West Nile virus is a mosquito-borne virus that can cause swelling and inflammation of the brain and spinal cord in many species, but especially horses, birds and humans. It’s related to the viruses that cause St. Louis encephalitis and Japanese encephalitis in horses.
“West Nile virus is spread by specific species of mosquitoes,” said Gerald Ollis, chief veterinarian with Alberta Agriculture.
“In Alberta, the species of mosquito of concern is Culex tarsalis, which becomes infected when feeding on infected wild birds. Wild birds are the primary reservoir of West Nile virus. Birds of the Corvidae family, including crows, blue jays, magpies and ravens, are susceptible to the effects of West Nile virus, as are some species of raptors.”
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More than 140 species of wild and domestic birds can be infected with West Nile virus, as well as bears, mountain sheep and goats, horses, mules, donkeys, cattle, alpacas, dogs and cats.However, the disease is rare in most of these species and only domestic geese, horses, mules and donkeys appear to be severely affected.
“Exposure of horses to West Nile virus was confirmed in 2002 in Manitoba, Saskatchewan, Ontario and Quebec,” said Ollis.
“In 2002, Quebec and Ontario reported the first human cases of West Nile virus in Canada. In 2003, (it) reached as far west as Alberta, where cases in mosquitoes, birds, horses and humans were confirmed. British Columbia is the only province in Canada to have never detected West Nile virus.”
In 2003, reported cases of the virus in Alberta numbered 272 in humans, 170 in horses and 439 in birds. In 2004, the number dropped significantly, with no human cases reported, four cases in horses and nine cases in birds.
“Experts suggest that the wet, cool summer Alberta experienced in 2004 was responsible for the lack of West Nile virus,” Ollis said.
He said horse owners should watch for signs of the virus. Most horses bitten by a mosquito infected with West Nile virus will not develop a clinical disease. They usually develop an asymptomatic infection, then recover and are none the worse for it.
“Horses that do become sick from infection with the virus exhibit symptoms common to nervous disorders.”
Ollis said the symptoms can include listlessness, a change in demeanor, drooping lips, muscle twitching, lack of co-ordination, weakness in the limbs, partial paralysis or an inability to get up. A fever is not always present. A veterinarian should examine horses with these signs because they are similar to those caused by western equine encephalitis, eastern equine encephalitis and rabies.
Horses exhibiting signs of West Nile virus, as with any nervous disorder, must be handled with caution. Treatment is primarily supportive therapy, intravenous fluids and nursing care to prevent secondary infections.
There is no specific treatment for horses affected with West Nile. Up to 35 percent showing clinical disease may die or have to be euthanized because of complications. Some recovered horses may exhibit permanent neurological deficiencies.
“There is no scientific evidence to indicate that West Nile virus can be transmitted directly from horses to other species, including humans,” Ollis added. “Therefore, quarantine of infected horses is not necessary. Detection of exposure to the virus in most horses is restricted to a blood test that identifies antibodies. Routine testing of horses is not recommended.”
Although the risk of disease in horses is low, the consequences can be severe. Preventive measures should be discussed with a veterinarian. These include:
- Minimizing exposure to mosquitoes by providing screened housing and avoiding outdoor activities during peak times of mosquito feeding, such as dawn and dusk.
- Using topical insect repellents or smudges.
- Reducing potential mosquito breeding sites by eliminating standing water, cleaning water troughs weekly and keeping grass short around buildings.
There are two West Nile virus vaccines licensed in Canada for use in horses and available from veterinarians, said Ollis. Vaccinated horses intended for export to the European Union or Japan require certification of vaccination. Consult the Canadian Food Inspection Agency for details.
