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  1. Summary
  2. The Virus Cycle
  3. Effects on Humans
  4. Prevention & Mosquito Control
 

The Virus Cycle

West Nile is normally a bird virus, and at last count at least 138 species of birds have been found to carry it. For most birds, the virus does no harm. However, in North America, members of the Corvidae (crow) family—crows, ravens, mag-pies and jays among others—suffer high mortality when infected with the virus. No one knows why this group of birds is affected so severely, but the appearance of dead crows is now used by health agencies as an ‘early warning system’ to identify areas in which the virus has become active. Long-term impacts of the virus on corvids are not yet known, but there are reports of reduced crow populations in some regions of the United States.

The virus usually cycles between birds and mosquitoes. The rapid spread of West Nile virus across North America is thought to be due primarily to transport by infected birds. A recent study has found that several bird species, including the blue jay, common grackle, house finch, house sparrow and American crow, are competent hosts for the virus. That is, they develop high levels of virus in their blood and are capable of infecting mosquitoes that feed on them.

Most mosquitoes feed on one type of animal, but some species may change their feeding behaviour depending on what hosts are available. When an infected mosquito feeds on a human or other mammal, it can transmit the virus. These mosquitoes are called ‘bridge vectors’, as they are responsible for moving the virus from birds to mammals and people, which in turn are referred to as ‘dead-end hosts’ – because they do not build up enough virus in their blood to pass the virus on to other mosquitoes.

Though people and mammals are not important in the life cycle of the virus, the virus can cause severe health problems for these ‘accidental’ hosts. There is now growing evidence that predatory birds such as hawks and owls, as well as scavengers such as crows and magpies, may also get the virus by feeding on infected prey.

Other studies have also found that infected crows can spread the virus to other crows through close contact. Researchers are actively looking at these modes of transmission to determine how important they are in the spread of the virus. Many mosquito species have been found to carry the virus, but members of the Culex genus are thought to be most responsible for spreading the virus among birds, and from birds to humans and mammals. In eastern and southern parts of North America, Culex pipiens (northern house mosquito) and Culex quinquefasciatus (southern house mosquito) are important vectors of the disease. At present, mosquito re-search has not yet identified the important species in the west, but Culex tarsalis has been implicated. This mosquito is known to spread western equine encephalitis, another virus with a life cycle similar to West Nile.

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