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Home > Health Topics > West Nile Virus > Controlling West Nile Virus

Controlling West Nile Virus

What is West Nile Virus?

West Nile Virus is a viral infection of birds that is transmitted from bird to bird by mosquitoes. Occasionally humans, horses, and other animals can become infected when they are bitten by an infected mosquito.

What is the Contra Costa Mosquito & Vector Control District?

The Contra Costa Mosquito & Vector Control District (CCMVCD) is a public health agency that is made up of certified trained technicians who carry out a countywide mosquito and vector control program.

The District conducts countywide surveillance for West Nile Virus and the mosquitoes that carry it. CCMVCD regularly inspects and treats bodies of water known to produce mosquitoes.

If surveillance information or the documentation of West Nile Virus indicates a threat to human health, the District will spray to control adult mosquitoes and to reduce the risk of disease transmission.

What insecticides will be sprayed for the control of adult mosquito?

The District will use botanical insecticides Pyrethrins, synthetic Pyrethroids and Piperonyl Butoxide in either ground or aerial spraying. In an emergency situation, Malathion or Naled may be used. Insecticides used for the control of adult mosquitoes are called adulticides.

The District will only use products approved by the Environmental Protection Agency.

All products will be applied by trained and certified technicians.

What is Piperonyl Butoxide?

Piperonyl Butoxide is a chemical that does not have pesticidal effects of its own but enhances pesticidal properties of other chemicals.

With out Piperonyl Butoxide insect's digestive system may weaken pesticides before they can take effect.

Using Piperonyl Butoxide reduces the amount of pesticide required.

What are Pyrethrins and Pyrethroids?

Pyrethrins are naturally occurring compounds with insecticidal properties that are found in pyrethrum, an extract derived from chrysanthemum flowers.

Pyrethroids are manufactured chemicals that are very similar in structure to pyrethrins.

How are these insecticides used in adult mosquito control?

Mosquito control professionals apply adulticides as an ultra low volume (ULV) spray. ULV sprayers dispense very fine aerosol droplets that stay in the air and kill adult mosquitoes on contact.

Pyrethroids used in mosquito control are typically mixed with piperonyl butoxide, a compound that enhances the effectiveness of the active ingredient.

Do these insecticide pose risks to human health?

The risks to human health are very low because mosquito adulticides are applied as ULV sprays. ULV applications involve small quantities of active ingredient in relation to the size of the area being sprayed, usually less than two ounces per acre. This minimizes exposure and risk to people and environment.

Are pregnant women and children at a higher risk level?

So far there is little evidence that exposure to these chemicals at common levels can significantly increase the risk to unborn babies or children. The EPA has indicated that when pesticides are used according to label directions adverse effects are not likely.

Do these insecticides pose risks to wildlife or the environment?

The insecticides used in mosquito control by the District do not pose unreasonable risks to wildlife or the environment. The low rates the insecticides are being applied at make them low in toxicity to mammals and practically nontoxic to birds.

Pyrethroids are toxic to fish and to bees. For that reason, EPA has established specific precautions on the label to reduce such risks.

Should I take steps to reduce exposure to pesticides during mosquito control spraying?

Generally there is no need to relocate during mosquito control spraying. Mosquito control pesticides pose low risks, however people with pre-existing respiratory or allergic conditions may want to avoid or minimize exposure.

Steps that can be taken to reduce exposure during spraying include:
  • Remain indoors
  • Close windows
  • Turn off window unit air conditioners
  • Keep children's toys indoors

People who suffer from chemical sensitivities or feel spraying may aggravate a preexisting condition may want to consult their physician or local health department.

Where can I get more information about West Nile Virus and Pesticides?

More information on the West Nile Virus can be obtained from Contra Costa Health Services by visiting our web site at or by calling 925-313-6740.

For more information on Contra Costa Mosquito and Vector Control District, call 925-685-9301 or visit their website at


Content provided by the Public Health Division of Contra Costa Health Services.

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