West Nile virus (WNV) belongs to the flavivirus family which includes, among others, the dengue, yellow fever, and Japanese encephalitis viruses.  WNV can cause mild to severe illness in human, other mammal (eg. horses, squirrels), and avian hosts.  The virus circulates and amplifies in birds, and is transmitted to humans primarily by Culex mosquitoes. Prior to 1999, WNV was known to occur only in Africa, Asia, and the Middle East. WNV was first detected in North America in New York in 1999, and has subsequently spread to 48 states, Canada, and Mexico.

History of WNV Activity in California

WNV first appeared in California in 2002 with the identification of one human case. In 2003, WNV activity was detected in six counties in southern California; three human WNV cases and one equine WNV case were reported and WNV activity was detected among dead birds, mosquito pools, and sentinel chickens.  In 2004, 58 counties detected WNV activity and 779 human cases were reported, with 28 WNV-associated fatalities. In 2005, 880 human cases and 19 fatalities were reported, and non-human WNV activity was detected in 54 counties.  

2006 Summary of WNV Activity in California 

In 2006, 54 counties in California reported WNV activity.  A total of 278 human cases were reported to the California Department of Health Services from 36 counties. Of the 278 cases, 190 (68%) were classified as West Nile fever, 83 (30%) were identified as neuroinvasive disease cases (i.e. encephalitis, meningitis, or acute flaccid paralysis), and five were of unknown clinical presentation.  There were seven reported WNV-associated fatalities. Non-human WNV activity reported included the following: 58 horse cases were reported from 23 counties--24 died or were euthanized; 832 WNV positive mosquito pools were detected from 33 counties; 640 WNV positive sentinel chickens were reported from 29 counties; 1,446 positive dead birds were reported from 53 counties; and 32 WNV positive tree squirrels were reported from nine counties.

Click here for information on current WNV activity in California.