West Nile virus in California

Abstract

West Nile virus (WNV) was first isolated in California during July 2003 from a pool of Culex tarsalis collected near El Centro, Imperial County. WNV transmission then increased and spread in Imperial and Coachella Valleys, where it was tracked by isolation from pools of Cx. tarsalis, seroconversions in sentinel chickens, and seroprevalence in free-ranging birds. WNV then dispersed to the city of Riverside, Riverside County, and to the Whittier Dam area of Los Angeles County, where it was detected in dead birds and pools of Cx. pipiens quinquefasciatus. By October, WNV was detected in dead birds collected from riparian corridors in Los Angeles, west to Long Beach, and through inland valleys south from Riverside to San Diego County. WNV was reported concurrently from Arizona in mid-August and from Baja, Mexico, in mid-November. Possible mechanisms for virus introduction, amplification, and dispersal are discussed.

Figure 1

Map of California showing locations where A) 9,731 mosquito pools were collected and B) 212 sentinel chicken flocks were located through November 1, 2003. Data are cumulative for 2003 and show negative, previously positive, and currently active sites as downloaded from http://www.vector.ucdavis.edu/. SLE, St. Louis encephalitis virus; WEE, western equine encephalitis virus; BUN, viruses in the California encephalitis virus complex, family Bunyaviridae; WN, West Nile virus.

Figure 2

Introduction of West Nile virus into California. Panels show the locations of positive mosquito pools, sentinel chicken flocks with >1 seroconversion, and positive dead birds during each month. Encircled in panel D are the locations of the three foci studied in depth during 2003.

Figure 3

Virus temporal dynamics in relation to Culex tarsalis in A) Imperial and B) Coachella Valleys. Shown are female (F) Cx. tarsalis collected per CO₂ trap night (TN). West Nile virus minimum infection rates (MIR) per 1,000 tested adjusted for differential sample sizes, and the number of sentinel chicken seroconversions per 2-week period.

Figure 4

Wild bird Flavivirus seroprevalence rates (Flavi pos %) in Coachella Valley during 2003. Shown are percentages of total serum samples that tested positive each month by enzyme immunoassay. Positives include infections caused by West Nile virus and St. Louis encephalitis.

Figure 5

Climate conditions in A) Coachella Valley and B) Los Angeles at National Oceanic and Atmospheric Administration weather stations. Of interest was the dramatic drop in maximum temperature during early November coincident with the end of transmission. Ppt, precipitation.

Figure 6

Virus temporal dynamics in relation to Culex abundance in the Whittier Narrows area of Los Angeles County. Shown are A) female Cx. tarsalis collected per CO₂ trap night (TN) and female Cx. p. quinquefasciatus collected per gravid TN, West Nile virus (WNV) minimum infection rates (MIR) per 1,000 Cx. p. quinquefasciatus tested, adjusted for differential sample sizes, and B) number of dead birds reported, tested, and positive for WNV in Los Angeles County.

Figure 7

Road map of California. Arrows indicate the points of entry of main U.S. highways into California from the East.