WEST NILE VIRUS-RELATED TRENDS IN AVIAN MORTALITY IN CALIFORNIA, USA, 2003-12

Changes in mosquito species and blood meal composition associated with adulticide applications

Although adulticide application is a pillar in the integrated management of nuisance and vector mosquitoes, non-target effects of adulticide applications within ecosystems are a substantial concern. However, the impacts of adulticide applications on non-target organisms are not necessarily detrimental, and in some cases, may provide benefits to certain organisms or wildlife. Here, we hypothesized that adulticide applications have beneficial non-target impacts on vertebrate wildlife through reduced biting pressure. To test this, we collected mosquitoes from ultra-low volume Permanone-treated (intervention) and untreated (reference) areas and assessed mosquito abundance and diversity, and abundance of blood-engorged female mosquitoes. We performed DNA barcoding analysis on mosquito blood meals to identify host species. Our results demonstrated a significant reduction in mosquito abundance by 58.9% in the intervention areas, taking into account the reduction in reference areas. Consequently, this decline led to a 64.5% reduction in the abundance of blood-engorged females. We also found a temporal dynamic of mosquito composition driven by mosquito control actions in which different mosquito species became dominant at treated sites while composition at reference areas remained similar during the same period. The present study suggests that the beneficial effects of mosquito control treatments for humans extend to other vertebrates, which represents an unstudied and rarely recognized non-target impact.

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.

Evaluation of the effectiveness of the California mosquito-borne virus surveillance & response plan, 2009–2018

Local vector control and public health agencies in California use the California Mosquito-Borne Virus Surveillance and Response Plan to monitor and evaluate West Nile virus (WNV) activity and guide responses to reduce the burden of WNV disease. All available data from environmental surveillance, such as the abundance and WNV infection rates in Culex tarsalis and the Culex pipiens complex mosquitoes, the numbers of dead birds, seroconversions in sentinel chickens, and ambient air temperatures, are fed into a formula to estimate the risk level and associated risk of human infections. In many other areas of the US, the vector index, based only on vector mosquito abundance and infection rates, is used by vector control programs to estimate the risk of human WNV transmission. We built models to determine the association between risk level and the number of reported symptomatic human disease cases with onset in the following three weeks to identify the essential components of the risk level and to compare California’s risk estimates to vector index. Risk level calculations based on Cx. tarsalis and Cx. pipiens complex levels were significantly associated with increased human risk, particularly when accounting for vector control area and population, and were better predictors than using vector index. Including all potential environmental components created an effective tool to estimate the risk of WNV transmission to humans in California.

Mosquito blood-feeding patterns and nesting behavior of American crows, an amplifying host of West Nile virus

Background

Although American crows are a key indicator species for West Nile virus (WNV) and mount among the highest viremias reported for any host, the importance of crows in the WNV transmission cycle has been called into question because of their consistent underrepresentation in studies of Culex blood meal sources. Here, we test the hypothesis that this apparent underrepresentation could be due, in part, to underrepresentation of crow nesting habitat from mosquito sampling designs. Specifically, we examine how the likelihood of a crow blood meal changes with distance to and timing of active crow nests in a Davis, California, population.

Methods

Sixty artificial mosquito resting sites were deployed from May to September 2014 in varying proximity to known crow nesting sites, and Culex blood meal hosts were identified by DNA barcoding. Genotypes from crow blood meals and local crows (72 nestlings from 30 broods and 389 local breeders and helpers) were used to match mosquito blood meals to specific local crows.

Results

Among the 297 identified Culex blood meals, 20 (6.7%) were attributable to crows. The mean percentage of blood meals of crow origin was 19% in the nesting period (1 May–18 June 2014), but 0% in the weeks after fledging (19 June–1 September 2014), and the likelihood of a crow blood meal increased with proximity to an active nest: the odds that crows hosted a Culex blood meal were 38.07 times greater within 10 m of an active nest than > 10 m from an active nest. Nine of ten crow blood meals that could be matched to a genotype of a specific crow belonged to either nestlings in these nests or their mothers. Six of the seven genotypes that could not be attributed to sampled birds belonged to females, a sex bias likely due to mosquitoes targeting incubating or brooding females.

Conclusion

Data herein indicate that breeding crows serve as hosts for Culex in the initial stages of the WNV spring enzootic cycle. Given their high viremia, infected crows could thereby contribute to the re-initiation and early amplification of the virus, increasing its availability as mosquitoes shift to other moderately competent later-breeding avian hosts.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04827-x.

West Nile virus in California, 2003-2018: A persistent threat

The California Arbovirus Surveillance Program was initiated over 50 years ago to track endemic encephalitides and was enhanced in 2000 to include West Nile virus (WNV) infections in humans, mosquitoes, sentinel chickens, dead birds and horses. This comprehensive statewide program is a function of strong partnerships among the California Department of Public Health (CDPH), the University of California, and local vector control and public health agencies. This manuscript summarizes WNV surveillance data in California since WNV was first detected in 2003 in southern California. From 2003 through 2018, 6,909 human cases of WNV disease, inclusive of 326 deaths, were reported to CDPH, as well as 730 asymptomatic WNV infections identified during screening of blood and organ donors. Of these, 4,073 (59.0%) were reported as West Nile neuroinvasive disease. California's WNV disease burden comprised 15% of all cases that were reported to the U.S. Centers for Disease Control and Prevention during this time, more than any other state. Additionally, 1,299 equine WNV cases were identified, along with detections of WNV in 23,322 dead birds, 31,695 mosquito pools, and 7,340 sentinel chickens. Annual enzootic detection of WNV typically preceded detection in humans and prompted enhanced intervention to reduce the risk of WNV transmission. Peak WNV activity occurred from July through October in the Central Valley and southern California. Less than five percent of WNV activity occurred in other regions of the state or outside of this time. WNV continues to be a major threat to public and wild avian health in California, particularly in southern California and the Central Valley during summer and early fall months. Local and state public health partners must continue statewide human and mosquito surveillance and facilitate effective mosquito control and bite prevention measures.

Emerging Diseases of Avian Wildlife

Climate change and the interaction with humans and domestic species influences disease in avian wildlife. This article provides updated information on emerging disease conditions such as the spread of an Asian tick, Haemaphysalis longicornis, and its associated diseases among migratory birds in the eastern United States; lymphoproliferative disease virus in wild turkeys in the United States; and salmonellosis, particularly among passerines, which has zoonotic potential. In addition, it includes updated information on West Nile virus, Wellfleet Bay virus, and avian influenza and is intended to serve as a complement to the current veterinary literature for veterinarians treating avian wildlife species.

The Role of Birds of Prey in West Nile Virus Epidemiology

Reported human cases of West Nile virus (WNV) in Europe increased dramatically in 2018. Lineage 1 strains had been circulating in Euro-Mediterranean countries since the early 1990s. The subsequent introduction of WNV lineage 2 has been responsible for the remarkable upsurge of European WNV outbreaks since 2004, including the dramatic increase in human cases observed since 2018. The virus exists in a natural cycle between mosquitoes and wild birds, with humans and horses acting as dead-end hosts. As the key vertebrate hosts in the transmission cycle of WNV, avian species have been the focus of surveillance across many countries. Raptors appear particularly susceptible to WNV infection, resulting in higher prevalence, and in some cases exhibiting neurological signs that lead to the death of the animal. In addition, birds of prey are known to play an important role as WNV reservoir and potentially amplifying hosts of infection. Importantly, raptor higher susceptibility/prevalence may indicate infection through predation of infected prey. Consequently, they are considered important target species when designing cost-effective surveillance for monitoring both seasonal WNV circulation in endemic countries and its emergence into new areas, where migrating raptors may play a critical role in virus introduction. This review summarizes the different aspects of the current knowledge of WNV infection in birds of prey and evaluates their role in the evolution of the epizootic that is spreading throughout Europe.

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.

Current Progress of Avian Vaccines Against West Nile Virus

Birds are the main natural host of West Nile virus (WNV), the worldwide most distributed mosquito-borne flavivirus, but humans and equids can also be sporadic hosts. Many avian species have been reported as susceptible to WNV, particularly corvids. In the case that clinical disease develops in birds, this is due to virus invasion of different organs: liver, spleen, kidney, heart, and mainly the central nervous system, which can lead to death 24–48 h later. Nowadays, vaccines have only been licensed for use in equids; thus, the availability of avian vaccines would benefit bird populations, both domestic and wild ones. Such vaccines could be used in endangered species housed in rehabilitation and wildlife reserves, and in animals located at zoos and other recreational installations, but also in farm birds, and in those that are grown for hunting and restocking activities. Even more, controlling WNV infection in birds can also be useful to prevent its spread and limit outbreaks. So far, different commercial and experimental vaccines (inactivated, attenuated, and recombinant viruses, and subunits and DNA-based candidates) have been evaluated, with various regimens, both in domestic and wild avian species. However, there are still disadvantages that must be overcome before avian vaccination can be implemented, such as its cost-effectiveness for domestic birds since in many species the pathogenicity is low or zero, or the viability of being able to achieve collective immunity in wild birds in freedom. Here, a comprehensive review of what has been done until now in the field of avian vaccines against WNV is presented and discussed.

A Recombinant Subviral Particle-Based Vaccine Protects Magpie (Pica pica) Against West Nile Virus Infection

The mosquito-borne West Nile virus (WNV) is a highly neurovirulent Flavivirus currently representing an emergent zoonotic concern. WNV cycles in nature between mosquito vectors and birds that act as amplifier hosts and play an essential role in virus ecology, being, thus, WNV a threat to many species. Availability of an efficient avian vaccine would benefit certain avian populations, both birds grown for hunting and restocking activities, as well as endangered species in captive breeding projects, wildlife reservations, and recreation installations, and would be useful to prevent and contain outbreaks. Avian vaccination would be also of interest to limit WNV spillover to humans or horses from susceptible bird species that live in urbanized landscapes, like magpies. Herein, we have addressed the efficacy of a single dose of a WNV recombinant subviral particle (RSP) vaccine in susceptible magpie (Pica pica). The protective capacity of the RSP-based vaccine was demonstrated upon challenge of magpies with 5 × 10³ plaque forming units of a neurovirulent WNV strain. A significant improvement in survival rates of immunized birds was recorded when compared to vehicle-inoculated animals (71.4 vs. 22.2%, respectively). Viremia, which is directly related to the capacity of a host to be competent for virus transmission, was reduced in vaccinated animals, as was the presence of infectious virus in feather follicles. Bird-to-bird transmission was recorded in three of six unchallenged (contact) magpies housed with non-vaccinated WNV-infected birds, but not in contact animals housed with vaccinated WNV-infected magpies. These results demonstrate the protective efficacy of the RSP-based vaccine in susceptible birds against WNV infection and its value in controlling the spread of the virus.

West Nile Virus in Hummingbirds in California, USA, 2005-17

Although West Nile virus (WNV) infects many avian species, the prevalence and impact of WNV in hummingbirds (Family Trochilidae) have not been described. We focused on hummingbird mortality and WNV infection using data from the state of California's Dead Bird Surveillance Program for the years 2005-17. The overall prevalence of WNV in tested carcasses was 10.1% (49/486), with annual prevalence ranging from 0-28%. Prevalence by county ranged from 0-40% while prevalence by species ranged from 0-12%. We demonstrated that hummingbirds were naturally infected with WNV and displayed temporal and spatial patterns of infection consistent with trends observed in other avian species.

Comparative fitness of West Nile virus isolated during California epidemics

West Nile virus (WNV) has been circulating in California since its first detection in 2003, causing repeated outbreaks affecting public, wildlife and veterinary health. Epidemics of WNV are difficult to predict due to the multitude of factors influencing transmission dynamics among avian and mosquito hosts. Typically, high levels of WNV amplification are required for outbreaks to occur, and therefore associated viral strains may exhibit enhanced virulence and mortality in competent bird species resulting in increased mosquito infection prevalence. In our previous study, most WNV isolates made from California during 2007-08 showed increased fitness when competed in House Finches (HOFI, Haemorhous mexicanus) and Culex tarsalis Coquillett mosquitoes against COAV997-5nt, a genetically marked recombinant virus derived from a 2003 California strain. Herein, we evaluated the competitive fitness of WNV strains isolated during California epidemics in 2004, 2005, 2007, 2011 and 2012 against COAV997-5nt. These outbreak isolates did not produce elevated mortality in HOFIs, but replicated more efficiently than did COAV997-5nt based on quantification of WNV RNA copies in sera, thereby demonstrating increased competitive fitness. Oral co-infections in Cx. tarsalis resulted in similar virus-specific infection and transmission rates, indicating that outbreak isolates did not have a fitness advantage over COAV997-5nt. Collectively, WNV isolates from outbreaks demonstrated relatively greater avian, but not vector, replicative fitness compared to COAV997-5nt, similar to previously characterized non-outbreak isolates of WNV. Our results indicated that ecological rather than viral factors may facilitate WNV amplification to outbreak levels, but monitoring viral phenotypes through competitive fitness studies may provide insight into altered replication and transmission potential among emerging WNV strains.

High susceptibility of magpie (Pica pica) to experimental infection with lineage 1 and 2 West Nile virus

West Nile virus (WNV), a zoonotic pathogen naturally transmitted by mosquitoes whose natural hosts are birds, has spread worldwide during the last few decades. Resident birds play an important role in flavivirus epidemiology, since they can serve as reservoirs and facilitate overwintering of the virus. Herein, we report the first experimental infection of magpie (Pica pica) with two strains of West Nile virus, lineages 1 (NY-99) and 2 (SRB Novi-Sad/12), which are currently circulating in Europe. Magpies were highly susceptible to WNV infection, with similar low survival rates (30% and 42.8%) for both lineages. All infected magpies developed viremia detectable at 3 days post-infection with titers above those necessary for successful transmission of WNV to a mosquito. Neutralizing antibodies were detected at all time points analyzed (from 7 to 17 days post-infection). WNV genome was detected in the brains and hearts of all magpies that succumbed to the infection, and, in some of the surviving birds. WNV-RNA was amplified from swabs (oral and cloacal) at 3, 6 and 7 days post-infection and feather pulps, from 3 to 17 days post-infection, of infected animals. Even more, infectious virus was recovered from swabs up to 7 days post-infection and from feather pulps up to 10 days post infection. Sham-infected control animals were negative for viremia, viral RNA, and antibodies. These results suggest that the magpie, which is one of the most abundant corvid species in Europe, could represent a source of WNV transmission for birds and humans. Our observations shed light on the pathogenesis, transmission, and ecology of WNV and can benefit the implementation of surveillance and control programs.

Birds play an important role in the epidemiology of flaviviruses such as West Nile virus (WNV) since birds are natural hosts and facilitate hibernation of the virus in periods of absence of mosquitoes that transmit the virus. Since it has been proposed that magpies play an important role in an endemic WNV cycle in human habitats in Europe, we conducted the first experimental infection of magpie with the two WNV lineages currently circulating in Europe. We observed high susceptibility of magpie to WNV infection with virus titers higher than those necessary for the successful transmission of WNV to a mosquito and often resulting in death. Likewise, we detected elevated titers of neutralizing antibodies in all the samples tested as well as the viral genome in the organs, oropharyngeal and cloacal swabs and feather pulps of the infected animals. Our results suggest that the magpie, which is one of the most abundant corvid species in Europe, could be a source of WNV transmission to other birds and humans, which expands the knowledge about WNV pathogenesis, transmission and ecology, that benefits monitoring and control programs.

LOW PREVALENCE OF WEST NILE VIRUS ANTIBODIES IN SELECT NORTHERN CALIFORNIA OWL SPECIES (2007-2014)

The objective of this study was to determine evidence of previous West Nile virus (WNV) infection in northern California owls. Owl serum samples were collected from birds presenting to a veterinary medical teaching hospital between 2007 and 2014 and were screened for the presence of WNV antibodies by an indirect enzyme immunoassay (EIA). Only one of 71 samples (1.41%) tested was positive by EIA and confirmed by a plaque reduction neutralization test; it was the most recent sample collected. The reason for the low prevalence of WNV in these California owls despite a high prevalence in sympatric avian species in the same region is unknown and should be a topic for further research.

Serosurvey for West Nile Virus Antibodies in Steller's Jays ( Cyanocitta stelleri ) Captured in Coastal California, USA

West Nile virus (WNV) was first detected in New York in 1999 and, during its expansion across the continental US, southern Canada, and Mexico, members of the Corvidae (ravens, crows, magpies, and jays) were frequently infected and highly susceptible to the virus. As part of a behavioral study of Steller's Jays ( Cyanocitta stelleri ) conducted from 2011-14 in the coastal California counties of San Mateo and Santa Cruz, 380 Steller's Jays were captured and tested for antibodies to WNV. Using the wild bird immunoglobulin G enzyme linked immunoassay, we failed to detect antibodies to WNV, indicating either that there was no previous exposure to the virus or that exposed birds had died.

Evaluation of Nucleic Acid Preservation Cards for West Nile Virus Testing in Dead Birds

The California West Nile virus (WNV) Dead Bird Surveillance Program (DBSP) is an important component of WNV surveillance in the state. We evaluated FTA™ and RNASound™ cards as an alternative method for sampling dead birds for WNV molecular testing as these cards allow for more cost effective, rapid, and safer diagnostic sampling than the shipment of bird carcasses. To evaluate accuracy of results among avian sampling regimes, Reverse-Transcription Polymerase Chain Reaction (RT-PCR) results from FTA™ and RNASound™ cards were compared with results from kidney tissue, brain tissue, or oral swabs in lysis buffer in 2012-2013. In addition, RT-PCR results were compared with results from oral swabs tested by rapid antigen tests (RAMP™ and VecTOR™). While test results from the cards were not as sensitive as kidney tissue testing, they were more likely to provide accurate results than rapid antigen tests, and detected WNV in corvids as well as in other passerines, raptors, and waterfowl. Overall, WNV RT-PCR cycle threshold (Ct) scores from the cards were higher than those from tissue testing, but both card products displayed high sensitivity and specificity. American Crow samples provided the highest sensitivity. The cards also proved to be easier and more convenient vehicles for collecting and shipping samples, and in 2014 our program launched use of RNASound™ cards in the DBSP. Both FTA™ and RNASound™ products displayed 96% agreement with tissue results and are an adequate alternative sampling method for WNV dead bird testing.

Field Methods and Sample Collection Techniques for the Surveillance of West Nile Virus in Avian Hosts

Avian hosts play an important role in the spread, maintenance, and amplification of West Nile virus (WNV). Avian susceptibility to WNV varies from species to species thus surveillance efforts can focus both on birds that survive infection and those that succumb. Here we describe methods for the collection and sampling of live birds for WNV antibodies or viremia, and methods for the sampling of dead birds. Target species and study design considerations are discussed.

Surveys for Antibodies Against Mosquitoborne Encephalitis Viruses in California Birds, 1996-2013

From 1996 through 2013, 54,546 individual birds comprising 152 species and 7 orders were banded, bled, and released at four study areas within California, from which 28,388 additional serum samples were collected at one or more recapture encounters. Of these, 142, 99, and 1929 birds from 41 species were positive for neutralizing antibodies against western equine encephalomyelitis virus (WEEV), St. Louis encephalitis virus (SLEV), or West Nile virus (WNV) at initial capture or recapture, respectively. Overall, 83% of the positive serum samples were collected from five species: House Finch, House Sparrow, Mourning Dove, California Quail, and Western Scrub-Jay. Temporal data supported concurrent arbovirus surveillance and documented the disappearance of birds positive for WEEV in 2008 and SLEV in 2003 and the appearance of birds positive for WNV after its invasion in 2003. Results of these serosurveys agreed well with the host selection patterns of the Culex vectors as described from bloodmeal sequencing data and indicated that transmission of WNV seemed most effective within urban areas where avian and mosquito host diversity was limited to relatively few competent species.