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West Nile Virus (WNV) first appeared in California in 2002 with the identification of one human case. In 2003, three human cases occurred in California and WNV activity was detected in six southern California counties. By 2004, WNV activity was observed in all 58 counties in California and 830 human infections were identified. Click Here for a summary of West Nile Virus in California in 2004.
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West Nile virus (WNV) is a mosquito-borne disease that is common in Africa, west Asia, the Middle East, and more recently, North America. Human infection with WNV may result in serious illness. Experts believe WNV is established as a seasonal epidemic in North America that flares up in the summer and continues into the fall.
WNV affects the central nervous system. However, symptoms vary:
West Nile virus was first detected in the United States in New York in 1999. Since then, WNV has spread to 48 states, and to Canada and Mexico. Last year there were 2,448 human cases of WNV detected in the United States, including 84 deaths. This is much lower than in 2003 when there were more than 10,000 human cases of WNV detected, including 262 deaths.
California is well prepared to detect, monitor, and respond to WNV through ongoing collaboration between over 100 public agencies. The California surveillance system includes human and horse case detection and testing of mosquitoes, sentinel chicken flocks, and dead birds for WNV.
Visit our How West Nile Virus is Transmitted page for more information.
People typically develop symptoms from 3 to14 days after they are bitten by an infected mosquito.
There is no specific treatment for WNV infection. In cases with milder symptoms, people experience fever and aches that pass on their own. In more severe cases, people may need to go to the hospital where they can receive supportive treatment including intravenous fluids, help with breathing, and nursing care.
Milder WNV illness improves without treatment, and people do not necessarily need to seek medical attention for this infection, though they may choose to do so. If you develop symptoms of severe WNV illness, such as unusually severe headaches or confusion, seek medical attention immediately. Pregnant women and nursing mothers are encouraged to talk to their doctor if they develop symptoms that could be WNV.
It is thought that once a person has recovered from WNV, they are immune for life to future infections with WNV. This immunity may decrease over time or with health conditions that compromise the immune system.
Find information about the who is at greatest risk of becoming severely ill from West Nile Virus on our Becoming Severely Ill from West Nile Virus page.
Find more information about how to prevent getting sick on our Avoiding Getting Sick from West Nile Virus page.
Yes, the public is encouraged to report dead birds because it helps the state monitor WNV activity. Birds play an important role in maintaining and spreading this virus. Mosquitoes acquire the virus from infected birds, and then transmit the virus to people. Evidence of the virus in dead birds is often the first indication that WNV has been introduced into a new region, or that transmission risk is high. Public reports of dead birds are provided to local mosquito control agencies who use this information to target WNV surveillance and control efforts. Some dead birds are tested for WNV. Dead birds can be reported via the website or by calling the hotline: 877-WNV-BIRD
State and local agencies conduct the following activities or provide the following services in conjunction with the statewide WNV prevention, surveillance and control program: