Chester County, Pennsylvania
County of Chester
313 West Market Street
West Chester, PA 19380
610.344.6000

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West Nile Virus


Spray Schedule
July 31st - Tredyffrin Township
June 18th


Positive Mosquito Pools for 2014
Downingtown Borough
West Goshen Township
Easttown Township
Tredyffrin Township
Uwchlan Township

Mosquito Control and why it's Important to YOU


What is West Nile Virus (WNV)?

West Nile virus is an arthropod-borne virus (arbovirus) most commonly spread by infected mosquitoes. West Nile virus can cause febrile illness, encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord).

How do people get WNV?

WNV is transmitted to humans through the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. The virus eventually gets into the mosquito's salivary glands. The virus may be injected into humans and animals by the bite of an infected mosquito, where it can multiply and possibly cause illness.

How prevalent is WNV in Chester County?

Chester County had one confirmed human case in 2012, the first since 2008.

In 2012, 146 Mosquito samples tested positive for WNV in Chester County, compared to 29 positive samples in 2011.

Pennsylvania WNV-Positive Mosquito Samples Data


How is Chester County controlling the mosquito population?




  • Breeding source elimination: Environmental health specialists respond to complaints of standing water, such as tires, pools, buckets and other artificial containers. Property owners are ordered to dump and drain all standing water to eliminate potential mosquito breeding areas.
  • Larval Control: Biological larvicide is used to treat areas of stagnant water that cannot be drained. Larvacide controls mosquitoes in the larval form and is an environmentally safe method of mosquito control.
  • Adult Control/ULV Application: ULV is generally conducted when high numbers of adult mosquitoes are identified. Adult mosquito control is conducted with a truck or ATV mounted sprayer Ultra Low Volume Sprayer.
  • Regulation/Enforcement: Chester County regulations prohibit property from being maintained in a condition conducive to the breeding of mosquitoes. Citations may be issued for failure to comply.

How does the Health Department decide which areas should be treated for mosquitoes?

  • Surveillance - traps are set weekly in several locations across the county. Traps are then collected and  captured mosquitoes are identified and tested for WNV. Traps are placed in known breeding areas such as past positive locations, parks, sewage treatment plants and areas of concern. Traps are also placed in response to complaints from county residents regarding high numbers of mosquito activity.
  • Dipping - areas of standing or stagnant water is checked for presence of mosquito larvae in a process called dipping. Samples are taken for ID purposes only. The presence of mosquito larvae in water will result in larval treatment with biological larvicide.

What type of mosquito control products are used for treatment?

  • Biological Larvicide - Bti and Bs
  • Growth regulator - Methoprene (Altosid)
  • Permanone RTU Insecticide (active ingredient is Permethrin 3.98%)

Comparative Acute Toxicities Graph
*An LD50 is a measure of toxicity. The longer the LD50 bar, the lower the toxicity. Compared to the active ingredient in other leading mosquito control products, Etofenprox, the active ingredient in Zenivex® products is the least toxic.

How do I know when my neighborhood is being sprayed?

Chester County residents are alerted at least 48 hours in advance of a treatment by the following methods:
How can I reduce the mosquito population on my property?

Remove or clean up any mosquito-breeding sites on your property. Stagnant water is a breeding ground for mosquitoes:
  • Do not leave trash cans outside uncovered.
  • Keep wading pools covered or emptied.
  • Get rid of old tires.
  • Wheelbarrows, pots, plastic containers, etc should be turned over so as to not collect water.
  • Do not allow water in bird baths to become stagnant.
  • Drill holes in the bottom of recycling containers to eliminate standing water.
  • Clean roof gutters, particularly if leaves from surrounding trees have a tendency to plug drains.
  • Clean and chlorinate swimming pools when not in use.

Homeowners can treat stagnant pools of water on their property by purchasing BTi products at local lawn and garden supply stores. This naturally-occurring bacterial product kills mosquito larvae but is safe for people, pets, aquatic life and plants.

How can I protect myself and my family from mosquito bites?

  • Make sure that screens fit tightly over doors and windows to keep mosquitoes out of your home.
  • When possible, reduce outdoor activities at dawn and dusk during the summer when mosquitoes are most active.
  • If you must be outdoors at dawn and dusk, consider wearing long sleeves, long pants and socks.
  • Use insect repellant that contains DEET, icoaridin, or oil of lemon eucalyptus. Be sure to follow manufacturer instructions before applying. Insect repellant is not recommended for infants under 2 months of age.

What do I do if I find a dead bird?

If you find a dead bird on your property, pick it up with a shovel and place it in a plastic bag. There is no evidence that West Nile virus can be acquired by handling dead birds, but it is best not to handle the dead bird with your bare hands. Wear plastic/latex gloves, or use a plastic bag wrapped around your hand if you must handle the bird. Seal the bag and place it in a second bag, seal, and then dispose of it in the trash or bury it away from water sources. Wash your hands with warm water and soap immediately after disposing of the bird.

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

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Chester County