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West Nile Virus


Centers for Disease Control West Nile Virus Activity by State as of August 26, 2014



Although the West Nile virus mainly infects birds, it can be transmitted to humans and other animals by certain species of mosquitoes that take blood meals from both birds and humans. The bite from just one infected mosquito is all that is needed to transfer the disease. 

 In humans, West Nile virus disease symptoms vary from no visible effect to flu-like symptoms, paralysis, or even death. All age groups are susceptible, but the elderly and those with weakened immune systems are more likely to experience severe forms of the disease. More on symptoms

Many cities and towns have excellent research based public mosquito control programs.  Even the best community mosquito control program will only reduce the risk from West Nile virus and thus wherever you live, personal protection against mosquito bites is your best safeguard.

The extreme weather conditions of 2014 from drought in the western United States to record flooding in the center part of the country has heightened concern relative to the virus.   These  extremes both have the potential to support populations of virus carrying mosquitoes.

Types of mosquitoes in the U.S. 

Asian Tiger Mosquito

Since 1999, in the United States more than 60 mosquito species have been found in West Nile virus positive mosquito pools.  These are pools from which West Nile virus was isolated, West Nile RNA detected, or West Nile antigen was detected using a variety of diagnostic tests.  Although a mosquito species found positive for the virus in nature may potentially be a carrier, further tests are usually needed to determine if the species is efficient in transmitting the virus to humans.  It is important to know which mosquitoes are efficient carriers of the virus in your state or local community.   

The summer of 2013 east coast and southern states started to see are large numbers of the Asian Tiger Mosquito.  The mosquito is known to be an aggressive daytime biter and is a carrier of the West Nile Virus.  For more information on this mosquito see these publications:

University of Florida -- Featured Creatures

University of Maryland -- Mosquitoes

 You may track identification of positive virus mosquito pools for your county and state at the link - 

                           Tracking positive mosquito pools in the United States


Why do mosquitoes bite? 

Adult female mosquitoes need blood to produce eggs and perpetuate their species. Although they can survive by feeding on sugary liquids, it is only after a blood meal that they start producing eggs. Only female mosquitoes bite. Male mosquitoes feed on sugars found in fruits and flowers.

Mosquitoes looking for a blood meal are mainly attracted to carbon dioxide in the breath, body heat, and sweat of humans. Lactic acid and numerous other scents emitted by the human skin have also been found attractive to mosquitoes. Individuals who produce more body heat, sweat, carbon dioxide, and lactic acid will be more attractive to mosquitoes.

Birds, horses, cattle, dogs, cats, rabbits, squirrels, and other animals are fed upon by mosquitoes. It is when mosquitoes bite different hosts that disease-causing microorganisms may be spread. 


Disease-causing microorganisms mosquitoes transmit 

Flowchart: West Nile Virus Transmission Cycle from the CDC web site, click on the image to visit the site and view the full-sized version.


  • Arboviruses

(short for arthropod-borne viruses) such as the West Nile virus, western equine encephalitis virus, and Saint Louis encephalitis virus are perhaps the most important microorganisms that mosquitoes transmit in the United States. In other parts of the world, mosquitoes are carriers of parasites that cause human malaria, filariasis, dengue fever, Japanese encephalitis, yellow fever, and others.

The West Nile virus is a disease that primarily affects birds. It may be transmitted by mosquitoes from infected birds to other hosts such as humans or horses. The West Nile virus has not been shown to be transmitted from human to human, horses to humans, or horses to horses. Thus, the West Nile virus is carried by various species of birds and spread by mosquitoes.



Most effective way to prevent bites and control mosquitoes at home 

Insect repellents are recommended as the best way for one to protect themselves from mosquito bites when involved in outdoor activities.  Repellents act by making a person "undesirable" for feeding or in other words the repellent masks the gases and scents known to be attractive to mosquitoes.

DEET and Picaridin are recommended to be applied to the skin and permethrin applied on the clothing. DEET and Picaridin repel mosquitoes while permethrin actually kills mosquitoes on contact.

Using DEET or Picaridin alone or permethrin alone will not be as effective as using the two in combination. However, using DEET or Picaridin alone may be sufficient for most outdoor activities such as going to the park, mowing the lawn, gardening, or relaxing in the backyard. Individuals who will be outdoors for an extended period of time, like hunters and campers, are encouraged to use the combination of DEET or Picaridin and permethrin.

There are alternative repellents such as combinations of soybean oil, geranium oil and coconut oil and lemon eucalyptus that have been shown to repel mosquitoes, but only for short periods of time. Before using any repellent read and follow all label directions.

Mosquitoes lay their eggs in standing water. Limit the number of places around your home for mosquitoes to breed by getting rid of items that hold water. Repair broken screens and windows on your home.

 Support your community mosquito control programs.


Last Updated:9/2/2014 10:52 AM

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