Overwintering Of West Nile Virus Research Articles

Spring temperature shapes West Nile virus transmission in Europe

West Nile Virus (WNV) is now endemic in many European countries, causing hundreds of human cases every year, with a high spatial and temporal heterogeneity. Previous studies have suggested that spring temperature might play a key role at shaping WNV transmission. Specifically, warmer temperatures in April-May might amplify WNV circulation, thus increasing the risk for human transmission later in the year. To test this hypothesis, we collated publicly available data on the number of human infections recorded in Europe between 2011 and 2019. We then applied generalized linear models to quantify the relationship between human cases and spring temperature, considering both average conditions (over years 2003-2010) and deviations from the average for subsequent years (2011-2019). We found a significant positive association both spatial (average conditions) and temporal (deviations). The former indicates that WNV circulation is higher in usually warmer regions while the latter implies a predictive value of spring conditions over the coming season. We also found a positive association with WNV detection during the previous year, which can be interpreted as an indication of the reliability of the surveillance system but also of WNV overwintering capacity. Weather anomalies at the beginning of the mosquito breeding season might act as an early warning signal for public health authorities, enabling them to strengthen in advance ongoing surveillance and prevention strategies.

Overwintering of West Nile Virus in the United States.

The establishment of a tropical virus such as West Nile (WNV; Flaviviridae: Flavivirus) within the temperate latitudes of the continental United States was unexpected and perhaps contingent, in part, upon the ability of this invasive virus to persist during winter when temperatures become too cold for replication and vector mosquito gonotrophic activity. Our Forum article reviews research examining possible overwintering mechanisms that include consistent reintroduction and local persistence in vector mosquitoes and avian hosts, mostly using examples from research conducted in California. We conclude that the transmission of WNV involves so many vectors and hosts within different landscapes that multiple overwintering pathways are possible and collectively may be necessary to allow this virus to overwinter consistently within the United States.

Overwintering of West Nile virus in a bird community with a communal crow roost

In temperate climates, transmission of West Nile virus (WNV) is detectable rarely during the coldest months (late fall through early spring), yet the virus has reappeared consistently during the next warm season. Several mechanisms may contribute to WNV persistence through winter, including bird-to-bird transmission among highly viremic species. Here we consider whether, under realistic scenarios supported by field and laboratory evidence, a winter bird community could sustain WNV through the winter in the absence of mosquitoes. With this purpose we constructed a deterministic model for a community of susceptible birds consisting of communally roosting crows, raptors and other birds. We simulated WNV introduction and subsequent transmission dynamics during the winter under realistic initial conditions and model parameterizations, including plausible contact rates for roosting crows. Model results were used to determine whether the bird community could yield realistic outbreaks that would result in WNV infectious individuals at the end of the winter, which would set up the potential for onward horizontal transmission into summer. Our findings strongly suggest that winter crow roosts could allow for WNV persistence through the winter, and our model results provide synthesis to explain inconclusive results from field studies on WNV overwintering in crow roosts.

Bionomics and Vector Potential of Culex thriambus (Diptera: Culicidae) Mosquitoes in Lake County, California.

California statewide West Nile virus (WNV) minimum infection rates in Culex thriambus Dyar mosquitoes are high; however, few specimens are submitted and tested each year, as their distribution seems limited to larval habitats along riparian systems. To evaluate the role of Cx. thriambus in the amplification, maintenance, and overwintering of WNV in Lake County, CA, the bionomics and vector potential of the species was investigated during 2014 and 2015. Culex thriambus was the most abundant mosquito species, with 1,153 adults and 7,624 immatures collected by vacuum aspiration and dip sampling, respectively, at the primary study site. Detection of WNV in four mosquito pools during September through November coincided with peak seasonality. Females entered and maintained a reproductive diapause during winter under field and seminatural conditions. Diapause was initiated in the majority of Cx. thriambus females by October and was terminated by 30 March. Some parous females (7.1%) and those in host-seeking arrest (7.1%) were collected throughout the winter period. An accrual of 679.51 degree-days (°D) was necessary for diapause termination under seminatural conditions. Culex thriambus females fed on 16 different avian species during spring and summer, and no mammalian feeds were detected. West Nile viral RNA was detected in four of 42 Cx. thriambus pools tested during June through November and infection rates ranged from 3.53–28.15/1,000 tested. In summary, WNV transmission may be increased along riparian corridors throughout California where Cx. thriambus mosquitoes remain relatively abundant.

The Contribution of Culex pipiens Complex Mosquitoes to Transmission and Persistence of West Nile Virus in North America

Mosquitoes within the Culex pipiens complex have been implicated as major vectors of West Nile virus (WNV) in North America due to their seasonal abundance, vector competence and high field infection rates. However, the role of Cx. p. pipiens complex mosquitoes in enzootic amplification of WNV among avian hosts and epidemic transmission to humans varies throughout its geographical distribution. In the northeastern United States, Cx. p. pipiens is recognized as the primary enzootic vector responsible for amplification of virus among wild bird populations. However, because this mosquito is strongly ornithophilic, its role in transmission to humans appears to be more limited in this region. In the north central and Mid-Atlantic States by contrast, Cx. p. pipiens shows an increased affinity for human hosts and has been incriminated as a key bridge vector. In southern regions of the United States, Culex p. quinquefasciatus are more opportunistic feeders, and are thought to be principal enzootic and epidemic vectors. In western regions of the United States where Culex tarsalis predominates, especially in rural areas, Cx. p. pipiens and Cx. p. quinquefasciatus play roles that are more limited and are recognized as secondary vectors. In the southwestern United States Cx. p. quinquefasciatus also appears to be the predominant vector in urban habitats, but only a secondary vector in more rural environs. The direct involvement of Cx. p. pipiens form molestus in WNV transmission is largely unknown, but human-biting Cx. p. pipiens are more likely to have a probability of genetic ancestry with Cx. p. pipiens form molestus. The detection of WNV from overwintering populations of diapausing Cx. p. pipiens and non-diapausing Cx. p. quinquefaciatus and their role in local overwintering of WNV are addressed.

Studies on Hibernating Populations of Culex pipiens from a West Nile Virus Endemic Focus in New York City: Parity Rates and Isolation of West Nile Virus

A 3-year study was undertaken to examine the parity status, survival, and prevalence of West Nile virus (WNV) in overwintering populations of Culex pipiens pipiens collected from a hibernaculum located in a WNV endemic region in New York City. Nearly 6,000 females were collected from December through April. Parity rates were highest among females collected in December and January, ranging from 12.3% to 21.9%, depending on the year. In each year of the study, the proportion of parous females declined significantly during the course of the winter; the percentage of parous females found in April ranged from 0.9% to 10%. Results provide unequivocal evidence that parous Cx. p. pipiens females from this region of the northeastern US enter hibernacula in the fall in comparatively high proportions not previously recognized for this species, and while these females experience significant mortality during the winter, some survived to April to emerge in the spring. The absence of any detectible blood remnants in overwintering females reaffirms that blood feeding does not occur among diapausing females during the winter. The possibility that a portion of the diapausing population may be autogenous as a result of hybridization with sympatric belowground populations of Cx. p. pipiens "form molestus" is discussed. A single isolation of WNV was obtained in Vero cell culture from a pool of 50 females collected on January 11, 2007, representing an infection prevalence of 0.07% in the overwintering population in 2007 (n = 1,370 mosquitoes, 33 pools). No isolations of WNV were made from mosquitoes collected in 2008 (n = 1,870 mosquitoes, 190 pools) or 2009 (n = 1,767 mosquitoes, 184 pools). Findings provide further evidence for local overwintering of WNV in diapausing Cx. p. pipiens, albeit at very low rates, consistent with the paucity of WNV-positive mosquitoes detected in June and early July despite the emergence of females from hibernacula in early May in this region.

Test of Recrudescence Hypothesis for Overwintering of West Nile Virus in Gray Catbirds

West Nile virus is a pathogen of concern for both human and wildlife health. Although many aspects of the ecology of West Nile virus are well understood, the mechanisms by which this and similar mosquito-borne viruses overwinter and become reinitiated each spring in temperate regions is not known. A thorough understanding of this mechanism is crucial to risk assessment and development of control strategies. One of the hypotheses to explain the mechanism by which this virus persists from year to year is the spring recrudescence of latent virus in avian reservoir hosts. Stress-related immunosuppression is implicated in the recrudescence of latent viruses in birds. We tested the spring recrudescence hypothesis in a controlled laboratory experiment using hatching-year gray catbirds (Dumatella carolinensis) captured in northern Ohio (July–August 2006). Catbirds (n = 60) were experimentally infected (September 2006) and later examined for the effects of immunosuppression through exogenous hormones and artificially induced migratory disposition. We found no effect of either testosterone or migratory behavior on infection status in any of the treatment birds. Moreover, we detected no viral RNA in the kidney, spleen, brain, or liver upon necropsy at 24 wk postinfection.

Test of Recrudescence Hypothesis for Overwintering of West Nile Virus in Gray Catbirds

West Nile virus is a pathogen of concern for both human and wildlife health. Although many aspects of the ecology of West Nile virus are well understood, the mechanisms by which this and similar mosquito-borne viruses overwinter and become reinitiated each spring in temperate regions is not known. A thorough understanding of this mechanism is crucial to risk assessment and development of control strategies. One of the hypotheses to explain the mechanism by which this virus persists from year to year is the spring recrudescence of latent virus in avian reservoir hosts. Stress-related immunosuppression is implicated in the recrudescence of latent viruses in birds. We tested the spring recrudescence hypothesis in a controlled laboratory experiment using hatching-year gray catbirds (Dumatella carolinensis) captured in northern Ohio (July-August 2006). Catbirds (n = 60) were experimentally infected (September 2006) and later examined for the effects of immunosuppression through exogenous hormones and artificially induced migratory disposition. We found no effect of either testosterone or migratory behavior on infection status in any of the treatment birds. Moreover, we detected no viral RNA in the kidney, spleen, brain, or liver upon necropsy at 24 wk postinfection.

Persistent West Nile virus infection in the house sparrow (Passer domesticus)

Long-term persistence of West Nile virus (WNV) infection within vertebrate reservoir hosts is a potential mechanism for overwintering of this (and other) arbovirus(es) at temperate latitudes. The house sparrow (Passer domesticus), an established amplifying host for WNV and other arboviruses, was used as a model to confirm chronicity of WNV infection in passerine birds and to evaluate the feasibility of two overwintering mechanisms: blood-borne infection of arthropod vectors (recrudescence) and oral infection of vertebrate reservoir hosts (ingestion of infected tissues through predation). WNV-inoculated sparrows were monitored for persistent infection for up to 2 years. Infectious virus persisted in tissues through 43 days, but not in sera beyond 6 days. Viral RNA persisted in tissues through 65 days. Chronicity of WNV infection in some tissues, but not blood, supports the predation mechanism of WNV overwintering, but not recrudescence. RNA persistence impacts interpretation and etiologic determination of avian mortality.

Field-Caught Culex erythrothorax Larvae Found Naturally Infected with West Nile Virus in Grand County, Utah

Culex erythrothorax larvae were collected from a bulrush-cattail marsh near Moab, Grand County, Utah, October 28, 2004, and found positive for West Nile virus (WNV) RNA by real-time reverse-transcriptase polymerase chain reaction (RT-PCR). This demonstrates WNV vertical transmission in this species in the wild and suggests that vertical transmission in overwintering Cx. erythrothorax larvae may contribute to WNV overwintering.

West Nile virus in the United States: guidelines for detection, prevention, and control.

The epidemic/epizootic of West Nile (WN) encephalitis in the northeastern United States in the summer and fall of 1999 was an unprecedented event, underscoring the ease with which emerging infectious pathogens can be introduced into new geographic areas in today's era of rapid transportation and increased movement of people, animals, and commodities. This epidemic/epizootic and the increased frequency of other exotic pathogens being imported into the United States raises the issue of whether local, state, and national public health agencies are prepared to deal with epidemics/epizootics of vector-borne infectious diseases. The overwintering of WN virus and the epizootic transmission in the summer of 2000 reinforces the need to rebuild the public health infrastructure to deal with vector-borne diseases in this country. This article summarizes guidelines for surveillance, prevention, and control of WN virus that were drafted in December 1999 to help prepare state and local health departments for monitoring WN virus activity in the spring and summer of 2000 and also summarizes the data collected from those surveillance systems through September 2000.