West Nile Virus

West Nile virus (WNV) is a member of the family Flaviviridae, which includes, among others, St. Louis encephalitis, dengue, yellow fever, and Japanese encephalitis viruses.  WNV can cause mild to severe illness in human, other mammal (e.g., 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. The following table shows the primary surveillance indicator results for WNY from 2003-2008:

 

Surveillance indicator
2003
2004
2005
2006
2007
2008
Human cases (fatal)
3(0)
779(29)
880(19)
278(7)
380(21)
438(15)
Horse cases
1
540
456
58
28
32
Dead bird positives
96
3,232
3,046
1,446
1,395
2,530
Mosquito pool positives
32
1,136
1,242
832
1,007
2,001
Sentinel chicken positives
70
809
1,053
640
510
584
Squirrel positives
--
49
48
32
26
32

 

2008 Summary of WNV Activity in California 

In 2008, 49 counties in California reported WNV activity.  A total of 438 human cases were reported to the California Department of Public Health  from 27 counties. Of the 438 cases, 15 (3.4%) were fatal. During the previous year, 21 deaths resulted from WNV infections. Non-human WNV activity reported included the following: 32 horse cases were reported from 14 counties; 2,001 WNV-positive mosquito pools were detected from 26 counties; 584 WNV-positive sentinel chickens were reported from 27 counties; 2,530 positive dead birds were reported from 46 counties; and 32 WNV positive tree squirrels were reported from 7 counties.

Click here for information on current WNV activity in California.