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West Nile Virus

What Is West Nile Virus?

West Nile Virus (WNV) is a disease carried by mosquitoes 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 in the fall.

When Was West Nile Virus First Found in North America?

WNV was first detected in the United States in New York in 1999. Since then, WNV has spread to 48 states, 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.

When Was West Nile Virus First Found in California?

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.

How Is West Nile Virus Detected and Monitored in California?

California is well prepared to detect, monitor and respond to WNV through ongoing collaboration between more than 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.

How Is West Nile Virus Transmitted?

  • Infected Mosquitoes. Most often, WNV is spread by the bite of an infected mosquito. Mosquitoes are WNV carriers that become infected when they feed on infected birds. Infected mosquitoes can then spread WNV to humans and other animals when they bite.
  • Transfusions, Transplants and Mother-to-Child. All donated blood is checked for WNV before being used. The risk of getting WNV through blood transfusions and organ transplants is very small, and should not prevent people who need surgery from having it. Transmission during pregnancy from mother to baby or transmission to an infant via breastfeeding is extremely rare.
  • Not Through Touching. WNV is not spread through casual contact such as touching or kissing a person with the virus or by breathing in the virus.

How Soon Do Infected People Get Sick?

People typically develop symptoms from 3 to 14 days after they are bitten by an infected mosquito.


For more information visit the California West Nile Virus Web site.​​

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