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West Nile Virus

What is West Nile virus?

West Nile virus is an infection spread by mosquitoes that may, in a very small number of cases, cause serious illness. The chance of contracting and becoming ill from West Nile virus is very low. Horses are also susceptible. For information regarding West Nile virus and horses, contact your veterinarian.

How do people get West Nile virus?

West Nile virus is spread to humans by the bite of an infected mosquito. Individuals cannot become infected through contact with another person carrying the virus.

How do mosquitoes get West Nile virus?

Mosquitoes become infected with West Nile virus by biting birds infected with the virus. Wild birds are the main hosts for the virus.

Can all mosquitoes carry the virus?

Only some species can carry the virus. And of those, only the female of the species is of concern. The female feeds on blood from birds to obtain protein for her eggs. In Ontario, the primary species that bites birds for blood is the Culex mosquito, commonly known as the northern house mosquito.
What is the incubation period in humans for West Nile virus infection?

Very few people will become infected with West Nile virus. In those who do, the time from infection to the onset of disease symptoms is usually three to 15 days.

What are the symptoms of West Nile virus?

In humans, most infections of the virus result in no symptoms. In a small number of cases, flu-like symptoms such as fever, frontal headache, muscle aches and, occasionally, skin rash may occur. In a very small number of people, particularly the elderly and those with weakened immune systems, additional symptoms such as neck stiffness, muscle weakness, stupor, disorientation and coma can occur. Rarely, the disease can be fatal. The time between infection and the onset of symptoms-the incubation period-is between three and 15 days. Most people infected with the virus show no symptoms at all.

Where did West Nile virus come from?

Until 1999, West Nile virus was found only in Africa, Eastern Europe, West Asia and the Middle East. In 1999, the first North American cases occurred in New York City. By 2000, the virus was found in birds in all but one county of New York State. In the summer of 2001 the virus was detected in birds in 12 jurisdictions of Southern Ontario, as well a number of other states. In 2002, Canada reported its first case of West Nile Virus in the human population.

What if I travel to areas where West Nile virus occurs?

Even in areas where mosquitoes carry the virus, very few mosquitoes-less than one per cent-are likely to be infected. The chance of being bitten by an infected mosquito is very small. As a safety precaution, though, it is still important to minimize exposure to mosquitoes in areas where West Nile virus activity has been documented.

How does West Nile virus cause illness in some people?

Once bitten by an infected mosquito, in a small number of cases, the West Nile virus multiplies in a person's blood system, crosses the blood-brain barrier and enters the brain. The virus then interferes with the normal functioning of the central nervous system and causes swelling and inflammation of the brain tissue.
Protecting yourself from mosquitoes:

Mosquito bites are an unavoidable nuisance, however you can help reduce your chance of being bitten by: 

  • wearing light-coloured long-sleeved shirts and long pants when out doors especially at sunrise, early evening and at night when mosquitoes are most active.
  • applying mosquito repellent containing DEET to exposed skin-following manufacturer's instructions.
  • spraying clothing with repellents to ward off mosquitoes that may bite through thin clothing.
  • making sure all windows and doors in your home have screens that are in good condition.

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