Experimental evaluation of infection, dissemination, and transmission rates for two West Nile virus strains in European Aedes japonicus under a fluctuating temperature regime

Altered thermal preferences of infected or immune-challenged Aedes aegypti and Aedes japonicus mosquitoes

Temperature is a critical factor shaping physiology, life cycle, and behaviour of ectothermic vector insects, as well as the development and multiplication of pathogens within them. However, the influence of pathogen infections on thermal preferences (behavioural thermoregulation) is not well-understood. The present study examined the thermal preferences of mosquitoes (Aedes aegypti and Ae. japonicus) infected with either Sindbis virus (SINV) or Dirofilaria immitis over 12 days post exposure (p.e.) or injected with a non-pathogenic Sephadex bead over 24 h in a thermal gradient (15-30 °C). SINV-infected Ae. aegypti preferred 5 °C warmer temperatures than non-infected ones at day 6 p.e., probably the time of highest innate immune response. In contrast, D. immitis-infected Ae. japonicus preferred 4 °C cooler temperatures than non-infected ones at day 9 p.e., presumably a stress response during the migration of third instar larvae from their development site to the proboscis. Sephadex bead injection also induced a cold preference in the mosquitoes but to a level that did not differ from control-injections. The cold preference thus might be a strategy to escape the risk of desiccation caused by the wound created by piercing the thorax. Further research is needed to uncover the genetic and physiological mechanisms underlying these behaviours.

Winged Threat on the Offensive: A Literature Review Due to the First Identification of Aedes japonicus in Poland

Genetic studies preceded by the observation of an unknown mosquito species in Mikołów (Poland) confirmed that it belongs to a new invasive species in Polish fauna, Aedes japonicus (Theobald, 1901), a known vector for numerous infectious diseases. Ae. japonicus is expanding its geographical presence, raising concerns about potential disease transmission given its vector competence for chikungunya virus, dengue virus, West Nile virus, and Zika virus. This first genetically confirmed identification of Ae. japonicus in Poland initiates a comprehensive review of the literature on Ae. japonicus, its biology and ecology, and the viral infections transmitted by this species. This paper also presents the circumstances of the observation of Ae. japonicus in Poland and a methodology for identifying this species.

Thermal preference of adult mosquitoes (Culicidae) and biting midges (Ceratopogonidae) at different altitudes in Switzerland

Mosquitoes (Diptera: Culicidae) and biting midges (Diptera: Ceratopogonidae) are among the most important vectors of human and veterinary pathogens. For modelling the distribution of these pathogens, entomological aspects are essential, which in turn are highly dependent on environmental factors, such as temperature. In this study, mosquitoes and biting midges were sampled in multiple microclimates at two low (360, 480 meters above sea level, m.a.s.l.) and two high (1250, 1530 m.a.s.l.) altitude locations in Switzerland. Sets of various traps (CO2 -baited CDC, LED-UV, resting boxes, oviposition cups) equipped with dataloggers were placed in transects at five sites with similar vegetation at each location. Only the CDC and the LED-UV traps collected enough insects for analyses. Taxonomic diversity was greater for mosquitoes but lower for biting midges at lower altitudes. Both mosquitoes and biting midges had a thermal preference. Culicoides preferred the traps with warmer microclimate, especially at lower altitudes, whereas mosquito preferences depended on the species, but not on altitude. Relative humidity had a significant positive impact on catches of biting midges but not mosquitoes. To obtain better data on thermal preferences of resting and ovipositing vectors in addition to host seeking individuals, new and improved collecting methods are needed.

The virome of the invasive Asian bush mosquito Aedes japonicus in Europe

The Asian bush mosquito Aedes japonicus is rapidly invading North America and Europe. Due to its potential to transmit multiple pathogenic arthropod-borne (arbo)viruses including Zika virus, West Nile virus, and chikungunya virus, it is important to understand the biology of this vector mosquito in more detail. In addition to arboviruses, mosquitoes can also carry insect-specific viruses that are receiving increasing attention due to their potential effects on host physiology and arbovirus transmission. In this study, we characterized the collection of viruses, referred to as the virome, circulating in Ae. japonicus populations in the Netherlands and France. Applying a small RNA-based metagenomic approach to Ae. japonicus, we uncovered a distinct group of viruses present in samples from both the Netherlands and France. These included one known virus, Ae. japonicus narnavirus 1 (AejapNV1), and three new virus species that we named Ae. japonicus totivirus 1 (AejapTV1), Ae. japonicus anphevirus 1 (AejapAV1) and Ae. japonicus bunyavirus 1 (AejapBV1). We also discovered sequences that were presumably derived from two additional novel viruses: Ae. japonicus bunyavirus 2 (AejapBV2) and Ae. japonicus rhabdovirus 1 (AejapRV1). All six viruses induced strong RNA interference responses, including the production of twenty-one nucleotide-sized small interfering RNAs, a signature of active replication in the host. Notably, AejapBV1 and AejapBV2 belong to different viral families; however, no RNA-dependent RNA polymerase sequence has been found for AejapBV2. Intriguingly, our small RNA-based approach identified an ∼1-kb long ambigrammatic RNA that is associated with AejapNV1 as a secondary segment but showed no similarity to any sequence in public databases. We confirmed the presence of AejapNV1 primary and secondary segments, AejapTV1, AejapAV1, and AejapBV1 by reverse transcriptase polymerase chain reaction (PCR) in wild-caught Ae. japonicus mosquitoes. AejapNV1 and AejapTV1 were found at high prevalence (87-100 per cent) in adult females, adult males, and larvae. Using a small RNA-based, sequence-independent metagenomic strategy, we uncovered a conserved and prevalent virome among Ae. japonicus mosquito populations. The high prevalence of AejapNV1 and AejapTV1 across all tested mosquito life stages suggests that these viruses are intimately associated with Ae. japonicus.

Estimating the Impact of Consecutive Blood Meals on Vector Competence of Aedes albopictus for Chikungunya Virus

The continuous expansion of Aedes albopictus in Europe and the increases in autochthonous arboviruses transmissions in the region urge a better understanding of the virus transmission dynamic. Recent work described enhanced chikungunya virus (CHIKV) dissemination in Aedes aegypti mosquitoes exposed to a virus-free blood meal three days after their infection with CHIKV. Our study investigated the impact of a second blood meal on the vector competence of Ae. albopictus from southern Switzerland infected with CHIKV. Seven-day-old Ae. albopictus females were exposed to CHIKV-spiked blood and incubated at constant (27 °C) and fluctuating (14-28 °C) temperatures. Four days post-infection (dpi), some of these females were re-fed with a non-infectious blood meal. Virus infectivity, dissemination, transmission rate, and efficiency were investigated at seven and ten dpi. No enhanced dissemination rate was observed among females fed a second time; however, re-fed females have shown higher transmission efficiency than those fed only once after seven days post-infection and incubated under a fluctuating temperature regime. Vector competence for CHIKV was confirmed in Ae. albopictus from southern Switzerland. We did not observe an increase in dissemination rates among mosquitoes fed a second time (second blood meal), regardless of the temperature regime.

Assessing vector competence of mosquitoes from northeastern France to West Nile virus and Usutu virus

West Nile virus (WNV) and Usutu virus (USUV) are two arthropod-borne viruses that circulate in mainland France. Assessing vector competence has only been conducted so far with mosquitoes from southern France while an increasingly active circulation of WNV and USUV has been reported in the last years. The main vectors are mosquitoes of the Culex genus and the common mosquito Culex pipiens. Here, we measure the vector competence of five mosquito species (Aedes rusticus, Aedes albopictus, Anopheles plumbeus, Culex pipiens and Culiseta longiareolata) present in northeastern France. Field-collected populations were exposed to artificial infectious blood meal containing WNV or USUV and examined at different days post-infection. We show that (i) Cx. pipiens transmitted WNV and USUV, (ii) Ae. rusticus only WNV, and (iii) unexpectedly, Ae. albopictus transmitted both WNV and USUV. Less surprising, An. plumbeus was not competent for both viruses. Combined with data on distribution and population dynamics, these assessments of vector competence will help in developing a risk map and implementing appropriate prevention and control measures.

Temperature preference of sugar- or blood-fed Aedes japonicus mosquitoes under semi-natural conditions

Mosquito-borne diseases pose a major burden on humans and animals. Temperature strongly influences the physiology and life cycle of mosquitoes and also the pathogens they transmit. Thermoregulatory behaviour of mosquitoes has been addressed in a few laboratory studies. Here, we expand such studies by investigating the thermal preference when resting of Aedes japonicus, an invasive and putative vector species of many pathogens, in a semi-field setup during summers in a temperate climate. Blood-fed or sugar-fed Ae. japonicus females were released in the late afternoon in a large outdoor cage containing three resting boxes. The next morning, temperature treatments were applied to the boxes, creating a "cool" (over all experiments around 18 °C), and a "warm" (around 35 °C) microhabitat in addition to an untreated "ambient" (around 26 °C) one. The mosquitoes resting within the three boxes were counted five times, every 2 h between 9h and 17h. The highest proportions of mosquitoes (e.g. up to 21% of blood-fed ones) were found in the cool box while both blood-fed and sugar-fed mosquitoes avoided the warm box. The mean resting temperatures of Ae. japonicus were below the ambient temperatures measured by a nearby meteorological station, and this was more pronounced at higher outdoor temperatures and in blood-fed as compared to sugar-fed mosquitoes. Thus, over all experiments with blood-fed mosquitoes, the calculated average resting temperature was 4 °C below the outdoor temperature. As mosquitoes prefer cooler resting places than temperatures measured by weather stations in summer, models to predict mosquito-borne disease outbreaks need to account for the thermoregulatory behaviour of mosquitoes, especially in the wake of climate change.

Invasive mosquito vectors in Europe: From bioecology to surveillance and management

Invasive mosquitoes (Diptera: Culicidae) play a key role in the spread of a number of mosquito-borne diseases worldwide. Anthropogenic changes play a significant role in affecting their distribution. Invasive mosquitoes usually take advantage from biotic homogenization and biodiversity reduction, therefore expanding in their distribution range and abundance. In Europe, climate warming and increasing urbanization are boosting the spread of several mosquito species of high public health importance. The present article contains a literature review focused on the biology and ecology of Aedes albopictus, Ae. aegypti, Ae. japonicus japonicus, Ae. koreicus, Ae. atropalpus and Ae. triseriatus, outlining their distribution and public health relevance in Europe. Bioecology insights were tightly connected with vector surveillance and control programs targeting these species. In the final section, a research agenda aiming for the effective and sustainable monitoring and control of invasive mosquitoes in the framework of Integrated Vector Management and One Health is presented. The WHO Vector Control Advisory Group recommends priority should be given to vector control tools with proven epidemiological impact.

dynamAedes: a unified modelling framework for invasive Aedes mosquitoes

Mosquito species belonging to the genus Aedes have attracted the interest of scientists and public health officers because of their capacity to transmit viruses that affect humans. Some of these species were brought outside their native range by means of trade and tourism and then colonised new regions thanks to a unique combination of eco-physiological traits. Considering mosquito physiological and behavioural traits to understand and predict their population dynamics is thus a crucial step in developing strategies to mitigate the local densities of invasive Aedes populations. Here, we synthesised the life cycle of four invasive Aedes species (Ae. aegypti, Ae. albopictus, Ae. japonicus and Ae. koreicus) in a single multi-scale stochastic modelling framework which we coded in the R package dynamAedes. We designed a stage-based and time-discrete stochastic model driven by temperature, photo-period and inter-specific larval competition that can be applied to three different spatial scales: punctual, local and regional. These spatial scales consider different degrees of spatial complexity and data availability by accounting for both active and passive dispersal of mosquito species as well as for the heterogeneity of the input temperature data. Our overarching aim was to provide a flexible, open-source and user-friendly tool rooted in the most updated knowledge on the species' biology which could be applied to the management of invasive Aedes populations as well as to more theoretical ecological inquiries.

Putative roles of mosquitoes (Culicidae) and biting midges (Culicoides spp.) as mechanical or biological vectors of lumpy skin disease virus

The stable fly Stomoxys calcitrans (Diptera: Muscidae) is considered as the main mechanical vector of the lumpy skin disease virus (LSDV). In addition, the mosquito species Aedes aegypti (Diptera: Culicidae) was shown to transmit the virus from donor to receptor animals. Retention of the virus for several days was shown for two additional tropical mosquito species and the biting midge Culicoides nubeculosus (Diptera: Ceratopogonidae). In the present study, viral retention for 10- or 7-days post feeding on virus-spiked blood through a membrane was shown for field-collected Aedes japonicus and laboratory-reared Culex pipiens, two widely distributed mosquito species in temperate regions. Viral DNA could be detected from honey-coated Flinders Technology Associates (FTA) cards and shedded faeces for 1 or 4 days after an infectious blood meal was given to Ae. aegypti. Virus increase over time and virus dissemination was observed in laboratory-reared C. nubeculosus, but the virus could be isolated from field-collected biting midges only from the day of exposure to the blood meal. Thus, mosquitoes might serve as mechanical vectors of LSDV in case of interrupted feeding. A putative biological virus transmission by Culicoides biting midges, as suspected from field observations, deserves further investigations.

First detection of WNV RNA presence in field-collected mosquitoes in Cyprus

West Nile virus (WNV) infections have increased over recent years to the extent that WNV has become one of the most widespread arboviruses in the world, with potential consequences for both human and animal health. While much is known about WNV and the vectors that transmit it from their primary hosts across continental Europe, little is known about the epidemiology of the disease on the island of Cyprus. In this study, the aim was to investigate the prevalence of WNV infection in potential mosquito vectors for the first time in the Republic of Cyprus, using WNV surveillance of mosquitoes. Mosquitoes were collected in 2019, during which an outbreak in humans had occurred, and sampled mosquitoes were then examined for WNV infection by testing them for the presence of WNV RNA. Of 126 mosquito pools tested, one pool, containing Culex pipiens mosquitoes sampled from the Nicosia district, was found to be positive for the presence of WNV RNA. The positive pool found in this study represents the first demonstration of WNV in mosquitoes in Cyprus and confirms that human cases in Cyprus are likely the result of transmission via local Culex mosquitoes.

A Survey on Native and Invasive Mosquitoes and Other Biting Dipterans in Northern Spain

PURPOSE

Haematophagous Diptera, such as mosquitoes (Culicidae), biting midges (Ceratopogonidae), and black flies (Simuliidae), are important insects for public and animal health due to their capacity to bite and transmit pathogens. Outdoor recreation areas are usually affected by biting species and provide suitable habitats to both adult and immature stages. This study aimed to determine the species diversity and larval sites of these Diptera groups in two golf courses.

METHODS

A multi-method collection approach using ultraviolet-CDC traps, human landing catches, collection in breeding sites, and ovitraps was implemented during summer 2020 in northern Spain. Insects were determined by morphological features accompanied by DNA barcoding.

RESULTS

A total of ten native mosquito species were recorded either as adults or as larval stages. The invasive species Aedes japonicus was collected only at egg or pupa stage in ovitraps. Culex pipiens s.l. and Culex torrentium were both common mosquito species accounting for 47.9% of the total larval site collections and their larvae might be found in a wide range of natural and artificial sites. Culiseta longiareolata specimens were also prominent (30.1% of the total) and occurred exclusively in man-made water-filled containers. A total of 13 Culicoides species were identified, 10 of which were captured by ultraviolet-CDC traps, particularly members of the Obsoletus complex (Culicoides obsoletus/Culicoides scoticus, 74.9%) and seven species by emergence traps, being the two most abundant C. kibunensis (44.8%) and C. festivipennis (34.9%). Simulium cryophilum was also collected hovering around the operator under field sampling.

CONCLUSION

A comprehensive representation of the blood-sucking Diptera fauna and their larval sites was obtained by the multi-method approach in two Spanish golf courses.

First Nationwide Monitoring Program for the Detection of Potentially Invasive Mosquito Species in Austria

Simple Summary

In the last years several alien mosquito species have been introduced into Austria. Those species pose a threat, as they—especially the Asian tiger mosquito (Aedes albopictus)—can transmit many pathogens. The aim of this study is a nationwide overview on the situation of alien mosquitoes in Austria. Using traps representing ideal breeding sites for those mosquitoes, we collected, counted and identified the species of the mosquito eggs laid in the traps. The Asian tiger mosquito was found at two sites, once in Tyrol, where this species has been reported before, and for the first time in the province of Lower Austria. The Asian bush mosquito (Aedes japonicus) was widespread and abundant in Austria. Although it was found in all provinces, the Asian bush mosquito was more often found in the South than the North and more eggs were collected in urban/industrial/transport areas than in (mostly) natural areas. Further, more eggs from the Asian bush mosquito were found in samples collected at higher daily mean temperatures, and fewer eggs in samples collected at higher daily maximum wind speeds. The results of this study will help to better understand the risk from alien mosquitoes to human health in Austria and will be useful to show future changes in the distribution of those species.

Abstract

In Austria, only fragmented information on the occurrence of alien and potentially invasive mosquito species exists. The aim of this study is a nationwide overview on the situation of those mosquitoes in Austria. Using a nationwide uniform protocol for the first time, mosquito eggs were sampled with ovitraps at 45 locations in Austria at weekly intervals from May to October 2020. The sampled eggs were counted and the species were identified by genetic analysis. The Asian tiger mosquito Aedes albopictus was found at two sites, once in Tyrol, where this species has been reported before, and for the first time in the province of Lower Austria, at a motorway rest stop. The Asian bush mosquito Aedes japonicus was widespread in Austria. It was found in all provinces and was the most abundant species in the ovitraps by far. Aedes japonicus was more abundant in the South than in the North and more eggs were found in habitats with artificial surfaces than in (semi-) natural areas. Further, the number of Ae. japonicus eggs increased with higher ambient temperature and decreased with higher wind speed. The results of this study will contribute to a better estimation of the risk of mosquito-borne disease in Austria and will be a useful baseline for a future documentation of changes in the distribution of those species.

Bloodmeal regulation in mosquitoes curtails dehydration-induced mortality, altering vectorial capacity

Mosquitoes readily lose water when exposed to any humidity less than that of near saturated air unless mitigated, leading to shifts in behavior, survival, distribution, and reproduction. In this study, we conducted a series of physiological experiments on two prominent species in the Culicinae subfamily: Culex pipiens, a vector of West Nile virus, and Aedes aegypti, a vector of yellow fever and Zika to examine the effects of dehydration. We exposed C. pipiens and A. aegypti to non-dehydrating conditions (saturated air), dehydrating conditions (air at a 0.89 kPa saturation vapor pressure deficit), several recovery conditions, as well as to bloodfeeding opportunities. We show that dehydrated mosquitoes increase bloodfeeding propensity, improve retention, and decrease excretion of a post-dehydration bloodmeal. In addition, mosquitoes that take a bloodmeal prior to dehydration exposure show increased survival over non-bloodfed counterparts. Dehydration-induced alterations in survival, reproduction, and bloodfeeding propensity of C. pipiens and A. aegypti resulted in marked changes to vectorial capacity. Ultimately, these results become increasingly important as drought intensifies in association with climate change and mosquitoes become more likely to experience arid periods.

Vector Competence of the Invasive Mosquito Species Aedes koreicus for Arboviruses and Interference with a Novel Insect Specific Virus

The global spread of invasive mosquito species increases arbovirus infections. In addition to the invasive species Aedes albopictus and Aedes japonicus, Aedes koreicus has spread within Central Europe. Extensive information on its vector competence is missing. Ae. koreicus from Germany were investigated for their vector competence for chikungunya virus (CHIKV), Zika virus (ZIKV) and West Nile virus (WNV). Experiments were performed under different climate conditions (27 ± 5 °C; 24 ± 5 °C) for fourteen days. Ae. koreicus had the potential to transmit CHIKV and ZIKV but not WNV. Transmission was exclusively observed at the higher temperature, and transmission efficiency was rather low, at 4.6% (CHIKV) or 4.7% (ZIKV). Using a whole virome analysis, a novel mosquito-associated virus, designated Wiesbaden virus (WBDV), was identified in Ae. koreicus. Linking the WBDV infection status of single specimens to their transmission capability for the arboviruses revealed no influence on ZIKV transmission. In contrast, a coinfection of WBDV and CHIKV likely has a boost effect on CHIKV transmission. Due to its current distribution, the risk of arbovirus transmission by Ae. koreicus in Europe is rather low but might gain importance, especially in regions with higher temperatures. The impact of WBDV on arbovirus transmission should be analyzed in more detail.

Temperature and food sources influence subadult development and blood-feeding response of Culicoides obsoletus (sensu lato) under laboratory conditions

BACKGROUND

Culicoides obsoletus (s.l.) is the most abundant Culicoides species in northern Europe and an important vector of bluetongue virus and Schmallenberg virus. Nevertheless, information on its subadult life stages remains scarce and no laboratory-reared colony exists.

METHODS

C. obsoletus (s.l.) adults were collected in Belgium and transferred to the laboratory in an attempt to establish a laboratory-reared colony. C. obsoletus (s.l.) were reared from eggs to adults at different temperatures (28 °C, 24 °C, 20/16 °C) and under different food regimes.

RESULTS

The most suitable temperature for rearing seemed to be 24 °C for most developmental parameters, but resulted in a biased 3:1 male/female sex ratio. The latter could be optimized to a 1:1 sex ratio when a 20/16 °C day/night temperature gradient was applied, but rearing at these low temperature conditions resulted in significantly lower egg hatching and pupation rates and a longer subadult development time. Independent of the rearing temperature, adding dung as an additional food source during larval development resulted in a significantly higher adult emergence rate and a decrease in subadult development time. Furthermore, blood-feeding rates of field-collected C. obsoletus (s.l.) were compared for different blood sources and feeding systems. The overall blood-feeding success was low and only successful with cotton pledgets (2.7% blood-fed midges) and through a membrane system with chicken skin (3.5% blood-fed midges). Higher feeding rates were obtained on cattle blood compared to sheep blood.

CONCLUSIONS

These results will help us to determine the necessary conditions to rear a viable laboratory colony of this important vector species, although further optimization is still required.

Wing Morphometrics of Aedes Mosquitoes from North-Eastern France

BACKGROUND

In the context of the increasing circulation of arboviruses, a simple, fast and reliable identification method for mosquitoes is needed. Geometric morphometrics have proven useful for mosquito classification and have been used around the world on known vectors such as Aedes albopictus. Morphometrics applied on French indigenous mosquitoes would prove useful in the case of autochthonous outbreaks of arboviral diseases.

METHODS

We applied geometric morphometric analysis on six indigenous and invasive species of the Aedes genus in order to evaluate its efficiency for mosquito classification.

RESULTS

Six species of Aedes mosquitoes (Ae. albopictus, Ae. cantans, Ae. cinereus, Ae. sticticus, Ae. japonicus and Ae. rusticus) were successfully differentiated with Canonical Variate Analysis of the Procrustes dataset of superimposed coordinates of 18 wing landmarks.

CONCLUSIONS

Geometric morphometrics are effective tools for the rapid, inexpensive and reliable classification of at least six species of the Aedes genus in France.

Dose dependence of susceptibility and transmission for an Argentinean West Nile virus strain in local Culex pipiens quinquefasciatus (Diptera: Culicidae)

BACKGROUND

West Nile virus (WNV) activity has been documented in the central region of Argentina since 2005, but its maintenance network has not been elucidated yet.

METHODS

We evaluated the susceptibility to WNV oral infection and transmission in a Culex pipiens quinquefasciatus mosquito population by means of a dose-dependent vector competence assay. Mosquitoes were orally infected with five different viral loads and evaluated for viral infection, dissemination and transmission.

RESULTS

The evaluated population was susceptible to WNV oral infection. Disseminated infections were detected in all the viral doses but transmission was only detected in the higher doses (6 and 7 log10 plaque-forming units/ml). A linear relationship between doses and transmission rates could be established.

CONCLUSIONS

A minimum transmission threshold suggests that the Cu. p. quinquefasciatus from Argentina could acquire WNV infection from local urban birds and transmit the virus.

Oviposition of Aedes japonicus japonicus (Diptera: Culicidae) and associated native species in relation to season, temperature and land use in western Germany

BACKGROUND

Aedes japonicus japonicus, first detected in Europe in 2000 and considered established in Germany 10 years later, is of medical importance due to its opportunistic biting behaviour and its potential to transmit pathogenic viruses. Its seasonal phenology, temperature and land use preference related to oviposition in newly colonised regions remain unclear, especially in the context of co-occurring native mosquito species.

METHODS

Focussing on regions in Germany known to be infested by Ae. japonicus japonicus, we installed ovitraps in different landscapes and their transition zones and recorded the oviposition activity of mosquitoes in relation to season, temperature and land use (arable land, forest, settlement) in two field seasons (May-August 2017, April-November 2018).

RESULTS

Ae. japonicus japonicus eggs and larvae were encountered in 2017 from June to August and in 2018 from May to November, with a markedly high abundance from June to September in rural transition zones between forest and settlement, limited to water temperatures below 30 °C. Of the three native mosquito taxa using the ovitraps, the most frequent was Culex pipiens s.l., whose offspring was found in high numbers from June to August at water temperatures of up to 35 °C. The third recorded species, Anopheles plumbeus, rarely occurred in ovitraps positioned in settlements and on arable land, but was often associated with Ae. japonicus japonicus. The least frequent species, Aedes geniculatus, was mostly found in ovitraps located in the forest.

CONCLUSIONS

The transition zone between forest and settlement was demonstrated to be the preferred oviposition habitat of Ae. japonicus japonicus, where it was also the most frequent container-inhabiting mosquito species in this study. Compared to native taxa, Ae. japonicus japonicus showed an extended seasonal activity period, presumably due to tolerance of colder water temperatures. Higher water temperatures and arable land represent distribution barriers to this species. The frequently co-occurring native species An. plumbeus might be useful as an indicator for potentially suitable oviposition habitats of Ae. japonicus japonicus in hitherto uncolonised regions. The results contribute to a better understanding of mosquito ecology and provide a basis for more targeted monitoring, distribution modelling and risk management of mosquitoes.

Diapause influenced oviposition behavior and physical egg hatch cues of Aedes atropalpus (Diptera: Culicidae): traits that may influence successful colonization of riverine rock pools

Mosquitoes have developed specialized oviposition strategies that allow them to develop in a wide variety of aquatic habitats. Environmentally cued hatching traits may also play an important role in the successful colonization of some larval habitats, but this subject has remained largely unexplored in Culicidae. Aedes atropalpus (Coquillett) is an autogenous rock pool specialist that may maintain unique adaptations for oviposition and egg hatching. We investigated the egg-laying strategies of Ae. atropalpus exposed to standard (non-diapausing) rearing conditions and diapause-inducing conditions and tested the impact of physical agitation on egg hatch rates by exposing floating and submerged eggs to physical agitation treatments. The results of the oviposition experiment indicate that Ae. atropalpus females primarily lay non-diapausing eggs directly onto the water surface and lay diapausing eggs directly on solid surfaces. The egg-hatching experiment demonstrated that physical agitation significantly increases Ae. atropalpus hatch rates. Floating and submerged eggs responded similarly to the agitation treatment. These data suggest that oviposition behaviors based on both egg diapause status and environmentally-cued hatching strategies may be important adaptations for Ae. atropalpus in riverine rock pools.

Assessing the role of two populations of Aedes japonicus japonicus for Zika virus transmission under a constant and a fluctuating temperature regime

BACKGROUND

Since the huge epidemic of Zika virus (ZIKV) in Brazil in 2015, questions were raised to understand which mosquito species could transmit the virus. Aedes aegypti has been described as the main vector. However, other Aedes species (e.g. Ae. albopictus and Ae. japonicus) proven to be competent for other flaviviruses (e.g. West Nile, dengue and yellow fever), have been described as potential vectors for ZIKV under laboratory conditions. One of these, the Asian bush mosquito, Ae. japonicus, is widely distributed with high abundances in central-western Europe. In the present study, infection, dissemination and transmission rates of ZIKV (Dak84 strain) in two populations of Ae. japonicus from Switzerland (Zürich) and France (Steinbach, Haut-Rhin) were investigated under constant (27 °C) and fluctuating (14-27 °C, mean 23 °C) temperature regimes.

RESULTS

The two populations were each able to transmit ZIKV under both temperature regimes. Infectious virus particles were detected in the saliva of females from both populations, regardless of the incubation temperature regime, from 7 days post-exposure to infectious rabbit blood. The highest amount of plaque forming units (PFU) (400/ml) were recorded 14 days post-oral infection in the Swiss population incubated at a constant temperature. No difference in terms of infection, dissemination and transmission rate were found between mosquito populations. Temperature had no effect on infection rate but the fluctuating temperature regime resulted in higher dissemination rates compared to constant temperature, regardless of the population. Finally, transmission efficiency ranged between 7-23% and 7-10% for the constant temperature and 0-10% and 3-27% under fluctuating temperatures for the Swiss and the French populations, respectively.

CONCLUSIONS

To the best of our knowledge, this is the first study confirming vector competence for ZIKV of Ae. japonicus originating from Switzerland and France at realistic summer temperatures under laboratory conditions. Considering the continuous spread of this species in the northern part of Europe and its adaptation at cooler temperatures, preventative control measures should be adopted to prevent possible ZIKV epidemics.

Mosquito Species Composition and Abundance in Quebec, Eastern Canada

Given current and projected changes in the climate, the composition of mosquito species is predicted to shift geographically with implications for the transmission dynamics of vector-borne pathogens. Many mosquito species are rarely collected in Canada and their history is poorly understood; thus assessing their potential role as vectors for pathogenesis is difficult. Mosquitoes were collected from four trapping sites in Quebec Province, Canada, from June to September during 2018 and 2019 using BG sentinel traps. From all morphologically identified female mosquitoes, at least one specimen was selected for identification confirmation using the DNA-barcoding technique. Sequences were subjected to alignment and a Neighbor-Joining (NJ) tree was created using Geneious software. In total, 2,752 female mosquitoes belonging to 20 species over five genera: including Aedes (Ae.), Anopheles (An.), Culex (Cx.), Culiseta (Cu.), Coquillettidia (Cq.) were collected. The predominant mosquito was found to be Ae. cinereus. The highest number of mosquito species was captured in July, followed by August, September, and then June. Five genera were characterized by a distinctive set of cytochrome oxidase I (COI) sequences that formed well-supported clusters in the NJ-tree. The presence of Ae.japonicus in Quebec provides an initial look at the distribution of mosquito species in eastern Canada, which may put Canadians at risk of a wider range of arboviruses.

Monitoring of alien mosquitoes in Western Austria (Tyrol, Austria, 2018)

Mosquitoes are of major importance to human and animal health due to their ability to transmit various pathogens. In Europe the role of mosquitoes in public health has increased with the introduction of alien Aedes mosquitoes such as the Asian tiger mosquito, Aedes albopictus; the Asian bush mosquito, Ae. japonicus; and Ae. koreicus. In Austria, Ae. japonicus has established populations in various regions of the country. Aedes albopictus is not known to overwinter in Austria, although isolated findings of eggs and adult female mosquitoes have been previously reported, especially in Tyrol. Aedes koreicus had not so far been found in Austria. Within the framework of an alien mosquito surveillance program in the Austrian province of Tyrol, ovitraps were set up weekly from May to October, 2018, at 67 sites– 17 in East Tyrol and 50 in North Tyrol. Sampling was performed at highways and at urban and rural areas. DNA obtained from mosquito eggs was barcoded using molecular techniques and sequences were analysed to species level. Eggs of alien Aedes species were found at 18 out of 67 sites (27%). Both Ae. albopictus and Ae. japonicus were documented at highways and urban areas in both East and North Tyrol. Aedes koreicus was found in East Tyrol. During this mosquito surveillance program, eggs of Ae. albopictus, Ae. japonicus, and Ae. koreicus were documented in the Austrian province of Tyrol. These findings not only show highways to be points of entry, but also point to possible establishment processes of Ae. japonicus in Tyrol. Moreover, Ae. koreicus was documented in Austria for the first time.

The importance of mosquitoes for Public Health in Europe increased dramatically with the introduction of alien species considered to be competent vectors of important human pathogens (e.g. dengue, chikungunya, and Zika viruses), which autochthonous mosquitoes are not yet known to transmit. The Asian tiger mosquito (Aedes albopictus), the Asian bush mosquito (Aedes japonicus), and Aedes koreicus are particularly of relevance, as they are expanding their range in Europe. Tyrol, a region in the Alps with main transport routes from Italy to Germany is of high relevance for the spread of potential invasive, alien Aedes mosquitoes. In this study, we demonstrate highways to be points of entry, and point to possible establishment of the Asian tiger mosquito and the East Asian bush mosquito in Tyrol (findings at highways and urban areas). Moreover, we report the first findings of Ae. koreicus in Austria, this species having most probably spread from neighbouring populations in Italy.

Thermal preferences of subtropical Aedes aegypti and temperate Ae. japonicus mosquitoes

Temperature is an important determinant affecting the capacity of disease vectors like mosquitoes (Culicidae) to transmit disease agents. Although the impact of temperature on vector-borne disease dynamics has been studied intensively, the actual temperature encountered by the vector in a heterogeneous landscape is rarely taken into account. If disease vectors have temperature preferences and therefore select specific microhabitats, this would substantially influence key life history traits that determine transmission intensity. The thermal preferences of subtropical Aedes aegypti and temperate Ae. japonicus mosquitoes were investigated in a temperature gradient set-up consisting of a Plexiglas box on top of an aluminium plate on two thermal regulators. Blood-fed (one day after feeding) and unfed (non-blood-fed) mosquitoes were released in small (15-20 °C, 20-25 °C, 25-30 °C) and large (15-30 °C, 30-45 °C) temperature gradients to assess their thermal preferences after 15 min. Additionally, the effect of humidity was investigated in a two-choice chamber setup. Both mosquito species avoided higher temperatures, pronouncedly dangerously high temperatures in the 30-45 °C gradient. At lower temperatures, blood-fed mosquitoes preferred the cooler sides of the 20-25 °C and 25-30 °C gradient, which were all below their rearing temperature. In the lowest gradient of 15-20 °C, no preferences were found. The thermal preference of unfed mosquitoes was similar to that of the blood-fed mosquitoes. No humidity preference or effect of humidity on temperature preferences was observed within the tested range (40-90%). The set-up allows for assessing the thermal preference of mosquitoes under controlled conditions. The observed preference of mosquitoes for cooler temperatures would increase their longevity and slow down pathogen development. If similar microhabitat selection is observed in the field, vector borne disease models should be adjusted accordingly.

Australian Aedes aegypti mosquitoes are susceptible to infection with a highly divergent and sylvatic strain of dengue virus type 2 but are unlikely to transmit it

Background

Humans are the primary hosts of dengue viruses (DENV). However, sylvatic cycles of transmission can occur among non-human primates and human encroachment into forested regions can be a source of emergence of new strains such as the highly divergent and sylvatic strain of DENV2, QML22, recovered from a dengue fever patient returning to Australia from Borneo. The objective of the present study was to evaluate the vector competence of Australian Aedes aegypti mosquitoes for this virus.

Methods

Four- to five-day-old mosquitoes from two strains of Ae. aegypti from Queensland, Australia, were fed a meal of sheep blood containing 10⁸ 50% cell culture infectious dose per ml (CCID₅₀/ml) of either QML22 or an epidemic strain of DENV serotype 2 (QML16) isolated from a dengue fever patient in Australia in 2015. Mosquitoes were maintained at 28 °C, 75% relative humidity and sampled 7, 10 and 14 days post-infection (dpi). Live virions in mosquito bodies (abdomen/thorax), legs and wings and saliva expectorates from individual mosquitoes were quantified using a cell culture enzyme-linked immunosorbent assay (CCELISA) to determine infection, dissemination and transmission rates.

Results

The infection and dissemination rates of the sylvatic DENV2 strain, QML22, were significantly lower than that for QML16. While the titres of virus in the bodies of mosquitoes infected with either of these viruses were similar, titres in legs and wings were significantly lower in mosquitoes infected with QML22 at most time points although they reached similar levels by 14 dpi. QML16 was detected in 16% (n = 25) and 28% (n = 25) of saliva expectorates at 10 and 14 dpi, respectively. In contrast, no virus was detected in the saliva expectorates of QML22 infected mosquitoes.

Conclusions

Australia urban/peri-urban Ae. aegypti species are susceptible to infection by the sylvatic and highly divergent DENV 2 QML22 but replication of QML22 is attenuated relative to the contemporary strain, QML16. A salivary gland infection or escape barrier may be acting to prevent infection of saliva and would prevent onward transmission of this highly divergent virus in Australia.

The invasive Asian bush mosquito Aedes japonicus found in the Netherlands can experimentally transmit Zika virus and Usutu virus

Background

The Asian bush mosquito Aedes japonicus is invading Europe and was first discovered in Lelystad, the Netherlands in 2013, where it has established a permanent population. In this study, we investigated the vector competence of Ae. japonicus from the Netherlands for the emerging Zika virus (ZIKV) and zoonotic Usutu virus (USUV). ZIKV causes severe congenital microcephaly and Guillain-Barré syndrome in humans. USUV is closely related to West Nile virus, has recently spread throughout Europe and is causing mass mortality of birds. USUV infection in humans can result in clinical manifestations ranging from mild disease to severe neurological impairments.

Methodology/Principal findings

In our study, field-collected Ae. japonicus females received an infectious blood meal with ZIKV or USUV by droplet feeding. After 14 days at 28°C, 3% of the ZIKV-blood fed mosquitoes and 13% of the USUV-blood fed mosquitoes showed virus-positive saliva, indicating that Ae. japonicus can transmit both viruses. To investigate the effect of the mosquito midgut barrier on virus transmission, female mosquitoes were intrathoracically injected with ZIKV or USUV. Of the injected mosquitoes, 96% (ZIKV) and 88% (USUV) showed virus-positive saliva after 14 days at 28°C. This indicates that ZIKV and USUV can efficiently replicate in Ae. japonicus but that a strong midgut barrier is normally restricting virus dissemination. Small RNA deep sequencing of orally infected mosquitoes confirmed active replication of ZIKV and USUV, as demonstrated by potent small interfering RNA responses against both viruses. Additionally, de novo small RNA assembly revealed the presence of a novel narnavirus in Ae. japonicus.

Conclusions/Significance

Given that Ae. japonicus can experimentally transmit arthropod-borne viruses (arboviruses) like ZIKV and USUV and is currently expanding its territories, we should consider this mosquito as a potential vector for arboviral diseases in Europe.

Arthropod-borne viruses (arboviruses) cause a high disease burden in humans and animals. Zika virus (ZIKV) causes microcephaly and Guillain-Barré syndrome in humans, whereas Usutu virus (USUV) induces high mortality in birds and neurological disease in humans. The spread of arboviruses such as ZIKV and USUV is determined by the presence of mosquitoes that can transmit these viruses from one vertebrate host to the next. Here, we investigate the risk of transmission of ZIKV and USUV by the Asian bush mosquito Aedes japonicus. This mosquito is invading Europe and is currently present in the Netherlands. We found that field-collected Ae. japonicus mosquitoes can experimentally transmit ZIKV and USUV. Of the orally infected mosquitoes, 3% (ZIKV) and 13% (USUV) showed virus-positive saliva after 14 days at 28°C. We also found that ZIKV and USUV activated the antiviral RNA interference immune response of Ae. japonicus. Moreover, a strong barrier in the mosquito midgut restricted virus dissemination, since 96% (ZIKV) and 88% (USUV) of the mosquitoes injected with ZIKV or USUV showed virus-positive saliva. Additionally, we discovered a narnavirus in Ae. japonicus. Given that Ae. japonicus can transmit ZIKV and USUV, we should consider this mosquito as a potential vector for arboviral diseases in Europe.

Comparison of vector competence of Aedes vexans Green River and Culex pipiens biotype pipiens for West Nile virus lineages 1 and 2

West Nile virus (WNV), a zoonotic arbovirus, has recently established an autochthonous transmission cycle in Germany. In dead-end hosts like humans and horses the WNV infection may cause severe symptoms in the central nervous system. In nature, WNV is maintained in an enzootic transmission cycle between birds and ornithophilic mosquitoes. Bridge vector species, such as members of the Culex pipiens complex and Aedes spp., also widely distributed in Germany, might transmit WNV to other vertebrate host species. This study determined and compared the vector competence of field-collected northern-German Cx. pipiens biotype pipiens and laboratory-reared Ae. vexans Green River (GR) for WNV lineage 1 (strain: Magpie/Italy/203204) and WNV lineage 2 (strain: "Austria") under temperatures typical for northern Germany in spring/summer and autumn. For assessment of vector competence, 7- to 14-day-old female mosquitoes were offered a WNV containing blood meal via Hemotek membrane feeding system or cotton-stick feeding. After incubation at 18°C respectively 24°C for 14 days engorged female mosquitoes were salivated and dissected for determination of infection, dissemination and transmission rates by reverse transcriptase quantitative real-time PCR (RT-qPCR). Both Ae. vexans GR and Cx. pipiens biotype pipiens were infected with both tested WNV strains and tested 14 days post-inoculation. Disseminated infections were detected only in Ae. vexans GR incubated at 18°C and in Cx. pipiens pipiens incubated at 24°C after infection with WNV lineage 1. Transmission of WNV lineage 1 was detected in Cx. pipiens pipiens incubated at 24°C. These results indicate that Cx. pipiens pipiens from Northern Germany may be involved in the transmission of WNV, also to dead-end hosts like humans and horses.

Locally temperature - driven mathematical model of West Nile virus spread in Germany

West Nile virus (WNV) is an arthropod-borne virus (arbovirus) transmitted by the bites of infected mosquitoes. WNV can also infect horses and humans, where it may cause serious illness and can be fatal. Birds are the natural reservoir, and humans, equines and probably other mammals are dead-end hosts. In 2018, WNV occurred for the first time in Germany, affecting birds and horses. Seroconversion of an exposed veterinarian has also been reported. It is therefore of importance to evaluate the circumstances, under which WNV may establish in Germany as a whole or in particular favourable regions. In our current work, we formulate a dynamic model to describe the spreading process of West Nile virus in the presence of migratory birds. To investigate the possible role of migratory birds in the dissemination of WNV in Germany, we include the recurring presence of migratory birds through a mechanistic ordinary differential equations (ODE) model system. We also perform a sensitivity analysis of the infection curves. Seasonal impacts are also taken into consideration. As result, we present an analytical expression for the basic reproduction number R₀. We find that after introducing WNV into Germany, R₀ will be above the critical value in many regions of the country. Furthermore, we observe that in the south of Germany, the disease reoccurs in the following season after the introduction. We include a potential distribution map associated with WNV cases in Germany to illustrate our findings in a spatial scale.

Mosquitoes of North-Western Europe as Potential Vectors of Arboviruses: A Review

Background: The intensification of trade and travel is linked to the growing number of imported cases of dengue, chikungunya or Zika viruses into continental Europe and to the expansion of invasive mosquito species such as Aedes albopictus and Aedes japonicus. Local outbreaks have already occurred in several European countries. Very little information exists on the vector competence of native mosquitoes for arboviruses. As such, the vectorial status of the nine mosquito species largely established in North-Western Europe (Aedes cinereus and Aedes geminus, Aedes cantans, Aedes punctor, Aedes rusticus, Anopheles claviger s.s., Anopheles plumbeus, Coquillettidia richiardii, Culex pipiens s.l., and Culiseta annulata) remains mostly unknown. Objectives: To review the vector competence of both invasive and native mosquito populations found in North-Western Europe (i.e., France, Belgium, Germany, United Kingdom, Ireland, The Netherlands, Luxembourg and Switzerland) for dengue, chikungunya, Zika, West Nile and Usutu viruses. Methods: A bibliographical search with research strings addressing mosquito vector competence for considered countries was performed. Results: Out of 6357 results, 119 references were related to the vector competence of mosquitoes in Western Europe. Eight species appear to be competent for at least one virus. Conclusions: Aedes albopictus is responsible for the current outbreaks. The spread of Aedes albopictus and Aedes japonicus increases the risk of the autochthonous transmission of these viruses. Although native species could contribute to their transmission, more studies are still needed to assess that risk.

Biological Adaptations Associated with Dehydration in Mosquitoes

Diseases that are transmitted by mosquitoes are a tremendous health and socioeconomic burden with hundreds of millions of people being impacted by mosquito-borne illnesses annually. Many factors have been implicated and extensively studied in disease transmission dynamics, but knowledge regarding how dehydration impacts mosquito physiology, behavior, and resulting mosquito-borne disease transmission remain underdeveloped. The lapse in understanding on how mosquitoes respond to dehydration stress likely obscures our ability to effectively study mosquito physiology, behavior, and vectorial capabilities. The goal of this review is to develop a profile of factors underlying mosquito biology that are altered by dehydration and the implications that are related to disease transmission.

Evidence of West Nile Virus (WNV) Circulation in Wild Birds and WNV RNA Negativity in Mosquitoes of the Danube Delta Biosphere Reserve, Romania, 2016

West Nile virus (WNV) is a zoonotic flavivirus whose transmission cycle in nature includes wild birds as amplifying hosts and ornithophilic mosquito vectors. Bridge vectors can transmit WNV to mammal species potentially causing West Nile Fever. Wild bird migration is a mode of WNV introduction into new areas. The Danube Delta Biosphere Reserve (DDBR) is a major stopover of wild birds migrating between Europe and Africa. The aim of this study was to investigate the presence of WNV in the DDBR during the 2016 transmission season in wild birds and mosquitoes. Blood from 68 wild birds (nine different species) trapped at four different locations was analyzed by competitive ELISA and Virus Neutralization Test (VNT), revealing positive results in 8/68 (11.8%) of the wild birds by ELISA of which six samples (three from juvenile birds) were confirmed seropositive by VNT. Mosquitoes (n = 6523, 5 genera) were trapped with CDC Mini Light traps at two locations and in one location resting mosquitoes were caught. The presence of WNV RNA was tested in 134 pools by reverse transcription quantitative PCR (RT-qPCR). None of the pools was positive for WNV-specific RNA. Based on the obtained results, WNV was circulating in the DDBR during 2016.

European Aedes caspius mosquitoes are experimentally unable to transmit Zika virus

BACKGROUND

Aedes caspius (Pallas, 1771) is a floodwater mosquito species widely distributed in the Western Palaearctic. As an anthropophilic species, its role as an arbovirus vector may be the key for understanding the transmission cycle of certain diseases in Europe such as Zika virus (ZIKV). Concerning vector competence for ZIKV, studies related to Ae. caspius are still scarce. ZIKV is an arbovirus that has provoked a widespread epidemic in the Pacific region (2007-2013) and in the Americas (2015-2016). ZIKV is associated with serious neurological injuries (e.g. microcephaly) and Guillain-Barré syndrome. Due to the ZIKV epidemics in the American continent, some viraemic travellers coming from endemic countries have been reported in Europe. More knowledge is therefore required to define the susceptibility of autochthonous mosquito species such as Ae. caspius for ZIKV in order to improve arbovirus surveillance and control programmes. In the present study, the vector competence of a European population of Ae. caspius was evaluated for two ZIKV lineages, the Suriname ZIKV strain (Asian lineage) and the MR766 ZIKV strain (African I lineage). Females were tested at 7, 14 and 21 days post-exposure (dpe) to infectious blood meals. An Ae. aegypti PAEA strain was used as a positive control.

RESULTS

Aedes caspius presented low susceptibility to ZIKV infection and the virus was only detected by RT-qPCR in body samples. Low viral loads were detected for the MR766 strain at 7 dpe and for the Suriname strain at 14 and 21 dpe. Aedes caspius was unable to produce a disseminated infection and virus transmission at any of the tested time points. Using Ae. aegypti PAEA strain, infection, dissemination and transmission rates were calculated for the Suriname ZIKV strain (Asian lineage) at each time point. For the MR766 ZIKV strain (African I lineage), while only infection rates were estimated at each time point, no dissemination or transmission were detected in either species.

CONCLUSIONS

The results of the present study reveal that the tested Ae. caspius population has a strong midgut escape barrier that limits the dissemination or transmission of the virus. As such, it seems unlikely that European Ae. caspius mosquitoes could be involved in ZIKV transmission if ZIKV was introduced into Europe. This information may help in designing a better strategy to European surveillance and control programmes for ZIKV.

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.

Culex torrentium: A Potent Vector for the Transmission of West Nile Virus in Central Europe

The continuous circulation of West Nile virus (WNV) in Central, South and East Europe and its recent detection in several dead birds and two horses in Germany highlights the need for information on WNV vector competence of mosquitoes from Central Europe. Therefore, three common Culex species (Culex pipiens biotype pipiens, Culex pipiens biotype molestus and Culex torrentium) from Germany were orally infected with WNV and kept at 18 °C, 21 °C, 24 °C or 27 °C for 14 or 21 days post infection (dpi). Thereafter viable WNV was present in the saliva in all tested taxa, but only at incubation temperatures of 24 °C or 27 °C and predominantly at the extended incubation period of 21 dpi. Highest transmission efficiency rates of 17 % (24 °C) and 24% (27 °C) were found for Cx. torrentium. Culex p. pipiens and Cx. p. molestus showed low transmission efficiencies with a maximum of only 3%. Consequently, temperatures above 21 °C support transmission of WNV, which matches the predominant distribution of human WNV cases around the Mediterranean Sea and in South-East Europe. Culex torrentium has been identified as a potent vector for WNV in Central and Northern Europe, which highlights the need for surveillance of mosquito-borne viruses north of the Alps.

Biochemical changes in saliva of cows with inflammation: A pilot study

Saliva contains a variety of compounds that can change in local and systemic pathologies including inflammation. Although changes in acute phase proteins and markers of oxidative stress in saliva during inflammation in humans and different animal species have been described, no data exist about possible changes during inflammation in analytes in saliva of cows. The aim of the present study was to evaluate changes in selected salivary biomarkers of stress, inflammation and immune system, and oxidative stress in cows with inflammation. For this purpose, bovine mastitis was used as model. Saliva and serum from 18 clinically healthy cows and 18 cows with clinical mastitis were used in the study. A panel of analytes integrated by alpha-amylase, cortisol, haptoglobin, adenosine deaminase, cholinesterase, total antioxidant capacity, lactate, and uric acid was measured in all samples and differences between the two groups of animals were evaluated. Significant increases in cortisol, alpha-amylase, uric acid, lactate and significant decreases in cholinesterase were detected in saliva of cows with mastitis. These results indicate that that cows with mastitis show changes in salivary biomarkers that reflect presence of stress, inflammation and oxidative stress in the animals.

Vector Competence of Culicoides sonorensis (Diptera: Ceratopogonidae) for Epizootic Hemorrhagic Disease Virus Serotype 2 Strains from Canada and Florida

Epizootic hemorrhagic disease virus (EHDV), an Orbivirus transmitted by Culicoides spp. vectors, is represented by seven serotypes and numerous strains worldwide. While studies comparing vector competence between serotypes exist, studies between viral strains are lacking. In this study, we examined the rates of infection, dissemination, and transmission of two strains of EHDV-2 orally fed to the known vector, Culicoides sonorensis Wirth & Jones. Culicoides sonorensis cohorts were fed an infectious blood meal containing EHDV-2 strains from either Alberta, Canada (Can-Alberta) or Florida (5.5 log₁₀ PFUe/mL) and tested for the vector’s susceptibility to infection and dissemination. In addition, transmission rates of the virus were assessed and compared using capillary tube and honey card methods. Our results show that the Florida strain had higher infection and dissemination rates than the Can-Alberta strain in spite of the Florida strain having significantly lower viral titers in C. sonorensis bodies, legs, and saliva than the Can-Alberta strain. Overall transmission rates were not significantly different between the two strains but varied significantly between the methods used. These findings suggest that the consequences of EHDV infection in C. sonorensis vary between virus strains and have huge implications in future vector competence studies involving Culicoides species and Orbiviruses.

The invasive mosquito Aedes japonicus japonicus is spreading in northeastern Italy

BACKGROUND

The invasive mosquito species, Aedes japonicus japonicus, was detected in northeastern Italy for the first time in 2015, at the border with Austria. After this finding, a more intensive monitoring was carried out to assess its distribution and to collect biological data. Herein, we report the results of four years (2015-2018) of activity.

METHODS

The presence of Ae. j. japonicus was checked in all possible breeding sites through collections of larvae. The monitoring started from the site of the first detection at the Austrian border and then was extended in all directions. The mosquitoes were identified morphologically and molecularly.

RESULTS

Aedes j. japonicus was found in 58 out of 73 municipalities monitored (79.5%). In total (2015-2018), 238 sampling sites were monitored and 90 were positive for presence of Ae. j. japonicus larvae (37.8%). The mosquito was collected mainly in artificial containers located in small villages and in rural areas. Cohabitation with other mosquito species was observed in 55.6% of the samplings.

CONCLUSIONS

Aedes j. japonicus is well established in Italy and in only four years has colonised two Italian Regions, displaying rapid spreading throughout hilly and mountainous areas. Colonization towards the south seems limited by climatic conditions and the occurrence of a large population of the larval competitor, Ae. albopictus. The further spread of Ae. j. japonicus has the potential to pose new threats of zoonotic agents (i.e. Dirofilaria spp. and West Nile virus) within areas at altitudes previously considered at negligible risk in Italy.

The Asian bush mosquito Aedes japonicus japonicus (Diptera: Culicidae) in Europe, 17 years after its first detection, with a focus on monitoring methods

After the first detection of the Asian bush mosquito Aedes japonicus japonicus in the year 2000 in France, its invasive nature was revealed in 2008 in Switzerland and Germany. In the following years, accumulating reports have shown that Ae. j. japonicus succeeded in establishing in several European countries. Surveillance efforts suggest that there are currently four populations in Europe, with the largest one, formed by the recent fusion of several smaller populations, ranging from West Germany, with extensions to Luxembourg and French Alsace, southwards to Switzerland and continuing westwards through Liechtenstein to western Austria. This paper summarises the present distribution of Ae. j. japonicus in Europe, based on published literature and hitherto unpublished findings by the authors, and critically reviews the monitoring strategies applied. A proposal for a more standardised monitoring approach is provided, aiming at the harmonisation of future data collections for improving the comparability between studies and the suitability of collected data for further research purposes, e.g. predictive modelling approaches.

Targeted surveillance reveals native and invasive mosquito species infected with Usutu virus

Background

The emergence of Usutu virus (USUV) in Europe was first reported in Austria, 2001, and the virus has since spread to many European countries. Initial outbreaks are marked by a mass die-off of European blackbirds (Turdus merula) and other bird species. During outbreaks, the virus has been detected in pools of Culex pipiens mosquitoes, and these mosquitoes are probably the most important enzootic vectors. Beginning in 2017, a second wave of blackbird deaths associated with USUV was observed in eastern Austria; the affected areas expanded to the Austrian federal states of Styria in the south and to Upper Austria in the west in 2018. We sampled the potential vector population at selected sites of bird deaths in 2018 in order to identify infected mosquitoes.

Results

We detected USUV RNA in 16 out of 19 pools of Cx. pipiens/Cx. torrentium mosquitoes at sites of USUV-linked blackbird mortality in Linz and Graz, Austria. A disseminated virus infection was detected in individuals from selected pools, suggesting that Cx. pipiens form pipiens was the principal vector. In addition to a high rate of infected Cx. pipiens collected from Graz, a disseminated virus infection was detected in a pool of Aedes japonicus japonicus.

Conclusions

We show herein that naturally-infected mosquitoes at foci of USUV activity are primarily Cx. pipiens form pipiens. In addition, we report the first natural infection of Ae. j. japonicus with USUV, suggesting that it may be involved in the epizootic transmission of USUV in Europe. Ae. j. japonicus is an invasive mosquito whose range is expanding in Europe.

Resource-Effective Serosurveillance for the Detection of West Nile Virus in Switzerland Using Abattoir Samples of Free-Range Laying Hens

West Nile virus (WNV) is an important zoonotic pathogen maintained in a natural transmission cycle between mosquitoes and birds as reservoir hosts. In dead-end hosts, such as humans, infection may result in fatal neurologic disease translating into disease and death-related suffering and increased health care costs. In humans, WNV may also be transmitted through blood transfusions and organ transplants. WNV is not present in Switzerland yet, but competent vector species (especially Culex pipiens and Aedes japonicus) are prevalent and an introduction of the virus, likely through wild birds, is expected at any time. Therefore, it is important for Switzerland to be prepared and establish a surveillance system for WNV to initiate increased prevention activities, such as the screening of blood and organ donations and public education activities in case virus circulation is detected. The long-term goal of these surveillance measures would be a reduced infection rate in humans resulting in less suffering and reduced health care costs. To provide the basis for a pragmatic and resource-effective WNV surveillance program, this study used aliquots of serum samples of free-range laying hens taken at the abattoir and collected in the frame of the ongoing Swiss Avian Influenza and Newcastle Disease monitoring program for a 2-year period. All 961 aliquots were analyzed using a commercial competitive WNV enzyme-linked immunosorbent assay (ELISA). The study allowed to set up sampling and laboratory routines as a basis for future WNV surveillance activities. At this stage there is no evidence for circulation of WNV in Switzerland.

Experimental transmission of Zika virus by Aedes japonicus japonicus from southwestern Germany

The invasive mosquito species Aedes japonicus japonicus (Ae. japonicus) is widely distributed in Central Europe and is a known vector of various arboviruses in the laboratory, including flaviviruses such as Japanese Encephalitis virus or West Nile virus. However, the vector competence of Ae. japonicus for the recently emerging Zika virus (ZIKV) has not been determined. Therefore, field-caught Ae. japonicus from Germany were orally infected with ZIKV and incubated at 21, 24, or 27 °C to evaluate the vector competence under climate conditions representative of the temperate regions (21 °C) in the species' main distribution area in Europe and of Mediterranean regions (27 °C). Aedes japonicus was susceptible to ZIKV at all temperatures, showing infection rates between 10.0% (21 °C) and 66.7% (27 °C). However, virus transmission was detected exclusively at 27 °C with a transmission rate of 14.3% and a transmission efficiency of 9.5%. Taking into account the present distribution of Ae. japonicus in the temperate regions of Central Europe, the risk of ZIKV transmission by the studied Ae. japonicus population in Central Europe has to be considered as low. Nevertheless, due to the species' vector competence for ZIKV and other mosquito-borne viruses, in combination with the possibility of further spread to Mediterranean regions, Ae. japonicus must be kept in mind as a potential vector of pathogens inside and outside of Europe.

Ongoing and emerging arbovirus threats in Europe

During the last decades, arboviruses that are endemic in Europe have expanded their geographic range and caused an increasing number of human outbreaks. These viruses include West Nile virus, which is expanding its area of circulation in central and southern Europe; Usutu virus, with increasing evidence of a role in human disease; tick-borne encephalitis virus, which is being detected in northern areas and at higher altitudes as a consequence of climate warming; Crimean-Congo hemorrhagic fever virus, which is endemic in Eastern Europe and the Middle East, but has been recently detected in Spain; other viruses, such as California encephalitis virus antigenic group, which circulate in northern and central Europe but whose relevance for human disease in largely unknown. In addition, the rise in global travel and trade has posed Europe to an increased risk of introduction and expansion of exotic arthropod vectors and autochthonous transmission of arboviruses, like dengue and chikungunya viruses, following new introductions from endemic areas. Implementation of integrated arbovirus surveillance programs has been crucial to adopt proper control measures. The identification of emerging outbreaks is however challenging and requires a high degree of awareness and laboratory capacity, especially for the most neglected but potentially threatening pathogens.

Vector competence of pre-alpine Culicoides (Diptera: Ceratopogonidae) for bluetongue virus serotypes 1, 4 and 8

Background

Bluetongue disease, caused by bluetongue virus serotype 8 (BTV-8), appeared for the first time in the northern part of Europe in 2006, and subsequently rapidly spread causing severe economic losses to the farming industry. The implicated vectors of BTV in Europe are Culicoides species within the subgenus Avaritia (C. chiopterus, C. dewulfi, C. obsoletus and C. scoticus). Epidemiological data from Switzerland have shown that BTV, whose spread was eliminated at an early stage by vaccination campaigns, had not been circulating among livestock at higher altitudes where other species dominate the Culicoides fauna. In this study, we investigated the extent that Culicoides spp. prevailing at higher altitudes (mainly C. grisescens) can act as vectors for BTV.

Methods

Culicoides were collected at farms in the pre-alpine region (two sites at 1550 m above sea level, masl, referred to as pre-alpine I; one site at 2030 masl, pre-alpine II) and, for comparative purposes, from the Swiss Plateau (one site, 650 masl). They were fed on bovine blood/BTV suspensions (BTV-1, 4 or 8) and incubated for eight days under a fluctuating temperature regime (13–25 °C, mean 19 °C), reflecting a mid-summer warm spell in the pre-alpine region. Susceptibility to BTV transmission was assessed from head homogenates by RT-qPCR and virus isolation.

Results

Overall, 9196 female Culicoides were exposed to the three BTV strains through an artificial membrane, with feeding rates of 14–27%. Survival rates of blood-engorged Culicoides females at eight days post-infection depended on both virus serotype and altitude of origin. Virus dissemination (C_q ≤ the cut-off value as determined by serial virus dilutions) was confirmed only for BTV-1 in C. scoticus (dissemination efficiency 22.5%; 9/40) and C. obsoletus (5.6%; 1/18) from the Swiss Plateau area. There was no strong evidence of susceptibility to infection for Culicoides from the pre-alpine area when fed with all BTV strains (BTV-1, 4 and 8).

Conclusions

This study confirms the susceptibility of C. scoticus and C. obsoletus to BTV-1 infection, including under cooler temperatures. Culicoides grisescens, which is highly abundant at higher altitudes, cannot be considered a potential vector under these temperature conditions.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3050-y) contains supplementary material, which is available to authorized users.