Information about West Nile Virus
What is West Nile Virus?
West Nile Virus (WNV) is a virus that causes a mild febrile illness
(with fever, headache, body aches) in most persons who acquire the
virus. Some individuals, however, particularly the elderly, may
develop permanent neurological damage due to encephalitis (inflammation
of the brain), possibly resulting in death.
West Nile virus was first isolated in the West Nile District of
Uganda in 1937. Since then it has spread through Africa, southern
Europe, the Middle East, Asia, Oceania and, most recently, North
America. It propagates in temperate zones of the world, and new
evidence suggests that it may over-winter in northern climates in
birds and animals. The disease is fatal in domestic and wild birds.
WNV was introduced into North America in 1999. In late summer of
1999 an outbreak of WNV occurred in New York City resulting in 62
cases of encephalitis, including 7 deaths. It has since spread inexorably
west and south through the U.S. and, more recently into Canada.
During 2002, more than 4,000 people in North America became ill
after becoming infected with WNV. This is the largest outbreak of
West Nile virus infection ever recorded.
Public Health Agency of Canada Case
Definitions for West Nile Virus
The first human cases of WNV in Canada occurred
in August, 2002. Since then, human test results in Canada have
confirmed WNV in all provinces except PEI and Newfoundland. The number of cases has fluctuated:
WNV Cases in Canada
| YEAR |
CASES |
DEATHS |
MAP LINK |
2002 |
414 |
14 |
|
2003 |
1481 |
14 |
|
2004 |
25 |
0 |
|
2005 |
225 |
10 |
|
2006 |
151 |
0 |
|
2007 |
2215 |
0 |
|
2008 |
0 |
0 |
|
Horse and dead bird testing results will no longer be reported
on this site. See the Public Health Agency of Canada links
below for more information on horse and bird results.
Etiologic Agent
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.
West Nile virus primarily infects birds (particularly crows, ravens,
blue jays and magpies), although it has been found in other mammals
such as horses and bears. Human cases are still very rare.
Transmission of WNV
Most people get West Nile virus from the bite of an infected mosquito.
Mosquitoes become infected from feeding on the blood of an infected
bird. Two weeks later the mosquito is capable of passing the virus
to animals and humans by biting them. Other blood-biting insects
do not appear to be able to spread West Nile virus.
In 2002 it became apparent that West Nile virus could be spread
in other ways, such as blood transfusions and organ transplants.
There is also evidence that pregnant women can pass on the virus
to their unborn babies and that the virus can be passed through
breast milk. Healthcare workers can get West Nile virus
via needlesticks or cuts. West Nile virus is not spread
by casual contact or by the respiratory route. It is not airborne.
There is no person-to-person spread, or animal-to-person spread.
There is no evidence of anyone getting WNV from handling infected
carcasses, however it is always recommended that gloves be worn
to handle any dead bird or animal remains, to prevent acquisition
of other types of infection. Special precautions are not
required for patients who have WNV encephalitis.
Symptoms of WNV
Most infections are mild and symptoms include the sudden onset
of fever, headache and body aches, nausea and/or vomiting, occasionally
a skin rash on the trunk of the body and swollen lymph glands. These
symptoms generally last only 3 to 6 days. More severe infection
may be marked by headache, high fever, neck stiffness, stupor, disorientation,
coma, tremors, convulsions, muscle weakness, paralysis and, rarely,
death. The incubation period of West Nile virus
encephalitis is usually 3 to 14 days.
Preventing WNV Infection
The best way to protect against WNV is to avoid mosquitoes:
- Consider staying indoors at dawn, dusk and in the early evening
- Install or repair window and door screens so that mosquitoes
cannot get indoors
- Place mosquito netting over infant carriers when you are outdoors
with infants
- When possible, wear long-sleeved shirts and long pants whenever
you are outdoors
- Apply insect repellent sparingly to exposed skin. Choose a repellent
that provides protection for the amount of time that you will
be outdoors
- To avoid helping mosquitoes breed in your environment, drain
standing water. Routinely empty water from flower pots, pet bowls,
clogged rain gutters, swimming pool covers, discarded tires, buckets,
barrels, cans and other items that collect water in which mosquitoes
can lay eggs.
Canada's Initiatives for the West Nile Season
Protecting the Blood System
Health Canada began testing blood donations for West Nile Virus
on June 17, 2003, using a test developed by the Canadian Blood Services
(ref.
Canadian Blood Services). On July 22, 2003 a unit of blood donated
in Saskatchewan became the first blood sample to test positive for
the virus. The blood was immediately withdrawn from circulation
and the donor was notified (ref.
Canadian Blood Services). On September 3, 2003 in response to
an increase in the number of human cases of WNV in Saskatchewan,
Canadian Blood Services announced a
voluntary withdrawal of all blood components collected in
the province since August 4, 2003. Canadian
Blood Services and Héma-Québec
have had contingency plans in place to ensure that Canadians have
enough safe blood available throughout the West Nile Virus season.
Travel Advisories [REF: PUBLIC Health AGENCY
OF Canada]
There are currently no travel restrictions due to West Nile virus.
Reporting and Submitting Dead Birds
The Canadian Cooperative Wildlife Health Centre has a wealth of
information regarding birds that carry WNV and how to submit suspicious
dead birds for testing:
Laboratory Testing for WNV [REF: CJID March/April 2003]
A large number of diagnostic tests are available for diagnosing
West Nile Virus disease. Serological procedures are the methods
of choice, while nucleic acid amplification assays are preferred
for screening donor blood and testing immunocompromised individuals.
Virus Isolation is difficult from blood, CSF or tissues
due to the low concentration of virus particles in these samples
and the transient nature of viremia. WNV has been isolated from
serum, CSF, liver and brain samples.
Serological assays include hemagglutination inhibition
(HI) assays and IgG/IgM ELISA assays (to detect antibodies to WNV
during acute and convalescent phases of illness) and the plaque-reduction
neutralization test (PRNT). Antibodies to other flaviviruses can
cross-react in the HI and ELISA tests, and the PRNT must be used
to differentiate between viruses. The diagnostic test of choice
is the IgM ELISA, which has a high sensitivity for serum and
CSF specimens collected during the acute phase of illness. The detection
of antibody in CSF usually indicates a current infection.
Nucleic acid amplification tests such as the polymerase
chain reaction (PCR) detects WNV RNA in CSF (preferred specimen),
blood or serum.
Preferred specimens for testing for WNV are an acute serum
sample collected within seven days of onset of illness for IgM ELISA
testing. CSF may also be collected for IgM antibodies. Convalescent
serum samples should also be taken 14 to 21 days after onset of
symptoms, to detect elevated titres (fourfold or greater) of antibody.
West
Nile Virus Diagnostic Test Criteria from Public Health Agency
of Canada.
WNV LINKS
| PUBLIC HEALTH AGENCY OF CANADA (PHAC) |
| |
| PROVINCIAL
INFORMATION |
| Ontario:
Alberta
British Columbia
Manitoba
New Brunswick
Nova Scotia
Prince Edward Island
Quebec
Saskatchewan
|
| CENTRES
FOR DISEASE CONTROL AND PREVENTION (CDC) |
| |
| OTHER
INFORMATION |
| |
WNV PUBLICATIONS
- MMWR
October 4, 2002 Investigations of West Nile Virus Infections
in Recipients of Organ Transplantation and Blood Transfusion -
Michigan, 2002
- MMWR
November 1, 2002 Public Health Dispatch: Investigations of
West Nile Virus Infections in Recipients of Blood Transfusions
- MMWR
December 20, 2002 Laboratory-Acquired West Nile Virus Infections
- United States 2002
- MMWR
December 20, 2002 Intrauterine West Nile Virus Infection
- United States 2002
- CDC,
2003 Epidemic/Epizootic West Nile Virus in the United States:
Guidelines for Surveillance, Prevention and Control
|
