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West Nile Virus (WNV)

For the most current information on West Nile Virus see mosquito-borne disease webpage.


Image of mosquito

West Nile virus (WNV) is a mosquito-borne infection that can cause serious illness, and in some cases, death. Delaware’s first human case of WNV was confirmed in 2002. During 2003, 17 human cases were confirmed, including 2 deaths. Additionally, during 2003, there were 63 cases in horses. Subsequent to 2003, there have been fewer than 5 human cases confirmed statewide.

The chances of a person becoming ill with WNV are small. Most people who are infected with the WNV will not have any type of illness. It is estimated that 20% of the people who become infected will develop West Nile fever: mild symptoms, including fever, headache, and body aches, occasionally with a skin rash and swollen lymph glands. In many individuals, these symptoms are so mild that they go unnoticed or undetected.


The symptoms of severe infection (West Nile encephalitis or meningitis) can include headache, high fever, neck stiffness, muscle weakness, stupor, disorientation, tremors, convulsions, paralysis, and coma. It is estimated that one in 150 persons infected with the West Nile virus will develop the more severe form of the disease. Prevention of mosquito-bites is the most important way to reduce your risk of mosquito-borne diseases such as WNV.


West Nile virus is spread by Culex mosquitoes, which are active during dawn and dusk. To help protect yourself from mosquito bites and WNV, avoid activity outdoors during early morning and evening, and wear long sleeves, pants, and socks during those times.

Baby mosquitoes need water to grow into flying adults, so if mosquitoes are biting you around your home, there is probably standing water in your yard. Check your yard for containers holding water. There are many uncommon items that hold water like the folds in tarps, rocks in drainage ditches, clogged rain gutters, corrugated pipes, and wheelbarrows.


  • Tip and toss containers that are holding water.
  • Turn containers and wheelbarrows over, so that they cannot collect water.
  • Change the water in bird baths.
  • Trim bushes and vegetation to reduce adult mosquito resting habitat.
  • Empty water from tarps that are covering items like boats, mulch, dirt, etc. 



  • Repair any tears or rips in window screens to prevent mosquitoes from entering your home.
  • Clean leaves and debris from gutters, which can hold water if they are clogged.
  • Recycle old tires, which are perfect habitats for mosquito larvae.
  • Vaccinate your horse(s). In 2003, a WNV vaccine was approved to protect horses.
  • Use netting over strollers, playpens, etc.
  • Consider using EPA-registered mosquito repellents and apply them according to the product label. More information on repellents can be found below.


Repellents used on your skin make it harder for mosquitoes, flies, or ticks to find you. Applying repellents when you are outdoors can reduce your risk for bug bites. The CDC recommends using EPA-registered repellents that have been tested for safety and effectiveness. Some common ingredients in EPA-registered products contain DEET, Picaridin, Permethrin, Oil of Lemon Eucalyptus (OLE), and IR3535. Permethrin is never applied to skin. Natural plant oils like peppermint, lemon grass, etc. have not been properly tested and may cause skin irritation. The EPA has developed a tool to find out what repellent is best for you.

If you decide to use any kind of repellent, carefully read and follow all label directions before each use. On the labels, you will find important information about how to apply the repellent, whether it can be applied to skin and/or clothing, special instructions for children, hazards to humans, physical or chemical hazards and first aid. For more information on repellent safety and common myths about repellents read NEVBD’s Insect Repellent Guide in English or Spanish.



Office of Infectious Disease Epidemiology

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