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Mosquitoes as Disease Carriers

Seventy-four mosquito species have been recorded in Canada. Mosquito species exhibit preferences for hosts (some species bite people, some species don’t) and for breeding sites. The relative importance of each species as a carrier of West Nile virus is not fully understood.

The Crow Family (Corvidae)

Crows, ravens, jays and magpies are very susceptible to the virus and often die when infected. Evidence suggests that crow die-offs precede an increased risk for human illness by two to six weeks. Monitoring of dead crows can provide an early warning signal that West Nile virus is active in an area.

Magpie	Raven	Gray Jay
Blue Jay	Crow

 

The Transmission

1. Mosquito to Bird
   The virus is most commonly transmitted by the bites of infected mosquitoes. Mosquitoes become infected when they bite birds infected with the virus. Approximately 326 bird species in North America have tested positive for infection. Birds of some species get ill and die while others become infected but do not show signs of the disease.
2. Bird to Bird
   Caged birds have transmitted the virus to each other directly. There is also growing evidence that raptors (hawks, owls) get the virus by eating infected prey. There is no evidence of increased mortality among waterfowl due to West Nile virus. The effects of the virus on waterfowl populations need to be monitored.
3. Mosquito to Human, Horse and Other Mammals
   While mammals may test positive for West Nile virus, they may never exhibit symptoms of being infected with the disease. Humans and horses can develop serious medical conditions. A vaccine exists for horses but not humans.

Mammals are dead-end hosts, meaning they do not transmit the virus back to mosquitoes.

Mosquito Life Cycle

<< Page 2 Virus History
Page 4 Infection Statistics >>