• View in gallery
    Figure 1.

    Monthly changes in (A) numbers of birds collected per month during 1996–2005 and (B) pools of mosquitoes collected and testing positive for West Nile virus (WNV) (WNV+) from 2004 to 2007 from Coachella Valley. Bird species remaining year round were indicated by dashed lines.

  • View in gallery
    Figure 2.

    Experimental infections of (A) Orange-crowned Warblers, (B) Yellow Warblers, and (C) Common Yellowthroats with the NY99 strain of West Nile virus (WNV). Shown are daily viremia estimates in days post infection in log10 plaque forming units per mL for each bird dying (filled points) or surviving (open points). Assay sensitivity 2.0 log10 plaque forming units (PFU)/mL.

  • View in gallery
    Figure 3.

    Experimental infections of (A) Orange-crowned Warblers and (B) Yellow-rumped Warblers with the KERN217 strain of SLEV. Shown are daily viremia estimates in days post infection in log10 plaque forming units per mL for each bird dying (filled points) or surviving (open points). Assay sensitivity 1.7 log10 PFU/mL.

  • 1.

    Rappole JH, Derrickson SR, Hubalek Z, 2000. Migratory birds and spread of West Nile virus in the Western Hemisphere. Emerg Infect Dis 6: 319328.

    • Search Google Scholar
    • Export Citation
  • 2.

    Rappole JH, Hubalek Z, 2003. Migratory birds and West Nile virus. J Appl Microbiol 94 (Suppl): 47S58S.

  • 3.

    Peterson AT, Vieglais DA, Andreasen JK, 2003. Migratory birds modeled as critical transport agents for West Nile virus in North America. Vector Borne Zoonotic Dis 3: 2737.

    • Search Google Scholar
    • Export Citation
  • 4.

    Dusek RJ, McLean RG, Kramer LD, Ubico SR, DuPuis AP, Ebel GD, Guptill SC, 2009. Prevalence of West Nile virus in migratory birds during spring and fall migration. Am J Trop Med Hyg 81: 11511158.

    • Search Google Scholar
    • Export Citation
  • 5.

    Malkinson M, Banet C, Weisman Y, Pokamunski S, King R, Drouet MT, Deubel V, 2002. Introduction of West Nile virus in the Middle East by migrating white storks. Emerg Infect Dis 8: 392397.

    • Search Google Scholar
    • Export Citation
  • 6.

    Lopez G, Jimenez-Clavero MA, Tejedor CG, Soriguer R, Figuerola J, 2008. Prevalence of West Nile virus neutralizing antibodies in Spain is related to the behavior of migratory birds. Vector Borne Zoonotic Dis 8: 615621.

    • Search Google Scholar
    • Export Citation
  • 7.

    Figuerola J, Jimenez-Clavero MA, Lopez G, Rubio C, Soriguer R, Gomez-Tejedor C, Tenorio A, 2008. Size matters: West Nile virus neutralizing antibodies in resident and migratory birds in Spain. Vet Microbiol 132: 3946.

    • Search Google Scholar
    • Export Citation
  • 8.

    Stamm DD, Newman RJ, 1963. Evidence of southward transport of arboviruses from the U.S. by migratory birds. Ann Microbiol 11: 123133.

  • 9.

    Lord RD, Calisher CH, 1970. Further evidence of southward transport of arboviruses by migratory birds. Am J Epidemiol 92: 7378.

  • 10.

    Calisher CH, Maness KS, Lord RD, Coleman PH, 1971. Identification of two South American strains of eastern equine encephalomyelitis virus from migrant birds captured on the Mississippi Delta. Am J Epidemiol 94: 172178.

    • Search Google Scholar
    • Export Citation
  • 11.

    Crans WJ, Cassamise DF, McNelly JR, 1994. Eastern equine encephalomyelitis in relation to the avian community of a coastal cedar swamp. J Med Entomol 31: 711728.

    • Search Google Scholar
    • Export Citation
  • 12.

    Brault AC, Powers AM, Chavez CL, Lopez RN, Cachon MF, Gutierrez LF, Kang W, Tesh RB, Shope RE, Weaver SC, 1999. Genetic and antigenic diversity among eastern equine encephalitis viruses from North, Central and South America. Am J Trop Med Hyg 61: 579586.

    • Search Google Scholar
    • Export Citation
  • 13.

    Reisen WK, Monath TP, 1989. Western equine encephalomyelitis, Monath TP, ed. The Arboviruses: Epidemiology and Ecology. Boca Raton, FL: CRC Press, 89138.

    • Search Google Scholar
    • Export Citation
  • 14.

    Reisen WK, Lundstrom JO, Scott TW, Eldridge BF, Chiles RE, Cusack R, Martinez VM, Lothrop HD, Gutierrez D, Wright S, Boyce K, Hill BR, 2000. Patterns of avian seroprevalence to western equine encephalomyelitis and St. Louis encephalitis viruses in California, USA. J Med Entomol 37: 507527.

    • Search Google Scholar
    • Export Citation
  • 15.

    Kramer LD, Chandler LJ, 2001. Phylogenetic analysis of the envelope gene of St. Louis encephalitis virus. Arch Virol 146: 23412355.

  • 16.

    Monath TP, Cropp CB, Lanciotti RS, Trent DW, 1980. Variation in virulence for mice and Rhesus monkeys among St. Louis encephalitis virus strains of different origin. J Virol Methods 29: 948962.

    • Search Google Scholar
    • Export Citation
  • 17.

    Kramer LD, Fallah HM, 1999. Genetic variation among isolates of western equine encephalomyelitis virus from California. Am J Trop Med Hyg 60: 708713.

    • Search Google Scholar
    • Export Citation
  • 18.

    Kramer LD, Presser SB, Hardy JL, Jackson AO, 1997. Genotypic and phenotypic variation of selected Saint Louis encephalitis viral strains in California. Am J Trop Med Hyg 57: 222229.

    • Search Google Scholar
    • Export Citation
  • 19.

    Reisen WK, Lothrop HD, Chiles RE, Cusack R, Green EG, Fang Y, Kensington M, 2002. Persistence and amplification of St. Louis encephalitis virus in the Coachella Valley of California, 2000–2001. J Med Entomol 39: 793805.

    • Search Google Scholar
    • Export Citation
  • 20.

    Reisen WK, Lothrop HD, Wheeler SS, Kensington M, Gutierrez A, Fang Y, Garcia S, Lothrop B, 2008. Persistent West Nile virus transmission and the displacement St. Louis encephalitis virus in southeastern California, 2003–2006. J Med Entomol 45: 494508.

    • Search Google Scholar
    • Export Citation
  • 21.

    Reisen WK, Hardy JL, Presser SB, Milby MM, Meyer RP, Durso SL, Wargo MJ, Gordon EW, 1992. Mosquito and arbovirus ecology in southeastern California, 1986–1990. J Med Entomol 29: 512524.

    • Search Google Scholar
    • Export Citation
  • 22.

    Reisen WK, Lothrop HD, Chiles RE, Madon MB, Cossen C, Woods L, Husted S, Kramer VL, Edman JD, 2004. West Nile virus in California. Emerg Infect Dis 10: 13691378.

    • Search Google Scholar
    • Export Citation
  • 23.

    Reisen WK, Carroll BD, Takahashi R, Fang Y, Garcia S, Martinez VM, Quiring R, 2009. Repeated West Nile virus epidemic transmission in Kern County, California, 2004–2007. J Med Entomol 46: 139157.

    • Search Google Scholar
    • Export Citation
  • 24.

    McCaughey K, Miles SQ, Woods L, Chiles RE, Hom A, Kramer VL, Jay-Russel M, Sun B, Reisen WK, Scott TW, Hui LT, Steinlein DB, Castro M, Houchin A, Husted S, 2003. The California West Nile virus dead bird surveillance program. Proc Mosq Vector Control Assoc Calif 71: 3843.

    • Search Google Scholar
    • Export Citation
  • 25.

    Chiles RE, Reisen WK, 1998. A new enzyme immunoassay to detect antibodies to arboviruses in the blood of wild birds. J Vector Ecol 23: 123135.

    • Search Google Scholar
    • Export Citation
  • 26.

    Ebel GD, DuPuis AP, Nicholas D, Young D, Maffei J, Kramer LD, 2002. Detection by enzyme-linked immunosorbent assay of antibodies to West Nile virus in birds. Emerg Infect Dis 8: 979982.

    • Search Google Scholar
    • Export Citation
  • 27.

    Kramer LD, Wolfe TM, Green EN, Chiles RE, Fallah H, Fang Y, Reisen WK, 2002. Detection of encephalitis viruses in mosquitoes (Diptera: Culicidae) and avian tissues. J Med Entomol 39: 312323.

    • Search Google Scholar
    • Export Citation
  • 28.

    Shi PY, Kauffman EB, Ren P, Felton A, Tai JH, DuPuis AP, Jones SA, Ngo KA, Nicholas DC, Maffei J, Ebel GD, Bernard KA, Kramer LD, 2001. High-throughput detection of West Nile virus RNA. J Clin Microbiol 39: 12641271.

    • Search Google Scholar
    • Export Citation
  • 29.

    Lanciotti RS, Kerst AJ, Nasci RS, Godsey MS, Mitchell CJ, Savage HM, Komar N, Panella NA, Allen BC, Volpe KE, Davis BS, Roehrig JT, 2000. Rapid detection of West Nile virus from human clinical specimens, field-collected mosquitoes, and avian samples by a TaqMan reverse transcriptase-PCR assay. J Clin Microbiol 38: 40664071.

    • Search Google Scholar
    • Export Citation
  • 30.

    Spence LP, 1980. St. Louis encephalitis in tropical America. Monath TP, ed. St. Louis Encephalitis. Washington, DC: Am. Publ. Hlth. Assoc., 451471.

    • Search Google Scholar
    • Export Citation
  • 31.

    Kramer LD, Styer LM, Ebel GD, 2008. A global perspective on the epidemiology of West Nile virus. Annu Rev Entomol 53: 6181.

  • 32.

    Reisen WK, Chiles RE, Green EN, Fang Y, Mahmood F, 2003. Previous infection protects finches from re-infection with St. Louis encephalitis virus. J Med Entomol 40: 300305.

    • Search Google Scholar
    • Export Citation
  • 33.

    Reisen WK, Chiles RE, Martinez VM, Fang Y, Green EN, 2004. Encephalitis virus persistence in California birds: experimental infections in mourning doves (Zenaidura macroura). J Med Entomol 3: 462466.

    • Search Google Scholar
    • Export Citation
  • 34.

    Nemeth NM, Oesterle PT, Bowen RA, 2009. Humoral immunity to West Nile virus is long-lasting and protective in the house sparrow (Passer domesticus). Am J Trop Med Hyg 80: 864869.

    • Search Google Scholar
    • Export Citation
  • 35.

    Nemeth N, Young G, Ndaluka C, Bielefeldt-Ohmann H, Komar N, Bowen R, 2009. Persistent West Nile virus infection in the house sparrow (Passer domesticus). Arch Virol 154: 783789.

    • Search Google Scholar
    • Export Citation
  • 36.

    Reisen WK, Fang Y, Lothrop HD, Martinez VM, Wilson J, O'Connor P, Carney R, Cahoon-Young B, Shafii M, Brault AC, 2006. Overwintering of West Nile virus in southern California. J Med Entomol 43: 344355.

    • Search Google Scholar
    • Export Citation
  • 37.

    Wheeler SS, Reisen WK, 2009. Persistent West Nile virus infections in avian hosts: a possible overwintering mechanism for WNV? Proc Mosq Vector Control Assoc Calif 77: 4243.

    • Search Google Scholar
    • Export Citation
  • 38.

    Lothrop HD, Lothrop BB, Gomsi DE, Reisen WK, 2008. Intensive early season adulticide applications decrease arbovirus transmission throughout the Coachella Valley, Riverside County, California. Vector Borne Zoonotic Dis 8: 475489.

    • Search Google Scholar
    • Export Citation
  • 39.

    Tesh RB, Siirin M, Guzman H, Travassos da Rosa AP, Wu X, Duan T, Lei H, Nunes MR, Xiao SY, 2005. Persistent West Nile virus infection in the golden hamster: studies on its mechanism and possible implications for other flavivirus infections. J Infect Dis 192: 287295.

    • Search Google Scholar
    • Export Citation
  • 40.

    Reisen WK, Chiles RE, Martinez VM, Fang Y, Green EN, 2003. Experimental infection of California birds with western equine encephalomyelitis and St. Louis encephalitis viruses. J Med Entomol 40: 968982.

    • Search Google Scholar
    • Export Citation
  • 41.

    Hom A, Marcus L, Kramer VL, Cahoon B, Glaser C, Cossen C, Baylis E, Jean C, Tu E, Eldridge BF, Carney R, Padgett K, Sun B, Reisen WK, Woods L, Husted S. 2005. Surveillance for mosquito-borne encephalitis virus activity and human disease, including West Nile virus, in California, 2004. Proc Mosq Vector Control Assoc Calif 73: 6677.

    • Search Google Scholar
    • Export Citation
  • 42.

    Reisen WK, Thiemann T, Barker CM, Lu H, Carroll B, Fang Y, Lothrop HD, 2010. Effects of warm winter temperature on the abundance and gonotrophic activity of Culex (Diptera: Culicidae) in California. J Med Entomol 47: 230237.

    • Search Google Scholar
    • Export Citation
  • 43.

    Reeves WC, 1961. Overwintering of anthropod-borne viruses. Prog Med Virol 3: 5978.

Past two years Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 302 140 32
PDF Downloads 95 50 3
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

 

 

Migratory Birds and the Dispersal of Arboviruses in California

William K. ReisenCenter for Vectorborne Diseases, Department of Pathology, Microbiology and Immunology, School of Veterinary Medicine, University of California, Davis, California

Search for other papers by William K. Reisen in
Current site
Google Scholar
PubMed
Close
,
Sarah S. WheelerCenter for Vectorborne Diseases, Department of Pathology, Microbiology and Immunology, School of Veterinary Medicine, University of California, Davis, California

Search for other papers by Sarah S. Wheeler in
Current site
Google Scholar
PubMed
Close
,
Sandra GarciaCenter for Vectorborne Diseases, Department of Pathology, Microbiology and Immunology, School of Veterinary Medicine, University of California, Davis, California

Search for other papers by Sandra Garcia in
Current site
Google Scholar
PubMed
Close
, and
Ying FangCenter for Vectorborne Diseases, Department of Pathology, Microbiology and Immunology, School of Veterinary Medicine, University of California, Davis, California

Search for other papers by Ying Fang in
Current site
Google Scholar
PubMed
Close

Each spring large numbers of neotropical migrants traversing the Pacific flyway pass through the Coachella Valley enroute to northern destinations, providing an opportunity to test the hypothesis that mosquito-borne encephalitis viruses are introduced annually into California by migratory birds. A total of 5,632 sera were collected from 43 species of migrants during spring (April–June), of which 34 (0.61%) comprised of 14 species tested positive by enzyme immunoassay; only 10 were confirmed by plaque reduction neutralization tests (PRNT). In addition, of 1,109 migrants comprised of 76 species that were reported dead by the public and necropsied, 126 (11%) were positive for West Nile virus (WNV) RNA; however, only three (0.7%) of 428 birds tested during the spring were positive. Limited experimental infection studies with WNV showed that Orange-crowned Warblers were highly susceptible and frequently died, whereas most Yellow Warblers survived. Our results indicated that birds entering California rarely exhibited a history of infection and that most birds probably became infected after entering California.

Introduction

The rapid dispersal and colonization of the New World by West Nile virus (WNV) has refocused attention on the role of migratory birds as vehicles for the rapid long distance movement of arboviruses.14 Proof of principal for long distance movement was shown recently in the Old World by the recovery of WNV from migrating storks5 and WNV antibody from other migrant species.6,7 Historically in the United States, large studies along the Mississippi and Atlantic flyways failed to document the introduction of encephalitis viruses by northbound migrants.811 In agreement, genetic studies have shown marked separation between North and South American strains of arboviruses, such as eastern equine encephalomyelitis (EEEV),12 western equine encephalomyelitis (WEEV),13,14 and St. Louis encephalitis (SLEV)15,16 viruses, suggesting limited genetic exchange. Temporal studies in California have shown intermittent genotype change among WEEV and SLEV isolates,1719 indicating possible introduction and replacement events. Neotropical migratory birds would seem to be the likely transport mechanism for such introductions.

The north shore of the Salton Sea in Coachella Valley is a unique area to study the introduction of arboviruses into western North America by northbound migrants. Typically, this is the first location in California and generally the western United States, where arboviral infections are discovered in mosquitoes each year,1921 and the Imperial Valley just south of this location is where WNV was first discovered west of the continental divide.22 Large numbers of neotropical migrants transiting the Pacific flyway use marshes at the north shore as a stopover site after traversing the Salton Sea and adjacent desert facilitating capture. In addition, arboviruses are active enzootically during most summers at wetlands surrounding the Salton Sea, indicating amplification after local overwintering or introduction.

From 1996 through 2007, we sampled both migratory and resident birds for the presence of antibody or viral infection at the north shore of the Salton Sea each spring to test the hypothesis that northbound migrants repeatedly introduce arboviruses into California. Similar data concurrently collected in Kern County at the southern end of the Central Valley provided an interesting comparison and a slightly different faunal composition. To assist in the interpretation of our field serology data, we experimentally infected Orange-crowned Warblers (Vermivora celata), Common Yellowthroats (Geothlypis trichas), Yellow Warblers (Dendroica petechia), and Yellow-rumped Warblers subspecies auduboni (Dendroica coronata auduboni) with either WNV or SLEV to measure viremia and mortality responses.

Materials and Methods

Bird collections.

Migratory and resident birds were captured while transiting sites on the northeastern (35.51, −115.92) and northwestern (33.46, −116.06) shores of the Salton Sea, Coachella Valley, Riverside County, CA, from 1996 through 2005. In addition, migratory transients were targeted during collections made during the springs of 2006 and 2007. Migrants and other birds also were collected during 1996 through 2008 in Kern County, primarily from wetlands at the Kern National Wildlife Refuge (35.73, −119.63) and the Kern River (35.32, −119.20). The ecology of these areas, descriptions of arbovirus activity, and bird sampling methods have been described previously.20,23 Briefly, birds were collected using multiple mist nets and grain-baited traps. Mist nets were 10 m long × 2.5 m tall and had 28-, 32-, or 60-mm mesh depending on the target species. Captured birds were banded with U.S. Geological Survey (USGS) bands, aged, sexed, and a 0.1 mL sample of blood collected from the jugular vein with 28-g needles. For antibody detection, blood samples were expelled into 0.9 mL of 0.9% saline solution, clarified through centrifugation, and stored at −70°C. During 2004, 2006, and 2007 blood samples from migratory species collected in Coachella Valley were expelled into 0.4 mL of virus diluent (phosphate buffered saline, 15% fetal bovine sera, antibiotics) and tested for infectious virus and antibody. Data from the California WNV Dead Bird Surveillance Program24 on neotropical migrants that were found dead by the public, sent for necropsy, and tested positive for WNV RNA were included for comparison.

Experimental infection.

Four warbler species commonly included in our field collections were experimentally infected with either WNV or SLEV. Birds were mist netted in either the Coachella Valley or Kern County, banded, and pre-bled to assure that they were serologically negative for WNV, SLEV, and WEEV, and transported to the Arbovirus Field Station. Birds were held in a mosquito-proofed and air conditioned facility, provided water ad libitum, and fed live meal worms, finely ground trout food (Aqua Max Carnivorous, Purina Mills, St. Louis, MO), and cut citrus. After an acclimation period of 7 to 14 d, birds were inoculated subcutaneously in the cervical region with »1,000 plaque forming units (PFU) of the NY99 strain of WNV or the Kern217 strain of SLEV. Birds were bled daily for 6 to 7 d by jugular puncture (50–100 μL of blood taken by 28-gauge syringe and expelled into 450 or 400 μL of virus diluent) to monitor viremia response. Additional birds inoculated with virus diluent served as maintenance and handling controls.

Virological and serological testing.

Wild bird sera were screened for antibodies to flaviviruses (WNV and SLEV) and WEEV using an enzyme immunoassay (EIA), with positives confirmed and identified by plaque reduction neutralization tests (PRNT).25,26 Those EIAs, with a positive/negative antigen well optical density ratio > 2.0, were considered to be confirmed if the sera neutralized > 80% of > 75 PFU of the specific virus tested for when grown on Vero cells in 6-well plates at a serum dilution of 1:20. To separate previous infection of WNV from SLEV, PRNT end-point titers had to be ≥ 4× the competing virus. To determine viremia, sera were tested by standard Vero cell plaque assay.27 Dead birds reported by the public were necropsied at the California Animal Health and Food Safety Laboratory at the University of California, Davis, and kidney samples sent to the Center for Vectorborne Diseases laboratory for testing for WNV RNA by quantitative reverse transcription-polymerase chain reaction (qRT-PCR)28 using previously published primers.29

Ethics.

The collection and infection of wild birds was done under University of California Davis IACUC Protocols 12880 and 12876, respectively, CA Resident Scientific Collection permit 801049-02 by the State of California Department of Fish and Game, Federal Fish and Wildlife permit MB082812-0, and Master Station Banding Marking and Salvage permit 22673 by the USGS. Infection of birds and testing for WNV and SLEV was done under BSL3 conditions approved by University of California Davis Environmental Health and Safety Biological Use Authorization protocol 0554 and U.S. Department of Agriculture (USDA) permit no. 47901.

Results

Field collections.

A total of 5,600 sera were collected from 43 species of neotropical migrants during spring (April–June) in Coachella Valley or Kern County, of which 34 migrants (0.61%) comprising 14 species tested EIA positive for arbovirus antibody (Table 1). Of the 34 EIA positives, 10 were confirmed by PRNT; i.e., had an EIA with a mean positive over negative antigen well optical density ratio > 2.0 and a PRNT ≥ 1:20. PRNT results that were ≥ 1:20, but could not be confirmed as either SLEV or WNV, were presented as flavivirus positive. Bullock's Orioles (10 EIA positive) and Ash-throated Flycatchers (4 EIA positive) were the spring migrant species most frequently positive for arboviral antibodies. In addition, 1,727 sera from the same 14 species were collected during the summer and/or fall months, of which 12 (0.69%) were EIA positive. This percentage positive was not statistically different (P > 0.05) than the percentage EIA positive for the spring migrants. Of these, 5 and 3 from 642 Common Yellowthroat, 0 and 3 from 42 Black-headed Grosbeak, and 0 and 1 from 182 Bullock's oriole sera were positive for WEEV or flavivirus antibodies, respectively; all other species listed in Table 1 tested negative. Only the 3 Black-headed Grosbeak sera were confirmed by PRNT to be SLEV antibody positive. In contrast, 2,987 (5.3%) sera from 56,174 birds collected over the entire study were positive for arboviral antibodies (Table 1), an order of magnitude greater than spring neotropical migrants. These non-neotropical migrant birds included winter, summer, and year-round residents, which were likely exposed locally to arboviruses.

Table 1

Neotropical migratory bird species collected in Coachella Valley (COAV) and Kern County (KERN) during spring (April–June) and tested for antibody against arboviruses*

SpeciesSiteTotalEIA resultsPRNT results
WEEV +FLAV +WEEV +SLEV +WNV +FLAV +
Lesser NighthawkCOAV33100
Chordeiles acutipennisKERN000
Western Wood-PeweeCOAV5700
Contopus virensKERN300
Ash-throated FlycatcherCOAV2900
Myiarchus cinerascensKERN37130020
Western KingbirdCOAV2500
Tyrannus verticalisKERN24202000
Warbling VireoCOAV276100
Vireo gilvusKERN1600
Tree SwallowCOAV000
Tachycineta bicolorKERN10600
Cliff SwallowCOAV000
Petrochelidon fulvaKERN22900
Swainson's ThrushCOAV9700
Catharus ustulatusKERN800
Hermit ThrushCOAV1300
Catharus guttatusKERN2951100
Orange-crowned WarblerCOAV17800
Vermivora celataKERN10300
Nashville WarblerCOAV47100
Vermivora ruficapillaKERN1400
Yellow WarblerCOAV66301010
Dendroica petechiaKERN6000
Townsend's WarblerCOAV1800
Dendroica townsendiiKERN300
MacGillivray's WarblerCOAV13902010
Oporomis tolmieiKERN1400
Common YellowthroatCOAV262120000
Geothlypis trichasKERN4300
Wilson's WarblerCOAV1,207110000
Wilsonia pusillaKERN11800
Yellow-breasted ChatCOAV3300
Icteria virensKERN100
Summer TanagerCOAV301010
Piranga rubraKERN000
Western TanagerCOAV74100
Piranga ludovicianaKERN5300
Black-headed GrosbeakCOAV39101
Pheucicus melanocephalusKERN891110
Blue GrosbeakCOAV1000
Guiraca caeruleaKERN5000
Lazuli BuntingCOAV4300
Passerina amoenaKERN2900
Yellow-headed BlackbirdCOAV8300
Xanthocephalus xanthocephalusKERN800
Bullock's OrioleCOAV441100
Icterus bullockiiKERN604261002
19 other speciesCOAV6900
KERN3300
Total migrantsCOAV3,442881030
KERN2,1585132022
Total Wild Birds [1996–2007]COAV26,7082016639111924888
[1997–2008]KERN29,4663861,73714011,407196

EIA = enzyme immunoassay; PRNT = plaque reduction neutralization test; WEEV = western equine encephalomyelitis; FLAV = flavivirus; WEEV = western equine encephalomyelitis; SLEV = St. Louis encephalomyelitis; WNV = West Nile virus. Not tested by PRNT.

Most of the common migrant species were collected during spring and fall (Figure 1A), but some Yellow-rumped Warblers and Common Yellowthroats remained as winter and year-round residents, respectively, in Coachella Valley. Residents could not be separated from migrant Common Yellowthroats except by time of capture, and therefore the resident portion of the population may have contributed to their slightly elevated seroprevalence in the Coachella Valley. The capture of migrants during spring was accompanied by the increase of WNV positive pools from mosquitoes collected along the Salton Sea (Figure 1B). The number of pools tested (mostly Culex tarsalis Coquillett) was related directly to the bimodal pattern of mosquito abundance at marshes along the Salton Sea. Virus amplification continued into June and the numbers of positive pools detected was highest during summer, decreasing in September as the migrants appeared again on their southbound flight.

Figure 1.
Figure 1.

Monthly changes in (A) numbers of birds collected per month during 1996–2005 and (B) pools of mosquitoes collected and testing positive for West Nile virus (WNV) (WNV+) from 2004 to 2007 from Coachella Valley. Bird species remaining year round were indicated by dashed lines.

Citation: The American Society of Tropical Medicine and Hygiene 83, 4; 10.4269/ajtmh.2010.10-0200

Recapture patterns of banded birds provided some indication of their survival and rate of movement through our study areas. Some abundant spring migrant species such as Wilson's and Yellow Warblers were never recaptured, and birds banded elsewhere also were not recovered in our collections. In addition, birds banded in Coachella Valley were never recaptured in Kern County or vice versa. In contrast, we frequently recaptured small-sized birds such as Orange-crowned Warblers (26 recaptures) and Common Yellow-throats (90 recaptures) that spent longer time periods within our study areas. The single seroconversion documented for a migrant was a male after hatching year Common Yellow-throat that tested negative on September 19, 2003 and then positive for flavivirus the following week; titers were too low to separate WNV from SLEV. Summer residents such as Bullock's Orioles (175 recaptures) and Western Kingbirds (27 recaptures) frequently were recaptured near nesting sites in Kern County. Overall, banded birds were recaptured on 12,590 occasions (22.4% of total), but only 379 of these (3%) were bird species listed in Table 1. Most of these were recaptured during summer or during the slower autumnal return, and not on their vernal northern flight. In marked contrast other species were frequently recaptured, for example, 3,092 White-crowned Sparrows, a winter resident, were captured in Coachella Valley during the study period, of which 561 (18.1%) were recaptures.

In an attempt to detect acute arbovirus infections during the spring of 2006 and 2007, blood samples from the Coachella Valley were collected in virus diluent, frozen immediately on dry ice, and tested for viremia by Vero cell plaque assay. Overall, 1,222 migrant and a few resident birds were tested for virus with negative findings (Table 2). Similar negative results were found for 196 warblers tested for virus in May 2004. Sera also were tested for antibody, of which 6 sera from migrants were arbovirus positive by EIA (Table 3); one Yellow Warbler and one Magillivray's Warbler were confirmed by PRNT to be WNV positive in 2007. In addition, during 2007 three House Finches and 1 Gambel's Quail also were confirmed to be seropositive for WNV; all were after hatching year resident bird species and most likely were infected during the previous transmission season.

Table 2

Resident and migratory birds collected in Coachella Valley during the spring of 2006 and 2007 and tested for viremia and antibody*

Common nameScientific name20062007Total
Gambel's QuailCallipepla gambelii1717
Mourning DoveZenaida macroura11
Common Ground-DoveColumbina passerina13720
Lesser NighthawkChordeiles acutipennis44
Western Wood-PeweeContopus sordidulus41418
Willow FlycatcherEmpidonax trailii88
Hammond's FlycatcherEmpidonax hammondii11
Dusky FlycatcherEmpidonax oberholseri44
Pacific-slope FlycatcherEmpidonax difficilis13435
Black PhoebeSayornis nigricans11
Ash-throated FlycatcherMyiarchus cinerascens21012
Western KingbirdTyrannus verticalis22
Cassin's VireoVireo cassinii22
Warbling VireoVireo gilvus126981
Northern Rough-winged SwallowStelgidopteryx serripennis22
Barn SwallowHirundo rustica11
VerdinAuriparus flaviceps4913
Cactus WrenCampylorhynchus brunneicapillus11
House WrenTroglodytes aedon22
Black-tailed GnatcatcherPolioptila melanura22
Swainson's ThrushCatharus ustulatus41721
Northern MockingbirdMimus polyglottos325
Orange-crowned WarblerVermivora celata53035
Nashville WarblerVermivora ruficapilla112
Yellow WarblerDendroica petechia73100173
Yellow-rumped WarblerDendroica coronata123
Townsend's WarblerDendroica townsendi2810
Townsend's X Hermit Warbler Hybrid112
Hermit WarblerDendroica occidentalis11
OvenbirdSeiurus aurocapillus11
MacGillivray's WarblerOporornis tolmiei373774
Common YellowthroatGeothlypis trichas61016
Wilson's WarblerWilsonia pusilla314189503
Yellow-breasted ChatIcteria virens178
Summer TanagerPiranga rubra22
Western TanagerPiranga ludoviciana112132
Clay-colored SparrowSpizella pallida11
Song SparrowMelospiza melodia235
Lincoln's SparrowMelospiza lincolnii156
White-crowned SparrowZonotrichia leucophrys33
Golden-crowned SparrowZonotrichia atricapilla11
Black-headed GrosbeakPheucticus melanocephalus1910
Blue GrosbeakGuiraca caerulea123
Lazuli BuntingPasserina amoena7310
Red-winged BlackbirdAgelaius phoeniceus11
Great-tailed GrackleQuiscalus mexicanus112
Brown-headed CowbirdMolothrus ater3710
Hooded OrioleIcterus cucullatus11
Bullock's OrioleIcterus bullockii22
House FinchCarpodacus mexicanus5252
Total5396831222

Grain-baited trap was operated only during 2007.

Table 3

Migratory birds collected in Coachella Valley during the spring of 2006 and 2007 that tested positive for antibody to WEEV or flaviviruses (Flav) by enzyme immunoassay (EIA) or plaque reduction neutralization assay (PRNT)

SpeciesVirusEIAPRNT
2006
Summer TanagerFlav3.1< 1:20
Western TanagerWEEV3.2< 1:20
Wilson's WarblerWEEV2.0< 1:20
Wilson's WarblerFlav2.3< 1:20
2007
MacGillivaray's WarblerWNV7.5> 1:80
Yellow WarblerWNV3.41:40

Dead bird testing.

From 2003 through 2007, a total of 27,194 birds comprising 227 species were reported dead by the public in California and tested for WNV RNA, of which 33% were WNV positive. Most of the birds tested were resident corvid species. Of the 1,109 birds comprised of 76 migratory species, 11% were positive for WNV RNA (Table 4); however, only 3 (0.7%) of 428 birds tested during spring were positive (none from Coachella Valley), agreeing well with our serological results. The percentage of migrants positive for WNV RNA during spring (0.7%) was significantly less (χ2 = 32.7, P < 0.001) than other species found dead and tested during spring (493 pos [8.4%] of 5,886 tested). In addition, the percentage of migrants positive for WNV during spring (0.7%) was significantly less (χ2 = 76.6, P < 0.001) than the percentage of migrants found dead and testing positive during summer (18.1%). The Western Tanager represented this data trend well, with 1 of 31 positive during spring, but 22 of 78 positive during summer (χ2 = 8.3, P < 0.004). Collectively, these data indicated that migrants were becoming infected after they entered California.

Table 4

Spring–fall transients found dead by public and submitted to the Dead Bird Program for testing, 2003–2007*

SpeciesWNV − springWNV + springWNV − summerWNV + summerGrand total
Bank Swallow11
Black-throated Gray Warbler11
California Gull819
Common Nighthawk314
Flamingo11
Merlin11
Peregrine Falcon11
Townsend's Warbler718
Tree Swallow718
Virginia Rail11112
Warbling Vireo516
Willow Flycatcher112
Hooded Oriole20222
Lesser Nighthawk527
Sora12214
Western Kingbird27229
Western Sandpiper325
Common Yellowthroat336
Hermit Thrush24327
House Wren8311
Osprey134
Barn Swallow19125449
Bullock's Oriole13417
Cliff Swallow18120443
Orange-Crowned Warbler13417
Wilson's Warbler17421
Band-tailed Pigeon15520
Swainson's Hawk12517
Yellow Warbler23629
Rufous Hummingbird9716
Swainson's Thrush55762
Black-headed Grosbeak10617123
Western Tanager3015622109
22 WNV neg. species summer4646
43 WNV neg. species spring361361
Grand total42835551231109

Dead birds turned in to the California dead bird program 2003–2007 and tested for West Nile virus (WNV). Only birds that tested positive for WNV at least once were included in the table. The spring period is March 1–June 15 and summer period is June 16–September 30.

Exact species unknown.

Experimental infections.

Four warbler species in three different genera within the family Parulidae were infected experimentally: Orange-crowned Warbler, Yellow Warbler, Common Yellowthroat, and Yellow-rumped Warbler subspecies audoboni (Figures 2 and 3). We had intended to use 6–8 birds per species per virus; however, we encountered difficulty adapting some species to captivity. We initially tried individual birds to learn holding and feeding methods and then tried to use these as “teacher” birds. Some species such as Orange-crowned Warbler adapted well, whereas others such as the Common Yellowthroat and Yellow-rumped Warbler did not. The single Common Yellowthroat used (Figure 2C) was our initial teacher bird, a second year male that survived more than 6 weeks in captivity; additional Common Yellowthroats and Yellow-rumped Warblers did not survive adaptation.

Figure 2.
Figure 2.

Experimental infections of (A) Orange-crowned Warblers, (B) Yellow Warblers, and (C) Common Yellowthroats with the NY99 strain of West Nile virus (WNV). Shown are daily viremia estimates in days post infection in log10 plaque forming units per mL for each bird dying (filled points) or surviving (open points). Assay sensitivity 2.0 log10 plaque forming units (PFU)/mL.

Citation: The American Society of Tropical Medicine and Hygiene 83, 4; 10.4269/ajtmh.2010.10-0200

Figure 3.
Figure 3.

Experimental infections of (A) Orange-crowned Warblers and (B) Yellow-rumped Warblers with the KERN217 strain of SLEV. Shown are daily viremia estimates in days post infection in log10 plaque forming units per mL for each bird dying (filled points) or surviving (open points). Assay sensitivity 1.7 log10 PFU/mL.

Citation: The American Society of Tropical Medicine and Hygiene 83, 4; 10.4269/ajtmh.2010.10-0200

Orange-crowned Warblers were the most susceptible of the three species tested for WNV, producing the highest mean maximum viremia (mean = 8.45, range = 7.6–10.5 log10 PFU/mL), longest duration viremia (max = 7 d), and most frequent mortality following infection (5 of 6 birds died by 7 d post infection) (Figure 2A). The single survivor had the lowest viremia among the 6 birds tested. Two control birds survived the experimental period, which included daily blood sampling. In marked contrast, four Yellow Warblers and the single Common Yellowthroat produced significantly lower (t = 2.79, degrees-of-freedom [df] = 9, P = 0.01) mean maximum viremias (mean = 6.76, range = 5.6 – 7.5 log10 PFU/mL), a shorter viremia period (3–4 d post infection), and a lower mortality rate (1 of 5 birds died) (Figure 2 B and C) than the Orange-crowned Warblers. The single Yellow Warbler that died on Day 6 post infection was viremic until death, although the viremia was markedly lower during the two days just before death (Figure 2B). One non-infected Yellow Warbler control survived handling and blood collection.

In marked contrast to infection with WNV, 4 of 4 Orange-crowned Warblers infected with SLEV produced peak viremias on Days 1–2 post infection, cleared their viremia by Day 3, and survived (Figure 3A). All birds were subsequently bled weekly for 6 weeks to follow antibody production. Antibody was readily detectable from Week 2–6 post infection, with mean EIA positive over negative antigen well ratio values ranging from 5.3 to 8.6. PRNT titers on Weeks 4 and 6 ranged from 1:20 to 1:40. In marked contrast, the single Yellow-rumped Warbler developed peak viremia between 5.3 and 7.4 log10 PFU/mL from Days 2–5 and then died (Figure 3B). Although our data were limited, they were interesting in that Orange-crowned Warblers, a warbler in the genus Vermivora, succumbed to WNV infection but survived SLEV, whereas warblers in the genus Dendroica survived WNV and one Yellow-rumped Warbler seemed to succumb from SLEV infection. Additional research is needed to improve these sample sizes and more fully understand the response of these warblers to flavivirus infection.

Discussion

West Nile virus infections in Cx. tarsalis mosquitoes collected at marshes along the NE margin of the Salton Sea in Coachella Valley, Riverside County, repeatedly were detected as early as April (Figure 1B) and usually were the first detection of encephalitis virus in California.1921 However, the source of these early season infections has yet to be determined. The recovery of multiple WNV-positive pools coincidental with the arrival of the first migrants made them suspect as the source of infection; however, there was no further increase in positive pools during May when most of the migrants passed through the Coachella Valley. In addition, over an 11-year period, few avian migrants entering California from southern overwintering locations were antibody positive, indicating a limited history of previous viral exposure. These data were consistent with the “disappearance” of WNV and other temperate arboviruses in the neotropics.30,31 Portions of some migrant populations such as the Common Yellowthroats remained in the Coachella Valley during summer (Figure 1A) and therefore may have been infected locally at that time as indicated by the single seroconversion during September 2003. Local infection also seemed evident from the dead bird data shown in Table 4.

Several migrants had low level EIA antibody that could not be confirmed by PRNT. These may be old infections, because we previously reported SLEV antibody decay in House Finches and Mourning Doves following experimental infections, with antibody detected by EIA in birds testing negative by PRNT the spring following the year of infection.32,33 In contrast, WNV produces long-lasting and elevated PRNT titers in House Sparrows and House Finches, perhaps stimulated by the intermittent release of viral antigen from long-lasting chronic infections.3437 During 2004, 2006, and 2007, we also tested these same migrant species for viremia to evaluate the notion that they may become infected during the northern migration and arrive viremic, but before becoming antibody positive. However, all these birds were negative by Vero cell plaque assay. This was unexpected because WNV was detected in Cx. tarsalis during early spring of both 2006 and 2007.38

Interestingly, some migrant species found dead by the public had kidney tissue collected at necropsy positive for WNV RNA by qRT-PCR. Dead birds reported as positive either had Ct scores < 30 for a primer set specific for the envelope region of the viral genome or if the Ct score was between 30 and 40, these infections were confirmed by qRT-PCR using a second primer set specific for the non-structual-NS3 gene of the WNV genome. Therefore, WNV-positive dead birds could have either succumbed during acute infection acquired locally or survived and maintained viral RNA in their kidney tissues. Previously, we reported evidence for persistent viral RNA in birds necropsied 6–8 weeks after experimental infection, and similar to hamsters,39 some of these tissues yielded infectious virus after C6/36 cell passage.36,40 One of the four Orange-crowned Warblers infected with SLEV had brain tissue positive at necropsy 6 weeks after experimental infection.

The percentage of all dead birds reported by the public that were positive for WNV at necropsy was an order of magnitude greater than the proportion of live birds that tested positive for antibody against WNV. Although all three species of warbler we experimentally infected were competent hosts for WNV, they differed markedly in the amplitude of their acute viremia and mortality rates. Orange-crowned Warblers readily succumbed to WNV, but not SLEV, experimental infection, and in agreement 11% of Orange-crowned Warblers found dead were WNV positive, whereas no live birds tested positive for WNV antibody. The single Common Yellowthroat experimentally infected with WNV survived, and in agreement several birds were found with flavivirus or WNV antibody in both Coachella and Kern County; none were found dead by the public and submitted for testing.

The general lack of evidence for previous or current arboviral infection in migratory birds entering California from southern overwintering sites indicated that migrants reported positive by the dead bird program probably were infected locally rather than on their overwintering grounds. The passage of these migrants through Coachella Valley after virus infection was detected in Cx. tarsalis may indicate that these birds acquired their infection locally and then dispersed virus northward from southern California into the Central Valley. In agreement, during the summer of 2004, WNV rapidly moved > 800 km northward from the Coachella Valley and Los Angeles basin into every county in California.41

Our current research36,42 has yet to resolve the primary mechanism(s) of WNV overwintering in California or possibly temperate North America, leaving us with the same suite of possibilities presented almost 50 years ago.43 Although a variety of these mechanisms have been explored since the arrival of WNV, all data were based on laboratory experiments or relatively rare field events that have been difficult to detect and repeat in nature. Spatial and temporal variation in the intensity of summer enzootic transmission levels undoubtedly has altered the magnitude of the overwintering virus population, thereby adding to the complexity of resolving the mechanism(s) of virus overwintering in nature.

Acknowledgments:

We especially thank the Coachella Valley (B Lothrop Manager) and Kern (R Quiring Manager) Mosquito and Vector Control Districts for continued logistical and fiscal support, and help with sampling the mosquitoes for virus testing. M. V. Armijos, B. D. Carroll, R. Cusak, H. D. Lothrop, J. Lündstrom, V. M. Martinez, and additional staff of the Arbovirus Unit, Center for Vectorborne Diseases (CVEC) assisted with the collection and bleeding of the birds. V. M. Martinez assisted with the experimental infections. R. E. Chiles, M. Shafii, S. Ashtari, M. Dannen, and the staff of the Arbovirus Laboratory, CVEC, tested dead birds and mosquitoes for viral infection. Dead bird reports were managed by the California Department of Public Health (S. Husted Senior Biologist), and necropsies were done at the California Animal Health and Food Safety laboratory at University of California Davis under the direction of L. Woods.

  • 1.

    Rappole JH, Derrickson SR, Hubalek Z, 2000. Migratory birds and spread of West Nile virus in the Western Hemisphere. Emerg Infect Dis 6: 319328.

    • Search Google Scholar
    • Export Citation
  • 2.

    Rappole JH, Hubalek Z, 2003. Migratory birds and West Nile virus. J Appl Microbiol 94 (Suppl): 47S58S.

  • 3.

    Peterson AT, Vieglais DA, Andreasen JK, 2003. Migratory birds modeled as critical transport agents for West Nile virus in North America. Vector Borne Zoonotic Dis 3: 2737.

    • Search Google Scholar
    • Export Citation
  • 4.

    Dusek RJ, McLean RG, Kramer LD, Ubico SR, DuPuis AP, Ebel GD, Guptill SC, 2009. Prevalence of West Nile virus in migratory birds during spring and fall migration. Am J Trop Med Hyg 81: 11511158.

    • Search Google Scholar
    • Export Citation
  • 5.

    Malkinson M, Banet C, Weisman Y, Pokamunski S, King R, Drouet MT, Deubel V, 2002. Introduction of West Nile virus in the Middle East by migrating white storks. Emerg Infect Dis 8: 392397.

    • Search Google Scholar
    • Export Citation
  • 6.

    Lopez G, Jimenez-Clavero MA, Tejedor CG, Soriguer R, Figuerola J, 2008. Prevalence of West Nile virus neutralizing antibodies in Spain is related to the behavior of migratory birds. Vector Borne Zoonotic Dis 8: 615621.

    • Search Google Scholar
    • Export Citation
  • 7.

    Figuerola J, Jimenez-Clavero MA, Lopez G, Rubio C, Soriguer R, Gomez-Tejedor C, Tenorio A, 2008. Size matters: West Nile virus neutralizing antibodies in resident and migratory birds in Spain. Vet Microbiol 132: 3946.

    • Search Google Scholar
    • Export Citation
  • 8.

    Stamm DD, Newman RJ, 1963. Evidence of southward transport of arboviruses from the U.S. by migratory birds. Ann Microbiol 11: 123133.

  • 9.

    Lord RD, Calisher CH, 1970. Further evidence of southward transport of arboviruses by migratory birds. Am J Epidemiol 92: 7378.

  • 10.

    Calisher CH, Maness KS, Lord RD, Coleman PH, 1971. Identification of two South American strains of eastern equine encephalomyelitis virus from migrant birds captured on the Mississippi Delta. Am J Epidemiol 94: 172178.

    • Search Google Scholar
    • Export Citation
  • 11.

    Crans WJ, Cassamise DF, McNelly JR, 1994. Eastern equine encephalomyelitis in relation to the avian community of a coastal cedar swamp. J Med Entomol 31: 711728.

    • Search Google Scholar
    • Export Citation
  • 12.

    Brault AC, Powers AM, Chavez CL, Lopez RN, Cachon MF, Gutierrez LF, Kang W, Tesh RB, Shope RE, Weaver SC, 1999. Genetic and antigenic diversity among eastern equine encephalitis viruses from North, Central and South America. Am J Trop Med Hyg 61: 579586.

    • Search Google Scholar
    • Export Citation
  • 13.

    Reisen WK, Monath TP, 1989. Western equine encephalomyelitis, Monath TP, ed. The Arboviruses: Epidemiology and Ecology. Boca Raton, FL: CRC Press, 89138.

    • Search Google Scholar
    • Export Citation
  • 14.

    Reisen WK, Lundstrom JO, Scott TW, Eldridge BF, Chiles RE, Cusack R, Martinez VM, Lothrop HD, Gutierrez D, Wright S, Boyce K, Hill BR, 2000. Patterns of avian seroprevalence to western equine encephalomyelitis and St. Louis encephalitis viruses in California, USA. J Med Entomol 37: 507527.

    • Search Google Scholar
    • Export Citation
  • 15.

    Kramer LD, Chandler LJ, 2001. Phylogenetic analysis of the envelope gene of St. Louis encephalitis virus. Arch Virol 146: 23412355.

  • 16.

    Monath TP, Cropp CB, Lanciotti RS, Trent DW, 1980. Variation in virulence for mice and Rhesus monkeys among St. Louis encephalitis virus strains of different origin. J Virol Methods 29: 948962.

    • Search Google Scholar
    • Export Citation
  • 17.

    Kramer LD, Fallah HM, 1999. Genetic variation among isolates of western equine encephalomyelitis virus from California. Am J Trop Med Hyg 60: 708713.

    • Search Google Scholar
    • Export Citation
  • 18.

    Kramer LD, Presser SB, Hardy JL, Jackson AO, 1997. Genotypic and phenotypic variation of selected Saint Louis encephalitis viral strains in California. Am J Trop Med Hyg 57: 222229.

    • Search Google Scholar
    • Export Citation
  • 19.

    Reisen WK, Lothrop HD, Chiles RE, Cusack R, Green EG, Fang Y, Kensington M, 2002. Persistence and amplification of St. Louis encephalitis virus in the Coachella Valley of California, 2000–2001. J Med Entomol 39: 793805.

    • Search Google Scholar
    • Export Citation
  • 20.

    Reisen WK, Lothrop HD, Wheeler SS, Kensington M, Gutierrez A, Fang Y, Garcia S, Lothrop B, 2008. Persistent West Nile virus transmission and the displacement St. Louis encephalitis virus in southeastern California, 2003–2006. J Med Entomol 45: 494508.

    • Search Google Scholar
    • Export Citation
  • 21.

    Reisen WK, Hardy JL, Presser SB, Milby MM, Meyer RP, Durso SL, Wargo MJ, Gordon EW, 1992. Mosquito and arbovirus ecology in southeastern California, 1986–1990. J Med Entomol 29: 512524.

    • Search Google Scholar
    • Export Citation
  • 22.

    Reisen WK, Lothrop HD, Chiles RE, Madon MB, Cossen C, Woods L, Husted S, Kramer VL, Edman JD, 2004. West Nile virus in California. Emerg Infect Dis 10: 13691378.

    • Search Google Scholar
    • Export Citation
  • 23.

    Reisen WK, Carroll BD, Takahashi R, Fang Y, Garcia S, Martinez VM, Quiring R, 2009. Repeated West Nile virus epidemic transmission in Kern County, California, 2004–2007. J Med Entomol 46: 139157.

    • Search Google Scholar
    • Export Citation
  • 24.

    McCaughey K, Miles SQ, Woods L, Chiles RE, Hom A, Kramer VL, Jay-Russel M, Sun B, Reisen WK, Scott TW, Hui LT, Steinlein DB, Castro M, Houchin A, Husted S, 2003. The California West Nile virus dead bird surveillance program. Proc Mosq Vector Control Assoc Calif 71: 3843.

    • Search Google Scholar
    • Export Citation
  • 25.

    Chiles RE, Reisen WK, 1998. A new enzyme immunoassay to detect antibodies to arboviruses in the blood of wild birds. J Vector Ecol 23: 123135.

    • Search Google Scholar
    • Export Citation
  • 26.

    Ebel GD, DuPuis AP, Nicholas D, Young D, Maffei J, Kramer LD, 2002. Detection by enzyme-linked immunosorbent assay of antibodies to West Nile virus in birds. Emerg Infect Dis 8: 979982.

    • Search Google Scholar
    • Export Citation
  • 27.

    Kramer LD, Wolfe TM, Green EN, Chiles RE, Fallah H, Fang Y, Reisen WK, 2002. Detection of encephalitis viruses in mosquitoes (Diptera: Culicidae) and avian tissues. J Med Entomol 39: 312323.

    • Search Google Scholar
    • Export Citation
  • 28.

    Shi PY, Kauffman EB, Ren P, Felton A, Tai JH, DuPuis AP, Jones SA, Ngo KA, Nicholas DC, Maffei J, Ebel GD, Bernard KA, Kramer LD, 2001. High-throughput detection of West Nile virus RNA. J Clin Microbiol 39: 12641271.

    • Search Google Scholar
    • Export Citation
  • 29.

    Lanciotti RS, Kerst AJ, Nasci RS, Godsey MS, Mitchell CJ, Savage HM, Komar N, Panella NA, Allen BC, Volpe KE, Davis BS, Roehrig JT, 2000. Rapid detection of West Nile virus from human clinical specimens, field-collected mosquitoes, and avian samples by a TaqMan reverse transcriptase-PCR assay. J Clin Microbiol 38: 40664071.

    • Search Google Scholar
    • Export Citation
  • 30.

    Spence LP, 1980. St. Louis encephalitis in tropical America. Monath TP, ed. St. Louis Encephalitis. Washington, DC: Am. Publ. Hlth. Assoc., 451471.

    • Search Google Scholar
    • Export Citation
  • 31.

    Kramer LD, Styer LM, Ebel GD, 2008. A global perspective on the epidemiology of West Nile virus. Annu Rev Entomol 53: 6181.

  • 32.

    Reisen WK, Chiles RE, Green EN, Fang Y, Mahmood F, 2003. Previous infection protects finches from re-infection with St. Louis encephalitis virus. J Med Entomol 40: 300305.

    • Search Google Scholar
    • Export Citation
  • 33.

    Reisen WK, Chiles RE, Martinez VM, Fang Y, Green EN, 2004. Encephalitis virus persistence in California birds: experimental infections in mourning doves (Zenaidura macroura). J Med Entomol 3: 462466.

    • Search Google Scholar
    • Export Citation
  • 34.

    Nemeth NM, Oesterle PT, Bowen RA, 2009. Humoral immunity to West Nile virus is long-lasting and protective in the house sparrow (Passer domesticus). Am J Trop Med Hyg 80: 864869.

    • Search Google Scholar
    • Export Citation
  • 35.

    Nemeth N, Young G, Ndaluka C, Bielefeldt-Ohmann H, Komar N, Bowen R, 2009. Persistent West Nile virus infection in the house sparrow (Passer domesticus). Arch Virol 154: 783789.

    • Search Google Scholar
    • Export Citation
  • 36.

    Reisen WK, Fang Y, Lothrop HD, Martinez VM, Wilson J, O'Connor P, Carney R, Cahoon-Young B, Shafii M, Brault AC, 2006. Overwintering of West Nile virus in southern California. J Med Entomol 43: 344355.

    • Search Google Scholar
    • Export Citation
  • 37.

    Wheeler SS, Reisen WK, 2009. Persistent West Nile virus infections in avian hosts: a possible overwintering mechanism for WNV? Proc Mosq Vector Control Assoc Calif 77: 4243.

    • Search Google Scholar
    • Export Citation
  • 38.

    Lothrop HD, Lothrop BB, Gomsi DE, Reisen WK, 2008. Intensive early season adulticide applications decrease arbovirus transmission throughout the Coachella Valley, Riverside County, California. Vector Borne Zoonotic Dis 8: 475489.

    • Search Google Scholar
    • Export Citation
  • 39.

    Tesh RB, Siirin M, Guzman H, Travassos da Rosa AP, Wu X, Duan T, Lei H, Nunes MR, Xiao SY, 2005. Persistent West Nile virus infection in the golden hamster: studies on its mechanism and possible implications for other flavivirus infections. J Infect Dis 192: 287295.

    • Search Google Scholar
    • Export Citation
  • 40.

    Reisen WK, Chiles RE, Martinez VM, Fang Y, Green EN, 2003. Experimental infection of California birds with western equine encephalomyelitis and St. Louis encephalitis viruses. J Med Entomol 40: 968982.

    • Search Google Scholar
    • Export Citation
  • 41.

    Hom A, Marcus L, Kramer VL, Cahoon B, Glaser C, Cossen C, Baylis E, Jean C, Tu E, Eldridge BF, Carney R, Padgett K, Sun B, Reisen WK, Woods L, Husted S. 2005. Surveillance for mosquito-borne encephalitis virus activity and human disease, including West Nile virus, in California, 2004. Proc Mosq Vector Control Assoc Calif 73: 6677.

    • Search Google Scholar
    • Export Citation
  • 42.

    Reisen WK, Thiemann T, Barker CM, Lu H, Carroll B, Fang Y, Lothrop HD, 2010. Effects of warm winter temperature on the abundance and gonotrophic activity of Culex (Diptera: Culicidae) in California. J Med Entomol 47: 230237.

    • Search Google Scholar
    • Export Citation
  • 43.

    Reeves WC, 1961. Overwintering of anthropod-borne viruses. Prog Med Virol 3: 5978.

Author Notes

*Address correspondence to William K. Reisen, Center for Vectorborne Diseases, University of California, Davis, CA 95616. E-mail: wkreisen@ucdavis.edu

Financial support: This research was funded, in part, by research grants RO1-AI39483, RO1-AI47855, and RO1-AI55607 from the National Institutes of Allergy and Infectious Diseases, NIH, ELC funding to the California Department of Public Health by the CDC, and by funds from the Coachella Valley MVCD.

Authors' addresses: William K. Reisen, Sarah S. Wheeler, Sandra Garcia, and Ying Fang, Center for Vectorborne Diseases, Department of Pathology, Microbiology and Immunology, School of Veterinary Medicine, University of California, Davis, CA, E-mails: wkreisen@ucdavis.edu, sswheeler@ucdavis.edu, sgarcia@ucdavis.edu, and ylfang@ucdavis.edu.

Save