West Nile virus (WNv):
- lives in a wide range of bird species
- can occasionally infect mammals including horses and humans
- generally does not affect other wildlife
Alberta formed a WNv response team in 2002, before the virus arrived in the province in 2003. The ongoing team includes representatives from provincial departments related to:
- municipal affairs
Bird surveillance program
Spread of WNv
Like any living organism, WNv is well adapted for its own survival. It can survive in many bird species. WNv can use a wide range of mosquito species to infect new individuals.
WNv arrived in North America on the US east coast in 1999 and quickly spread among local bird populations. Many infected birds were migratory and carried the virus to southern climates. Viral transmission continued as these birds intermingled with local mosquitoes and overwintering birds of the same or different species.
During spring migration, the virus was carried northward by infected birds that spread out across the continent. By the end of 2001, WNv was in most states east of the Mississippi River and in southern Ontario. By the end of 2004, it was found in most provinces and states.
The WNv distribution reflects the primary bird migration pathways. The virus sequentially entered the following flyways:
The virus continued to spread across North America and now occurs throughout:
- Canada (except British Columbia and the maritimes)
- the continental United States
- Central America
- South America
WNv in Alberta
WNv arrived in Alberta in 2003. Local bird populations had never been exposed to the virus and had no natural immunity or resistance to infection. Members of the crow family (Corvidae) were particularly susceptible and many died as a result of infection. They included:
The dead birds became an early warning system to show where and when the virus was active in the province. Health professionals, veterinarians and the public used the information to assess the risk of possible infection.
What we learned
When and where WNv occurs
WNv returns each July and August to the Grassland Natural Region of southeastern Alberta. This area has conditions that favour Culex tarsalis, the only mosquito that effectively transmits WNv in Alberta. The extent of the affected area varies slightly depending on local weather patterns and associated populations of this mosquito.
The number of dead corvids (crow family) each summer has declined significantly since the virus first appeared in 2003. Yet the populations of crows and magpies remained stable. It is likely that bird species exposed to WNv adapted to it in the ecosystem and developed protective immunity.
Reports on WNv wild bird testing from 2002 to 2009 can be found at: Alberta West Nile virus Wild Bird Surveillance.
News releases from 2003 to 2004 include:
- First bird found in Alberta with West Nile virus in 2004
- Alberta prepared for return of West Nile virus
- Focus continues on reducing risk to humans as second bird tests positive for West Nile virus
- First bird with West Nile virus found in Alberta
Wildlife information bulletins on WNV include:
- Wildlife Info Bulletin #3: West Nile virus in Alberta – What Happened in 2004?
- Wildlife Info Bulletin #1: Sage-Grouse and West Nile virus in Alberta
Birds and raptors
Birds are the natural habitat for WNv. All bird species are likely susceptible to infection. The virus has been identified in more than 225 species of wild and captive bird species in North America. The virus can also occasionally be transmitted to a few species of mammals and reptiles.
The U.S. National Wildlife Health Center is investigating if some raptor species may be more susceptible than bird species. The raptor species include some owl and hawk species common in Alberta. To date, there is not enough information to make conclusions about the risk to these species.
Infection does not necessarily cause illness. The large majority of infected birds do not show any ill effects of having WNv. Crows and their relatives are the exception. In these species the virus can cause illness and death in many individual birds.
Mammals in general are very resistant to WNv infection. Although wild mammals may be bitten by infected mosquitoes, they generally produce an immune response that prevents infection. In blood samples, antibodies may be found but no virus that could be passed on to mosquitoes. Some individuals, however, may not develop immunity and could become sick or die.
Horses are an exception to the general rule for mammals. They are more susceptible to WNv and closely related viruses. For information about West Nile virus in horses, visit: West Nile virus (WNv) in Horses Surveillance
We can safely predict that WNv will reappear each year in July and August in southeastern Alberta. To avoid infection, people should continue to avoid being bitten by mosquitoes. Similarly, horse owners should provide appropriate protection for their horses.
Dead bird handling
There was only one instance of direct transfer of WNv from birds to humans. It involved accidental infection of lab technicians who were handling many heavily infected birds.
If you find a dead bird, you can:
- leave it alone
- bury it
- wrap it in plastic and put it in the garbage
When dealing with any wildlife found dead:
- always wear gloves
- use a double plastic bag inverted over your hand or a stick to move the dead animal into a container
Dead bird clusters
Clusters of dead birds found in small areas over short time frames should be reported to a Fish and Wildlife office. While not likely to be WNv, the cause of death may be another disease that should be investigated. To find the Fish and Wildlife office closest to you, see:
Bird feeders and birdbaths
Bird feeders are not a concern for WNV transmission or human infection. But you should change the water in bird baths regularly and drain stagnant water in your yard to reduce mosquito breeding.
There have been ongoing problems with salmonella infections at bird feeders. You should disinfect feeders and birdbaths regularly with weak bleach. Since salmonella bacteria can infect humans, you should wear gloves and wash thoroughly. For more information, see the Avian Salmonellosis Fact Sheet at: Wildlife Diseases.
Hunters, like all outdoor recreationists, should take precautions against being bitten by mosquitoes. Mosquitoes are common in wetland habitats and are active at dawn and dusk. Health officials advise that there is no WNV risk from handling hunter-killed birds in Alberta.
There is no evidence of WNV risk to persons handling live passerines (perching birds) or waterfowl in Alberta. The majority of birds are not infected. Those that are don’t carry enough virus to pose a threat to human health. Anyone handling live birds can always increase their protection by wearing gloves. Bird banders should take precautions to avoid mosquito bites.
Bird rehabilitation facilities
There are growing concerns that captive or recuperating birds may be at increased risk of West Nile infection. Facility operators in the southern regions of the province would be well advised to:
- screen the facility
- take other precautions to protect the birds from mosquitoes
A vaccine for birds looks somewhat promising, but is not available at this time. Some facilities have used the horse vaccine on individual birds of critically endangered species. But there is no evidence that it provides any protection to birds.
To connect with Fish and Wildlife:
To connect with the Alberta Environment and Parks Information Centre:
Main Floor, Forestry Building
9920 108 Street
Edmonton, Alberta T5K 2M4