Comparative fitness of West Nile virus isolated during California epidemics

Epidemic versus endemic West Nile virus dead bird surveillance in California: Changes in sensitivity and focus

Since 2003, the California West Nile virus (WNV) dead bird surveillance program (DBSP) has monitored publicly reported dead birds for WNV surveillance and response. In the current paper, we compared DBSP data from early epidemic years (2004-2006) with recent endemic years (2018-2020), with a focus on specimen collection criteria, county report incidence, bird species selection, WNV prevalence in dead birds, and utility of the DBSP as an early environmental indicator of WNV. Although fewer agencies collected dead birds in recent years, most vector control agencies with consistent WNV activity continued to use dead birds as a surveillance tool, with streamlined operations enhancing efficiency. The number of dead bird reports was approximately ten times greater during 2004-2006 compared to 2018-2020, with reports from the Central Valley and portions of Southern California decreasing substantially in recent years; reports from the San Francisco Bay Area decreased less dramatically. Seven of ten counties with high numbers of dead bird reports were also high human WNV case burden areas. Dead corvid, sparrow, and quail reports decreased the most compared to other bird species reports. West Nile virus positive dead birds were the most frequent first indicators of WNV activity by county in 2004-2006, followed by positive mosquitoes; in contrast, during 2018-2020 mosquitoes were the most frequent first indicators followed by dead birds, and initial environmental WNV detections occurred later in the season during 2018-2020. Evidence for WNV impacts on avian populations and susceptibility are discussed. Although patterns of dead bird reports and WNV prevalence in tested dead birds have changed, dead birds have endured as a useful element within our multi-faceted WNV surveillance program.

Viral fitness of MHV-68 viruses harboring drug resistance mutations in the protein kinase or thymidine kinase

Murine γ-herpesvirus-68 (MHV-68), genetically and biologically related to human γ-herpesviruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus, can be easily propagated in vitro allowing drug resistance studies. Previously, we described specific changes in MHV-68 protein kinase (PK) or thymidine kinase (TK) associated with resistance to various purine or pyrimidine nucleoside analogues, respectively. To investigate how specific TK and PK mutations affect viral replication capacity, we performed dual infection competition assays in which wild-type and drug-resistant virus compete in absence or presence of antivirals in Vero cells. The composition of the mixed viral population was analyzed using next-generation sequencing and relative fitness of seven MHV-68 PK or TK mutants was calculated based on the frequency of viral variants at the time of infection and after 5-days growth. A MHV-68 mutant losing the PK function due to a 2-nucleotide deletion was less fit than the wild-type virus in absence of antivirals, consistent with the essential role of viral PKs during lytic replication, but overgrew the wild-type virus under pressure of purine nucleosides. TK mutant viruses, with frameshift or missense mutations, grew equal to wild-type virus in absence of antivirals, in accordance with the viral TK function only being essential in non-replicating or in TK-deficient cells, but were more fit when treated with pyrimidine nucleosides. Moreover, TK missense mutant viruses also increased fitness under pressure of antivirals other than pyrimidine nucleosides, indicating that MHV-68 TK mutations might influence viral fitness by acting on cellular and/or viral functions that are unrelated to nucleoside activation.

Bird Species Involved in West Nile Virus Epidemiological Cycle in Southern Québec

Despite many studies on West Nile Virus (WNV) in the US, including the reservoir role of bird species and the summer shifts of the Culex mosquito, feeding from birds to mammals, there have been few equivalent studies in the neighboring regions of Canada where WNV is endemic. Here, a priority list of bird species likely involved in WNV transmission in the greater Montréal area is constructed by combining three sources of data: (i) from WNV surveillance in wild birds (2002–2015); (ii) blood meal analysis of Culex pipiens–restuans (CPR), the primary enzootic vectors of WNV in the region, collected from surveillance in 2008 and 2014; (iii) literature review on the sero-prevalence/host competence of resident birds. Each of these data sources yielded 18, 23 and 53 species, and overall, 67 different bird species were identified as potential WNV amplifiers/reservoirs. Of those identified from CPR blood meals, Common starlings, American robins, Song sparrows and House sparrows ranked the highest and blood meal analysis demonstrated a seasonal shift in feed preference from birds to mammals by CPR. Our study indicates that there are broad similarities in the ecology of WNV between our region and the northeastern US, although the relative importance of bird species varies somewhat between regions.

Influence of herd immunity in the cyclical nature of arboviruses

We review and contrast the evidence for an effect of amplifying host herd immunity on circulation and human exposure to arboviruses. Herd immunity of short-lived West Nile virus avian amplifying hosts appears to play a limited role in levels of enzootic circulation and spillover infections of humans, which are not amplifiers. In contrast, herd immunity of nonhuman primate hosts for enzootic Zika, dengue, and chikungunya viruses is much stronger and appears to regulate to a large extent the periodicity of sylvatic amplification in Africa. Following the recent Zika and chikungunya pandemics, human herd immunity in the Americas quickly rose to ∼50% in many regions, although seroprevalence remains patchy. Modeling from decades of chikungunya circulation in Asia suggests that this level of herd immunity will suppress for many years major chikungunya and Zika epidemics in the Americas, followed by smaller outbreaks as herd immunity cycles with a periodicity of up to several decades.

On the Fly: Interactions Between Birds, Mosquitoes, and Environment That Have Molded West Nile Virus Genomic Structure Over Two Decades

West Nile virus (WNV) was first identified in North America almost 20 yr ago. In that time, WNV has crossed the continent and established enzootic transmission cycles, resulting in intermittent outbreaks of human disease that have largely been linked with climatic variables and waning avian seroprevalence. During the transcontinental dissemination of WNV, the original genotype has been displaced by two principal extant genotypes which contain an envelope mutation that has been associated with enhanced vector competence by Culex pipiens L. (Diptera: Culicidae) and Culex tarsalis Coquillett vectors. Analyses of retrospective avian host competence data generated using the founding NY99 genotype strain have demonstrated a steady reduction in viremias of house sparrows over time. Reciprocally, the current genotype strains WN02 and SW03 have demonstrated an inverse correlation between house sparrow viremia magnitude and the time since isolation. These data collectively indicate that WNV has evolved for increased avian viremia while house sparrows have evolved resistance to the virus such that the relative host competence has remained constant. Intrahost analyses of WNV evolution demonstrate that selection pressures are avian species-specific and purifying selection is greater in individual birds compared with individual mosquitoes, suggesting that the avian adaptive and/or innate immune response may impose a selection pressure on WNV. Phylogenomic, experimental evolutionary systems, and models that link viral evolution with climate, host, and vector competence studies will be needed to identify the relative effect of different selective and stochastic mechanisms on viral phenotypes and the capacity of newly evolved WNV genotypes for transmission in continuously changing landscapes.