOW IS WNV DIAGNOSED AND TREATED?

WNV is diagnosed by testing blood.  WNV antibodies can be found in the blood and the virus isolated and identified.  The main treatment is supportive therapy to reduce the severity of symptoms.

HOW CAN YOU PROTECT YOUR HORSES?
Prevention of WNV includes reduction of mosquito breeding sites on farms and vaccination.  Eliminating standing pools of water and maintaining clean watering troughs may reduce mosquito breeding sites.  Mosquito repellent products that are harmless to horses are available through feed mills and can be added to pools of water that are difficult to remove.  The Canadian Food Inspection Agency had conditionally approved a vaccine to help prevent or reduce WNV infection in horses.  Horse owners need to be aware that, if they vaccinate for WNV, their horses may be ineligible for export to countries that require negative test results for WNV.

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