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Vector Control Program: West Nile Virus Prevention & Mosquito Control

The West Nile Virus Prevention Program is working diligently to reduce mosquito populations using safe integrated pest management techniques and providing environmental health information to prevent 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

First detected in the United States in New York City in 1999, West Nile virus (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, we have found infected birds and mosquitoes in every county in California.

Several species of mosquito can transmit the virus to people. Even where the virus is known to be present in birds, only a small proportion of mosquitoes will harbor the virus. Most people who are bitten by a mosquito with the virus 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. West Nile virus is not normally spread from person to person.

During 2004 there were 830 confirmed cases of human illness in California caused by West Nile virus, resulting in 28 deaths. Through mid-August 2005, there have been about 175 recorded cases of illness in California. West Nile virus is expected to affect Northern California harder this year and Sacramento has seen a significant number of cases of disease. Public health officials expect that more cases will appear in northern counties this summer and fall—the peak time for mosquito activity. So far, only one San Francisco resident has developed WNV disease. Investigations indicate that the case was most likely not acquired locally. The current California case count can be found at

Environmental Health Services

The Environmental Health Section (EHS) coordinates and monitors the City’s preparation and response to West Nile Virus. A key prevention goal is to eliminate or treat mosquito breeding sources (standing water) with biological or least-toxic larvacides. Fortunately, cool, foggy, windy weather in 2005 has kept mosquito activity relatively low; still, EH certified vector technicians and the City’s pest control contractor investigate all reports of mosquito activity.

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
  • Airport
  • MUNI
  • Port
  • Department of Public Works

A great deal of ground work was done during 2004 to prepare for West Nile virus. A new network of contacts was built within and between San Francisco’s departments, and with the larger community of stakeholders: our universities and schools, medical providers, property owners, community groups and residents. Key agencies have developed response plans, provided staff education, and developed a media campaign including posters on MUNI buses.

San Francisco has augmented its resources to control mosquitoes for 2005 and 2006. The Public Utility Commission routinely inspects and controls mosquitoes breeding in public catch basins and sewers, with monthly treatments from late spring to early fall.

The State of California recently awarded the SFDPH Environmental Health Mosquito Control program a grant 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.

Because West Nile Virus is transmitted by mosquito bites, preventing mosquito bites can also prevent disease in San Francisco. 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.
  • 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.

The San Francisco Health Code requires property owners in San Francisco to maintain their properties free of conditions that allow mosquitoes to breed.