An Integrative Eco-Epidemiological Analysis of West Nile Virus Transmission

Is Culex modestus a New Usutu virus vector?

Usutu virus is an emerging pathogen transmitted by mosquitoes. Culex modestus mosquitoes are widespread in Europe, but their role in disease transmission is poorly understood. Recent data from a single infectious mosquito suggested that Culex modestus could be an unrecognized vector for Usutu virus. In this study, our aim was to corroborate this finding using a larger sample size. We collected immature Culex modestus from a reedbed pond in Flemish Brabant, Belgium, and reared them in the laboratory until the third generation. Adult females were then experimentally infected with Usutu virus in a blood meal and incubated at 25 °C for 14 days. The presence of Usutu virus in the saliva, head and body of each female was determined by plaque assay and quantitative real-time reverse-transcription polymerase chain reaction (qRT-PCR). The transmission efficiency was 54% (n = 15/28), confirming that Belgian Culex modestus can experimentally transmit Usutu virus.

Culex modestus: the overlooked mosquito vector

Culex (Barraudius) modestus (Ficalbi 1889) are found in temperate regions across Europe, Asia, and Northern Africa. These mosquitoes thrive during the summer and prefer to breed in permanent vegetative habitats such as rice paddies and marshes. Culex modestus feed on a wide range of bird species but are highly attracted to humans, which makes them a potential 'bridge' vector for enzootic pathogens. There is compelling evidence that Culex modestus is an efficient vector for West Nile virus, potentially capable of causing epidemics in humans and other mammals. This species is also a likely vector for Usutu virus, avian malaria (Plasmodium spp.), and parasitic heartworms (Dirofilaria spp.). Culex modestus can be morphologically identified at the larval and adult stages, and a distinctive phenotype of this species is their ability to overwinter. Despite the widespread establishment of this mosquito species and their role as vectors for human pathogens, we lack sufficient knowledge on this species to implement and evaluate targeted vector control measures. Since Culex modestus can be considered a potential public health threat, there is a need for a better understanding of this mosquito species.

Spatial and temporal dynamics of West Nile virus between Africa and Europe

It is unclear whether West Nile virus (WNV) circulates between Africa and Europe, despite numerous studies supporting an African origin and high transmission in Europe. We integrated genomic data with geographic observations and phylogenetic and phylogeographic inferences to uncover the spatial and temporal viral dynamics of WNV between these two continents. We focused our analysis towards WNV lineages 1 (L1) and 2 (L2), the most spatially widespread and pathogenic WNV lineages. Our study shows a Northern-Western African origin of L1, with back-and-forth exchanges between West Africa and Southern-Western Europe; and a Southern African origin of L2, with one main introduction from South Africa to Europe, and no back introductions observed. We also noticed a potential overlap between L1 and L2 Eastern and Western phylogeography and two Afro-Palearctic bird migratory flyways. Future studies linking avian and mosquito species susceptibility, migratory connectivity patterns, and phylogeographic inference are suggested to elucidate the dynamics of emerging viruses.

Comparison of West Nile Virus Disease in Humans and Horses: Exploiting Similarities for Enhancing Syndromic Surveillance

West Nile virus (WNV) neuroinvasive disease threatens the health and well-being of horses and humans worldwide. Disease in horses and humans is remarkably similar. The occurrence of WNV disease in these mammalian hosts has geographic overlap with shared macroscale and microscale drivers of risk. Importantly, intrahost virus dynamics, the evolution of the antibody response, and clinicopathology are similar. The goal of this review is to provide a comparison of WNV infection in humans and horses and to identify similarities that can be exploited to enhance surveillance methods for the early detection of WNV neuroinvasive disease.

Vector role and human biting activity of Anophelinae mosquitoes in different landscapes in the Brazilian Amazon

BACKGROUND

Environmental disturbance, deforestation and socioeconomic factors all affect malaria incidence in tropical and subtropical endemic areas. Deforestation is the major driver of habitat loss and fragmentation, which frequently leads to shifts in the composition, abundance and spatial distribution of vector species. The goals of the present study were to: (i) identify anophelines found naturally infected with Plasmodium; (ii) measure the effects of landscape on the number of Nyssorhynchus darlingi, presence of Plasmodium-infected Anophelinae, human biting rate (HBR) and malaria cases; and (iii) determine the frequency and peak biting time of Plasmodium-infected mosquitoes and Ny. darlingi.

METHODS

Anopheline mosquitoes were collected in peridomestic and forest edge habitats in seven municipalities in four Amazon Brazilian states. Females were identified to species and tested for Plasmodium by real-time PCR. Negative binomial regression was used to measure any association between deforestation and number of Ny. darlingi, number of Plasmodium-infected Anophelinae, HBR and malaria. Peak biting time of Ny. darlingi and Plasmodium-infected Anophelinae were determined in the 12-h collections. Binomial logistic regression measured the association between presence of Plasmodium-infected Anophelinae and landscape metrics and malaria cases.

RESULTS

Ninety-one females of Ny. darlingi, Ny. rangeli, Ny. benarrochi B and Ny. konderi B were found to be infected with Plasmodium. Analysis showed that the number of malaria cases and the number of Plasmodium-infected Anophelinae were more prevalent in sites with higher edge density and intermediate forest cover (30-70%). The distance of the drainage network to a dwelling was inversely correlated to malaria risk. The peak biting time of Plasmodium-infected Anophelinae was 00:00-03:00 h. The presence of Plasmodium-infected mosquitoes was higher in landscapes with > 13 malaria cases.

CONCLUSIONS

Nyssorhynchus darlingi, Ny. rangeli, Ny. benarrochi B and Ny. konderi B can be involved in malaria transmission in rural settlements. The highest fraction of Plasmodium-infected Anophelinae was caught from midnight to 03:00 h. In some Amazonian localities, the highest exposure to infectious bites occurs when residents are sleeping, but transmission can occur throughout the night. Forest fragmentation favors increases in both malaria and the occurrence of Plasmodium-infected mosquitoes in peridomestic habitat. The use of insecticide-impregnated mosquito nets can decrease human exposure to infectious Anophelinae and malaria transmission.

Establishment of Culex modestus in Belgium and a Glance into the Virome of Belgian Mosquito Species

Culex modestus mosquitoes are considered potential transmission vectors of West Nile virus and Usutu virus. Their presence has been reported across several European countries, including one larva detected in Belgium in 2018. In this study, mosquitoes were collected in the city of Leuven and surrounding areas in the summers of 2019 and 2020. Species identification was performed based on morphological features and partial sequences of the mitochondrial cytochrome oxidase subunit I (COI) gene. The 107 mosquitoes collected in 2019 belonged to eight mosquito species, Culex pipiens (24.3%), Cx. modestus (48.6%), Cx. torrentium (0.9%), Culiseta annulata (0.9%), Culiseta morsitans (0.9%), Aedes sticticus (14.0%), Aedes cinereus (9.3%), and Anopheles plumbeus (0.9%), suggesting the presence of an established Cx. modestus population in Belgium. The collection of Cx. modestus mosquitoes at the same locations in 2020 confirmed their establishment in the region. Haplotype network analysis of the COI sequences for Cx. modestus showed that the Belgian population is rather diverse, suggesting that it may have been established in Belgium for some time. The Belgian Cx. modestus population was most closely related to populations from the United Kingdom and Germany. Characterization of the virome of the collected mosquitoes resulted in the identification of at least 33 eukaryotic viral species. Nine (nearly) complete genomes belonging to 6 viral species were identified, all of which were closely related to known viruses. In conclusion, here, we report the presence of Cx. modestus in the surrounding areas of Leuven, Belgium. As this species is considered to be a vector of several arboviruses, the implementation of vector surveillance programs to monitor this species is recommended.

IMPORTANCECulex modestus mosquitoes are considered to be a potential “bridge” vector, being able to transmit pathogens between birds as well as from birds to mammals, including humans. In Belgium, this mosquito species was considered absent until the finding of one larva in 2018 and subsequent evidence of a large population in 2019 to 2020 described here. We collected mosquitoes in the summers of 2019 and 2020 in the city of Leuven and surrounding areas. The mosquito species was identified by morphological and molecular methods, demonstrating the presence of Cx. modestus in this region. The ability of mosquitoes to transmit pathogens can depend on several factors, one of them being their natural virus composition. Therefore, we identified the mosquito-specific viruses harbored by Belgian mosquitoes. As Cx. modestus is able to transmit viruses such as West Nile virus and Usutu virus, the establishment of this mosquito species may increase the risk of virus transmission in the region. It is thus advisable to implement mosquito surveillance programs to monitor this species.

Individual, household and environmental factors associated with arboviruses in rural human populations, Brazil

Landscape change is one of the foremost drivers of the emergence of infectious diseases. Exploring demographic, household and environmental conditions under which infectious diseases occur may inform strategies to prevent disease emergence in human populations. We collected blood samples from 523 humans and explore factors for arbovirus emergence in Bahia, Brazil. The overall arbovirus seroprevalence was 65.2%, with the genus Flavivirus most prevalent (64.4%). Based on monotypic reactions, the population had contact with five arbovirus: Dengue 3, Ilheus, Oropouche, Caraparu and Eastern equine encephalitis virus. To our knowledge, this is the first study reporting exposure to Oropouche, Caraparu and Eastern equine encephalitis virus in human populations in Bahia, Northeast of Brazil. The best model fit demonstrated that household and environmental variables were more predictive of the risk of arbovirus exposure than demographic variables. The presence of forest and free-living monkeys in the areas close to the communities had a protective effect for the human population (i.e. lower seroprevalence). The dilution effect is considered as one explanation for this finding. These results highlight the important ecological role of wildlife-friendly agriculture.

Deconstructing the Impact of Malaria Vector Diversity on Disease Risk

AbstractRecent years have seen significant progress in understanding the impact of host community assemblage on disease risk, yet diversity in disease vectors has rarely been investigated. Using published malaria and mosquito surveys from Kenya, we analyzed the relationship between malaria prevalence and multiple axes of mosquito diversity: abundance, species richness, and composition. We found a net amplification of malaria prevalence by vector species richness, a result of a strong direct positive association between richness and prevalence alongside a weak indirect negative association between the two, mediated through mosquito community composition. One plausible explanation of these patterns is species niche complementarity, whereby less competent vector species contribute to disease transmission by filling spatial or temporal gaps in transmission left by dominant vectors. A greater understanding of vector community assemblage and function, as well as any interactions between host and vector biodiversity, could offer insights to both fundamental and applied ecology.

The knowns and unknowns of West Nile virus in Europe: what did we learn from the 2018 outbreak?

Introduction: West Nile virus (WNV) is a mosquito-borne human and animal pathogen with nearly worldwide distribution. In Europe, the virus is endemic with seasonal regional outbreaks that have increased in frequency over the last 10 years. A massive outbreak occurred across southern and central Europe in 2018 with the number of confirmed human cases increasing up to 7.2-fold from the previous year, and expanding to include previously virus-free regions.Areas covered: This review focuses on potential causes that may explain the 2018 European WNV outbreak. We discuss the role genetic, ecological, and environmental aspects may have played in the increased activity during the 2018 transmission season, summarizing the latest epidemiological and virological publications.Expert opinion: Optimal environmental conditions, specifically increased temperature, were most likely responsible for the observed outbreak. Other factors cannot be ruled out due to limited available information, including factors that may influence host/vector abundance and contact. Europe will likely experience even larger-scale outbreaks in the coming years. Increased surveillance efforts should be implemented with a focus on early-warning detection methods, and large-scale host and vector surveys should continue to fill gaps in knowledge.

Predicting West Nile virus transmission in North American bird communities using phylogenetic mixed effects models and eBird citizen science data

BACKGROUND

West Nile virus (WNV) is a mosquito-transmitted disease of birds that has caused bird population declines and can spill over into human populations. Previous research has identified bird species that infect a large fraction of the total pool of infected mosquitoes and correlate with human infection risk; however, these analyses cover small spatial regions and cannot be used to predict transmission in bird communities in which these species are rare or absent. Here we present a mechanistic model for WNV transmission that predicts WNV spread (R0) in any bird community in North America by scaling up from the physiological responses of individual birds to transmission at the level of the community. We predict unmeasured bird species' responses to infection using phylogenetic imputation, based on these species' phylogenetic relationships with bird species with measured responses.

RESULTS

We focused our analysis on Texas, USA, because it is among the states with the highest total incidence of WNV in humans and is well sampled by birders in the eBird database. Spatio-temporal patterns: WNV transmission is primarily driven by temperature variation across time and space, and secondarily by bird community composition. In Texas, we predicted WNV R0 to be highest in the spring and fall when temperatures maximize the product of mosquito transmission and survival probabilities. In the most favorable months for WNV transmission (April, May, September and October), we predicted R0 to be highest in the "Piney Woods" and "Oak Woods & Prairies" ecoregions of Texas, and lowest in the "High Plains" and "South Texas Brush County" ecoregions. Dilution effect: More abundant bird species are more competent hosts for WNV, and predicted WNV R0 decreases with increasing species richness. Keystone species: We predicted that northern cardinals (Cardinalis cardinalis) are the most important hosts for amplifying WNV and that mourning doves (Zenaida macroura) are the most important sinks of infection across Texas.

CONCLUSIONS

Despite some data limitations, we demonstrate the power of phylogenetic imputation in predicting disease transmission in heterogeneous host communities. Our mechanistic modeling framework shows promise both for assisting future analyses on transmission and spillover in heterogeneous multispecies pathogen systems and for improving model transparency by clarifying assumptions, choices and shortcomings in complex ecological analyses.

Prevalence and molecular epidemiology of West Nile and Usutu virus infections in Croatia in the 'One health' context, 2018

In 2018, Croatia reported the largest outbreak of West Nile virus (WNV) infections as well as the re-occurrence of human Usutu virus (USUV) infections. For the first time, fatal WNV and USUV infections were detected in wild birds. We analysed epidemiological characteristics and molecular epidemiology of WNV and USUV infections detected during 2018 transmission season. From April to November, 178 patients with neuroinvasive disease and 68 patients with febrile disease were tested for WNV and USUV. Viral RNA was detected in cerebrospinal fluid (CSF) and urine samples using a real-time RT-PCR. Positive samples were tested by nested RT-PCR and nucleotide sequencing. IgM/IgG antibodies were detected in serum/CSF samples using ELISA with confirmation of cross-reactive samples by virus neutralization test (VNT). WNV neuroinvasive disease was confirmed in 54 and WNV fever in seven patients from 10 continental Croatian counties. Areas affected in 2018 were those in which cases occurred in previous seasons, while in three areas human cases were reported for the first time. Phylogenetic analysis of six strains from patients residing in different geographic areas showed circulation of WNV lineage 2. In three patients, neuroinvasive USUV infection was confirmed by RT-PCR or VNT. Sequence analysis of one detected strain revealed USUV Europe 2 lineage. During the same period, a total of 2,574 horse and 1,069 poultry serum samples were tested for WNV antibodies using ELISA. Acute asymptomatic WNV infection (IgM antibodies) was documented in 20/0.7% horses. WNV IgG antibodies were found in 307/11.9% horses and in 125/12.7% poultry. WNV RNA was detected in two goshawks and USUV RNA was detected in one blackbird from north-western Croatia. In the Zagreb area, 3,670 female mosquitoes were collected. One Culex pipiens pool collected in July tested positive for USUV RNA. Our results highlight the importance of continuous multidisciplinary 'One health' surveillance of these emerging arboviruses.

Flaviviruses in migratory passerines during spring stopover in a desert oasis

Bird migration has long been hypothesized as the main mechanism for long-distance dispersal of flaviviruses, but the role of migratory birds in flaviviruses spillover is not well documented. In this study, we investigated the eco-epidemiology of West Nile virus (WNV) and Usutu virus (USUV) in trans-Saharan passerines during their spring stopover in southern Tunisian oases. To do, we combined oral swab analysis and serological tools to assess whether migratory birds could be reaching these stopover sites while infectious or have been previously exposed to viruses. All sampled birds tested negative for oral swab analysis. However, anti-WNV and anti-USUV antibodies were detected in 32% and 1% of tested birds, respectively. Among WNV-seropositive species, the Golden oriole (Oriolus oriolus) showed the highest anti-WNV occurrence probability. In this species, anti-WNV occurrence was twice larger in males than females. Inter-specific and intraspecific morphological, physiological and behavioural differences could explain these results. Although our findings did not show evidence for passerines migrating while infectious, they did not exclude an existing enzootic WNV transmission cycle in Tunisian oases. Further investigations including larger samples of migratory birds are needed for a better understanding of this issue.

West Nile virus transmission and human infection risk in Veneto (Italy): a modelling analysis

An intensified and continuous West Nile virus (WNV) spread across northern Italy has been observed since 2008, which caused more than one hundred reported human infections until 2016. Veneto is one of the Italian regions where WNV is considered endemic, and the greatest intensity of circulation was observed during 2013 and 2016. By using entomological data collected across the region in those years, we calibrated a temperature-driven mathematical model through a Bayesian approach that simulates the WNV infection in an avian population with seasonal demography. We considered two alternative routes of life cycle re-activation of the virus at the beginning of each vector breeding season: in the first one the virus is maintained by infected birds, in the other by diapausing mosquitoes previously infected. Afterwards, we computed seasonal risk curves for human infection and quantified how they translate into reported symptomatic cases. According to our results, WNV is more likely to be re-activated each year via previously infected mosquitoes. The highest probability of human infection is expected to occur in August, consistently with observations. Our epidemiological estimates can be of particular interest for public health authorities, to support decisions in term of designing efficient surveillance plans and preventive measures.