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.