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California West Nile Virus Home > 2005 Summary 04/21/06

West Nile virus activity was detected in 54/58 California counties in 2005.

  • In 2005 there were 935 human WNV infections from 40 counties in California.
  • 101 of the 935 infections were first detected in asymptomatic individuals through screening done at blood banks - 42 later became symptomatic. Eight of the 101 presumptively viremic blood donors did not confirm in follow-up testing, and have been removed from the totals.
  • Four asymptomatic individuals were detected via local public health agencies.
  • Of the 880 WNV cases with symptoms, 305 were classified as West Nile neuroinvasive disease, 534 were classified as West Nile fever cases, and 41 were of unknown clinical presentation.
  • There were 19 West Nile virus-related fatalities reported in California in 2005 from Amador, Butte, Fresno, Kings, Madera, Plumas, Riverside, Sacramento, San Bernardino, San Joaquin, Stanislaus, and Tulare counties.
Equines (Horses)
  • In 2005, 456 horses from 43 counties were infected with WNV. Of the horses with infections, 200 have either died or been euthanized.
Dead Birds
Mosquito Pools and Sentinel Chickens
  • In 2005, 1,242 mosquito pools from 29 counties tested positive for WNV.
  • 1,053 chickens from 31 counties tested positive for WNV/SLE ***antibodies (seroconverted) in 2005.

* Horse data is only updated on Fridays

** A mosquito pool is a collection of approximately 50 mosquitoes that are tested together for West Nile virus.

*** WNV and SLE (St. Louis encephalitis) are closely related viruses in the family Flaviviridae and preliminary testing cannot distinguish between these two viruses.  Further testing is conducted as needed to determine if the sentinel chickens are antibody positive to WNV or SLE.

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