West Nile virus hit California harder than ever last year, with a record 561 cases of neuroinvasive disease -- the most serious types of the illness -- reported from the mosquito-borne virus, according to federal health data released Thursday.
The number of these serious California cases was 83 percent higher than the previous record number reported in the state in 2005, according to the U.S. Centers for Disease Control and Prevention.
Most of those cases — 70 percent — were reported from Los Angeles and Orange counties, which recorded 15 West Nile deaths last year. Statewide, 31 people died of West Nile disease in the state in 2014.The West Nile virus is spread by the bite of an infected mosquito. Mosquitoes can become infected when they feed on birds carrying the virus. Most people infected will have no symptoms, but about one in five will develop a fever with other symptoms. Even fewer people will develop the serious neuroinvasive disease tracked by the CDC, including West Nile-caused meningitis and encephalitis, but most of them will require hospitalization.
Public health experts say there’s no single explanation for why some West Nile virus seasons are more severe than others.