Overwintering of West Nile Virus in Southern California

Abstract

West Nile virus (family Flaviviridae, genus Flavivirus, WNV) invaded southern California during 2003, successfully overwintered, amplified to epidemic levels, and then dispersed to every county in the state. Although surveillance programs successfully tracked and measured these events, mechanisms that allowed the efficient overwintering and subsequent amplification of WNV have not been elucidated. Our current research provided evidence for three mechanisms whereby WNV may have persisted in southern California during the winters of 2003-2004 and 2004-2005: 1) continued enzootic transmission, 2) vertical transmission by Culex mosquitoes, and 3) chronic infection in birds. WNV was detected in 140 dead birds comprising 32 species, including 60 dead American crows, thereby verifying transmission during the November-March winter period. Dead American crows provide evidence of recent transmission because this species always succumbs rapidly after infection. However, WNV RNA was not detected concurrently in 43,043 reproductively active female mosquitoes comprising 11 species and tested in 1,258 pools or antibody in sera from 190 sentinel chickens maintained in 19 flocks. Although efficient vertical transmission by WNV was demonstrated experimentally for Culex tarsalis Coquillett infected per os, 369 females collected diapausing in Kern County and tested in 32 pools were negative for WNV. Vertical transmission was detected in Culex pipiens quinquefasciatus Say adults reared from field-collected immatures collected from Kern County and Los Angeles during the summer transmission period. Chronic infection was detected by finding WNV RNA in 34 of 82 birds that were inoculated with WNV experimentally, held for >6 wk after infection, and then necropsied. Frequent detection of WNV RNA in kidney tissue in experimentally infected birds >6 wk postinfection may explain, in part, the repeated detection of WNV RNA in dead birds recovered during winter, especially in species such as mourning doves that typically do not die after experimental infection. In summary, our study provides limited evidence to support multiple modes of WNV persistence i n southern California. Continued transmission andvertical transmission by Culex p. quinquefasciatus Say seem likely candidates for further study.

Environmental Factors and Risk Areas of West Nile Virus in Southern California, 2007–2009

“…According to the PDSI values as shown in Based on the results of environmental factor analysis, the positive role of temperature detected in the environmental factor analysis is consistent with the finding reported by Reisen et al [41,42]. Temperature must be above the minimum temperature (14.3°C) required for virus replication in mosquitoes for WNV to be disseminated throughout the year, especially in the cold months [42].…”

“…Temperature must be above the minimum temperature (14.3°C) required for virus replication in mosquitoes for WNV to be disseminated throughout the year, especially in the cold months [42]. A temperature study in southern California showed that mean temperatures in Coachella Valley in Riverside County and the hilly parts of Los Angeles County were mainly above the minimum in the coldest months of years 2003-2005, while temperatures in Long Beach, Los Angeles and Shafter, Kern County floated around the minimum in the same period of time, and the differences increase during summer [41]. The results also show that areas with lower elevations tended to be more susceptible to WNV invasion.…”

“…The west coast and central states have become the most prevalent regions for WNV epidemics. Its dissemination varies seasonally in response to the characteristics of environmental and socioeconomic conditions and climate [2,32,41,42]. Nonhuman WNV surveillance can be used as an early warning for WNV activity [10,35].…”

West Nile Virus Antibodies in Permanent Resident and Overwintering Migrant Birds in South-Central Kansas

“…Three mechanisms have been proposed for the overwintering of WNV: (1) continued enzootic transmission, (2) chronic infections in living birds, and (3) vertical transmission of virus (Reisen et al 2006). Of these, within-winter seroconversion of an individual bird is consistent with the first two mechanisms.…”

“…We report on the results of epitope-blocking ELISAs detecting WNV antibodies in overwintering migrant and permanent resident bird species in south-central Kansas during the winters of 2003-04 through 2005-06. Serial sampling was used to detect within-winter seroconversion of overwintering passerines, that could provide preliminary support for a role of enzootic transmission and/or chronic infection as overwintering mechanisms (Reisen et al 2006). In addition, many avian species were captured and assayed in the first general survey of seropositivity in this geographic region.…”