Field-based assessments of the seasonality of Culex pipiens sensu lato in England: an important enzootic vector of Usutu and West Nile viruses

BACKGROUND

Usutu virus (USUV), which is closely related to West Nile virus (WNV), sharing a similar ecology and transmission cycle, was first reported in the UK in the southeast of England in 2020. Both USUV and WNV are emerging zoonotic viruses hosted by wild birds. The 2020 finding of USUV in England raised awareness of this virus and highlighted the importance of understanding the seasonality of Culex pipiens sensu lato (Cx. pipiens s.l.), the main enzootic vector of these viruses. Zoos are prime locations for trapping mosquitoes because of their infrastructure, security, and range of vertebrate hosts and aquatic habitats.

METHODS

Three independent zoo-based case studies at four locations that cover the seasonality of Cx. pipiens s.l. in England were undertaken: (i) London Zoo (Zoological Society London [ZSL]) and surrounding areas, London; (ii) Chester Zoo (Cheshire); (ii) Twycross Zoo (Leicestershire); and (iv) Flamingo Land (zoo; North Yorkshire). Various adult mosquito traps were used to catch adult Cx. pipiens s.l. across seasons.

RESULTS

High yields of Cx. pipiens s.l./Culex torrentium were observed in Biogents-Mosquitaire and Center for Disease Control and Prevention Gravid traps in all studies where these traps were used. Mosquito counts varied between sites and between years. Observations of adult Cx. pipiens s.l./Cx. torrentium abundance and modelling studies demonstrated peak adult abundance between late July and early August, with active adult female Cx. pipiens s.l./Cx. torrentium populations between May and September.

CONCLUSIONS

The information collated in this study illustrates the value of multiple mosquito monitoring approaches in zoos to describe the seasonality of this UK vector across multiple sites in England and provides a framework that can be used for ongoing and future surveillance programmes and disease risk management strategies.

Impact of larvae of the Asian tiger mosquito Aedes albopictus on larvae of the Culex pipiens complex from Germany in laboratory co-breeding studies

Since the first detection of the Asian tiger mosquito Aedes albopictus (Diptera: Culicidae) eggs in Germany in 2007, several populations of this species have established in Germany. Although colloquially Ae. albopictus is called an 'invasive species', it is not considered 'invasive' and therefore to be controlled according to the European Union (EU) Environmental and Nature Protection Act since evidence of displacement of native species is missing. To test the competitive potential of Ae. albopictus towards mosquito species native to Germany, laboratory experiments were conducted with larvae of this species and indigenous Cx. pipiens complex species/biotypes. First instar larvae of Ae. albopictus and of one of the native taxa were exposed to different temperatures and fed with different food sources. The ratio of individuals developing into adults as well as the time the larvae needed for development were taken as a measure of competitive outcome. In addition, the size of emerging adults was compared between control and experimental groups. Regarding developmental time, no significant differences were found between treatments and controls while significant differences were found regarding developmental rate and average wing size of individuals. Because no evidence of competitive repression of the native species was found, Ae. albopictus cannot be included in the EU list of invasive species.

Overwintering Culex torrentium in abandoned animal burrows as a reservoir for arboviruses in Central Europe

Culex pipiens s.s./Culex torrentium belong to the most widespread mosquito taxa in Europe and are the main vectors of Sindbis, West Nile and Usutu virus. The adult overwintering females can act as reservoir for these arthropod-borne viruses (arboviruses), thus contributing to their local persistence when transmission cycles are interrupted during the winter. However, the main overwintering sites of Cx. torrentium are unknown. In a study from 2017, 3455 Cx. pipiens s.s./Cx. torrentium specimens were collected from abandoned animal burrows in Poznan, Poland. These specimens were retrospectively identified to species-level with a PCR assay, which revealed Cx. torrentium as dominant species (> 60%). Motivated by these results, we conducted a field study from February to July 2022 to systematically analyse the overwintering site patterns of Cx. pipiens s.s./Cx. torrentium. Mosquitoes were sampled using pipe traps in abandoned animal burrows (n = 20) and with aspirators in nearby anthropogenic overwintering sites (n = 23). All Cx. pipiens s.s./Cx. torrentium were screened for Flaviviridae RNA. In total, 4710 mosquitoes of five different taxa were collected from anthropogenic sites. 3977 of them were identified as Cx. p. pipiens/Cx. torrentium (Cx. p. pipiens: 85%, Cx torrentium: 1%, pools with both species: 14%). In contrast, only Cx. p. pipiens/Cx. torrentium (1688 specimens) were collected from animal burrows dominated by Cx. torrentium (52%), followed by pools with both species (40%) and Cx. p pipiens (8%). A single pool of 10 Cx. torrentium specimens collected from an animal burrow was positive for Usutu virus. The detection of Usutu virus demonstrates that Cx. torrentium can act as winter reservoir for arboviruses. Abandoned animal burrows may by the primary overwintering site for the species and should be considered in future surveillance programmes, when sampling overwintering 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.

Sindbis Virus Infection in Non-Blood-Fed Hibernating Culex pipiens Mosquitoes in Sweden

A crucial, but unresolved question concerning mosquito-borne virus transmission is how these viruses can remain endemic in regions where the transmission is halted for long periods of time, due to mosquito inactivity in, e.g., winter. In northern Europe, Sindbis virus (SINV) (genus alphavirus, Togaviridae) is transmitted among birds by Culex mosquitoes during the summer, with occasional symptomatic infections occurring in humans. In winter 2018–19, we sampled hibernating Culex spp females in a SINV endemic region in Sweden and assessed them individually for SINV infection status, blood-feeding status, and species. The results showed that 35 out of the 767 collected mosquitoes were infected by SINV, i.e., an infection rate of 4.6%. The vast majority of the collected mosquitoes had not previously blood-fed (98.4%) and were of the species Cx. pipiens (99.5%). This is the first study of SINV overwintering, and it concludes that SINV can be commonly found in the hibernating Cx. pipiens population in an endemic region in Sweden, and that these mosquitoes become infected through other means besides blood-feeding. Further studies on mosquito ecology and viral interactions are needed to elucidate the mechanisms of the persistence of these viruses over winter.

First records of Anopheles (Anopheles) plumbeus Stephens, 1828 and Culex (Culex) torrentium Martini, 1925 (Diptera: Culicidae) in Galicia (NW Spain)

We present the first records of Anopheles (Anopheles) plumbeus Stephens, 1828 and Culex (Culex) torrentium Martini, 1925 in the autonomous region of Galicia (NW Spain), obtained through the Rede Galega de Vixilancia de Vectores (ReGaViVec). This entomological surveillance network, which was initiated in 2017 by the Xunta de Galicia in collaboration with the University of Vigo and the University of Santiago de Compostela, aims to detect the arrival of invasive vectors as well as to improve the knowledge about mosquito populations (Diptera: Culicidae) in the Galician territory. This study shows the first findings of these species in Galicia, which have been reported in six different locations throughout the region: five in the province of Pontevedra and one in the province of Lugo. The 51 captured specimens were collected at different stages of development between July and September, 2018 and 2019 by using specialized traps arranged in favorable regions for the settlement and development of culicids.

Surveillance of mosquito vectors in Southern Sweden for Flaviviruses and Sindbis virus

There are three human pathogenic bird-viruses transmitted by Culex mosquitoes in Europe: the alphavirus Sindbis and the flaviviruses West Nile virus and Usutu virus. Cases of Sindbis fever occur in the north while the flaviviruses are reported from southern Europe. In this study, 7933 Culex pipiens/torrentium mosquitoes from southern Sweden were screened by RTqPCR for these viruses. None of the mosquitoes were positive for viral RNA. The importance of mosquito species composition is discussed as a potential explanation to the lack of detection of mosquito-borne viruses in southern Sweden. However, continued surveillance of mosquitoes for Flaviviruses would be valuable as an early warning for public health awareness.

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.

DNA-aided identification of Culex mosquitoes (Diptera: Culicidae) reveals unexpected diversity in underground cavities in Austria

Subterranean cavities serve as resting places and hibernation shelters for mosquitoes. In Europe, members of the genus Culex are often the most abundant insects on cave walls. Culex pipiens L., the common house mosquito, exists in two physically very similar, yet genetically and ecologically distinct biotypes (or forms, 'f.'), namely Cx. pipiens f. pipiens and Cx. pipiens f. molestus. Autogeny and stenogamy of the latter form have been interpreted as adaptations to underground habitats. The epigean occurrence of the two biotypes and their hybrids was recently examined in Eastern Austria, but the hypogean distribution of the Cx. pipiens complex and morphologically similar non-members such as Cx. torrentium is unknown. Considering the key role of Culex mosquitoes in the epidemiology of certain zoonotic pathogens, the general paucity of data on species composition and relative abundance in subterranean shelters appears unfortunate.For a first pertinent investigation in Austria, we collected mosquitoes in four eastern federal states. Based on analyses of the ACE2 gene and the CQ11 microsatellite locus, 150 female and three male mosquitoes of the genus Culex, two females of the genus Culiseta and a single female of the genus Anopheles were determined to species level or below. In our catches, Cx. pipiens f. pipiens exceeded the apparent abundance of the purportedly cave-adapted Cx. pipiens f. molestus many times over. Records of Cx. hortensis and Cx. territans, two species rarely collected in Austria, lead us to infer that underground habitats host a higher diversity of culicine mosquitoes than previously thought.

Wide recognition of Culex pipiens and lack of detection of Culex torrentium through biomolecular differentiation of mosquitoes in the Emilia-Romagna region, Northern Italy

The Culex pipiens complex includes species with reported differences in vector competence for arthropod-borne viruses, many of which are of significant importance to human health such as the West Nile virus and the Sindbis virus. This group of mosquitoes is difficult to distinguish morphologically; particularly as adult females. In Europe, the two species of the complex, Culex pipiens Linnaeus 1758 and Culex torrentium Martini 1925, are often found sympatrically. With the aim to characterize the presence and spread of both species in the Emilia-Romagna region, Northern Italy, mosquitoes of the complex - collected during the West Nile virus surveillance plans - were tested by multiplex real-time PCR for the detection of the two species Cx. pipiens and Cx. torrentium. A total of 24 165 mosquitoes, collected between 2012 and 2014 from 105 sites, and sorted in 204 pools, were tested. All tested pools were found to be composed of Cx. pipiens, whereas Cx. torrentium was not detected. These results indicate a likely absence of Cx. torrentium mosquitoes within the surveyed territory, whereas Cx. pipiens is widely distributed in the area mentioned. This is in line with previous reports, which describe a pre-alpine distribution of Cx. torrentium in Italy.

Physico-Chemical Characteristics of Culex pipiens sensu lato and Culex torrentium (Diptera: Culicidae) Breeding Sites in Germany

Culex pipiens sensu lato (L.) and Culex torrentium Martini, 1925 are two widely distributed mosquito species in Europe. Due to difficulties in morphological discrimination, the current knowledge on differences in the breeding ecology is fragmentary. Therefore, this study evaluated the relation between the presence-absence of both species at various types of breeding habitats in response to physico-chemical parameters, using a recently developed molecular assay that allows reliable species-specific typing of larvae and pupae. The results revealed that the two species often occur in sympatry at the studied breeding sites, and there were no substantial differences concerning presence-absence of the two species with regard to the various environmental parameters investigated.

The arbovirus vector Culex torrentium is more prevalent than Culex pipiens in northern and central Europe

Two species of arbovirus vector, Culex torrentium and Culex pipiens (Diptera: Culicidae), occur in several European countries, but difficulties in their accurate identification and discrimination have hampered both detailed and large-scale distribution and abundance studies. Using a molecular identification method, we identified to species 2559 larvae of Cx. pipiens/torrentium collected from 138 sites in 13 European countries ranging from Scandinavia to the Mediterranean coast. In addition, samples of 1712 males of Cx. pipiens/torrentium collected at several sites in the Czech Republic were identified to species based on the morphology of their hypopygia. We found that the two species occur together in large areas of Europe, and that Cx. torrentium dominates in northern Europe and Cx. pipiens dominates south of the Alps. The transition in dominance occurs in central Europe, where both species are roughly equally common. There was a strong correlation between the length of the growing season at different sites and occurrences of the two species. As the growing season increases, the proportion and detection of Cx. torrentium decrease, whereas those of Cx. pipiens increase. The present findings have important consequences for the interpretation of the results of studies on major enzootic and link-vectors of mosquito-borne bird-associated viruses (i.e. Sindbis, West Nile and Usutu viruses), especially in central Europe and Scandinavia.

The use of morphometric wing characters to discriminate female Culex pipiens and Culex torrentium

The reliability of the length of wing radial vein r(2/3) as a character for the morphological discrimination of the two potential arbovirus vectors Culex pipiens s.s. and Cx. torrentium from Germany was reassessed, after this character had been neglected for more than 40 years. Additionally, multivariate morphometric analyses were applied to evaluate wing shape variation between both species. Although high-throughput molecular tools are now available to differentiate the two species, a simple, low-cost routine alternative may be useful in the absence of a molecular laboratory, such as under semi-field conditions. A thin-plate splines transformation confirmed that primarily the shrinkage of vein r(2/3) is responsible for the wing differences between the two species. In the bivariate analysis, the r(2/3)/r3 indices of Cx. pipiens s.s. and Cx. torrentium were 0.185 and 0.289, respectively, resulting in a correct classification of more than 91% of all tested specimens. Using the absolute length of vein r(2/3) alone still allowed for more than 90% accurate discrimination. Furthermore, classification accuracy of linear discriminant analysis exceeded 97%.