West Nile Virus

What it is…

West Nile Virus (WNV) is a mosquito-borne virus that can, in rare cases, cause encephalitis or inflammation of the brain. WNV was discovered in 1937 in the West Nile district of Uganda. West Nile Virus was first identified in 1999 in the United States. Since the first identification, WNV has been established as a seasonal epidemic in North America. Infections for WNV flare up in the summer and into the fall.

What it does…

A bite from an infected mosquito spreads WNV. Contact with infected people or animals does not spread WNV. All people are at risk of getting WNV. There is a higher chance of obtaining WNV if you are exposed to mosquito bites during the summer months. People over the age of 50 are more likely to develop serious symptoms of WNV if they do get sick.

What to expect…

Approximately 80 percent of people who are infected with WNV will not show any symptoms at all, but there is no way to know in advance if you will develop an illness or not. Up to 20 percent of the people who become infected will have symptoms which can include fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms can last for as short as a few days to as long as several weeks. Milder WNV illness improves on its own, and people do not need to seek medical attention for this infection though they may choose to do so.

About 1 in 150 people infected with WNV will develop severe illness. The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis. These symptoms may last several weeks, and neurological effects may be permanent. If you develop symptoms of severe WNV illness, such as unusually severe headaches or confusion, seek medical attention immediately.

What to do…

Individuals should take precautions to avoid mosquito bites to decrease their chance of being infected. Use mosquito repellant when outside. Wear a long-sleeved shirt and long pants, especially between dusk until dawn. Ensure that your home has good screens on windows to keep mosquitos out. Drain anything that holds water twice a week, such as a watering can, bird baths, or empty flowerpots.

Stay informed…

For up-to-date information on WNV including signs/symptoms, spread, prevention, treatment, and more, go to Centers for Disease Control and Prevention website or seek other credible sources. For any questions regarding you or your family’s health, consult your chosen primary care provider.

Center for Disease Control and Prevention. (2023) West Nile Virus. 

Pennsylvania Department of Environmental Protection. West Nile Virus Control Program. 

Lancaster/Lebanon Mosquito-Borne Disease Control Program. 


In 2000, West Nile virus appeared for the first time in Pennsylvania in birds, mosquitoes and a horse.  To combat the spread of West Nile virus, which is transmitted by mosquitoes, Pennsylvania has developed a comprehensive network.  This network, which covers 40 counties, includes trapping mosquitoes, collecting dead birds and monitoring horses, people and, in years past, sentinel chickens.   Please explore the PA West Nile Virus Control Program Website to find out more about how you can help, to learn about West Nile virus or the latest surveillance update from  your area. 

Have a general question or in need of mosquito control?
  • Click here to visit the Lancaster/Lebanon County Mosquito Borne Disease Program to get in contact with the team. 
  • Email info@lccd.com and ask for information about their Mosquito-Borne Disease Control Program. 
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