West Nile Virus Hits California Hard

By Chindi Peavey

San Mateo County Mosquito Abatement District

The year 2004 marks the 100th anniversary of mosquito control programs in San Mateo County. This anniversary is especially relevant in light of the arrival of West Nile virus (a mosquito-borne disease) to Northern California.

Mosquito control in the county began in 1904, when the Burlingame Improvement Club asked entomologists at the University of California’s Agricultural Experiment Station to evaluate the mosquito problem. They found that 95 percent of the problem was caused by salt marsh mosquitoes (Aedes dorsalis and Aedes squamiger). The community raised $2,000 and hired a crew to carry out control work, which consisted of repairing tide gates, constructing ditches to increase tidal circulation, and applying oil to remaining pockets of standing water. The city of San Mateo and the County Board of Supervisors passed an ordinance forbidding the maintenance of standing water and giving the Board of Health authority to force property owners to clean up mosquito breeding sources on their land.

Even so, mosquito control campaigns remained spasmodic for several years. Residents were quoted as saying “life was unbearable” and as late as 1912 a well-known resort in San Mateo, the Peninsula Hotel, was forced to close in midseason and property values were plummeting.

Finally the California Legislature passed the Mosquito Abatement Act in 1915. A part of the California Health and Safety Code, this legislation laid the foundation for organized mosquito control in California. Two mosquito control districts were formed on the San Francisco Peninsula: the Three Cities Mosquito Abatement District, encompassing Burlingame, Hillsborough, and San Mateo, and the Pulgas Mosquito Abatement District just to the south. The two districts merged in 1953 to form the San Mateo County Mosquito Abatement District (MAD). A ballot measure passed in January 2004 expanded the district’s boundaries to include all of San Mateo County. 

The district’s mosquito control program has evolved over time to its present day emphasis on integrated pest management and control of larvae with biorational materials. After World War II, DDT was widely used to control mosquitoes throughout the state. Resistance to DDT was detected by 1959, which prompted a switch to other classes of insecticides. During the 1960s and 1970s, the San Mateo County MAD and others throughout the state participated in field trials of various biological materials for control of mosquito larvae. Effective biological materials became commercially available in the late 1970s and have now replaced more toxic materials. Today the district controls mosquito larvae using two bacteria and a hormone that prevent larvae from developing to the adult stage.

Although salt marsh mosquitoes continue to be a large part of the district’s focus, with the increase in urban development, control of northern house mosquitoes (Culex pipiens) has become equally important. House mosquitoes are so named for their propensity to enter homes and feed on humans as they sleep.

West Nile Virus in San Mateo County

This work has taken on even greater importance with the arrival of West Nile virus. House mosquitoes are among the three most efficient vectors of this virus and the most frequently encountered mosquito in this county during summer and fall. Two other vectors of West Nile virus also occur in San Mateo County: culex tarsalis (the encephalitis mosquito), which develops in freshwater marshes and agricultural sources; and culex erythrothorax (the tule mosquito), which develops in freshwater marshes with dense stands of emergent vegetation such as cattails or tules.

West Nile virus (WNV) can cause serious illness in humans and horses, and many species of birds. It was first discovered in a 50-year-old woman in the West Nile district of Uganda in 1937. Besides the United States, outbreaks have occurred in Africa, Europe, the Middle East, and Asia. WNV arrived in the United States in 1999 in the Queens neighborhood of New York City. It may have been introduced through importation of an infected bird or mosquito. In that first year, 62 human cases were reported in the United States, seven of them fatal. The virus spread rapidly across the country, resulting in 9,862 human cases and 264 deaths in 2003. Thus far in 2004, there have been 1,865 human cases with 59 deaths. The number is expected to climb before the mosquito season ends in November.

West Nile Virus affects the nervous system and can cause severe illness and even death in a small percentage of those infected. Only 20 percent of people exposed to WNV will develop symptoms and most will have a flu-like syndrome including fever, severe muscle aches, and headache. One out of 150 who develop symptoms will suffer more serious disease, such as stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis. These symptoms sometimes can last several weeks, and some effects may be permanent. Patients with a high fever lasting more than three days or any of the neurological signs mentioned should see a doctor immediately.

Thirty percent of horses infected with WNV die. There is a vaccine available for horses; check with your veterinarian.

WNV was first detected in California in August 2003. By the end of mosquito season, there had been three human cases and virus had been detected in 96 dead wild birds. That year the virus was restricted to Southern California. With the arrival of cool weather, viral activity ceased in November. Dead birds positive for WNV began appearing again in February 2004, and in July and August the virus spread rapidly to the rest of the state. Thus far, the virus has been detected in 56 of the state’s 58 counties. California led the nation this year in numbers of human cases, with 558 infections and 15 fatalities. Most of these occurred in Southern California. However, next year the number of cases and fatalities in Northern California is expected to rise.

The virus was first detected in San Mateo County in July of 2004. To date, the virus has been detected in 12 dead wild birds, primarily blue jays and crows. No human cases have been reported in this county.

West Nile virus is transmitted to humans through the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds. Humans and horses do not have enough virus circulating in their blood to pass the infection on to a feeding mosquito. A few patients have acquired WNV through blood transfusions and organ transplants. Blood banks now screen for the virus and several cases without disease symptoms have been detected through this process.

Although a case of virus transmission from mother to child through breast milk has occurred, physicians are not recommending the cessation of breast-feeding in mothers with a positive blood test. The benefits of breast-feeding far outweigh the risks of spreading the virus to the baby.

In summary, the best method of preventing WNV infection is by avoiding mosquito bites. This can be done by repairing screens on windows, wearing repellants such as DEET, and dressing in long pants and shirts with long sleeves. Mosquitoes are attracted to dark colors and will bite less readily through light colored clothing. Communitywide mosquito control is the best way to reduce the risk of WNV, and San Mateo County is fortunate to have a well-established preventative program that covers the entire county.

You can help by eliminating standing water on your own property and by calling if you are experiencing mosquito problems (SMC MAD 650-344-8592). The district will send a technician out to investigate mosquito complaints. It is helpful to provide a specimen of the mosquitoes biting you. Report a dead bird to the California Department of Health Services (877-968-2473). 

Ms. Peavey is a vector ecologist with the San Mateo County Mosquito Abatement Department.