West Nile Virus
(WNV · West Nile Encephalitis)
In this condition factsheet:
The Facts on West Nile Virus
West Nile virus is an illness that spreads from mosquitoes to humans. A
mosquito becomes infected when it feeds on the blood of a bird that is carrying
West Nile virus. About two weeks later, the mosquito is capable of spreading
the virus to people and animals while biting for a blood meal. The virus is
not spread from person to person, and cannot be spread directly from infected
animals, such as birds, horses or pets to people.
West Nile virus originated in the West Nile region of Uganda in 1937, and was
confined to Africa, the Indian subcontinent and parts of the Middle East and
Europe for decades. It then sparked alarm after being detected in 1999 in New
York City, where seven people died. Canada had its first confirmed infection
in a bird in 2001. In September 2002, the first confirmed human cases of West
Nile virus were reported in parts of Quebec and Ontario. That year, 426 Canadians
became ill after being infected with the virus. By 2003, West Nile virus had
spread to 7 Canadian provinces: Nova Scotia, New Brunswick, Quebec, Ontario,
Manitoba, Saskatchewan, and Alberta. Ten deaths were linked to West Nile virus,
and more than 1300 Canadians became ill after being infected. Experts are not
sure how the virus is able to spread so quickly. Health Canada says it is possible
that West Nile virus will spread to new parts of Canada, including British Columbia,
in 2004.
The virus's continuing spread indicates that it is probably here to stay. West
Nile virus affects the central nervous system and infection usually results
in mild flu like symptoms or no symptoms at all. However, in severe cases, infection
with West Nile virus can be fatal. Affected provinces are developing aggressive
strategies to tackle the problem, including surveillance programs to track the
location and numbers of infected mosquitoes and birds.
Causes of West Nile Virus
Mosquitoes become infected with the virus and spread the disease by biting
birds or humans. Sometimes the virus spreads from mosquitoes to horses and
other animals. The virus is stored in the mosquito's salivary glands, and infected
mosquitoes transmit West Nile virus to humans and animals while biting to take
blood. There is evidence that the virus can also be transmitted through blood
transfusions and during organ transplants. However, the risk of transmission
through these procedures is quite low. Health Canada works closely with Canadian
Blood Services and Héma-Québec, the only blood operators in Canada,
to screen the blood supply for West Nile and other infectious diseases. Evidence
also suggests that West Nile can be transmitted from a pregnant woman to her
unborn child as well as through breast milk.
Of the 74 known species of mosquitoes in Canada, West Nile virus has been found
in 10 species. Not all species are found in all parts of the country. In a given
area, it is estimated that less than 1% of mosquitoes carry the West Nile virus.
Scientists don't know for sure which mosquito species actually transmit West
Nile to people through their bites. The uncertainty means that no one can tell
health officials which mosquito larvae to kill in order to avoid a repeat of
the previous summer.
West Nile virus belongs to a group of similar viruses including others that
are less severe, such as dengue fever and St. Louis encephalitis.
This means that several other viruses can cause similar symptoms, and they must
be ruled out before a diagnosis of West Nile virus can be confirmed.
Symptoms and Complications of West Nile Virus
Overall, most doctors believe that the risk of people becoming seriously
ill with West Nile is extremely small. West Nile virus affects the central
nervous system. About 80% of those who contract the virus may not show symptoms
at all. When infection does cause illness, symptoms vary from person to person
and will usually appear within 2 days to 15 days. Up to 20% of those infected
may develop West Nile fever, which features mild flu-like symptoms. It is characterized
by fever, headache, muscle aches, nausea, vomiting, and sometimes swollen lymph
glands or a skin rash on the chest, stomach and back. Health Canada and the
U.S. Centers for Disease Control and Prevention estimate that fewer than 1%
of those infected develop severe symptoms, and fewer than that experience life-threatening
complications. People over 50, people with chronic health conditions (such as
cancer, diabetes, alcoholism, or heart disease), and those with weakened immune
systems are more likely to have serious health effects from West Nile virus.
About one out of every 150 infected people will develop severe illness. Severe
symptoms include high fever, headache, neck stiffness, confusion, tremors (shaking),
and convulsions (seizures). Infection with West Nile Virus can result in encephalitis
(an inflammation of the brain) or meningitis (an inflammation of the
linings that cover the brain). West Nile virus can cause the brain or spinal
cord to swell and block the flow of blood to the brain. This could lead to a
coma, paralysis, or even death. The neurological effects may be permanent in
some people. Scientists do not know why some people recover quickly while others
face long-term health problems. These problems may include:
- physical effects such as long-term muscle weakness and paralysis, fatigue
and headache
- confusion, depression, and problems with concentration and memory
- difficulties in performing daily tasks such as preparing meals, and shopping
Even though the majority of people with mild flu-like symptoms may not need
blood tests for West Nile, consult your doctor if you have the following symptoms:
- confusion
- convulsions or seizures
- extreme swelling or infection at the site of a mosquito bite
- fever
- muscle weakness
- severe headache
- stiff neck
- sudden sensitivity to light or an inability to perform routine tasks
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