November 22,

West Nile Virus

West Nile Virus

What is the West Nile virus?

West Nile virus is mainly spread to people through the bite of an infected mosquito. Mosquitoes transmit the virus after becoming infected by feeding on the blood of birds which carry the virus. The disease has been and continues to be a risk in Canada every year.

How is it spread?

Mosquitoes become infected with West Nile virus when they feed on the blood of infected birds. Birds such as crows, blue jays, and sparrows are particularly susceptible to the virus. The virus is carried in the mosquito’s salivary glands and it is transmitted when mosquitoes bite humans and animals. The virus is not contagious and is not spread from person-to–person contact except in extreme cases such as blood transfusions.

In general, temperature is the major influence on the level of WNV activity. If the weather remains warm, with enough rain, Ontario could experience more WNV activity. Mild winter conditions allow more mosquitoes that are carrying the virus to survive into the next year. If the season turns out to be hot, with little rain, then Ontario would likely see lower numbers as there would be fewer pools of stagnant water for mosquitoes to breed in. If the weather is cool, but there is enough rain, Ontario could see some cases but not as high as with hot weather. There is no way to predict how many people will be infected by the West Nile virus in any given year.

Who is at risk?

People of all ages can become infected with West Nile virus however most will have a mild illness. Of those who are infected, 80% have no symptoms. About 20% of those infected will experience West Nile fever, which may consist of fever, headache, muscle ache and rash. The most serious illnesses caused by West Nile virus are encephalitis and meningitis, which only occur in approximately 1% of infections. The risk of severe illness increases with age.  Those with chronic diseases, weakened or developing immune systems are also at greater risk for severe illness.

West Nile Virus Transmission Cycle

In nature, West Nile virus cycles between mosquitoes (especially Culex species) and birds. Some infected birds, can develop high levels of the virus in their bloodstream and mosquitoes can become infected by biting these infected birds. After about a week, infected mosquitoes can pass the virus to more birds when they bite. Mosquitoes with West Nile virus also bite and infect people, horses and other mammals. However, humans, horses and other mammals are ‘dead end’ hosts. This means that they do not develop high levels of virus in their bloodstream, and cannot pass the virus on to other biting mosquitoes.

Click here for a visual of the West Nile Virus Transmission Cycle

What are the symptoms of West Nile virus?

West Nile virus is usually a mild disease with signs and symptoms that may include fever, headache, muscle ache, swollen lymph nodes, and skin rash.  In some cases the disease may progress to a more severe stage with signs and symptoms that can include disorientation, coma, and even death. In its most serious state, West Nile virus infection can cause an inflammation in the brain, otherwise known as encephalitis.

How is West Nile virus treated?

There are no antibiotics that are effective against the infection and there is no vaccine which humans can use to prevent themselves from becoming infected with the virus. Suspected cases are often hospitalized where supportive treatment can be provided.


Last modified on Jul 08, 2016