Mosquitoes spreading disease

How did the West Nile virus spread so quickly during the 2002 Ontario epidemic? One view is that the virus had a spell of "good luck" - strong genes, bird hosts with no natural defenses against a foreign invader, and the favourable environmental conditions of the long and dry summers.

How it spreads so efficiently to humans is also still unclear. Experts suspect that the virus spreads as follows:

  1. In the spring, the West Nile virus lives at low levels among birds like crows, blue jays, or house sparrows. Bird-biting mosquito species take in the virus by ingesting blood from an infected bird, and then spread it by biting other birds.
  2. Other virus-carrying mosquito species, alone or in various combinations, then pass the virus from infected birds to people.
  3. For a significant human outbreak of West Nile, the virus has to first spread widely among birds to some unknown level high enough to be transmitted to people through the mosquitoes at step 2.

Health Canada has put together a surveillance, education, prevention, and response strategy for West Nile virus. Provincial, national, and local authorities will keep track of the virus as it appears in birds, horses, mosquitoes, and humans. In some locations, pesticides may be used to control mosquito populations and to reduce the risk of spreading the virus to people.

There are two types of pesticides used to control mosquitoes: larvicides and adulticides. Larvicides are added to standing water, where mosquitoes breed. They kill the mosquito larvae before they become blood-feeding adults. Adulticides are sprayed into the air or applied to vegetation where mosquitoes rest. They work by killing adult mosquitoes. Larvicides are preferred to adulticides because they target the mosquitoes right in their breeding areas and often reduce overall pesticide use.

The risk of becoming infected with the West Nile virus is greatest during mosquito season. In Canada, this can start as early as mid-April and last until late September or October. To help reduce the risk of West Nile virus in affected areas, early detection of the virus in mosquitoes and animals is important, pesticide use should be exercised when needed, and avoiding mosquito bites is key.

Here are some tips on avoiding mosquito bites:

  • Avoid spending too much time outdoors when mosquito activity is greatest (i.e., at dawn and dusk).
  • Wear light-coloured clothing - mosquitoes are attracted to dark colours.
  • If possible, cover as much of your exposed skin as possible - wear long sleeves, closed shoes, and a hat, and tuck your pants into your socks. You could also consider putting elastics around the wrists of your shirt to prevent bugs from travelling upwards.
  • Use insect repellents - ask your doctor or pharmacist to recommend a safe and effective product.
  • Keep mosquitoes out of your home by sealing any leaks in windows, screens, and doors.

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