Overwintering of West Nile virus in Southern California

William Reisen, Ying Fang, Hugh D. Lothrop, Vincent M. Martinez, Jennifer Wilson, Paul O'Connor, Ryan Carney, Barbara Cahoon-Young, Marzieh Shafii, Aaron Brault

Research output: Contribution to journalArticle

107 Scopus citations

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 in southern California. Continued transmission and vertical transmission by Culex p. quinquefasciatus Say seem likely candidates for further study.

Original languageEnglish (US)
Pages (from-to)344-355
Number of pages12
JournalJournal of Medical Entomology
Volume43
Issue number2
DOIs
StatePublished - Nov 20 2006

Keywords

  • Culex pipiens quinquefasciatus
  • Culex tarsalis
  • Overwintering
  • Southern california
  • West Nile virus

ASJC Scopus subject areas

  • Parasitology
  • veterinary(all)
  • Insect Science
  • Infectious Diseases

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  • Cite this

    Reisen, W., Fang, Y., Lothrop, H. D., Martinez, V. M., Wilson, J., O'Connor, P., Carney, R., Cahoon-Young, B., Shafii, M., & Brault, A. (2006). Overwintering of West Nile virus in Southern California. Journal of Medical Entomology, 43(2), 344-355. https://doi.org/10.1603/0022-2585(2006)043[0344:OOWNVI]2.0.CO;2