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West Nile Virus - Information for Physicians

Dr Steve Whitehead
Deputy Medical Health Officer
For Saskatoon District Health Region.

July 17th 2003

1. Background

West Nile Virus (WN virus) gets its name from the West Nile Province of Uganda where the disease was first isolated in 1937. Today it is most commonly found in Africa, West Asia, Europe, and the Middle East. It was first identified in North America in New York City in 1999 and since then has spread rapidly across the continent.

The first reports of West Nile Virus in Canada came in August 2001, when the virus was found in dead birds and mosquito pools in southern Ontario. Canada had its first human cases in 2002 after people tested positive for WN virus in parts of Quebec and Ontario. In addition 2 people from Alberta became infected, but these cases were thought to be travel related. Altogether over 400 people were infected in Canada in 2002.

The virus was also found in birds, horses or mosquitoes in Nova Scotia, Quebec, Ontario, Manitoba and Saskatchewan. In 2002 in Saskatchewan a total of 44 birds and 10 horses tested positive but there were no human cases. Given this situation it is highly likely there will be human cases of WN virus in Saskatchewan during 2003.

2. Transmission of West Nile Virus

West Nile Virus is an arbovirus, meaning it is transmitted between vertebrates by mosquitoes and possibly other arthropod vectors. It belongs to the Flaviviridae family of viruseswhich includes the yellow fever, Japanese encephalitis, dengue, Saint Louis encephalitis and Western Equine Encephalitis viruses.

Wild birds are considered the principal vertebrate host and mammals (including humans) the dead end hosts. The virus can infect a wide variety of mammals (dogs, camels, ruminants, rabbits, pigs and bats), but important clinical infection has been detected mainly in humans, horses and birds.

Fugure - West Nile Virus Transmission Cycle

Over 125 species of bird have been shown to be infected by WN virus in North America. The crow family of birds (Corvids) has been shown to be particularly sensitive to the virus and have high death rates if infected. Evidence suggests that an increase in the number of crows dying precedes an increased risk of human illness by 2 - 6 weeks.


There are some 41 different species of mosquito recorded in Saskatchewan, several species of which are known to carry Western Equine and St. Louis encephalitis viruses and could be carriers of WN virus.

Culex Tarsalis is the species most likely to transmit the virus. It breeds in many different settings and has a wide range of feeding hosts including birds and mammals. It is also known to take multiple feeds. The highest numbers of Culex Tarsalis mosquitos are expected to appear in late summer and will contribute to greatest risk of WN virus transmission at that time. Culex Tarsalis occurs most frequently in prairie Saskatchewan and can account for 25% mosquitoes in that area.

Other Recognized Routes of Transmission

Whilst transmission through mosquitoes is by far the most important route, other routes of possible transmission of the virus to humans have been described:

3. Clinical Presentation and Diagnosis Risk of disease

Most people bitten by a mosquito in an area where there is WN virus will not become infected. It is suggested that only 1% of mosquitoes are likely to be able to transmit the disease. In a seroprevalence study in Queens following the New York epidemic of 1999, only 3% of the population tested showed serological evidence of exposure to the disease.

Of those people who do become infected 80% will have no symptoms and 20% will demonstrate mild disease with the sudden onset of an acute non specific flu like illness characterized by high fever. This typically occurs 3 to 6 days after the bite of an infected mosquito but this can be up to 15 days.

About 1 in 150 people infected develop serious disease such as meningitis or encephalitis with symptoms including headache, fever, neck stiffness, muscle weakness, confusion. About 5 to10% of this seriously affected group can die.

Patients with neurological disease typically have a fever of 1 - 7 days before they develop neurological symptoms. Neurological symptoms are similar to those with other flaviviruses and depend on which part of the nervous system is affected; the meninges (meningitis) the brain parenchyma ( encephalitis) or the spinal cord (myelitis) An unusual feature of the outbreak in North America ,that has not been seen elsewhere, is a polio - like flaccid paralysis in patients who are otherwise fully conscious.

Disease is usually more severe in the elderly, the very young and those with weakened immune systems.


There is no specific treatment, medication, or cure for WN virus. Serious cases are treated with supportive therapies to ease symptoms and prevent secondary infections. These cases may require hospital or nursing care.

Long-term effects of WN virus

Because WN virus is an emerging disease in North America, the long-term effects are not fully understood. The following long term effects have been described:

It is thought that infection with WN virus confers lifelong immunity

Vaccine for WN virus?

A vaccine is available for use in horses, but at this time there is no licensed vaccine to protect people against WN virus and it is not anticipated one will be available in Canada for several years.


Physicians should consider WNV especially in patients presenting with fever (often of sudden onset) and severe headache or other symptoms of meningitis or encephalitis or acute flaccid paralysis and a history of travel or residence in an area where WN virus has been detected.

The Saskatchewan provincial laboratory is using an IgM test for antibodies for to WN virus and will turn around results within 3 days. Some sera will be sent to the National Microbiology laboratory in Winnipeg for confirmation of WNV. The lab point out that as WNV occurs during the enterovirus season physicians should consider sending throat swab and stool specimen as well as CSF on patients with meningitis or encephalitis for enterovirus culture.

Testing for WN virus will also be available through the RUH laboratory in Saskatoon in co-operation with the Provincial Laboratory.

4. Surveillance of WN virus in Saskatchewan 2003

Saskatchewan Health and health regions are committed to a graduated response to tackle WN virus and reduce risk from the disease. This will depend on a clear picture of the disease within the Province. A surveillance system is in place the components of which include:

Physicians have already been alerted to consider the diagnosis of WN virus in people presenting with fever (often of sudden onset) and severe headache or other symptoms of meningitis or encephalitis or acute flaccid paralysis and a history of travel or residence in an area where WN virus has been detected. If surveillance suggests that the likelihood of infection increased this human surveillance approach will become more active.

5. Action to be Taken to Reduce Risk

Information from the surveillance system will inform decisions about appropriate action to take to reduce risk of transmission of the disease.

Of utmost importance are personal protective measures to prevent mosquito bites. These include the use of appropriate clothing and insect repellents, especially DEET based products which have recently been reviewed by the Pest Management Regulatory Agency (PMRA). They have produced new guidelines on the use of DEET products to reduce mosquito bites.

Individuals can also be encouraged to take action to reduce mosquito breeding sites in their yards. Any small collection of water is a potential breeding site for mosquitos.

At a municipal level programs of environmental engineering to reduce breeding sites and larviciding, treating potential breeding grounds to kill mosquitoes at the larval stage of development are important.

Finally it is recognized that in extreme circumstances with an intense epizootic and evidence of transmission to the human population, adulticiding, the ultra low volume (ULV) spraying of Malathion might be necessary to interrupt transmission. The use of Malathion in this context has recently been reviewed by the PMRA.

6. Further Information

The following websites have some excellent information about WN virus and its current status in Canada:

1. Saskatchewan Health WN virus website gives a good overview of the approach being taken to reduce risk from the disease in the Province and useful links to other relevant websites

2. The Canadian Cooperative Wildlife Health Centre website describes the approach being taken to bird surveillance and has up to date data on the findings of this surveillance

3. The Health Canada West Nile Virus monitor website gives further information about the results of surveillance across the country