Vector Control Program: West Nile Virus Prevention & Mosquito Control
The West Nile Virus Prevention Program reduces mosquito populations using safe integrated pest management techniques and provides environmental health information so that all citizens can participate in preventing the spread of West Nile Virus in the City and County of San Francisco.
This program handles calls for assistance regarding mosquito activity in San Francisco and offers assistance to tenants and property owners in identifying and controlling potential and actual breeding sites. In addition, our staff includes state certified vector control specialists trained to apply and give advice on the control of pests and the use of pesticides.
West Nile Virus History & Information
West Nile virus (WNV) was first detected in the United States in 1999. WNV is an arthropod-borne virus (arbovirus) which is primarily transmitted by mosquitoes from bird to bird. People and other animals become infected through the bite of a mosquito carrying the virus.
The infection is fatal to most birds, so the presence of dead birds can mean that the virus is circulating. Infected dead birds were first found in San Francisco in August 2004. By 2005, infected birds and mosquitoes were identified in every county in California.
Several species of mosquito can transmit the virus to people, however only a small proportion of mosquitoes harbor the virus even when a larger proportion of the bird population is known to be infected. Most people who are bitten by a mosquito with WNV will not develop disease. About 80% of persons who do become infected will have no symptoms at all and will acquire long-term immunity to the virus. About 20% of infected persons may experience flu-like symptoms that might include aches, fever, fatigue and rash, which may last three weeks. Fewer than 1% of infected persons develop life-threatening neurological symptoms and require hospitalization for supportive care. Almost all fatalities have occurred in persons older than 50. Persons who are immune-compromised may be at greater risk of serious illness. At this time there is no vaccine or specific treatment for WNV disease. WNV is not normally spread from person to person.
The current California case count can be found at www.westnile.ca.gov.
Environmental Health Branch
The Environmental Health Branch (EHB) coordinates and monitors the City’s preparation and response to West Nile Virus; EHB certified vector technicians and the City’s pest control contractor investigate all reports of mosquito activity. Fortunately the cool, foggy, windy weather typical of San Francisco keeps mosquito activity relatively low, and our work to eliminate or treat mosquito breeding sources (standing water) with biological or least-toxic larvacides further limits the threat from West Nile Virus and other mosquito-borne diseases..
The Department of Public Health (DPH) also has partnerships with all local agencies, departments and institutions to craft creative and cost-effective strategies for mosquito control and for raising awareness among our citizens of the importance of avoiding mosquito bites. Some key public agencies include:
- San Francisco Public Utilities Commission
- Department of the Environment
- Recreation and Parks Department
- Department of Public Works
- San Francisco Unified School District
SFDPH Environmental Health Mosquito Control program has received grant support from the State of California to pay for additional mosquito control technicians, equipment and supplies.
How to prevent mosquito breeding
There are simple and practical steps San Francisco residents can take to prevent West Nile Virus illness. One step is to prevent mosquito breeding and sheltering in your yard. Mosquitoes breed by laying their eggs in standing water. These hatch into larvae (wigglers), and will mature into adult mosquitoes in about a week in warm weather. Where possible, residents should:
- Drain all standing water from the property, such as saucers below flower pots, hot tub covers, wading pools, hollow stumps and trash containers. Remove tires and car parts or store them indoors.
- Stock permanent ponds with fish that eat mosquito larvae. Pumps that circulate water are also effective.
- Clean out clogged roof gutters in the spring and fall, and maintain drains clear of leaf litter.
- Cut back overgrown vegetation, especially if it is growing in the shade, and do not over water your yard. Keep grass cut short and let the ground and the soil in potted plants dry on the surface before watering. Keep your ground clear of leaf litter.
- Use non-chemical insecticides such as Bti "dunks" that can be purchased at garden supply stores and used in pools of standing water that cannot be drained.
The San Francisco Public Utilities Commission routinely inspects and controls mosquitoes breeding in public catch basins and storm drains, with monthly treatments from late spring to early fall.
The San Francisco Health Code requires property owners in San Francisco to maintain their properties free of conditions that allow mosquitoes to breed.
How to prevent mosquito bites
West Nile Virus is transmitted by mosquito bites, so preventing mosquito bites is a critical element to preventing disease. Here are some simple ways to protect yourself and your family from bites.
- Screens: make sure that doors and windows have tight fitting screens. Repair or replace those with tears or holes.
- Bed nets: Place mosquito nets over beds and cribs.
- Dawn and dusk: avoid outdoor activity at dawn and dusk. In most areas the mosquito season is from May to October, but in the Bay Area the season may extend almost year round.
- Protective clothing: wear long sleeve shirts and long pants when mosquitoes are active.
- Repellents: DEET and permethrin products are most effective but must be used with caution, especially around children.
Making a Complaint
The Environmental Health Branch will pick up dead birds that are in good condition for West Nile Virus testing. The EHB will NOT pick up rock doves (pigeons) or water fowl. To report a dead bird for WNV testing, please call 1-877-968-2473 or 1-877-WNVBIRD.
The Environmental Health Branch investigates complaints about standing water and other conditions that may allow mosquito breeding. For complaints about these issues call or visit 311 to have your complaint recorded and routed correctly to Environmental Health.