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

Is it time to consider west Nile and Usutu viruses endemic in central Italy?

West Nile (WNV) and Usutu (USUV) viruses co-circulated in a region of Central Italy (Lazio) in 2018, as evidenced by the detection of WNV in the nervous tissues of symptomatic horses and USUV in blood donors and mosquito pools. To assess whether these viruses were endemic in the region, we analysed: 1) diapausing Culex pipiens mosquitoes collected during the winter seasons 2022-2023 and 2023-2024, 2) Cx. pipiens mosquitoes collected during the adult activity period from April to November in 2022 and 2023 across 4 provinces, and 3) sera from 52 horses and tissues from 537 birds. Field-collected Cx. pipiens, including both diapausing and non-diapausing individuals, were tested in pools for WNV and USUV using real-time RT-PCR. Serum samples from horses were tested with two WNV ELISA assays, IgM and IgG, while bird tissues were tested for both viruses via real-time RT-PCR. A total of 18,834 Cx. pipiens females were collected, including 9,812 mosquitoes during the winter seasons and 9,022 during the adult activity periods. Mosquitoes were tested in 623 pools, with all pools of diapausing mosquitoes testing negative for both viruses and 12 pools of non-diapausing mosquitoes positive to USUV. The WNV IgG positivity of 7 horse sera, which were negative at the beginning of the study period, was not confirmed by the virus neutralization test. All tissue samples were negative for WNV and USUV. Since WNV and USUV were not detected in diapausing mosquitoes, there was no evidence of the two viruses endemicity in the study area.

Characterization of overwintering sites (hibernacula) of the West Nile vector Culex pipiens in Central Italy

BACKGROUND

In cool-temperate regions, mosquitoes face winter conditions that hinder their development. To cope with cold temperatures, species like Culex pipiens, a major vector of West Nile virus (WNV), diapause as adult females in overwintering shelters known as hibernacula. This study aimed to identify and characterize the overwintering sites of Cx. pipiens in central Italy, analyzing the environmental factors influencing the abundance of the two biological forms, Cx. pipiens pipiens and Cx. pipiens molestus.

METHODS

Field surveys were conducted in Lazio and Tuscany over two consecutive winters (2022/2023 and 2023/2024). Overwintering mosquitoes were collected from different hibernacula types, including natural caves, artificial cavities and buildings. Environmental variables such as temperature, humidity, light intensity and vapor pressure deficit (VPD) were recorded in the entrance and inner rooms of each hibernaculum. Mosquito species and Cx. pipiens forms were identified through morphological and molecular analyses. A beta regression model was applied to assess the relationship between environmental factors and Cx. pipiens abundance. Redundancy analysis (RDA) was used to explore the impact of small- and landscape-scale variables on biological forms distribution.

RESULTS

Culex pipiens presence was confirmed in 24 hibernacula and represented the most abundant species, with Anopheles maculipennis s.l., Culiseta annulata and Culex hortensis accounting for 0.4% of the collected individuals. Comparing the entrance and inner rooms, a higher abundance of Cx. pipiens s.s. was observed in the darker environments, characterized by a humidity of 50-75%, a temperature of 10-20 °C and a VPD of 0.3-0.8 kPa. Inside the inner rooms, Cx. pipiens females preferred lower temperatures, light intensity and humidity, combined with higher VPD. The RDA highlighted that Cx. pipiens pipiens was associated with low temperatures and VPD and high humidity levels, preferring semi-natural areas. Culex pipiens molestus was positively associated with artificial areas. Hybrids were observed in several types of hibernacula.

CONCLUSIONS

This study provides insights into the overwintering ecology of Cx. pipiens in southern Europe, highlighting the environmental factors driving its abundance. These results may inform future vector control strategies aimed at reducing mosquito populations and limiting WNV diffusion in temperate regions.

Usutu virus continues to spread across Europe: first report of multiple molecular detections of the USUV Africa 2 and Africa 3 lineages in free-living and captive birds in Poland, July-November 2023

The growing importance of the Usutu virus (USUV) as an emerging zoonotic viral pathogen motivated Poland to investigate the epidemiological status of USUV infections among native bird species. Consequently, out of the 357 birds tested, 34 avian individuals from different geographical areas of the country were confirmed to be USUVpositive by RT‒PCR between July and November 2023. The phylogenetic analysis revealed the first recorded occurrence of the USUV Africa 2 and Africa 3 genetic lineages in Poland.

Overwintering of Usutu virus in mosquitoes, The Netherlands

Analyses of mosquito-borne virus outbreaks have revealed the presence of similar virus strains over several years. However, it remains unclear how mosquito-borne viruses can persist over winter, when conditions are generally unfavorable for virus circulation. One potential route for virus persistence is via diapausing mosquitoes. We therefore studied whether Usutu virus (USUV), West Nile virus (WNV) and/or Sindbis virus (SINV) can be identified in diapausing mosquitoes in The Netherlands. Mosquito collections were carried out in November 2022 in hibernacula located in two areas with previously observed WNV and/or USUV activity. A total of 4857 mosquitoes, belonging to four species (groups) (Culex pipiens/torrentium, Culiseta annulata, Anopheles maculipennis s.l. and Culex territans), were collected. WNV-, USUV- and SINV-screening using a multiplex real-time RT-PCR assay was carried out on mono-specific mosquito pools. One Culex pipiens/torrentium pool tested positive for USUV RNA. Whole genome sequencing and subsequent phylogenetic analysis revealed that the virus belongs to USUV lineage Africa 3 and clusters with other USUV sequences derived from The Netherlands in 2022. This finding confirms our hypothesis of the potential of local overwintering of USUV in diapausing mosquitoes in The Netherlands.

Phenotypical aspects of Culex pipiens biotype pipiens during diapause: Lipid utilization, body size, insemination, and parity

In temperate regions, female Culex pipiens biotype pipiens mosquitoes undergo diapause in winter. Diapausing biotype pipiens mosquitoes are potentially important winter reservoirs for mosquito-borne viruses, such as West Nile virus (WNV), Usutu virus (USUV), and Sindbis virus (SINV). Mosquitoes in diapause have not taken a bloodmeal prior to winter. Therefore, they do not become infected by taking an infectious bloodmeal and as a consequence, vertical transmission is considered the primary mechanism of mosquito-borne virus overwintering. Prior to winter, biotype pipiens mosquitoes build up fat reserves, which they utilize throughout winter. Furthermore, earlier studies have indicated that larger body size is correlated with increased survival during winter. However, not much is known about lipid utilization and body size of wild biotype pipiens mosquitoes in diapause. Therefore, we performed monthly collections of diapausing biotype pipiens mosquitoes in two consecutive winters (2020/2021 and 2021/2022) in bunkers of the New Hollandic Waterline in the Netherlands. Every month, we checked the proportion of inseminated and parous females via microscopy. In addition, we measured wing length as proxy for body size, and assessed total lipid content. Furthermore, we monitored indoor temperature in the overwintering locations. We show that the overwintering sites in our study provide relatively stable environments, in which temperatures rarely drop below 0 °C. The vast majority of biotype pipiens females were inseminated (84.1 %) and nulliparous (97.5 %). We detected differences in body size between but not within the two years of sampling. Additionally, we detected a difference in lipid content between the sampling years. We confirm that the vast majority of diapausing biotype pipiens females are inseminated and nulliparous. This indicates that they did not blood feed prior to winter, which underscores the likeliness of vertical transmission being the primary mechanism behind virus overwintering. The detected difference in body size between years can most likely be attributed to differences in summer conditions the mosquitoes were exposed to as larvae, although this needs confirmation. The difference in lipid depletion could not be explained by differences in climatic conditions. To shed more light on the links between climatic conditions, body size, lipid depletion and the consequences for mosquito population dynamics and arbovirus transmission, future experimental work, for example by arbovirus exposure followed by artificially induced diapause, is desired.

Host attraction and host feeding patterns indicate generalist feeding of Culex pipiens s.s. and Cx. torrentium

BACKGROUND

Mosquito host feeding patterns are an important factor of the species-specific vector capacity determining pathogen transmission routes. Culex pipiens s.s./Cx. torrentium are competent vectors of several arboviruses, such as West Nile virus and Usutu virus. However, studies on host feeding patterns rarely differentiate the morphologically indistinguishable females.

METHODS

We analyzed the host feeding attraction of Cx. pipiens and Cx. torrentium in host-choice studies for bird, mouse, and a human lure. In addition, we summarized published and unpublished data on host feeding patterns of field-collected specimens from Germany, Iran, and Moldova from 2012 to 2022, genetically identified as Cx. pipiens biotype pipiens, Cx. pipiens biotype molestus, Cx. pipiens hybrid biotype pipiens × molestus, and Cx. torrentium, and finally put the data in context with similar data found in a systematic literature search.

RESULTS

In the host-choice experiments, we did not find a significant attraction to bird, mouse, and human lure for Cx. pipiens pipiens and Cx. torrentium. Hosts of 992 field-collected specimens were identified for Germany, Iran, and Moldova, with the majority determined as Cx. pipiens pipiens, increasing the data available from studies known from the literature by two-thirds. All four Culex pipiens s.s./Cx. torrentium taxa had fed with significant proportions on birds, humans, and nonhuman mammals. Merged with the data from the literature from 23 different studies showing a high prevalence of blood meals from birds, more than 50% of the blood meals of Cx. pipiens s.s. were identified as birds, while up to 39% were human and nonhuman mammalian hosts. Culex torrentium fed half on birds and half on mammals. However, there were considerable geographical differences in the host feeding patterns.

CONCLUSIONS

In the light of these results, the clear characterization of the Cx. pipiens s.s./Cx. torrentium taxa as ornithophilic/-phagic or mammalophilic/-phagic needs to be reconsidered. Given their broad host ranges, all four Culex taxa could potentially serve as enzootic and bridge vectors.

Detection of Usutu virus in a house martin bug Oeciacus hirundinis (Hemiptera: Cimicidae): implications for virus overwintering in a temperate zone

The family Cimicidae comprises ectoparasites feeding exclusively on the blood of endothermic animals. Cimicid swallow bugs specifically target swallow birds (Hirundinidae) and their nestlings in infested nests. Bugs of the genus Oeciacus are commonly found in mud nests of swallows and martins, while they rarely visit the homes of humans. Although-unlike other cimicid species-the house martin bug Oeciacus hirundinis has never been reported as a vector of zoonotic pathogens, its possible role in arbovirus circulation in continental Europe is unclear. Samples of O. hirundinis were therefore collected from abandoned house martin (Delichon urbicum) nests in southern Moravia (Czech Republic) during the 2021/2022 winter season and checked for alpha-, flavi- and bunyaviruses by RT-PCR. Of a total of 96 pools consisting of three adult bugs each, one pool tested positive for Usutu virus (USUV)-RNA. Phylogenetic analysis showed that the virus strain was closely related to Italian and some Central European strains and corresponded to USUV lineage 5. The detection of USUV in O. hirundinis during wintertime in the absence of swallows raises the question for a possible role of this avian ectoparasite in virus overwintering in Europe.

Ixodes ricinus as potential vector for Usutu virus

Usutu virus (USUV) is an emerging flavivirus that is maintained in an enzootic cycle with mosquitoes as vectors and birds as amplifying hosts. In Europe, the virus has caused mass mortality of wild birds, mainly among Common Blackbird (Turdus merula) populations. While mosquitoes are the primary vectors for USUV, Common Blackbirds and other avian species are exposed to other arthropod ectoparasites, such as ticks. It is unknown, however, if ticks can maintain and transmit USUV. We addressed this question using in vitro and in vivo experiments and field collected data. USUV replicated in IRE/CTVM19 Ixodes ricinus tick cells and in injected ticks. Moreover, I. ricinus nymphs acquired the virus via artificial membrane blood-feeding and maintained the virus for at least 70 days. Transstadial transmission of USUV from nymphs to adults was confirmed in 4.9% of the ticks. USUV disseminated from the midgut to the haemocoel, and was transmitted via the saliva of the tick during artificial membrane blood-feeding. We further explored the role of ticks by monitoring USUV in questing ticks and in ticks feeding on wild birds in the Netherlands between 2016 and 2019. In total, 622 wild birds and the Ixodes ticks they carried were tested for USUV RNA. Of these birds, 48 (7.7%) carried USUV-positive ticks. The presence of negative-sense USUV RNA in ticks, as confirmed via small RNA-sequencing, showed active virus replication. In contrast, we did not detect USUV in 15,381 questing ticks collected in 2017 and 2019. We conclude that I. ricinus can be infected with USUV and can transstadially and horizontally transmit USUV. However, in comparison to mosquito-borne transmission, the role of I. ricinus ticks in the epidemiology of USUV is expected to be minor.

Mosquitoes from Europe Are Able to Transmit Snowshoe Hare Virus

Snowshoe hare virus (SSHV) is a zoonotic arthropod-borne virus (arbovirus) circulating in colder areas of the Northern Hemisphere. SSHV is maintained in an enzootic cycle between small mammals and mosquitoes, assumably of the genera Aedes and Culiseta. Symptoms of SSHV human infection can range from asymptomatic to severe neuroinvasive disease. Studies on SSHV transmission are limited, and there is no information available on whether mosquitoes of the genus Culex are able to transmit SSHV. Therefore, we investigated six mosquito species via salivation assay for their vector competence. We demonstrated that SSHV can be transmitted by the abundant European Culex species Cx. pipiens biotype pipiens, Cx. pipiens biotype molestus, and Cx. torrentium with low transmission efficiency between 3.33% and 6.67%. Additionally, the invasive species Ae. albopictus can also transmit SSHV with a low transmission efficiency of 3.33%. Our results suggest that local transmission of SSHV after introduction to Europe seems to be possible from a vector perspective.