West Nile Virus

What is West Nile Virus (WNV)?

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WNV is a mosquito-borne virus that can cause fever, encephalitis (inflammation of the brain), or meningitis (inflammation of the lining of the brain and spinal cord).

How is it spread?

WNV is most commonly spread through the bite of an infected mosquito. WNV can be spread through blood transfusions, organ transplants, and from mother to baby during pregnancy, delivery, or breastfeeding but this is very rare. It is not transmitted from person to person, or from person to animal.

What are the symptoms?

Most people (70-80%) infected with WNV do not develop any symptoms.

If present, WNV symptoms usually appear 2-14 days after the mosquito bite. Approximately 1 in 5 people infected will develop a fever and possibly headache, body aches, joint pain, vomiting, diarrhea, or rash. Most people with these symptoms recover completely, but fatigue and weakness can last for weeks or months.

Less than 1% of people infected will develop serious neurologic illness such as encephalitis or meningitis (inflammation of the brain or surrounding tissues). Recovery from severe illness may take weeks or months. Some of the neurologic effects may be permanent. Only about 10% of people who develop neurologic infection due to WNV will die.

Serious illness can occur in people of any age. However, people over 60 years of age are at the greatest risk for serious illness. People with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and people who have received organ transplants are also at greater risk for serious illness.

See your health care provider if you have symptoms of WNV.

Who is at risk?

Anyone living in an area where mosquitoes are infected with WNV is at risk. WNV has been detected in all states except Alaska and Hawaii. The risk of infection is highest for people who work outside or participate in outdoor activities because of greater exposure to mosquitoes.

How can it be prevented?

All residents can help prevent WNV and other mosquito-borne diseases by making you and your home a Bite-Free Zone.

In addition, the Chester County Health Department’s Mosquito Borne Disease program monitors mosquito populations to control the spread of West Nile Virus, and investigates West Nile Virus infections in residents. Mosquito controls include EPA-registered products approved to kill mosquito larvae in bodies of standing water that cannot be drained, and adult mosquitos in areas that have high mosquito activity and multiple mosquito samples testing positive for WNV. Adult spraying is done as a last resort after exhausting all other mosquito control strategies. The Chester County Health Department notifies residents and municipal staff in designated spray areas 48 hours ahead of time through a press release, and also notifies registered beekeepers and residents who are listed as hypersensitive and in or within 500 feet of a designated spray area. People concerned about exposure to mosquito control products can reduce their potential for exposure by staying indoors when their neighborhood is being sprayed.

Is there a vaccine or treatment?

There is no vaccine or specific treatment for WNV infection.

People with mild symptoms of WNV infection usually recover on their own. Over-the-counter pain relievers can be used to reduce fever and relieve some symptoms. People with severe illness usually need to be hospitalized to receive supportive treatment, such as intravenous fluids, pain medication, and nursing care.

How can I find out when a mosquito control spray is being conducted in my neighborhood?

The Chester County Health Department notifies residents of sprays at least 48 hours ahead of time through the following channels:

For more information, call 610-344-6752 or email [email protected].



The Chester County Health Department is a member of the Environmental Protection Agency’s Pesticide Environmental Stewardship Program. This program requires participants to affirm that environmental stewardship is an integral part of their integrated pest management (IPM) practice, use current, comprehensive information regarding the life cycle of mosquitoes within their IPM program, educate the community on the benefits of IPM, and demonstrate a commitment to pesticide risk reduction activities.