St. Louis Encephalitis Virus

St. Louis encephalitis virus (SLE), a member of the flavivirus family, was the most important mosquito-borne arbovirus in North America up until 1999, when West Nile virus (WNV) was introduced into the United States. Wild birds are the maintenance and amplifying hosts of SLE, which is transmitted among birds and to humans by primarily Culex mosquitoes. Human infection with SLE can result in mild to severe illness, with case-fatality rates ranging from 3%-30%.

SLE was first recognized in 1933 in St. Louis, Missouri, where it caused over 1000 human cases and 200 fatalities. The virus is found throughout the U.S., Canada, and nothern Mexico, and causes periodic epidemics of encephalitis, with outbreaks generally occuring between August and October. An average of 128 cases are reported annually in the U.S. The most recent outbreak occured in 1999, in New Orleans, Louisiana, with 20 reported cases.

SLE in California

Since 1945, 597 human cases of SLE have been reported in California. The most recent outbreaks occurred in 1984 and 1989 in the Los Angeles Basin (26 cases) and the southern San Joaquin Valley (29 cases), respectively. The last human case reported was in 1997, from Los Angeles County. SLE activity has not been detected in mosquito pools or sentinel chickens since 2003.

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.

 

CA Arbovirus Bulletins

Last 28 Days Positive Samples in California

As part of California's vector control efforts, samples from a variety of sources are regularly tested for the presence of West Nile virus, St. Louis encephalitis and Western equine encephalomyelitis. The map below is provided to illustrate the ongoing efforts to detect and control the these diseases. Use the layer checkboxes below the map to enable or disable the various layers.

[wmap id="survCAMap"][width]100%[/width] [height]400px[/height] [center]37.16031654673677,-119.2236328125[/center] [scale]5[/scale] [align]center[/align] [togglesize]Large[/togglesize] [basetoggle]on[/basetoggle] [scaletoggle]on[/scaletoggle] [basetype]Map[/basetype] [drag]yes[/drag] [overlays toggle="true"] [markers id="mosqpool" name="Mosquito Pools" display="true"] [icon]dot_black_red_16[/icon] [file] [source]http://www.calsurv.org/sites/calsurv.org/files/etc/PositiveMosquitoPools.csv[/source] [display]agency,city,county,species,collection date,test date,positive for[/display] [latitude]latitude[/latitude] [longitude]longitude[/longitude] [/file] [/markers][markers id="sentinel" name="Sentinel Chickens" display="true"] [icon]dot_black_blue_16[/icon] [file] [source]http://www.calsurv.org/sites/calsurv.org/files/etc/PositiveSentinelChickens.csv[/source] [display]agency,city,county,collection date,test date,positive for[/display] [latitude]latitude[/latitude] [longitude]longitude[/longitude] [/file] [/markers][/overlays] [/wmap]
All map data are updated every 30 minutes. All data coordinates are randomized estimations within half a degree of the true collection site.

Malaria Outbreaks in California Since 1945

Malaria is not endemic to California. But on occasion, imported malaria resulted in local outbreaks. The map below illustrates those transmissions where 10 or more cases resulted.

[wmap id="poolCAMap"] [width]100%[/width] [height]400px[/height] [center]37.16031654673677,-119.2236328125[/center] [scale]5[/scale] [align]center[/align] [togglesize]Large[/togglesize] [basetoggle]on[/basetoggle] [scaletoggle]on[/scaletoggle] [basetype]Map[/basetype] [drag]yes[/drag] [overlays] [markers] [icon]blue[/icon] [file] [source]http://www.calsurv.org/sites/calsurv.org/files/etc/malaria_map.csv[/source] [display]location,nearest_city,county,probable_vector,first_case,last_case,number_of_cases,description,reference[/display] [latitude]latitude[/latitude] [longitude]longitude[/longitude] [/file] [/markers][/overlays] [/wmap]
For terms and definitions used when describing human malaria cases, please look at Terms Associated with Human Malaria Cases