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West Nile Virus Home > Ecology and Virology > Entomology


Arthropod-borne viruses (termed "arboviruses") are viruses that are maintained in nature through biological transmission between susceptible vertebrate hosts by blood-feeding arthropods (mosquitoes, sand flies, ceratopogonids "no-see-ums", and ticks). Vertebrates can become infected when an infected arthropod bites them to take a blood meal. The term 'arbovirus' has no taxonomic significance.

The arboviral encephalitides are zoonotic, being maintained in complex life cycles involving a nonhuman primary vertebrate host and a primary arthropod vector. These cycles usually remain undetected until humans encroach on a natural focus, or the virus escapes this focus via a secondary vector or vertebrate host as the result of some ecologic change. Humans and domestic animals can develop clinical illness but usually are incidental or "dead-end" hosts because they do not produce significant viremia (circulating virus), and thus do not contribute to the transmission cycle.

In the United States, West Nile virus is transmitted by infected mosquitoes, primarily members of the Culex species.

Image: Culex mosquito laying eggs.

Image: Culex mosquito laying eggs.
(View enlarged image.)

Arboviral encephalitis can be prevented in two major ways:

  1. Personal protective measures to reduce contact with mosquitoes and
  2.  Public health measures to reduce the population of infected mosquitoes in the environment.

Personal protection measures include reducing time outdoors, particularly in early morning and evening hours, wearing long pants and long sleeved shirts, and applying mosquito repellent to exposed skin areas and clothing.

Public health measures include elimination of larval habitats or spraying of insecticides to kill juvenile (larvae) and adult mosquitoes. The combination of mosquito control methods selected for use in a control program depends on the time of year, the type of mosquitoes to be controlled, and the habitat structure. In emergency situations, wide area aerial spraying is used to quickly reduce the number of adult mosquitoes. In many states, aerial spraying may be available as a means to control nuisance mosquitoes. Such resources can be redirected to areas of virus activity when necessary.

Financing of aerial spraying costs during disease outbreaks is often provided by state or local emergency funds. Federal funding of emergency spraying is rare and almost always is associated with a natural disaster such as flood or hurricane.

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