[HideMe]Mosquitoborne Diseases
Mosquitoes are primary vectors of serious infectious diseases in California. They transmit several arboviruses of public health importance, filarial worms causing dog heartworm, and sporadically and focally, human malarial parasites.
Photo credit Edman/McClelland collection
[HideMe]Arboviruses
Worldwide, there are several thousand recognized arboviruses. The term arbovirus was coined many years ago to include viruses that were transmitted to vertebrate animals by arthropods. Mosquitoes, ticks, and biting flies of various kinds are all vectors. In California, the most important viruses transmitted to humans and other vertebrates are mosquitoborne. Western equine encephalomyelitis virus (abbreviated WEE) is one of the important mosquitoborne viral diseases in California. For information on specific California mosquitoborne viruses, select the name of the virus from the menu at the right.
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) 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.
WNV Reports and Presentations
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.
[HideMe]Malaria
Malaria is no longer endemic in California. However, the disease is diagnosed in California residents every year, primarily as a result of contracting the disease in malarious areas outside the USA, and rarely, because of small focal outbreaks occurring as a result of importation of a case into an area when and where anopheline mosquitoes are prevalent.
Terms to describe the methods of acquiring human malaria cases have been developed by the World Health Organization and adopted by the Malaria Branch of the Centers for Disease Control and Prevention. They can be studied by selecting this link: Malaria Terms.
A thorough discussion of malaria occurring in areas where it is no longer endemic can be found by visiting the CDC malaria website.
Over the past several decades, California has had more cases of locally-acquired malaria than any other US state. To read an article summarizing subject of locally-transmitted malaria in the USA, click this link: Locally-transmitted malaria.
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