Easy as Health
- Health Information for a Better Life
What is
West Nile virus?
West Nile virus belongs to a family of viruses called Flaviviridae.
It is spread by mosquitoes that have fed on the blood of infected
birds. West Nile virus is closely related to the viruses that cause
Dengue fever, Yellow fever and St. Louis encephalitis.
Where did West Nile virus come from?
The virus was first isolated in 1937 in the West Nile district of
Uganda. Since then, there have been outbreaks in Egypt, Israel,
South Africa, and in parts of Europe, Asia and North America. The
first recorded outbreak in North America happened in New York City
in 1999.
How did West Nile virus get to North America?
No one knows for sure how the virus made its way to North America.
There are many theories, including one that suggests the virus
arrived on this continent through an infected migratory or imported
bird, or through an infected mosquito that was transported to North
America by accident.
During 2002, more than 4,000 people in North America became ill
after being infected with WN virus. This is the largest outbreak of
West Nile virus infection ever recorded.
How do people get infected with West Nile virus?
The evidence shows that most people infected with West Nile virus
got it from the bite of an infected mosquito. A mosquito becomes
infected when it feeds on the blood of a bird that is infected with
the virus. About two weeks later, the mosquito becomes capable of
passing the virus to people and animals by biting them. There is
some evidence that female mosquitoes may be able to transfer WN
virus to their offspring, but researchers are still looking into
this possibility.
In 2002, scientists discovered that people could be infected with
West Nile virus in other ways, including blood transfusions and
organ/tissue transplants. The risk of getting WN virus this way is
considered to be quite low. There is also evidence that pregnant
women can pass the virus to their unborn babies and that the virus
may be passed through breast milk. In addition, laboratory workers
who handle infected specimens can get WN virus through needle
punctures or cuts.
There is no evidence to suggest that people can get WN virus by
touching or kissing someone who is infected, or from being around a
health care worker who has treated an infected person. Likewise,
there is no evidence to date that the virus can pass from infected
animals (horses, pets, etc.) to people.
Who is more likely to get sick after being infected with WN
virus?
Evidence shows that many people infected with WN virus have mild
symptoms, or no symptoms at all. People with weaker immune systems
and people with chronic diseases, are at greater risk for serious
health effects. While persons of any age or health status can be at
risk of serious health effects associated withWest Nile virus
infection, the overall risk of serious health effects increases with
age.This is why it is so important to avoid mosquito bites if the
virus is active in your neighbourhood, town or region.
When is the risk greatest?
The risk of becoming infected is greatest during mosquito season. In
Canada, this can start as early as mid-April and last until the
first hard frost in late September or October.
What can people do to reduce their risk of WN virus infection?
If WN virus is found in birds, mosquitoes, horses or people in your
area, you can take steps to reduce the risk of getting mosquito
bites.
For example, wear protective clothing when outdoors, and use insect
repellents that contain DEET or other approved ingredients. Insect
screens on doors and windows will reduce the chance of mosquitoes
entering your home; make sure screens have no holes and fit tightly.
Also, take steps to reduce mosquito populations around your home and
property. For example, reduce standing water around your home e.g.,
dispose of old car tires, empty bird baths twice a week, and empty
pool covers regularly.
For most Canadians, the risk of WN virus infection is low, and the
risk of serious health effects from the virus is even lower.
However, anyone exposed to mosquitoes in an area where WN virus has
been detected is at risk for infection. Anyone who becomes infected
is at risk for serious health effects.
If a person is infected with West Nile virus, does that person
develop lifelong immunity to future infection by the virus?
It is assumed that immunity will be lifelong; however, it may
decrease in later years.
What types of birds carry West Nile virus?
The virus has been found in more than 150 bird species in North
America. Some species may have no obvious signs of illness when
infected. Others, such as crows, blue jays, magpies and ravens, get
sick more often and can die.
How do I report dead birds?
Reporting dead birds is the first thing you can do to help identify
West Nile virus in your community. (see our next article)
What types of mosquitoes spread West Nile virus?
There are 74 known species of mosquitoes in Canada. West Nile virus
infection has been found in 10 of these, but it is more common in
species that feed on birds. Examples include Culex pipiens, Culex
restuans and Culex tarsalis.
Not all of the 74 different species in Canada are found in all parts
of the country. The actual numbers of mosquitoes in different areas
will vary according to the time of year. Temperature and rainfall
can also have a dramatic effect on mosquito populations.
It is estimated that fewer than 1% of mosquitoes in any given area
are infected with WN virus. This means the risk of being bitten by
an infected mosquito is low. However, it could happen to anyone in
areas where WN virus is active. The best way to reduce your risk is
to avoid getting mosquito bites.
Where was WN virus activity reported in North America during
2002?
There were reports of WN virus in more than 40 states in the United
States last year. The so-called hot spots, with the highest numbers
of confirmed human cases, were Illinois, Michigan, Ohio, Louisiana
and Indiana. For more details, including case counts and maps, visit
the Centers for Disease Control and Prevention West Nile virus Web
site.
In Canada, there was WN virus in five provinces (Nova Scotia,
Quebec, Ontario, Manitoba, Saskatchewan), including confirmed cases
of human infection in Ontario and Quebec.
