North American Culex pipiens and Culex quinquefasciatus are competent vectors for Usutu virus

Update on the composition and distribution of the mosquito fauna (Diptera: Culicidae) in Cabo Verde, a country at risk for mosquito-borne diseases

Mosquitoes play a critical role as vectors of pathogens affecting both humans and animals. Therefore, understanding their biodiversity and distribution is crucial to developing evidence-based vector control strategies. The current study updated the composition and distribution of mosquito species through a comprehensive survey of all municipalities of Cabo Verde. From October 2017 to September 2018, mosquito larvae and pupae were collected from 814 aquatic habitats. Anopheles gambiae (Giles, 1902) and Culex pipiens (Linnaeus, 1758) complexes were subjected to PCR-based techniques for sibling species identification. Ten mosquito species from 5 genera were identified: Aedes aegypti (Linnaeus, 1762), Aedes caspius (Pallas, 1771), Anopheles arabiensis (Patton, 1905), Anopheles pretoriensis (Theobald, 1903), Culex bitaeniorhynchus (Giles, 1901), Cx. pipiens, Culex quinquefasciatus (Say, 1823), Culex tritaeniorhynchus (Giles, 1901), Culiseta longiareolata (Macquart, 1838), and Lutzia tigripes (de Grandpre & de Charmoy, 1901). Santiago Island reported the highest number of species (n = 8). Ae. aegypti and Cx. quinquefasciatus were the most widely distributed species across the country. An. arabiensis was the sole species identified within the An. gambiae complex. The findings from our study will help guide health policy decisions to effectively control mosquito-borne diseases.

Mosquito (Diptera: Culicidae) populations in contrasting areas of the western regions of Burkina Faso: species diversity, abundance and their implications for pathogen transmission

BACKGROUND

Mosquitoes (Diptera: Culicidae) can have a significant negative impact on human health. The urbanization of natural environments and their conversion for agricultural use, as well as human population growth, may affect mosquito populations and increase the risk of emerging or re-emerging mosquito-borne diseases. We report on the variety and number of adult mosquitoes found in four environments with varying degrees of human impact (rural, urban, rice fields, and forest) located in a savannah zone of West Africa.

METHODS

Mosquitoes were collected from two regions (Hauts-Bassins and Sud-Ouest) of Burkina Faso during five periods between August 2019 and June 2021. Sampling sites were grouped according to environment. Mosquitoes were collected using BG-Sentinel traps and double net traps, and Prokopack Aspirators. Statistical analyses were performed using R software version 4.1.2. Logistic regression, using generalised mixed linear models, was used to test the effect of environment on mosquito abundance and diversity. Alpha diversity analysis was also performed, using the vegan package.

RESULTS

A total of 10,625 adult mosquitoes were collected, belonging to 33 species and five genera: Culex, Aedes, Anopheles, Mansonia, and Ficalbia. The most dominant species were Culex quinquefasciatus, Anopheles gambiae sensu lato and Aedes aegypti. Alpha diversity was similar in the two regions. Habitat had a significant effect on mosquito species richness, the Shannon index and the Simpson index. The rural environment had the highest species richness (n = 28) followed by the forest environment (n = 24). The highest number of mosquitoes (4977/10,625) was collected in the urban environment.

CONCLUSIONS

The species composition of the mosquito populations depended on the type of environment, with fewer species in environments with a high human impact such as urban areas and rice fields. Due to the diversity and abundance of the mosquito vectors, the human populations of all of the environments examined are considered to be at potential risk of mosquito-borne diseases.

Blood meal source identification and RNA virome determination in Japanese encephalitis virus vectors collected in Ishikawa Prefecture, Japan, show distinct avian/mammalian host preference

In Asia, Culex mosquitoes are of particular interest because of their role in maintaining endemic mosquito-borne viral diseases, including the Japanese encephalitis virus (JEV). Nonetheless, host-feeding preferences, along with naturally infecting RNA viruses in certain Culex species, remain understudied. In this study, selected blood-fed mosquitoes were processed for avian and mammalian blood meal source identification. Concurrently, cell culture propagation and high-throughput sequencing (HTS) approaches were used to determine the RNA virome of Culex mosquitoes collected in Ishikawa Prefecture, Japan. The identification of blood meal sources from wild-caught Culex spp. revealed that Culex (Culex) tritaeniorhynchus Giles, 1901, has a robust preference toward wild boar (62%, 26/42), followed by heron (21%, 9/42). The other two species, Culex (Oculeomyia) bitaeniorhynchus Giles, 1901, and Culex (Culex) orientalis Edwards, 1921, showed a distinct preference for avian species, including migratory birds. From the HTS results, 34 virus sequences were detected, four of which were newly identified virus sequences of unclassified Aspiviridae, Qinviridae, Iflaviridae, and Picornaviridae. The absence of observable cytopathic effects in mammalian cells and phylogenetic analysis suggested that all identified virus sequences were insect-specific. Further investigations involving other mosquito populations collected in different areas are warranted to explore previously unknown vertebrate hosts that may be linked to JEV dispersal in nature.

Horses as Sentinels for the Circulation of Flaviviruses in Eastern-Central Germany

Since 2018, autochthonous West Nile virus (WNV) infections have been regularly reported in eastern-central Germany. While clinically apparent infections in humans and horses are not frequent, seroprevalence studies in horses may allow the tracing of WNV and related flaviviruses transmission, such as tick-borne encephalitis virus (TBEV) and Usutu virus (USUV), and consequently help to estimate the risk of human infections. Hence, the aim of our study was to follow the seropositive ratio against these three viruses in horses in Saxony, Saxony Anhalt, and Brandenburg and to describe their geographic distribution for the year 2021. In early 2022, i.e., before the virus transmission season, sera from 1232 unvaccinated horses were tested using a competitive pan-flavivirus ELISA (cELISA). In order to estimate the true seropositive ratio of infection with WNV, TBEV, and USUV for 2021, positive and equivocal results were confirmed by a virus neutralization test (VNT). In addition, possible risk factors for seropositivity using questionnaires were analyzed using logistic regression based on questionnaires similar to our previous study from 2020. In total, 125 horse sera reacted positive in the cELISA. Based on the VNT, 40 sera showed neutralizing antibodies against WNV, 69 against TBEV, and 5 against USUV. Three sera showed antibodies against more than one virus, and eight were negative based on the VNT. The overall seropositive ratio was 3.3% (95% CI: 2.38-4.40) for WNV, 5.6% (95% CI: 4.44-7.04) for TBEV, and 0.4% (95% CI: 0.14-0.98) for USUV infections. While age and number of horses on the holding were factors predicting TBEV seropositivity, no risk factors were discovered for WNV seropositivity. We conclude that horses are useful sentinels to determine the flavivirus circulation in eastern-central Germany, as long as they are not vaccinated against WNV.

Environmentally-Friendly Pesticidal Activities of Callicarpa and Karomia Essential Oils from Vietnam and Their Microemulsions

There is an ongoing interest to identify alternative pesticidal agents to avoid the chronic problems associated with synthetic pesticides. Essential oils have shown promise as botanical pest control agents. In the present study, the essential oils of four members of the Lamiaceae (Callicarpa candicans, C. erioclona, C. macrophylla, and Karomia fragrans; Vietnamese names: Nàng nàng, Tu châu lông mem, Tu châu lá to and Cà diện, respectively), obtained from wild populations in Vietnam, have been obtained by hydrodistillation and analyzed by gas chromatography-mass spectrometry. The essential oils were formulated into microemulsions and the essential oils and their microemulsions were screened for mosquito larvicidal activity against Aedes aegypti, Aedes albopictus, Culex quinquefasciatus, and for molluscicidal activity against Pomacea canaliculata. Atractylone and (E)-caryophyllene dominated the volatiles of C. candicans (CCEO) and C. erioclona (CEEO), while the major component in C. macrophylla (CMEO) and K. fragrans (KFEO) was (E)-caryophyllene. The essential oils and microemulsions of both C. candicans and C. erioclona exhibited excellent larvicidal activity against all three mosquito species (Ae. aegypti, Ae. albopictus, and Cx. quinquefasciatus) with LC₅₀ values <10 μg/mL. Additionally, the larvicidal activity of the microemulsions were significantly improved compared with their free essential oils, especially for C. candicans and C. erioclona. All four essential oils and their microemulsions showed excellent molluscicidal activity with LC₅₀ <10 μg/mL. In most cases, the essential oils and microemulsions showed greater pesticidal activity against target organisms than the non-target freshwater fish, Oreochromis niloticus. The in silico studies on physicochemical and ADMET properties of the major components in the studied essential oils were also investigated and most of the compounds possessed a favorable ADMET profile. Computational modeling studies of the studied compounds demonstrated a favorable binding interaction with the mosquito odorant-binding protein target and support atractylone, β-selinene, and caryophyllene oxide as potential inhibitors. Based on the observed pesticidal activities of the essential oils and their microemulsions, the Callicarpa species and K. fragrans should be considered for potential cultivation and further exploration as botanical pesticidal agents.

Usutu virus: An emerging flavivirus with potential threat to public health in Africa: Nigeria as a case study

Usutu virus (USUV) is an arthropod-borne virus (arbovirus) of the flaviviridae family (genus Flavivirus) which belong to the Japanese encephalitis virus complex. Culex mosquitoes have been implicated in the transmission of this pathogen. The major susceptible hosts of USUV are migratory birds, thereby potentiating its ability to spread from one region to another globally. Nigeria has the largest economy in Africa with a significant percentage of the gross domestic product relying on the agricultural and animal production industry. This review explores the zoonotic potentials of the virus in Africa, especially Nigeria, with special focus on the devastating sequelae this might lead to in the future if necessary precautionary policies are not enacted and adopted to bolster the surveillance system for mosquito-borne viruses.

Human Usutu Virus Infections in Europe: A New Risk on Horizon?

The Usutu virus (USUV), a neurotropic mosquito-borne flavivirus discovered in 1959 in South Africa, has spread over the last twenty years across the European continent. This virus follows an enzootic cycle involving mosquitoes and birds. This caused epizootics with significant bird mortality in Europe in 2016 and 2018. It can also occasionally infect humans and other mammals, including horses and bats, which act as incidental or dead-end hosts. The zoonotic risk associated with this succession of avian epizootics in Europe deserves attention, even if, to date, human cases remain exceptional. Human infection is most often asymptomatic or responsible for mild clinical symptoms. However, human Usutu infections have also been associated with neurological disorders, such as encephalitis and meningoencephalitis. One of the major complexities of the study of USUV pathogenesis is the presence of a great diversity of lineages which could co-circulate spatiotemporally. In this review we discuss several aspects of the circulation of Usutu virus in humans in Europe, the neurological disorders associated, involved viral lineages, and the issues and questions raised by their circulation.

North American House Sparrows Are Competent for Usutu Virus Transmission

Usutu virus (USUV, Flaviviridae) is an emerging mosquito-borne virus that has been implicated in neuroinvasive disease in humans and epizootic deaths in wild birds. USUV is maintained in an enzootic cycle between ornithophilic mosquitoes, primarily Culex spp., and wild birds, predominantly passerine species. However, limited experimental data exist on the species competent for USUV transmission. Here, we demonstrate that house sparrows are susceptible to multiple USUV strains. Our study also revealed that Culex quinquefasciatus mosquitoes are susceptible to USUV, with a significantly higher infection rate for the Netherlands 2016 USUV strain compared to the Uganda 2012 USUV strain at 50% and 19%, respectively. To assess transmission between avian host and mosquito vector, we allowed mosquitoes to feed on either juvenile chickens or house sparrows inoculated with USUV. Both bird models transmitted USUV to C. quinquefasciatus mosquitoes. Linear regression analyses indicated that C. quinquefasciatus infection rates were positively correlated with avian viremia levels, with 3 to 4 log₁₀ PFU/mL representing the minimum avian viremia threshold for transmission to mosquitoes. Based on the viremia required for transmission, house sparrows were estimated to more readily transmit the Netherlands 2016 strain compared to the Uganda 2012 strain. These studies provide insights on a competent reservoir host of USUV. IMPORTANCE Usutu virus (USUV) is a zoonotic mosquito-borne virus that can cause neuroinvasive disease, including meningitis and encephalitis, in humans and has resulted in hundreds of thousands of deaths in wild birds. The perpetuation of USUV in nature is dependent on transmission between Culex spp. mosquitoes and various avian species. To date, few experimental data exist for determining which bird species are important for the maintenance of USUV. Our studies showed that house sparrows can transmit infectious Usutu virus, indicating their role as a competent host species. By identifying reservoir species of USUV, we can predict areas of USUV emergence and mitigate its impacts on global human and wildlife health.

Culex Mosquito Piwi4 Is Antiviral against Two Negative-Sense RNA Viruses

Culex spp. mosquitoes transmit several pathogens concerning public health, including West Nile virus and Saint Louis encephalitis virus. Understanding the antiviral immune system of Culex spp. mosquitoes is important for reducing the transmission of these viruses. Mosquitoes rely on RNA interference (RNAi) to control viral replication. While the siRNA pathway in mosquitoes is heavily studied, less is known about the piRNA pathway. The piRNA pathway in mosquitoes has recently been connected to mosquito antiviral immunity. In Aedes aegypti, Piwi4 has been implicated in antiviral responses. The antiviral role of the piRNA pathway in Culex spp. mosquitoes is understudied compared to Ae. aegypti. Here, we aimed to identify the role of PIWI genes and piRNAs in Culex quinquefasciatus and Culex tarsalis cells during virus infection. We examined the effect of PIWI gene silencing on virus replication of two arboviruses and three insect-specific viruses in Cx. quinquefasciatus derived cells (Hsu) and Cx. tarsalis derived (CT) cells. We show that Piwi4 is antiviral against the La Crosse orthobunyavirus (LACV) in Hsu and CT cells, and the insect-specific rhabdovirus Merida virus (MERDV) in Hsu cells. None of the silenced PIWI genes impacted replication of the two flaviviruses Usutu virus (USUV) and Calbertado virus, or the phasivirus Phasi-Charoen-like virus. We further used small RNA sequencing to determine that LACV-derived piRNAs, but not USUV-derived piRNAs were generated in Hsu cells and that PIWI gene silencing resulted in a small reduction in vpiRNAs. Finally, we determined that LACV-derived DNA was produced in Hsu cells during infection, but whether this viral DNA is required for vpiRNA production remains unclear. Overall, we expanded our knowledge on the piRNA pathway and how it relates to the antiviral response in Culex spp mosquitoes.

Flavivirus Persistence in Wildlife Populations

A substantial number of humans are at risk for infection by vector-borne flaviviruses, resulting in considerable morbidity and mortality worldwide. These viruses also infect wildlife at a considerable rate, persistently cycling between ticks/mosquitoes and small mammals and reptiles and non-human primates and humans. Substantially increasing evidence of viral persistence in wildlife continues to be reported. In addition to in humans, viral persistence has been shown to establish in mammalian, reptile, arachnid, and mosquito systems, as well as insect cell lines. Although a considerable amount of research has centered on the potential roles of defective virus particles, autophagy and/or apoptosis-induced evasion of the immune response, and the precise mechanism of these features in flavivirus persistence have yet to be elucidated. In this review, we present findings that aid in understanding how vector-borne flavivirus persistence is established in wildlife. Research studies to be discussed include determining the critical roles universal flavivirus non-structural proteins played in flaviviral persistence, the advancement of animal models of viral persistence, and studying host factors that allow vector-borne flavivirus replication without destructive effects on infected cells. These findings underscore the viral–host relationships in wildlife animals and could be used to elucidate the underlying mechanisms responsible for the establishment of viral persistence in these animals.

Seasonal Phenological Patterns and Flavivirus Vectorial Capacity of Medically Important Mosquito Species in a Wetland and an Urban Area of Attica, Greece

Seasonal patterns of mosquito population density and their vectorial capacity constitute major elements to understand the epidemiology of mosquito-borne diseases. Using adult mosquito traps, we compared the population dynamics of major mosquito species (Culex pipiens, Aedes albopictus, Anopheles spp.) in an urban and a wetland rural area of Attica Greece. Pools of the captured Cx. pipiens were analyzed to determine infection rates of the West Nile virus (WNV) and the Usutu virus (USUV). The data provided were collected under the frame of the surveillance program carried out in two regional units (RUs) of the Attica region (East Attica and South Sector of Attica), during the period 2017-2018. The entomological surveillance of adult mosquitoes was performed on a weekly basis using a network of BG-sentinel traps (BGs), baited with CO₂ and BG-Lure, in selected, fixed sampling sites. A total of 46,726 adult mosquitoes were collected, with larger variety and number of species in East Attica (n = 37,810), followed by the South Sector of Attica (n = 8916). The collected mosquitoes were morphologically identified to species level and evaluated for their public health importance. Collected Cx. pipiens adults were pooled and tested for West Nile virus (WNV) and Usutu virus (USUV) presence by implementation of a targeted molecular methodology (real-time PCR). A total of 366 mosquito pools were analyzed for WNV and USUV, respectively, and 38 (10.4%) positive samples were recorded for WNV, while no positive pool was detected for USUV. The majority of positive samples for WNV were detected in the East Attica region, followed by the South Sector of Attica, respectively. The findings of the current study highlight the WNV circulation in the region of Attica and the concomitant risk for the country, rendering mosquito surveillance actions and integrated mosquito management programs as imperative public health interventions.

Mosquito Saliva Modulates Japanese Encephalitis Virus Infection in Domestic Pigs

Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus that is the leading cause of pediatric viral encephalitis in Asia. Japanese encephalitis virus is transmitted by Culex species mosquitoes that also vector several zoonotic flaviviruses. Despite the knowledge that mosquito saliva contains molecules that may alter flavivirus pathogenesis, whether or not the deposition of viruses by infected mosquitoes has an impact on the kinetics and severity of JEV infection has not been thoroughly examined, especially in mammalian species involved in the enzootic transmission. Most JEV pathogenesis models were established using needle inoculation. Mouse models for West Nile (WNV) and dengue (DENV) viruses have shown that mosquito saliva can potentiate flavivirus infections and exacerbate disease symptoms. In this study, we determined the impact of mosquito salivary components on the pathogenesis of JEV in pigs, a species directly involved in its transmission cycle as an amplifying host. Interestingly, co-injection of JEV and salivary gland extract (SGE) collected from Culex quinquefasciatus produced milder febrile illness and shortened duration of nasal shedding but had no demonstrable impact on viremia and neuroinvasion. Our findings highlight that mosquito salivary components can differentially modulate the outcomes of flavivirus infections in amplifying hosts and in mouse models.

Rift Valley Fever and West Nile virus vectors in Morocco: Current situation and future anticipated scenarios

Rift Valley Fever (RVF) and West Nile virus (WNV) are two important emerging Arboviruses transmitted by Aedes and Culex mosquitoes, typically Ae. caspius, Ae. detritus and Cx. pipiens in temperate regions. In Morocco, several outbreaks of WNV (1996, 2003 and 2010), affecting horses mostly, have been reported in north-western regions resulting in the death of 55 horses and one person cumulatively. Serological evidence of WNV local circulation, performed one year after the latest outbreak, revealed WNV neutralizing bodies in 59 out of 499 tested participants (El Rhaffouli et al., 2012). The country also shares common borders with northern Mauritania, where RVF is often documented. Human movement, livestock trade, climate changes and the availability of susceptible mosquito vectors are expected to increase the spread of these diseases in the country. Thus, in this study, we gathered a data set summarizing occurrences of Ae. caspius, Ae. detritus and Cx. pipiens in the country, and generated model prediction for their potential distribution under both current and future (2050) climate conditions, as a proxy to identify regions at-risk of RVF and WNV probable expansion. We found that the north-western regions (where the population is most concentrated), specifically along the Atlantic coastline, are highly suitable for Ae. caspius, Ae. detritus and Cx. pipiens, under present-day conditions. Future model scenarios anticipated possible range changes for the three mosquitoes under all climatic assumptions. All of the studied species are prospected to gain new areas that are currently not suitable, even under the most optimist scenario, thus placing additional human populations at risk. Our maps and predictions offer an opportunity to strategically target surveillance and control programmes. Public health officials, entomological surveillance and control delegation must augment efforts and continuously monitor these areas to reduce and minimize human infection risk.

Modeling and mapping the habitat suitability and the potential distribution of Arboviruses vectors in Morocco

Mosquitoes transmit several agents of diseases and the presence of different species represents a threat to animal and public health. Aedes and Culex mosquitoes are of particular concern giving their potential vector competence for Arbovirus transmission. In Morocco, the lack of detailed information related to their spatial distribution raises major concerns and hampers effective vector surveillance and control. Using maximum entropy (Maxent) modeling, we generated prediction models for the potential distribution of Arboviruses vectors (Aedes aegypti, Ae. vexans, Ae. caspius, Ae. detritus, and Culex pipiens) in Morocco, under current climatic conditions. Also, we investigated the habitat suitability for the potential occurrence and establishment of Ae. albopictus and Ae. vittatus recorded only once in the country. Prediction models for these last two species were generated considering occurrence datasets from close countries of the Mediterranean Basin, where Ae. albopictus is well established, and from a worldwide database for the case of Ae. vittatus (model transferability). With the exception of Ae. vittatus, the results identify potential habitat suitability in Morocco for all mosquitos considered. Existing areas with maximum risk of establishment and high potential distribution were mainly located in the northwestern and central parts of Morocco. Our results essentially underline the assumption that Ae. albopictus, if not quickly controlled, might find suitable habitats and has the potential to become established, especially in the northwest of the country. These findings may help to better understand the potential distribution of each species and enhance surveillance efforts in areas identified as high risk.

Effect of blood source on vector competence of Culex pipiens biotypes for Usutu virus

Background

Infectious blood meal experiments have been frequently performed with different virus-vector combinations to assess the transmission potential of arthropod-borne (arbo)viruses. A wide variety of host blood sources have been used to deliver arboviruses to their arthropod vectors in laboratory studies. The type of blood used during vector competence experiments does not always reflect the blood from the viremic vertebrate hosts in the field, but little is known about the effect of blood source on the experimental outcome of vector competence studies. Here we investigated the effect of avian versus human blood on the infection and transmission rates of the zoonotic Usutu virus (USUV) in its primary mosquito vector Culex pipiens.

Methods

Cx. pipiens biotypes (pipiens and molestus) were orally infected with USUV through infectious blood meals containing either chicken or human whole blood. The USUV infection and transmission rates were determined by checking mosquito bodies and saliva for USUV presence after 14 days of incubation at 28 °C. In addition, viral titers were determined for USUV-positive mosquito bodies and saliva.

Results

Human and chicken blood lead to similar USUV transmission rates for Cx. pipiens biotype pipiens (18% and 15%, respectively), while human blood moderately but not significantly increased the transmission rate (30%) compared to chicken blood (17%) for biotype molestus. USUV infection rates with human blood were consistently higher in both Cx. pipiens biotypes compared to chicken blood. In virus-positive mosquitoes, USUV body and saliva titers did not differ between mosquitoes taking either human or chicken blood. Importantly, biotype molestus had much lower USUV saliva titers compared to biotype pipiens, regardless of which blood was offered.

Conclusions

Infection of mosquitoes with human blood led to higher USUV infection rates as compared to chicken blood. However, the blood source had no effect on the vector competence for USUV. Interestingly, biotype molestus is less likely to transmit USUV compared to biotype pipiens due to very low virus titers in the saliva.

Lantana camara Essential Oils from Vietnam: Chemical Composition, Molluscicidal, and Mosquito Larvicidal Activity

Lantana camara is a troublesome invasive plant introduced to many tropical regions, including Southeast Asia. However, the plant does hold promise as a source of essential oils that may be explored for potential use. Fresh water snails such as Pomacea canaliculata, Gyraulus convexiusculus, and Tarebia granifera can be problematic agricultural pests as well as hosts for parasitic worms. Aedes and Culex mosquitoes are notorious vectors of numerous viral pathogens. Control of these vectors is of utmost importance. In this work, the essential oil compositions, molluscicidal, and mosquito larvicidal activities of four collections of L. camara from north-central Vietnam have been investigated. The sesquiterpene-rich L. camara essential oils showed wide variation in their compositions, not only compared to essential oils from other geographical locations (at least six possible chemotypes), but also between the four samples from Vietnam. L. camara essential oils showed molluscicidal activities comparable to the positive control, tea saponin, as well as other botanical agents. The median lethal concentrations (LC₅₀ ) against the snails were 23.6-40.2 μg/mL (P. canaliculata), 7.9-29.6 μg/mL (G. convexiusculus), and 15.0-29.6 μg/mL (T. granifera). The essential oils showed good mosquito larvicidal activities with 24-h LC₅₀ values of 15.1-29.0 μg/mL, 26.4-53.8 μg/mL, and 20.8-59.3 μg/mL against Ae. aegypti, Ae. albopictus, and Cx. quinquefasciatus, respectively. The essential oils were more toxic to snails and mosquito larvae than they were to the non-target water bug, Diplonychus rusticus (24-h LC₅₀ =103.7-162.5 μg/mL). Sesquiterpene components of the essential oils may be acting as acetylcholinesterase (AChE) inhibitors. These results suggest that the invasive plant, L. camara, may be a renewable botanical pesticidal agent.

Vector Competence of Florida Culicoides insignis (Diptera: Ceratopogonidae) for Epizootic Hemorrhagic Disease Virus Serotype-2

Epizootic hemorrhagic disease virus (EHDV; family Reoviridae, genus Orbivirus) is an arthropod-borne virus of ungulates, primarily white-tailed deer in North America. Culicoides sonorensis, the only confirmed North American vector of EHDV, is rarely collected from Florida despite annual virus outbreaks. Culicoides insignis is an abundant species in Florida and is also a confirmed vector of the closely related Bluetongue virus. In this study, oral challenge of C. insignis was performed to determine vector competence for EHDV serotype-2. Field-collected female midges were provided bovine blood spiked with three different titers of EHDV-2 (5.05, 4.00, or 2.94 log₁₀PFUe/mL). After an incubation period of 10 days or after death, bodies and legs were collected. Saliva was collected daily from all females from 3 days post feeding until their death using honey card assays. All samples were tested for EHDV RNA using RT-qPCR. Our results suggest that C. insignis is a weakly competent vector of EHDV-2 that can support a transmissible infection when it ingests a high virus titer (29% of midges had virus positive saliva when infected at 5.05 log₁₀PFUe/mL), but not lower virus titers. Nevertheless, due to the high density of this species, particularly in peninsular Florida, it is likely that C. insignis plays a role in the transmission of EHDV-2.

American Aedes japonicus japonicus, Culex pipiens pipiens, and Culex restuans mosquitoes have limited transmission capacity for a recent isolate of Usutu virus

Usutu virus (USUV; Flavivirus) has caused massive die-offs in birds across Europe since the 1950s. Although rare, severe neurologic disease in humans has been reported. USUV is genetically related to West Nile virus (WNV) and shares an ecological niche, suggesting it could spread from Europe to the Americas. USUV's risk of transmission within the United States is currently unknown. To this end, we exposed field-caught Aedes japonicus, Culex pipiens pipiens, and Culex restuans-competent vectors for WNV-to a recent European isolate of USUV. While infection rates for each species varied from 7%-21%, no dissemination or transmission was observed. These results differed from a 2018 report by Cook and colleagues, who found high dissemination rates and evidence of transmission potential using a different USUV strain, U.S. mosquito populations, temperature, and extrinsic incubation period. Future studies should evaluate the impact of these experimental conditions on USUV transmission by North American mosquitoes.

German Culex pipiens biotype molestus and Culex torrentium are vector-competent for Usutu virus

BACKGROUND

Usutu virus (USUV) is a rapidly spreading zoonotic arbovirus (arthropod-borne virus) and a considerable threat to the global avifauna and in isolated cases to human health. It is maintained in an enzootic cycle involving ornithophilic mosquitoes as vectors and birds as reservoir hosts. Despite massive die-offs in wild bird populations and the detection of severe neurological symptoms in infected humans, little is known about which mosquito species are involved in the propagation of USUV.

METHODS

In the present study, the vector competence of a German (i.e. "Central European") and a Serbian (i.e. "Southern European") Culex pipiens biotype molestus laboratory colony was experimentally evaluated. For comparative purposes, Culex torrentium, a frequent species in Northern Europe, and Aedes aegypti, a primarily tropical species, were also tested. Adult female mosquitoes were exposed to bovine blood spiked with USUV Africa 2 and subsequently incubated at 25 °C. After 2 to 3 weeks saliva was collected from each individual mosquito to assess the ability of a mosquito species to transmit USUV.

RESULTS

Culex pipiens biotype molestus mosquitoes originating from Germany and the Republic of Serbia and Cx. torrentium mosquitoes from Germany proved competent for USUV, as indicated by harboring viable virus in their saliva 21 days post infection. By contrast, Ae. aegypti mosquitoes were relatively refractory to an USUV infection, exhibiting low infection rates and lacking virus in their saliva.

CONCLUSIONS

Consistent with the high prevalences and abundances of Cx. pipiens biotype molestus and Cx. torrentium in Central and Northern Europe, these two species have most likely played a historic role in the spread, maintenance, and introduction of USUV into Germany. Identification of the key USUV vectors enables the establishment and implementation of rigorous entomological surveillance programs and the development of effective, evidence-based vector control interventions.

Structural and antigenic investigation of Usutu virus envelope protein domain III

The mosquito-borne flavivirus Usutu virus (USUV) has recently emerged in birds and humans in Europe. Symptoms of a USUV infection resemble those of West Nile virus (WNV); further, the close antigenic relationship of domain III (DIII) of the USUV and WNV envelope (E) proteins has prevented the development of a reliable serological test to distinguish USUV from WNV. To begin to address this deficiency, we identified ten different sequence groups of DIII from 253 complete and 80 partial USUV genome sequences. We solved the DIII structures of four groups, including that of the outlying CAR-1969 strain, which shows an atypical DIII structure. Structural comparisons of the USUV DIII groups and the DIII of WNV bound to the neutralizing antibody E16 revealed why the E16 failed to neutralize all USUV strains tested except for USUV CAR-1969. The analyses allowed predictions to be made to engineer an antibody specific for USUV CAR-1969.

In Vitro and In Vivo Models to Study the Zoonotic Mosquito-Borne Usutu Virus

Usutu virus (USUV), a mosquito-borne zoonotic flavivirus discovered in South Africa in 1959, has spread to many European countries over the last 20 years. The virus is currently a major concern for animal health due to its expanding host range and the growing number of avian mass mortality events. Although human infections with USUV are often asymptomatic, they are occasionally accompanied by neurological complications reminiscent of those due to West Nile virus (another flavivirus closely related to USUV). Whilst USUV actually appears less threatening than some other emergent arboviruses, the lessons learned from Chikungunya, Dengue, and Zika viruses during the past few years should not be ignored. Further, it would not be surprising if, with time, USUV disperses further eastwards towards Asia and possibly westwards to the Americas, which may result in more pathogenic USUV strains to humans and/or animals. These observations, inviting the scientific community to be more vigilant about the spread and genetic evolution of USUV, have prompted the use of experimental systems to understand USUV pathogenesis and to boost the development of vaccines and antivirals. This review is the first to provide comprehensive coverage of existing in vitro and in vivo models for USUV infection and to discuss their contribution in advancing data concerning this neurotropic virus. We believe that this paper is a helpful tool for scientists to identify gaps in the knowledge about USUV and to design their future experiments to study the virus.

SARS-CoV-2 failure to infect or replicate in mosquitoes: an extreme challenge

This research addresses public speculation that SARS-CoV-2 might be transmitted by mosquitoes. The World Health Organization has stated “To date there has been no information nor evidence to suggest that the new coronavirus could be transmitted by mosquitoes”. Here we provide the first experimental data to investigate the capacity of SARS-CoV-2 to infect and be transmitted by mosquitoes. Three widely distributed species of mosquito; Aedes aegypti, Ae. albopictus and Culex quinquefasciatus, representing the two most significant genera of arbovirus vectors that infect people, were tested. We demonstrate that even under extreme conditions, SARS-CoV-2 virus is unable to replicate in these mosquitoes and therefore cannot be transmitted to people even in the unlikely event that a mosquito fed upon a viremic host.

Usutu Virus Infection of Embryonated Chicken Eggs and a Chicken Embryo-Derived Primary Cell Line

Usutu virus (USUV) is a mosquito-borne flavivirus, closely related to the West Nile virus (WNV). Similar to WNV, USUV may cause infections in humans, with occasional, but sometimes severe, neurological complications. Further, USUV can be highly pathogenic in wild and captive birds and its circulation in Europe has given rise to substantial avian death. Adequate study models of this virus are still lacking but are critically needed to understand its pathogenesis and virulence spectrum. The chicken embryo is a low-cost, easy-to-manipulate and ethically acceptable model that closely reflects mammalian fetal development and allows immune response investigations, drug screening, and high-throughput virus production for vaccine development. While former studies suggested that this model was refractory to USUV infection, we unexpectedly found that high doses of four phylogenetically distinct USUV strains caused embryonic lethality. By employing immunohistochemistry and quantitative reverse transcriptase-polymerase chain reaction, we demonstrated that USUV was widely distributed in embryonic tissues, including the brain, retina, and feather follicles. We then successfully developed a primary cell line from the chorioallantoic membrane that was permissive to the virus without the need for viral adaptation. We believe the future use of these models would foster a significant understanding of USUV-induced neuropathogenesis and immune response and allow the future development of drugs and vaccines against USUV.

Usutu virus: A new threat?

Usutu virus (USUV) is an emerging arbovirus that was first isolated in South Africa in 1959. This Flavivirus is maintained in the environment through a typical enzootic cycle involving mosquitoes and birds. USUV has spread to a large part of the European continent over the two decades mainly leading to substantial avian mortalities with a significant recrudescence of bird infections recorded throughout Europe within the few last years. USUV infection in humans is considered to be most often asymptomatic or to cause mild clinical signs. Nonetheless, a few cases of neurological complications such as encephalitis or meningoencephalitis have been reported. USUV and West Nile virus (WNV) share many features, like a close phylogenetic relatedness and a similar ecology, with co-circulation frequently observed in nature. However, USUV has been much less studied and in-depth comparisons of the biology of these viruses are yet rare. In this review, we discuss the main body of knowledge regarding USUV and compare it with the literature on WNV, addressing in particular virological and clinical aspects, and pointing data gaps.

Comprehensive response to Usutu virus following first isolation in blood donors in the Friuli Venezia Giulia region of Italy: Development of recombinant NS1-based serology and sensitivity to antiviral drugs

Surveillance of Usutu virus is crucial to prevent future outbreaks both in Europe and in other countries currently naïve to the infection, such as the Americas. This goal remains difficult to achieve, notably because of the lack of large-scale cohort studies and the absence of commercially available diagnostic reagents for USUV.

This work started with the first identification of USUV in a blood donor in the Friuli Venezia Giulia (FVG) Region in Northern-Eastern Italy, which is endemic for West Nile virus. Considering that only one IgG ELISA is commercially available, but none for IgM, a novel NS1 antigen based IgG/M ELISA has been developed. This assay tested successfully for the detection of Usutu virus in blood donors with the identification of a second case of transmission and high levels of exposure. Furthermore, two pan-flavivirus antiviral drugs, that we previously characterized to be inhibitors of other flavivirus infectivity, were successfully tested for inhibition of Usutu virus with inhibitory concentrations in the low micromolar range.

To conclude, this work identifies North-Eastern Italy as endemic for Usutu virus with implications for the screening of transfusion blood. A novel NS1-based ELISA test has been implemented for the detection of IgM/G that will be of importance as a tool for the diagnosis and surveillance of Usutu virus infection. Finally, Usutu virus is shown to be sensitive to a class of promising pan-flavivirus drugs.

Tropical viruses transmitted by ticks or mosquitoes are becoming a health threat in areas of the world that were previously naïve to these infections. Usutu virus is a mosquito-borne virus that is circulating in Europe causing massive outbreaks in birds. Transmission to humans is documented, with some reports of severe neurological disease. However, the real size of transmission to humans suffers from lack of data due to insufficient surveillance. The first confirmed case of human USUV infection in an asymptomatic blood donor from North-Eastern Italy is hereby demonstrated by molecular assays and virus isolation. Specific Usutu virus serology has also been developed taking advantage of the NS1 viral antigen, which is tested on a number of blood donors demonstrating a high level of Usutu positivity. These findings confirm the human transmission in the region and offer a novel tool for specific Usutu virus surveillance. Finally, two drugs that were previously shown to have a wide spectrum of activity towards members of this family of viruses are shown to inhibit also Usutu virus, opening the way to a novel class antivirals.

Chemical Compositions, Mosquito Larvicidal and Antimicrobial Activities of Essential Oils from Five Species of Cinnamomum Growing Wild in North Central Vietnam

Members of the genus Cinnamomum (Lauraceae) have aromatic volatiles in their leaves and bark and some species are commercially important herbs and spices. In this work, the essential oils from five species of Cinnamomum (C. damhaensis, C. longipetiolatum, C. ovatum, C. polyadelphum and C. tonkinense) growing wild in north central Vietnam were obtained by hydrodistillation, analyzed by gas chromatography and screened for antimicrobial and mosquito larvicidal activity. The leaf essential oil of C. tonkinense, rich in β-phellandrene (23.1%) and linalool (32.2%), showed excellent antimicrobial activity (MIC of 32 μg/mL against Enterococcus faecalis and Candida albicans) and larvicidal activity (24 h LC₅₀ of 17.4 μg/mL on Aedes aegypti and 14.1 μg/mL against Culex quinquefasciatus). Cinnamomum polyadelphum leaf essential oil also showed notable antimicrobial activity against Gram-positive bacteria and mosquito larvicidal activity, attributable to relatively high concentrations of neral (11.7%) and geranial (16.6%). Thus, members of the genus Cinnamomum from Vietnam have shown promise as antimicrobial agents and as potential vector control agents for mosquitoes.

New Insights into the Susceptibility of Immunocompetent Mice to Usutu Virus

Usutu virus (USUV) is a mosquito-borne flavivirus that shares many similarities with the closely related West Nile virus (WNV) in terms of ecology and clinical manifestations. Initially distributed in Africa, USUV emerged in Italy in 1996 and managed to co-circulate with WNV in many European countries in a similar mosquito-bird life cycle. The rapid geographic spread of USUV, the seasonal mass mortalities it causes in the European avifauna, and the increasing number of infections with neurological disease both in healthy and immunocompromised humans has stimulated interest in infection studies to delineate USUV pathogenesis. Here, we assessed the pathogenicity of two USUV isolates from a recent Belgian outbreak in immunocompetent mice. The intradermal injection of USUV gave rise to disorientation and paraplegia and was associated with neuronal death in the brain and spinal cord in a single mouse. Intranasal inoculation of USUV could also establish the infection; viral RNA was detected in the brain 15 days post-infection. Overall, this pilot study probes the suitability of this murine model for the study of USUV neuroinvasiveness and the possibility of direct transmission in mammals.

Usutu virus disease: a potential problem for North America?

Usutu virus is an emerging mosquito-borne flavivirus initially identified in South Africa in 1959 that is now circulating throughout parts of Africa, Europe, and the Middle East. It is closely related to West Nile virus, and has similar vectors, amplifying bird hosts, and epidemiology. Usutu virus infection can occur in humans and may be asymptomatic or cause systemic (e.g., fever, rash, and hepatitis) or neuroinvasive (e.g., meningitis and encephalitis) disease. Given few reported cases, the full clinical spectrum is not known. No anti-viral treatment is available, but it can be largely prevented by avoiding mosquito bites. Because of similar mosquitoes, birds, and climate to Europe, the potential for introduction to North America is possible.

Multiple Lineages of Usutu Virus (Flaviviridae, Flavivirus) in Blackbirds (Turdus merula) and Mosquitoes (Culex pipiens, Cx. modestus) in the Czech Republic (2016-2019)

Usutu virus (USUV) is a flavivirus (Flaviviridae: Flavivirus) of an African origin transmitted among its natural hosts (diverse species of birds) by mosquitoes. The virus was introduced multiple times to Europe where it caused mortality of blackbirds (Turdus merula) and certain other susceptible species of birds. In this study, we report detection of USUV RNA in blackbirds, Culex pipiens and Cx. modestus mosquitoes in the Czech Republic, and isolation of 10 new Czech USUV strains from carcasses of blackbirds in cell culture. Multiple lineages (Europe 1, 2 and Africa 3) of USUV were found in blackbirds and mosquitoes in the southeastern part of the country. A single USUV lineage (Europe 3) was found in Prague and was likely associated with increased mortalities in the local blackbird population seen in this area in 2018. USUV genomic RNA (lineage Europe 2) was detected in a pool of Cx. pipiens mosquitoes from South Bohemia (southern part of the country), where no major mortality of birds has been reported so far, and no flavivirus RNA has been found in randomly sampled cadavers of blackbirds. The obtained data contributes to our knowledge about USUV genetic variability, distribution and spread in Central Europe.

Usutu Virus: An Arbovirus on the Rise

The Usutu virus (USUV) is a flavivirus that is drawing increasing attention because of its potential for emergence. First isolated in Africa, it was introduced into Europe where it caused significant outbreaks in birds, such as in Austria in 2001. Since then, its geographical distribution has rapidly expanded, with increased circulation, especially in the last few years. Similar to West Nile virus (WNV), the USUV enzootic transmission cycle involves Culex mosquitoes as vectors, and birds as amplifying reservoir hosts, with humans and other mammals likely being dead-end hosts. A similarity in the ecology of these two viruses, which co-circulate in several European countries, highlights USUV’s potential to become an important human pathogen. While USUV has had a severe impact on the blackbird population, the number of human cases remains low, with most infections being asymptomatic. However, some rare cases of neurological disease have been described, both in healthy and immuno-compromised patients. Here, we will discuss the transmission dynamics and the current state of USUV circulation in Europe.