Culex pipiens - Factsheet for experts

Disclaimer: The information contained in this fact sheet is intended for the purpose of general information and should not substitute individual expert advice or judgement of healthcare professionals.

Hazard associated with vector species

Culex pipiens is a species complex native to Europe that is known as a pest in urban environments. Since the early 20th century, campaigns have been organised in many European cities to control the species [1,2]. The species shows high ecological plasticity, which gives a complex picture in terms of trophic behaviour and vectorial capacities.

Females of Cx. pipiens feed on a variety of vertebrate hosts and may therefore contribute to the amplification cycle of West Nile Virus (WNV) among birds, and also the occasional spill-over of viruses to human and other mammal populations [3,4,25]. Thus, Cx. pipiens mosquitoes are major vectors of both West Nile and Usutu Viruses (USUV) in Europe. They are also able to transmit several other arboviruses, while also acting as vectors for filarial worms (e.g. canine dirofilariasis) and plasmodia that cause avian malaria [25,43,45].

Entomology

Species name/classification: Culex (Culex) pipiens Linnaeus, 1758

Common name: (Common) house mosquito, Northern house mosquito

Synonyms and other names in use: Culex pipiens (biotype) pipiens, Culex pipiens (biotype) molestus

Culex pipiens is a polytypic species that is member of a species complex (or assemblage) and has a sibling species. The Pipiens Assemblage consists of Cx. pipiens, Cx. quinquefasciatus, and Cx. australicus; hybridisation (introgression) occurs between Cx. pipiens and Cx. quinquefasciatus, the product of which in Asia is known under the denomination pallens [5]. The taxon Cx. pipiens is considered to be a plastic species with two forms (or biotypes) and their hybrids in Europe: the pipiens form and the molestus form. Culex torrentium is regarded to be a sibling species of Cx. pipiens [5]. The other closely-related species, Cx. globocoxitus, is coupled at higher taxonomic level (i.e. subgroup Pipiens).

Morphological characters and similar species

Culex pipiens is a medium-size mosquito (4-10 mm), brownish-coloured overall, without any obvious specific pattern. It can be morphologically distinguished from other European mosquitoes (except those species from the Pipiens subgroup) based on a brownish colouration of its body, a proboscis (biting mouthpart) with only dark scales dorsally, legs without pale and dark ring patterns, an abdomen with a rounded tip and dorsal segments bearing yellowish basal bands [6,7].

Culex pipiens can morphologically be accurately differentiated from its sibling species Cx. torrentium only by examination of the male genitalia. Thus, only molecular tools allow females of these species to be identified with certainty [8,9].

A few tiny characteristics have been described to distinguish the pipiens and molestus forms as well as Cx. quinquefasciatus [5], but none of these are easy to judge, and the occurrence of hybrids complicates the picture. Typical specimens of Cx. quinquefasciatus show abdominal basal bands in the shape of a half-moon, whereas in Cx. pipiens they are more regular, thinner and often reduced and connected to larger lateral patches [7]. Here too, only molecular tools can guarantee an accurate identification of the species and hybrids [9,10].

Other European Culex species could be confused with Cx. pipiens since they also have regular basal abdominal bands: Cx. perexiguus and Cx. univittatus for which the abdominal bands are formed by white scales (yellowish for Cx. pipiens) and Cx. laticinctus, which has a larger abdominal band (covering 1/2 to 2/3 of the segment, whereas the same band covers less than 1/2 of the segment for Cx. pipiens) [6,7].

Ecology and ethology

Populations of Cx. pipiens show pronounced plasticity and disparities in their morphology and ethology, which has led to varying descriptions under specific species names that are now all considered to be junior synonyms to Cx. pipiens (which is the first published ‘senior’ synonym).

Females lay their eggs on water surfaces in batches as egg rafts containing around 200 eggs. These eggs are non-dormant and the larvae hatch rapidly as soon as the embryonic development is complete. The duration of the development depends on temperature. Thus the eggs hatch after only one day at 30°C, after three days at 20°C, ten days at 10°C, and below 7°C, embryonic development cannot be completed [7]. The larvae develop into adults within a few weeks, depending on the temperature (6-7 days at 30 °C, 21-24 days at 15 °C) [7]. They can inhabit nearly every type of water source. Larvae of Cx. pipiens can be found in temporary or (semi-)permanent water sources, ponds with vegetation, rice fields, along river edges in still zones, in areas prone to inundation, in puddles and ruts, occasionally even in water-filled tree-holes. The larvae also frequently occur in man-made water bodies, such as flooded cellars, construction sites, road drains and pits, water barrels, metal tanks, ornamental ponds and any type of container (e.g. in gardens or cemeteries). They can breed in clear water but also in water polluted by organic matter, and can even tolerate a small amount of salinity (e.g. coastal marshes or rock pools). The species can complete several generations per year depending on climatic conditions [7]. Larvae can be found from mid-spring until the first frosts, and the species is abundant in summer and autumn [2]. Only mated females overwinter in shelters free of deep frost, such as cellars, caves, bunkers, or ground burrows. Females bite any warm-blooded vertebrates at night, indoors or outdoors, and rest indoors for blood digestion. Reactivation of diapausing/overwintering females occurs in spring when the temperature and photoperiod increase [11]. Adults do not disperse far from their breeding site - usually less than 500 metres [7]. In nature, females are more abundant at canopy level than at ground level, where other mosquito species occur more frequently [7,12].

There are differences between the pipiens and the molestus classic forms. Females of the pipiens form mainly bite birds (i.e. ornithophilic), feed outdoors (i.e. exophagic) and rest outdoors (i.e. exophilic). They need a blood meal to lay their first batch of eggs (i.e. anautogenous), and have an obligatory winter diapause at the adult stage. Larvae are mainly found in clear water (both natural and artificial bodies). The molestus form is characterised by females biting mainly humans and other mammals, indoors (i.e. endophagic) or outdoors, frequently resting indoors (i.e. endophilic), being able to lay a first batch of eggs (approximately 40 eggs only) without taking a blood meal (i.e. autogenous) and having no obligatory diapause. Larvae are mainly found in water containing high amounts of organic matter, generally but not always underground (e.g. in sewage and subway systems, flooded cellars, pits, catch basins and wastewater basins) [2,5,7,13]. Therefore, the form molestus occurs more frequently in human environments and can reproduce throughout the winter in dark and warm urban habitats containing water or overwinter in unheated cellars or similar man-made shelters. Adults can mate in confined spaces without swarming which is not the case for adults of the pipiens form, making it very difficult to breed them in the laboratory. Overall, the populations of the pipiens and the molestus forms are clear and differentiated to the north of the Alps, but south of the Alps the characteristics of populations are less clear, due to changes in their behaviour throughout the season, differing weather conditions, and probably also due to hybridisation [14,15].

Geographical distribution

Culex pipiens is native to Africa, Asia and Europe although nowadays it is widely distributed. It lives in temperate regions of Europe, Asia, Africa, Australia, and North and South America [5], whereas Cx. quinquefasciatus is found at low-to-moderate elevations throughout the tropical, subtropical, and warm temperate regions of the world [5], and Cx. torrentium occurs in northern Europe down to a latitude of 46°N and further south only at high elevations [16,17].

Culex pipiens is found in all European countries, except Iceland and Faroe Islands, and in all Middle Eastern and North African countries. There are no known established populations of Cx. quinquefasciatus in these countries except in Iraq and Kuwait [18]. A recent description of its occurrence in Turkey [19] and a reported hybrid population in Greece [20] still need to be substantiated.

There is no evidence of a recent spread of Cx. pipiens into new areas, but its presence in the Americas, Asia and Australia is due to historical introductions via ships and subsequent spread [5]. The reported presence of Cx. pipiens might have been overestimated in parts of Europe (north of the Alps) due to possible confusion with Cx. torrentium [17].

Epidemiology and transmission of pathogens

Since Cx. pipiens is broadly distributed and bites a wide range of hosts, the species enters into contact with a wide range of pathogens. Consequently, adult populations show natural infection, some vector competences and even significant vectorial capacities with several pathogens. In particular, Cx. pipiens appears to be a major vector of both West Nile and Usutu Viruses, canine dirofilarial worms, and avian malaria parasites in Europe.

West Nile Virus (WNV)

Culex pipiens females have often been found infected by WNV in nature. In Europe, this was the case in the Czech Republic, Portugal, Romania, and Russia [21], in Spain [22], and more recently in Croatia, Greece, Italy and Serbia [23,24]. Laboratory experiments have demonstrated that European populations of Cx. Pipiens, form pipiens and form molestus, are both susceptible to WNV infection and capable of transmission, with the level depending on the virus lineage [25]. Based on its vector competence and the frequency of natural infection, Cx. pipiens is considered to be a major vector in WNV transmission: it plays a role as an amplificatory vector in the enzootic cycle (from birds to birds), as a bridge vector in the epizootic cycle (from birds to mammals) and as a reservoir [13,26-29]. In particular, its role is crucial in areas where other putative vector species (e.g. Cx. modestus or Cx. perexiguus/univittatus) have very low densities or are absent. In Italy, the abundance of Cx. pipiens (estimated as a monthly average of sampled mosquitoes per station) was highest inside the WNV circulation area (compared to a non-affected area), although a close relationship between mosquito abundance and viral circulation was not observed [28]. In Serbia, a strong significant correlation was found between the vector index (which includes the number of mosquitoes) and the number of West Nile neuro-invasive human cases at NUTS3 level [30]. These data also suggest that the mosquito density must exceed a certain threshold to support virus circulation and ensure transmission to humans. According to the data from outside the WNV circulation area for June and July in Italy, a monthly average value of 300 Cx. pipiens specimens collected per trap-night seems to correspond to the minimal risk threshold for human cases in the described surveillance system [28]. Nevertheless, to clarify the role of Cx. pipiens in the maintenance and transmission of WNV it is necessary to understand the bionomics of the different forms, its populations and hybrids, which could all play particular roles. In fact, the Cx. pipiens form pipiens, which mainly feeds on birds, probably plays an important role in the enzootic cycle, whereas the form molestus and hybrids which feed on both birds and mammals are more likely to play the role of a bridge vector in the epizootic cycle [31].

Usutu virus (USUV)

Cx. pipiens mosquitoes have often been found naturally infected with USUV during recent outbreaks in Europe (as with WNV), and experimental infections have demonstrated vector competence of both pipiens and molestus forms [25]. Another similarity with WNV is the species’ host preferences and abundance in outbreak areas, and the high frequency of natural infection compared to other species, suggesting that Cx. pipiens plays a major role as a vector of USUV in temperate regions [32,33].

Rift Valley fever virus (RVFV)

Culex pipiens mosquitoes have been found infected in nature in Egypt [34]. Vector competence was demonstrated at moderate-to-high levels [21,35-37]. Based on these findings, Cx. pipiens must be considered a potential RVFV vector in Europe [21].

Japanese encephalitis virus (JEV)

Natural infection of Cx. pipiens x quinquefasciatus (nominal form pallens) by JEV has been observed in China and Cx. pipiens form molestus is considered to be a moderately competent vector, based on laboratory infection [21].

Sindbis virus (SINV)

Due to the virus’ significant ability to infect and disseminate in the mosquito, natural infections observed in the field, and adult abundance and biting behaviour, Cx. pipiens is considered to be a moderately efficient vector of SINV. However, its sibling species Cx. torrentium may play a more important role in the enzootic cycle [36,38].

Tahyna virus (TAHV)

Females of Cx. pipiens have been found infected by TAHV in nature in Romania [39] and laboratory experiments have shown some vector competence [40]. The species could therefore act as a vector in a European context [38].

Other viruses

Culex pipiens females have recently been found infected by Batai virus in Germany [41], but the vector competence of the species is unknown. They have shown a medium susceptibility to infection and dissemination of the Saint Louis encephalitis virus, but are incompetent in acquiring equine encephalitis viruses (Eastern equine encephalitis virus, Venezuelan equine encephalitis virus and Western equine encephalitis virus) [36]. Finally, both pipiens and molestus forms show no infection at all for Zika virus [42].

Dirofilarial worms

Culex pipiens (both pipiens and molestus forms) is considered to be a vector of both Dirofilaria immitis and D. repens for dogs and possibly humans, based on its vector competence as evidenced in laboratory experiments, and its abundance and biting activity in dirofilariosis foci across Europe [43,44].

Avian malaria

Several mosquito species are considered to be vectors of avian malaria parasites (Plasmodium relictum, Plasmodium vaughani) and there is growing evidence that Cx. pipiens is a major vector in the northern hemisphere [45,46].

Overall, in nature, transmission of a pathogen by mosquitoes is dependent not only on vector competence but also on factors describing the intensity of interaction between the vector, the pathogen and the host in the local environment [35]. Therefore, vector and host densities, geographic distribution, longevity, dispersal and feeding preferences have to be considered to determine the vectorial capacity of a mosquito population and its role in transmission.

Public health control measures

In cities where wastewater is poorly managed, Cx. pipiens can be a serious nuisance. In large cities around the Mediterranean basin and also in urbanised areas further north in Europe (e.g. Paris and its surroundings) [2,47], control measures are frequently implemented against the species.

Vector management is often the primary option to prevent and control outbreaks of diseases such as West Nile Virus infection. However, its implementation is complex and needs to be supported by integrated multidisciplinary surveillance systems (i.e. systems surveying both the vector presence/activity and the pathogen introduction/circulation) and to be organised within the framework of risk assessment and predefined response plans [48-50]. The impact of a vector control programme depends on multiple factors and the identification of the best combination of vector control measures and methods is therefore not always straightforward [51].

The first step would be to design a pathogen/vector-targeted surveillance system. Culex pipiens can be surveyed actively or passively. A passive surveillance system is mainly based on contributions from the community, which is invited to report nuisance and if possible to send in specimens collected through larval sampling or adult catches [52]. However, passive surveillance can only act as a supplement to active surveillance which is more targeted in its aims and based on appropriate methods to collect data of a specific value (e.g. abundance, seasonal activity, breeding site and other ecological characteristics) and provide samples for pathogen screening [53]. Classic methods of larval sampling can be used, from March to November, to collate data on presence, distribution, seasonal dynamics, and breeding site locations and productivity, which are essential for guiding larval control measures. Although catching adults when they land on humans or in host-baited catches is better for providing data on the biting rate, resting catches are better for providing information on overwintering behaviour. Adult trapping also provides information on the abundance, seasonal dynamic and virus circulation (mainly from May to October). Different types of traps can be used, the most efficient being CO2-baited traps, which are better for abundance and assessment of the impact of control measures. Water-enticed gravid traps are better for targeting pathogen-infected mosquitoes. More guidance can be found in ECDC guidelines [53].

Suppression of Cx. pipiens populations may be based on environmental management measures; treatment of the larval breeding sites and/or treatment of adult populations during an outbreak. Environmental management measures include management of wastewater, cleaning flooded cellars and ventilation spaces, and removal of (man-made) water containers. The latter may also control the container-breeding invasive Aedes species in areas where the species are sympatric. Community participation, and therefore targeted communication, is essential to reduce larval breeding sites on private properties. Mosquito populations can be suppressed by applying bio-insecticides against larvae to avoid the emergence of the adults from breeding sites that cannot be eliminated. Here, the active substances registered on the EU market are limited to two microbial bio-insecticides: Bti (Bacillus thurengiensis israelensis) and Lsph (Lysinibacillus sphaericus). These bio-insecticides are highly specific, targeting mosquito larvae (Culicidae) but no other organisms developing at mosquito breeding sites. They can also be combined efficiently as a single formula. There are also a few insect growth regulators (e.g. diflubenzuron and pyriproxyfen) on the market. Finally, monomolecular films can be used to suffocate larvae and pupae but only in non-natural habitats. The chemical insecticides, which can be used against adult mosquitoes (pyrethroids), are not specific and will also have an impact on other fauna. Moreover, the populations normally recover quickly, with newly emerged adults. Therefore, adult mosquitoes are best targeted only during outbreaks, in order to kill the infected mosquitoes. Any regular use of chemical insecticides and insect growth regulators should be followed up with a quality control inspection and an assessment of the target population’s susceptibility to the active substance. This will enable adequate mitigation measures to be taken, where necessary, to prevent insecticide resistance [53,54]. Mass trapping is also a possibility but there is, as yet, no evidence of its efficiency in dealing with the nuisance and controlling an outbreak. More guidance on mosquito control can be found in the literature [7,51].

Personal protective measures can be effective in reducing the risk of mosquito bites [7,51]. In relation to Cx. pipiens, this specifically includes the use of mosquito bed nets (preferably insecticide-treated nets), sleeping or resting in screened or air-conditioned rooms, and the use of mosquito repellent in accordance with the instructions indicated on the product label while being outdoors at night.

Key areas of uncertainty

The house mosquito Cx. pipiens is undoubtedly among the most well-known mosquito species in Europe and is a significant vector of pathogens affecting both human and animals. However, the transmission of the various disease pathogens that this mosquito is involved in are complex. In fact, several other mosquito species besides Cx. pipiens are involved in the transmission cycles, playing different roles at various capacity levels. Moreover, Cx. pipiens is a species complex with members (species, forms/biotypes, and hybrids) showing differences in their biology and behaviour, and thus potentially having a variety of roles or capacity levels in transmission (e.g. form pipiens is more likely to be an enzootic vector and form molestus is more likely to be an epizootic vector of WNV). Finally, the complexity of the situation is increased by the fact that the traits used to characterise the two forms pipiens and molestus have varied over time and depending on the location. Therefore, any surveillance and control measures should be adapted according to the knowledge of Cx. pipiens in the local environmental context.

Further reading

Books and reviews

Vinogradova EB. Culex pipiens mosquitoes: taxonomy, distribution, ecology, physiology, genetics, applied importance and control. Pensoft: Sofia. 2000.

Becker N, Petric D, Zgomba M, Boase C, Madon M, Dahl C, et al. Mosquitoes and their control. Springer: Heidelberg, Dordrecht, New York. 2010.

Pages N, Huber K, Cipriani M, Chevallier V, Conraths FJ, Goffredo M, et al. Scientific review on mosquitoes and mosquito-borne diseases. EFSA Supporting Publications. 2009;6(8):7E.

Bellini R, Zeller H, Van Bortel W. A review of the vector management methods to prevent and control outbreaks of West Nile Virus infection and the challenge for Europe. Parasites & Vectors. 2014;7:323.

Community information

Aranda C. The common house mosquito: an unwanted companion in our lives. Barcelona Institute for Global Health. 2017. https://www.isglobal.org/en/healthisglobal/-/custom-blog-portlet/el-mosquito-comun-un-companero-indeseable-en-nuestras-vidas/5083982/8301

Mosquito surveillance

European Centre for Disease Prevention and Control. Guidelines for the surveillance of native mosquitoes in Europe. Technical Report. Stockholm: ECDC. 2014. doi 10.2900/37227. https://ecdc.europa.eu/sites/portal/files/media/en/publications/Publications/surveillance-of%20native-mosquitoes%20-guidelines.pdf

Mosquito control

European Mosquito Control Association (EMCA) and WHO Regional Office for Europe. Guidelines for the control of mosquitoes of public health important in Europe. 2013. https://www.emca-online.eu/emca/who-guidelines

European Centre for Disease Prevention and Control. Vector control with a focus on Aedes aegypti and Aedes albopictus mosquitoes: literature review and analysis of information. Technical Report. Stockholm: ECDC. 2017. https://ecdc.europa.eu/sites/portal/files/documents/Vector-control-Aedes-aegypti-Aedes-albopictus.pdf

Kampen H, Schaffner F. Chapter 11. Mosquitoes. In: Bonnefoy X., Kampen H., Sweeney K., editors. Public health significance of urban pests. Geneva: WHO Regional Office for Europe; 2008:347-386. http://www.euro.who.int/en/publications/abstracts/public-health-significance-of-urban-pests

Insecticide resistance

Insecticide Resistance Action Committee (IRAC). Prevention and management of insecticide resistance in vectors of public health importance. Second edition. 2011. https://www.irac-online.org/documents/irm-vector-manual/?ext=pdf

Risk assessment and prevention plans for West Nile Virus

European Centre for Disease Prevention and Control. West Nile Virus risk assessment tool. Technical Report. Stockholm: ECDC. 2013. https://www.ecdc.europa.eu/sites/default/files/media/en/publications/Publications/west-nile-virus-risk-assessment-tool.pdf

Anonymous. Guide de procédures de lutte contre la circulation du virus West Nile en France métropolitaine. Paris: Ministère des affaires sociales et de la santé; Ministère de l’écologie, du développement durable et de l’énergie; Ministère de l’agriculture, de l’agroalimentaire et de la forêt. 2012. [In French.]

Anonymous. Piano nazionale integrato di prevenzione, sorveglianza e risposta ai virus West Nile e Usutu – 2019. Roma: Ministero della Salute. 2019. [In Italian.] http://www.trovanorme.salute.gov.it/norme/renderNormsanPdf?anno=2019&codLeg=68806&parte=1%20&serie=null

References