West Nile Virus Surveillance Research Articles

Remote sensing of temperature-dependent mosquito and viral traits predicts field surveillance-based disease risk.

Mosquito-borne diseases contribute substantially to the global burden of disease, and are strongly influenced by environmental conditions. Ongoing and rapid environmental change necessitates improved understanding of the response of mosquito-borne diseases to environmental factors like temperature, and novel approaches to mapping and monitoring risk. Recent development of trait-based mechanistic models has improved understanding of the temperature dependence of transmission, but model predictions remain challenging to validate in the field. Using West Nile virus (WNV) as a case study, we illustrate the use of a novel remote sensing-based approach to mapping temperature-dependent mosquito and viral traits at high spatial resolution and across the diurnal cycle. We validate the approach using mosquito and WNV surveillance data controlling for other key factors in the ecology of WNV, finding strong agreement between temperature-dependent traits and field-based metrics of risk. Moreover, we find that WNV infection rate in mosquitos exhibits a unimodal relationship with temperature, peaking at ~24.6-25.2°C, in the middle of the 95% credible interval of optimal temperature for transmission of WNV predicted by trait-based mechanistic models. This study represents one of the highest resolution validations of trait-based model predictions, and illustrates the utility of a novel remote sensing approach to predicting mosquito-borne disease risk.

Public health contributions of entomological surveillance of West Nile virus (WNV) and other mosquito-borne arboviruses in a context of climate change.

Climate change is likely to increase the risk of human transmission of arboviruses endemic to Canada, including West Nile virus (WNV), Eastern equine encephalitis virus (EEEV) and California serogroup virus (CSV), calling for enhanced surveillance, including entomological surveillance targeting mosquito vectors. A scoping review was carried out to document the public health contributions of entomological surveillance of arboviruses of importance in Canada. The Ovid® and EBSCO platforms and the grey literature were searched to identify documents published between 2009 and 2023, in English or French, dealing with entomological surveillance of arboviruses of interest, conducted annually for human health purposes under the aegis of a government authority, with specified public health objectives and actions. The 42 selected publications mainly reported two public health objectives of adult mosquito surveillance: early warning of viral circulation and assessment of the level of risk of human transmission. Recommended actions included clinical preparedness, risk communication, promotion of personal protection measures and vector control. The main objectives of immature mosquito surveillance were to identify sites with high larval densities, in order to reduce/eliminate them and target the application of larvicides. In a context of climate change favouring the spread of arboviruses, this study highlights the potential public health contributions of regular entomological surveillance of endemic arboviruses of importance in Canada. It helps support concrete actions to protect the health of the population from the risks of arboviral transmission.

West Nile virus surveillance using sentinel birds: results of eleven years of testing in corvids in a region of northern Italy.

The natural transmission cycle of West Nile virus (WNV) involves birds as primary hosts and mosquitoes as vectors, but this virus can spread to mammals, human beings included. Asymptomatic infected donors pose a risk to the safety of blood transfusions and organ transplants, as WNV can be transmitted through these medical procedures. Since 2009, the region of Emilia-Romagna in northern Italy has been implementing an integrated surveillance system in order to detect WNV circulation in the environment at an early stage. Here we report the results of the two components of the surveillance system, the active testing of corvids and humans, and demonstrate that bird surveillance alone improves a surveillance system based solely on human case detection. As WNV risk reduction measures are applied on a provincial basis, we assessed the ability of this surveillance system component to detect virus circulation prior to the notification of the first human case for each province. Overall, 99 epidemic seasons were evaluated as a result of 11 years (2013-2023) of surveillance in the nine provinces of the region. In this period, 22,314 corvids were tested for WNV and 642 (2.9%) were found to be infected. WNV was generally first detected in birds in July, with sample prevalence peaks occurring between August and September. During the same period, 469 autochthonous human cases were notified, about 60% of which were reported in August. WNV was detected 79 times out of the 99 seasons considered. The virus was notified in birds 73 times (92.4%) and 60 times (75.9%) in humans. WNV was first or only notified in birds in 57 seasons (72.1%), while it was first or only notified in humans in 22 seasons (27.8%). Active surveillance in corvids generally allows the detection of WNV before the onset of human cases. Failure of virus detection occurred mainly in seasons where the number of birds tested was low. Our results show that active testing of a minimum of 3.8 corvids per 100 km2 provides a satisfactory timeliness in the virus detection, but for early detection of WNV it is crucial to test birds between mid-June and mid-August.

An update on the ecology, seasonality and distribution of Culex modestus in England

Abstract Culex (Barraudius) modestus is an important vector of West Nile virus (WNV) in Europe and it has the potential to play a bridge vector role in future WNV transmission in the UK. Here we provide an update on the known distribution of the species in England based on adult and larval data, characterise the preferred Cx. modestus larval habitats, and present adult and larval data from sites where the species is known to occur. Culex modestus is primarily found in the south-east of England, particularly in North Kent, the Thames Estuary, and along the Essex coast, and as far east as Orford Ness, Suffolk, and in Rainham Marshes, Essex, in the West. Adult numbers peak in mid-late July, with larval numbers highest in late August / early September. Preferred habitats in North Kent are warm, shallow, narrow ditches, with an abundance of marginal, submerged, and floating vegetation. Such environmental data on the distribution, seasonality and habitat preference of Cx. modestus are critical for informing WNV surveillance programmes, identifying at risk areas (associated with this species) and providing information for a targeted control strategy in the event of WNV transmission.

Automated molecular detection of West Nile Virus in mosquito pools using the Panther Fusion system

West Nile Virus (WNV) is an arthropod-borne virus that is spread through mosquito vectors. WNV emerged in the US in 1999 and has since become endemic in the US, causing the most domestically acquired arboviral disease in the country. Mosquito surveillance for WNV is useful to monitor arboviral disease burden over time and across different locations. RT-qPCR is the preferred method for WNV surveillance, but these methods are labor-intensive. The Panther Fusion System has an Open Access feature that allows for laboratory-developed tests (LDTs) to run on a fully automated system for nucleic acid extraction, RT-qPCR, and result generation. This study demonstrates the successful optimization of a WNV multiplex LDT (assay targets: ENV and NS1 genes) for high-throughput environmental surveillance testing of mosquito pool homogenates on the Panther Fusion System. Analytical sensitivity of the assay was 186 and 744 copies/PCR reaction for the ENV and NS1 targets, respectively. To assess the performance of this assay, a total of 80 mosquito pools were tested, including 60 previously tested pools and 20 spiked negative mosquito pools. Among the 60 previously tested specimens, the Panther Fusion WNV LDT demonstrated 100% positive and negative agreement with the CDC West Nile RT-qPCR assay. The Panther Fusion WNV LDT also detected all 20 spiked specimens. The Panther Fusion WNV LDT assay was successfully developed and optimized for high throughput testing with similar performance to the previously used CDC West Nile RT-qPCR assay.

Effects of climatic and environmental factors on mosquito population inferred from West Nile virus surveillance in Greece

Mosquito-borne diseases’ impact on human health is among the most prominent of all communicable diseases. With limited pool of tools to contrast these diseases, public health focus remains preventing mosquito-human contacts. Applying a hierarchical spatio-temporal Bayesian model on West Nile virus (WNV) surveillance data from Greece, we aimed to investigate the impact of climatic and environmental factors on Culex mosquitoes’ population. Our spatio-temporal analysis confirmed climatic factors as major drivers of WNV-transmitting-Culex mosquitoes population dynamics, with temperature and long periods of moderate-to-warm climate having the strongest positive effect on mosquito abundance. Conversely, rainfall, high humidity, and wind showed a negative impact. The results suggest the presence of statistically significant differences in the effect of regional and seasonal characteristics, highlighting the complex interplay between climatic, geographical and environmental factors in the dynamics of mosquito populations. This study may represent a relevant tool to inform public health policymakers in planning preventive measures.

Surveillance, prevention and control of West Nile virus and Usutu virus infections in the EU/EEA

Surveillance, prevention and control of West Nile virus and Usutu virus infections in the EU/EEA

Serological and molecular surveillance of West Nile virus in domesticated mammals of peninsular Malaysia.

West Nile virus is a mosquito-borne neurotropic pathogen with a wide host range that constitutes a significant risk to public and animal health. There is limited information on WNV infection in domesticated mammals in Malaysia; however, current reports indicate infections in birds, macaques, bats and pigs from Malaysia. In this study, 203 serum samples from cattle, goats, and horses were tested for the presence of anti-WNV IgG using a competitive enzyme-linked immunosorbent assay (c-ELISA). Additionally, using one-step RT-PCR, nasopharyngeal swabs were analyzed for WNV RNA from all 203 animals in this study. The WNV seroprevalence was 32.53% (27/83) at 95% CI (0.2342-0.4319) in cattle, 48.27% (14/29) at 95% CI (0.3139-0.6557) in goats and 53.84% (49/91) at 95% CI (0.4366-0.6373) in horses. Cross-reactive JEV antibodies were detected in two cattle and 34 horses. None of the cattle or goats tested positive for WNV RT-PCR. Seven horses were positive for WNV RT-PCR, a molecular prevalence of 7.69% (7/91) at 95% CI (0.0353-0.1528). This is the first reported detection of WNV in domesticated mammals of Malaysia, a significant addition to the growing evidence that WNV is being transmitted from vectors to susceptible hosts in Malaysia.

Long-term serological surveillance for West Nile and Usutu virus in horses in south-West Spain

West Nile virus (WNV) is a re-emerging zoonotic pathogen with increasing incidence in Europe, producing a recent outbreak in 2020 in Spain with 77 human cases and eight fatalities. However, the factors explaining the observed changes in the incidence of WNV in Europe are not completely understood. Longitudinal monitoring of WNV in wild animals across Europe is a useful approach to understand the eco-epidemiology of WNV in the wild and the risk of spillover into humans. However, such studies are very scarce up to now. Here, we analysed the occurrence of WNV and Usutu virus (USUV) antibodies in 2102 samples collected between 2005 and 2020 from a population of feral horses in Doñana National Park. The prevalence of WNV antibodies varied between years, with a mean seroprevalence of 8.1% (range 0%–25%) and seasonally. Climate conditions including mean minimum annual temperatures and mean rainy days per year were positively correlated with WNV seroprevalence, while the annual rainfall was negatively. We also detected the highest incidence of seroconversions in 2020 coinciding with the human outbreak in southern Spain. Usutu virus-specific antibodies were detected in the horse population since 2011. The WNV outbreak in humans was preceded by a long period of increasing circulation of WNV among horses with a very high exposure in the year of the outbreak. These results highlight the utility of One Health approaches to better understand the transmission dynamics of zoonotics pathogens.

Arbovirus surveillance on the Mexico-USA border: West Nile virus identification in various species of field mosquitoes.

West Nile virus (WNV) has been documented in human and/or mosquito samples near the border with Mexico in El Paso, Texas, and Doña Ana County, New Mexico. However, on the Mexican side of the border, particularly in the State of Chihuahua, no such cases of WNV-infected mosquitoes have been documented. We tested 367 mosquitoes of four species (Culex quinquefasciatus, Cx. tarsalis, Aedes aegypti, and Aedes (Ochlerotatus) epactius) and found a high rate of WNV-positivity, including the first record of Ae. (Ochlerotatus) epactius infection with WNV. These results call for intensifying WNV surveillance efforts on the border between the United States and Mexico, with particular emphasis on vector control and monitoring of the species included in this study.

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.

Circulation of West Nile virus in mosquitoes approximate to the migratory bird stopover in West Coast Malaysia.

Being a tropical country with a conducive environment for mosquitoes, mosquito-borne illnesses such as dengue, chikungunya, lymphatic filariasis, malaria, and Japanese encephalitis are prevalent in Malaysia. Recent studies reported asymptomatic infection of West Nile virus (WNV) in animals and humans, but none of the studies included mosquitoes, except for one report made half a century ago. Considering the scarcity of information, our study sampled mosquitoes near migratory bird stopover wetland areas of West Coast Malaysia located in the Kuala Gula Bird Sanctuary and Kapar Energy Venture, during the southward migration period in October 2017 and September 2018. Our previous publication reported that migratory birds were positive for WNV antibody and RNA. Using a nested RT-PCR analysis, WNV RNA was detected in 35 (12.8%) out of 285 mosquito pools consisting of 2,635 mosquitoes, most of which were Culex spp. (species). Sanger sequencing and phylogenetic analysis revealed that the sequences grouped within lineage 2 and shared 90.12%-97.01% similarity with sequences found locally as well as those from Africa, Germany, Romania, Italy, and Israel. Evidence of WNV in the mosquitoes substantiates the need for continued surveillance of WNV in Malaysia.

Development of Toehold Switches as a Novel Ribodiagnostic Method for West Nile Virus.

West Nile virus (WNV) is an emerging neurotropic RNA virus and a member of the genus Flavivirus. Naturally, the virus is maintained in an enzootic cycle involving mosquitoes as vectors and birds that are the principal amplifying virus hosts. In humans, the incubation period for WNV disease ranges from 3 to 14 days, with an estimated 80% of infected persons being asymptomatic, around 19% developing a mild febrile infection and less than 1% developing neuroinvasive disease. Laboratory diagnosis of WNV infection is generally accomplished by cross-reacting serological methods or highly sensitive yet expensive molecular approaches. Therefore, current diagnostic tools hinder widespread surveillance of WNV in birds and mosquitoes that serve as viral reservoirs for infecting secondary hosts, such as humans and equines. We have developed a synthetic biology-based method for sensitive and low-cost detection of WNV. This method relies on toehold riboswitches designed to detect WNV genomic RNA as transcriptional input and process it to GFP fluorescence as translational output. Our methodology offers a non-invasive tool with reduced operating cost and high diagnostic value that can be used for field surveillance of WNV in humans as well as in bird and mosquito populations.

Species of mosquitoes present in Abruzzo and Molise and their possible role as vector of Usutu and West Nile viruses.

In 2019, entomological survey on mosquitoes was carried out in Abruzzo and Molise regions in central Italy to obtain data on local mosquito fauna. Collection sites were selected based on a previous ecoregion classification of the territory. From 2019 to 2021 virological surveillance for West Nile virus (WNV) and Usutu virus (USUV) on mosquitoes was carried out in the same regions, selecting ecoregions where virus circulation and vector presence were more likely, all mosquitoes were collected and identified, and the female mosquitoes were sorted in 3046 pools and tested for the presence of WNV and USUV by Real-time PCR. All pools tested negative for WND, while USUV was detected in 7 pools of Aedes caspius collected in Molise region, 17 pools of Culex pipiens s.l. (2 collected in Molise, 15 in Abruzzo), and 1 pool of Culiseta longiareolata collected in Molise. These results suggests the presence of an USUV enzootic cycle, maintained by Culex pipiens s.l. and Aedes caspius in both Italian regions, as well as providing a useful picture in terms of species presence and abundance for both regions. Ecoregions proved to be a very valuable tool in determining high risk areas for vector borne diseases.

Molecular Surveillance of West Nile Virus in Mosquitoes and Sentinel Chickens.

Arboviral surveillance is a critical step in any effective mosquito control program. Surveillance aids in the early detection of pathogen transmission as well as establishes a baseline of transmission activity. Two of the most practical forms of arboviral surveillance is through the use of testing mosquito pools for the presence of pathogen and screening sentinel chickens for pathogen exposure. This chapter describes the process for each of these methods for West Nile virus.

Rapid increase in neuroinvasive West Nile virus infections in humans, Italy, July 2022.

As in 2018, when a large West Nile virus (WNV) epidemic occurred, the 2022 vector season in Italy was marked by an early onset of WNV circulation in mosquitoes and birds. Human infections were limited until early July, when we observed a rapid increase in the number of cases. We describe the epidemiology of human infections and animal and vector surveillance for WNV and compare the more consolidated data of June and July 2022 with the same period in 2018.

2021 AMCA Presidential Address.

2021 AMCA Presidential Address.

West Nile Virus Surveillance in Sentinel Chickens and Mosquitoes in Panama City Beach, Florida, from 2014 To 2020.

Over 20 years since its introduction, the West Nile virus (WNV) continues to be the leading cause of arboviral disease in the USA. In Panama City Beach (Bay County, FL), WNV transmission is monitored using sentinel chickens and testing mosquito pools for presence of viral RNA. In the current work, we monitored WNV transmission from 2014 to 2020 through weekly serology sampling of sentinel chickens; mosquito populations through biweekly mosquito collections by suction traps (1 m and 9 m) and weekly gravid trap collections; and mosquito infection rates using a reverse transcriptase-polymerase chain reaction (RT-PCR) assay. Samples were sent to the Bureau of Public Health Laboratories (Tampa, FL) for testing presence/absence of WNV via RT-PCR assay. Our results indicated that canopy surveillance could augment ground collections, providing greater proportions of Culex mosquitoes with less bycatch compared with ground collections. Serology indicated 94 seroconversions to WNV in the study area from 2014 to 2020. The most active year was 2016, which accounted for 32% (n = 30) of all seroconversions reported during the study period. We detected 20 WNV-positive mosquito pools from Culex quinquefasciatus during 2014-17; mosquito infection rates ranged from 2.02 to 23.81 per thousand (95% CI). Climate data indicated anomalously high precipitation in 2014-19 preceding WNV transmission. Data analyzed herein indicate utility in year-round continuous and diversified surveillance methodologies. This information is needed to properly calibrate future models that could assist with predicting transmission events of WNV in Panama City Beach, FL.

West Nile Virus Occurrence and Ecological Niche Modeling in Wild Bird Species and Mosquito Vectors: An Active Surveillance Program in the Peloponnese Region of Greece.

West Nile Virus (WNV) is maintained in nature in a bird-mosquito cycle and human infections follow a seasonal pattern, favored by climatic conditions. Peloponnese Region, located in Southern Greece, initiated an active WNV surveillance program to protect public health during 2019–2020. The project included monitoring of avian hosts and mosquito vectors, while sampling locations were prioritized after consideration of WNV circulation in birds, mosquitos and humans during previous seasons. Biological materials were collected from 493 wild birds of 25 species and 678 mosquito pools, which were molecularly screened for WNV presence. In this case, 14 environmental variables were associated with WNV detection in wild birds and mosquitos by using two separate MaxEnt models. Viral RNA was not detected in the target species during 2019, although in 2020, it was reported on 46 wild birds of ten species and 22 mosquito pools (Culex pipiens and Aedes albopictus). Altitude and land uses were significant predictors for both models and in fact, suitable conditions for virus occurrence were identified in low altitude zones. Bird- and mosquito-based surveillance systems yielded similar results and allowed for targeted vector control applications in cases of increased virus activity. Human cases were not reported on Peloponnese in 2020.

Pathogenesis of West Nile Virus Lineage 2 in Domestic Geese after Experimental Infection

West Nile virus (WNV) is an emerging infectious pathogen circulating between mosquitoes and birds but also infecting mammals. WNV has become autochthonous in Germany, causing striking mortality rates in avifauna and occasional diseases in humans and horses. We therefore wanted to assess the possible role of free-ranging poultry in the WNV transmission cycle and infected 15 goslings with WNV lineage 2 (German isolate). The geese were monitored daily and sampled regularly to determine viremia, viral shedding, and antibody development by molecular and serological methods. Geese were euthanized at various time points post-infection (pi). All infected geese developed variable degrees of viremia from day 1 to day 10 (maximum) and actively shed virus from days 2 to 7 post-infection. Depending on the time of death, the WN viral genome was detected in all examined tissue samples in at least one individual by RT-qPCR and viable virus was even re-isolated, except for in the liver. Pathomorphological lesions as well as immunohistochemically detectable viral antigens were found mainly in the brain. Furthermore, all of the geese seroconverted 6 days pi at the latest. In conclusion, geese are presumably not functioning as important amplifying hosts but are suitable sentinel animals for WNV surveillance.