Potential vectors of equine arboviruses in the UK

iNaturalist community observations provide valuable data on human-mosquito encounters

Mosquitoes (Diptera: Culicidae) and the pathogens they transmit represent a threat to human and animal health. Low-cost and effective surveillance methods are necessary to enable sustainable monitoring of mosquito distributions, diversity, and human interactions. This study examined the use of iNaturalist, an online, community-populated biodiversity recording database, for passive mosquito surveillance in the United Kingdom (UK) and Ireland, countries under threat from the introduction of invasive mosquitoes and emerging mosquito-borne diseases. The Mozzie Monitors UK & Ireland iNaturalist project was established to collate mosquito observations in these countries. Data were compared with existing long-term mosquito UK datasets to assess representativeness of seasonal and distribution trends in citizen scientist-recorded observations. The project collected 738 observations with the majority recorded 2020-2022. Records were primarily associated with urban areas, with the most common species Culex pipiens and Culiseta annulata significantly more likely to be observed in urban areas than other species. Analysis of images uploaded to the iNaturalist project also provided insights into human-biting behavior. Our analyses indicate that iNaturalist provides species composition, seasonal occurrence, and distribution figures consistent with existing datasets and is therefore a useful surveillance tool for recording information on human interactions with mosquitoes and monitoring species of concern.

UK mosquitoes are competent to transmit Usutu virus at native temperatures

Usutu virus (USUV) is an emerging zoonotic virus transmitted primarily by Culex mosquitoes. Since its introduction into Europe from Africa during the late 20th century, it has caused mortality within populations of passerine birds and captive owls, and can on occasion lead to disease in humans. USUV was first detected in the UK in 2020 and has become endemic, having been detected in either birds and/or mosquitoes every subsequent year. Importantly, the vector competence of indigenous mosquitoes for the circulating UK (London) USUV strain at representative regional temperatures is still to be elucidated. This study assessed the vector competence of five field-collected mosquito species/biotypes, Culex pipiens biotype molestus, Culex pipiens biotype pipiens, Culex torrentium, Culiseta annulata and Aedes detritus for the London USUV strain, with infection rates (IR) and transmission rates (TR) evaluated between 7 and 28 days post-infection. Infection and transmission were observed in all species/biotypes aside from Ae. detritus and Cx. torrentium. For Cx. pipiens biotype molestus, transmission potential suggests these populations should be monitored further for their role in transmission to humans. Furthermore, both Cx. pipiens biotype pipiens and Cs. annulata were shown to be competent vectors at 19 °C indicating the potential for geographical spread of the virus to other UK regions.

A difference in larval mosquito size allows a biocontrol agent to target the invasive species

As the global temperature rises in the coming decades, Aedes albopictus is expected to invade and establish in South East England, where Culex pipiens is currently the most common native mosquito species. Biocontrol measures that use local cyclopoid copepods against Ae. albopictus may be compromised if the copepods prefer alternate Cx. pipiens prey. In this study, I assessed the predation efficiency of Megacyclops viridis copepods against Ae. albopictus larvae from France and larvae that hatched from egg rafts of Cx. pipiens collected in South East England. The experiments were conducted at 15 and 25°C, which are representative of present and future summer temperatures in South East England. Ae. albopictus larvae that survived the course of the experiment in the predator-absent controls were significantly smaller than Cx. pipiens larvae that survived in the absence of predation. The background mortality of Cx. pipiens larvae increased with the 10-degree increase in temperature, and the smaller size of surviving Cx. pipiens larvae at 25°C, relative to survivors at 15°C, suggests that larger Cx. pipiens larvae were more likely to die at the higher temperature setting. Across all experimental treatments, the ratio of copepod body length to mean prey length, based on larval lengths of survivors from the corresponding predator-absent controls, was a significant predictor of the copepod's predation efficiency. Adding temperature setting to the predation efficiency model as a predictor did not improve model fit. Within the mixed prey treatments, the predation efficiency of M. viridis was 34.5 percentage points higher against Ae. albopictus prey than against Cx. pipiens prey. The higher predation efficiency that M. viridis exhibited against invasive Ae. albopictus prey, likely due to the smaller size of these larvae, supports the future use of M. viridis as a biocontrol agent in the United Kingdom.

Blood-feeding ecology of mosquitoes in two zoological gardens in the United Kingdom

BACKGROUND

Zoological gardens contain unique configurations of exotic and endemic animals and plants that create a diverse range of developing sites and potential sources of blood meals for local mosquitoes. This may imply unusual interspecific pathogen transmission risks involving zoo vertebrates, like avian malaria to captive penguins. Understanding mosquito ecology and host feeding patterns is necessary to improve mosquito control and disease prevention measures in these environments.

METHODS

Mosquito sampling took place in Chester Zoo for 3 years (2017, 2018, and 2019) and for 1 year in Flamingo Land (2017) using different trapping methods. Blood-fed mosquitoes were identified and their blood meal was amplified by PCR, sequenced, and blasted for host species identification.

RESULTS

In total, 640 blood-fed mosquitoes were collected [Culex pipiens (n = 497), Culiseta annulata (n = 81), Anopheles maculipennis s.l. (n = 7), An. claviger (n = 1), and unidentifiable (n = 55)]. Successful identification of the host species was achieved from 159 blood-fed mosquitoes. Mosquitoes fed on birds (n = 74), non-human mammals (n = 20), and humans (n = 71). There were mixed blood meals from two hosts (n = 6). The proportions of blood-fed mosquitoes varied across sampling seasons and sites within the zoos. The use of resting traps and aspiration of vegetation were more efficient techniques for capturing blood-fed mosquitoes than traps for host-seeking or gravid mosquitoes. By relating the locations of zoo vertebrates to where fed mosquitoes were trapped, the minimum travelling distances were calculated (13.7 to 366.7 m). Temperature, precipitation, relative humidity, proximity to zoo vertebrate exhibits, and vegetation level were found to be significantly associated with the proportion of captured blood-fed mosquitoes by generalized linear modelling.

CONCLUSIONS

Mosquito feeding behaviour in zoos is mainly influenced by time, location (sampling area), temperature, and host availability, which highlights the value of mosquito monitoring in complex settings to plan control strategies and potentially reduce inherent disease transmission risks for humans and threatened zoo vertebrates.

Laboratory transmission potential of British mosquitoes for equine arboviruses

BACKGROUND

There has been no evidence of transmission of mosquito-borne arboviruses of equine or human health concern to date in the UK. However, in recent years there have been a number of outbreaks of viral diseases spread by vectors in Europe. These events, in conjunction with increasing rates of globalisation and climate change, have led to concern over the future risk of mosquito-borne viral disease outbreaks in northern Europe and have highlighted the importance of being prepared for potential disease outbreaks. Here we assess several UK mosquito species for their potential to transmit arboviruses important for both equine and human health, as measured by the presence of viral RNA in saliva at different time points after taking an infective blood meal.

RESULTS

The following wild-caught British mosquitoes were evaluated for their potential as vectors of zoonotic equine arboviruses: Ochlerotatus detritus for Venezuelan equine encephalitis virus (VEEV) and Ross River virus (RRV), and Culiseta annulata and Culex pipiens for Japanese encephalitis virus (JEV). Production of RNA in saliva was demonstrated at varying efficiencies for all mosquito-virus pairs. Ochlerotatus detritus was more permissive for production of RRV RNA in saliva than VEEV RNA. For RRV, 27.3% of mosquitoes expectorated viral RNA at 7 days post-infection when incubated at 21 °C and 50% at 24 °C. Strikingly, 72% of Cx. pipiens produced JEV RNA in saliva after 21 days at 18 °C. For some mosquito-virus pairs, infection and salivary RNA titres reduced over time, suggesting unstable infection dynamics.

CONCLUSIONS

This study adds to the number of Palaearctic mosquito species that demonstrate expectoration of viral RNA, for arboviruses of importance to human and equine health. This work adds to evidence that native mosquito species should be investigated further for their potential to vector zoonotic mosquito-borne arboviral disease of equines in northern Europe. The evidence that Cx. pipiens is potentially an efficient laboratory vector of JEV at temperatures as low as 18 °C warrants further investigation, as this mosquito is abundant in cooler regions of Europe and is considered an important vector for West Nile Virus, which has a comparable transmission ecology.

Surveillance of Mosquitoes (Diptera, Culicidae) in a Northern Central Region of Spain: Implications for the Medical Community

Mosquitoes are important to public and animal health due to their capacity to transmit diseases. Since the Zika virus was declared a pandemic by the WHO in 2016, and it has been recorded in different regions of Mediterranean Area (included Spain), the Government of La Rioja (Northern Spain) through the Center of Rickettsiosis and Arthropod-Borne Diseases, implemented an entomological surveillance programme of mosquitoes in La Rioja and in a close area of Navarra. This surveillance extended to some of the pathogens that they can transmit. Here we describe the framework of the initial surveillance programme for the detection of mosquitoes and associated human pathogens. We outline the benefits and the limitation of the programme to date, and explore how greater benefits can be achieved, for example using a One Health approach. Entomological surveillance has been carried out with BG-Sentinel traps, human bait technique and other methods such as collecting adults in resting places or immature stages by dipping in several wetlands. Since Aedes albopictus, vector of arbovirus such as Dengue, Chikungunya, and Zika, has not been detected yet in the region, the entomological programme included the surveillance of this exotic species using ovitraps in the most important cities. Morphological identification was supported using the mitochondrial cytochrome C oxidase subunit I and the internal transcribed spacer 2 genes analysis. In 2016 and 2017, more than 6,000 mosquitoes were collected. The mosquito's community included 21 species associated with six genera: Anopheles (n = 4), Aedes (n = 5), Culex (n = 6), Culiseta (n = 4), Uranotaenia (n = 1) and Coquillettidia (n = 1). Eleven species represent new records for La Rioja and Navarra regions. Several species were collected biting humans and a great proportion of the sampled mosquito population are competent vectors of several pathogens, such as West Nile virus. Sequences closely related to mosquito-only flavivirus have been detected in 0.34% of analysed pools. At the same time, the epidemiological surveillance emphasis is placed in the early detection of mosquito-borne diseases in primary health and emergency services. The surveillance programme represents a relevant and necessary assessment of the risk of pathogen transmission in a region, and it allows for the establishment of the appropriate preventive measures.

Survey of UK horse owners' knowledge of equine arboviruses and disease vectors

Increased globalisation and climate change have led to concern about the increasing risk of arthropod-borne virus (arbovirus) outbreaks globally. An outbreak of equine arboviral disease in northern Europe could impact significantly on equine welfare, and result in economic losses. Early identification of arboviral disease by horse owners may help limit disease spread. In order to determine what horse owners understand about arboviral diseases of horses and their vectors, the authors undertook an open, cross-sectional online survey of UK horse owners. The questionnaire was distributed using social media and a press release and was active between May and July 2016. There were 466 respondents, of whom 327 completed the survey in full. High proportions of respondents correctly identified photographic images of biting midges (71.2 per cent) and mosquitoes (65.4 per cent), yet few were aware that they transmit equine infectious diseases (31.4 per cent and 35.9 per cent, respectively). Of the total number of respondents, only 7.4 per cent and 16.2 per cent correctly named a disease transmitted by biting midges and mosquitoes, respectively. Only 13.1 per cent and 12.5 per cent of participants identified specific clinical signs of African horse sickness (AHS) and West Nile virus (WNV), respectively. This study demonstrates that in the event of heightened disease risk educational campaigns directed towards horse owners need to be implemented, focussing on disease awareness, clinical signs and effective disease prevention strategies.

The challenges posed by equine arboviruses

Equine populations worldwide are at increasing risk of infection by viruses transmitted by biting arthropods, including mosquitoes, biting midges (Culicoides), sandflies and ticks. These include the flaviviruses (Japanese encephalitis, West Nile and Murray Valley encephalitis), alphaviruses (eastern, western and Venezuelan encephalitis) and the orbiviruses (African horse sickness and equine encephalosis). This review provides an overview of the challenges faced in the surveillance, prevention and control of the major equine arboviruses, particularly in the context of these viruses emerging in new regions of the world.

Blood-feeding patterns of native mosquitoes and insights into their potential role as pathogen vectors in the Thames estuary region of the United Kingdom

BACKGROUND

The range of vertebrate hosts on which species of mosquito blood-feed is an important parameter for identifying potential vectors and in assessing the risk of incursion and establishment of vector-borne pathogens. In the United Kingdom, studies of mosquito host range have collected relatively few specimens and used techniques that could only broadly identify host species. This study conducted intensive collection and analysis of mosquitoes from a grazing marsh environment in southeast England. This site provides extensive wetland habitat for resident and migratory birds and has abundant human nuisance biting mosquitoes. The aim was to identify the blood-feeding patterns of mosquito species present at the site which could contribute to the transmission of pathogens.

METHODS

Twice-weekly collections of mosquitoes were made from Elmley Nature Reserve, Kent, between June and October 2014. Mosquitoes were collected using resting boxes, by aspiration from man-made structures and using a Mosquito Magnet Pro baited with 1-octen-3-ol. Blood-fed specimens were classified according to the degree of blood meal digestion using the Sella scale and vertebrate origin determined using sequencing of a fragment of the mitochondrial cytochrome C oxidase subunit I gene. Mosquitoes that were morphologically cryptic were identified to species level using multiplex PCR and sequencing methods.

RESULTS

A total of 20,666 mosquitoes of 11 species were collected, and 2,159 (10.4%) were blood-fed (Sella scale II-VI); of these 1,341 blood-fed specimens were selected for blood meal analysis. Vertebrate origin was successfully identified in 964 specimens (72%). Collections of blood-fed individuals were dominated by Anopheles maculipennis complex (73.5%), Culiseta annulata (21.2%) and Culex pipiens form pipiens (10.4%). Nineteen vertebrate hosts comprising five mammals and 14 birds were identified as hosts for mosquitoes, including two migratory bird species. Feeding on birds by Culex modestus and Anopheles atroparvus populations in England was demonstrated.

CONCLUSIONS

This study expands the vertebrate host range of mosquitoes in the Thames estuary region of the UK. Feeding on both resident and migratory bird species by potential arbovirus vectors including Cx. pipiens f. pipiens and Cx. modestus indicates the potential for enzootic transmission of an introduced arbovirus between migratory and local bird species by native mosquito species.