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Mosquito bite
As children play hide-and-seek on a hot summer night, a mosquito, Culex, hides and seeks as well.
Book
Updated distribution maps of predominant Culex mosquitoes across the Americas
Background
Estimates of the geographical distribution of
Culex
mosquitoes in the Americas have been limited to state and provincial levels in the United States and Canada and based on data from the 1980s. Since these estimates were made, there have been many more documented observations of mosquitoes and new methods have been developed for species distribution modeling. Moreover, mosquito distributions are affected by environmental conditions, which have changed since the 1980s. This calls for updated estimates of these distributions to understand the risk of emerging and re-emerging mosquito-borne diseases.
Methods
We used contemporary mosquito data, environmental drivers, and a machine learning ecological niche model to create updated estimates of the geographical range of seven predominant
Culex
species across North America and South America:
Culex erraticus
,
Culex nigripalpus
,
Culex pipiens
,
Culex quinquefasciatus
,
Culex restuans
,
Culex salinarius
, and
Culex tarsalis
.
Results
We found that
Culex
mosquito species differ in their geographical range. Each
Culex
species is sensitive to both natural and human-influenced environmental factors, especially climate and land cover type. Some prefer urban environments instead of rural ones, and some are limited to tropical or humid areas. Many are found throughout the Central Plains of the USA.
Conclusions
Our updated contemporary
Culex
distribution maps may be used to assess mosquito-borne disease risk. It is critical to understand the current geographical distributions of these important disease vectors and the key environmental predictors structuring their distributions not only to assess current risk, but also to understand how they will respond to climate change. Since the environmental predictors structuring the geographical distribution of mosquito species varied, we hypothesize that each species may have a different response to climate change.
Graphical abstract
Journal Article
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.
Graphical Abstract
Journal Article
Wolbachia Enhances West Nile Virus (WNV) Infection in the Mosquito Culex tarsalis
Novel strategies are required to control mosquitoes and the pathogens they transmit. One attractive approach involves maternally inherited endosymbiotic Wolbachia bacteria. After artificial infection with Wolbachia, many mosquitoes become refractory to infection and transmission of diverse pathogens. We evaluated the effects of Wolbachia (wAlbB strain) on infection, dissemination and transmission of West Nile virus (WNV) in the naturally uninfected mosquito Culex tarsalis, which is an important WNV vector in North America. After inoculation into adult female mosquitoes, Wolbachia reached high titers and disseminated widely to numerous tissues including the head, thoracic flight muscles, fat body and ovarian follicles. Contrary to other systems, Wolbachia did not inhibit WNV in this mosquito. Rather, WNV infection rate was significantly higher in Wolbachia-infected mosquitoes compared to controls. Quantitative PCR of selected innate immune genes indicated that REL1 (the activator of the antiviral Toll immune pathway) was down regulated in Wolbachia-infected relative to control mosquitoes. This is the first observation of Wolbachia-induced enhancement of a human pathogen in mosquitoes, suggesting that caution should be applied before releasing Wolbachia-infected insects as part of a vector-borne disease control program.
Journal Article
Climate Change Influences on the Global Potential Distribution of the Mosquito Culex quinquefasciatus, Vector of West Nile Virus and Lymphatic Filariasis
Rapid emergence of most vector-borne diseases (VBDs) may be associated with range expansion of vector populations. Culex quinquefasciatus Say 1823 is a potential vector of West Nile virus, Saint Louis encephalitis virus, and lymphatic filariasis. We estimated the potential distribution of Cx. quinquefasciatus under both current and future climate conditions. The present potential distribution of Cx. quinquefasciatus showed high suitability across low-latitude parts of the world, reflecting the current distribution of the species. Suitable conditions were identified also in narrow zones of North Africa and Western Europe. Model transfers to future conditions showed a potential distribution similar to that under present-day conditions, although with higher suitability in southern Australia. Highest stability with changing climate was between 30°S and 30°N. The areas present high agreement among diverse climate models as regards distributional potential in the future, but differed in anticipating potential for distribution in North and Central Africa, southern Asia, central USA, and southeastern Europe. Highest disparity in model predictions across representative concentration pathways (RCPs) was in Saudi Arabia and Europe. The model predictions allow anticipation of changing distributional potential of the species in coming decades.
Journal Article
Transmission of West Nile and five other temperate mosquito-borne viruses peaks at temperatures between 23°C and 26°C
The temperature-dependence of many important mosquito-borne diseases has never been quantified. These relationships are critical for understanding current distributions and predicting future shifts from climate change. We used trait-based models to characterize temperature-dependent transmission of 10 vector–pathogen pairs of mosquitoes (
Culex pipiens
,
Cx. quinquefascsiatus
,
Cx. tarsalis
, and others) and viruses (West Nile, Eastern and Western Equine Encephalitis, St. Louis Encephalitis, Sindbis, and Rift Valley Fever viruses), most with substantial transmission in temperate regions. Transmission is optimized at intermediate temperatures (23–26°C) and often has wider thermal breadths (due to cooler lower thermal limits) compared to pathogens with predominately tropical distributions (in previous studies). The incidence of human West Nile virus cases across US counties responded unimodally to average summer temperature and peaked at 24°C, matching model-predicted optima (24–25°C). Climate warming will likely shift transmission of these diseases, increasing it in cooler locations while decreasing it in warmer locations.
Journal Article
How Zoophilic Japanese Encephalitis Vector Mosquitoes Feed on Humans
Japanese encephalitis virus (JEV) is the most frequent cause of mosquito-borne encephalitis in Asian countries. Several culicine species are potential vectors. The primary JEV vectors feed mainly on cows (a dead-end host for JEV), pigs (an amplifying host), and, occasionally, humans (a dead-end host). It is essential to determine blood-feeding patterns to understand the transmission cycle of the disease. Here we review blood-feeding characteristics of the primary JEV vectors Culex tritaeniorhynchus, Culex vishnui, and Culex gelidus based on experimental works and field surveys conducted in Asian countries. Several studies showed that these JEV vectors have an innate preference for cows; however, the former two species often showed higher rates of blood-feeding on pigs than on cows, probably because pigs are more abundant than cows. On the other hand, the latter species Cx. gelidus fed mostly on cows. Thus, the first two species showed higher plasticity to compromise host availability than the last. By reviewing the available articles and based on our relevant studies, it may be deduced that JEV transmission cannot be reduced by zooprophylaxis. We emphasize the need of keeping cows away from the human residences to dampen the human risk of JEV. These primary JEV vector species exhibit pre-biting resting. The adaptive significance of this behavior remains to be unexplored, but it may have a function to avoid defensive attack of host animals. Application of recent quantitative analysis of gene expression in this phase may enable us to come up with novel vector control strategies.
Journal Article
West Nile Virus Mosquito Vectors in North America
In North America, the geographic distribution, ecology, and vectorial capacity of a diverse assemblage of mosquito species belonging to the genus Culex determine patterns of West Nile virus transmission and disease risk. East of the Mississippi River, mostly ornithophagic Culex pipiens L. complex mosquitoes drive intense enzootic transmission with relatively small numbers of human cases. Westward, the presence of highly competent Culex tarsalis (Coquillett) under arid climate and hot summers defines the regions with the highest human risk. West Nile virus human risk distribution is not uniform geographically or temporally within all regions. Notable geographic ‘hotspots' persist with occasional severe outbreaks. Despite two decades of comprehensive research, several questions remain unresolved, such as the role of non-Culex bridge vectors, which are not involved in the enzootic cycle, but may be involved in virus transmission to humans. The absence of bridge vectors also may help to explain the frequent lack of West Nile virus ‘spillover’ into human populations despite very intense enzootic amplification in the eastern United States. This article examines vectorial capacity and the eco-epidemiology of West Nile virus mosquito vectors in four geographic regions of North America and presents some of the unresolved questions.
Journal Article
The Emergence of Japanese Encephalitis Virus in Australia in 2022: Existing Knowledge of Mosquito Vectors
In early 2022, the Japanese encephalitis virus (JEV) was identified as the cause of stillborn and mummified piglets in pig farms in southeastern Australia. Human cases and additional pig farms with infected piglets were subsequently identified across a widespread area encompassing four states. To inform surveillance and control programs, we synthesized existing information on Australian vectors of JEV, much of which was generated in response to incursions of JEV into the northern state of Queensland between 1995 and 2005. Members of the Culex sitiens subgroup, particularly Culex annulirostris, should be considered the primary vectors of JEV in Australia, as they yielded >87% of field detections of JEV, were highly efficient laboratory vectors of the virus, readily fed on pigs and birds (the key amplifying hosts of the virus) when they were available, and are widespread and often occur in large populations. Three introduced species, Culex quinquefasciatus, Culex gelidus and Culex tritaeniorhynchus may also serve as vectors, but more information on their geographical distribution, abundance and bionomics in the Australian context is required. Mosquitoes from other genera, such as Aedes and Verrallina, whilst considered relatively poor vectors, could play a regional or supplemental role in transmission, especially facilitating vertical transmission as a virus overwintering mechanism. Additional factors that could impact JEV transmission, including mosquito survival, dispersal and genetics, are also discussed. Possible directions for investigation are provided, especially in the context of the virus emerging in a region with different mosquito fauna and environmental drivers than northern Australia.
Journal Article
Exploring the range of Culex mosquitoes in Western Argentinean Patagonia, unveiling the presence of Culex pipiens bioform pipiens in South America
Culicids in Argentinean Patagonia are characterized by low species diversity and adaptation to extreme environmental conditions, yet few studies have been conducted in the region. To further assess the occurrence of Culicidae in Western Patagonia, and in particular the presence of
Culex pipiens
bioforms at the southernmost extent of their distribution, immature and adult specimens were collected aboveground across various land uses located in shrubland, steppe, and deciduous forest between 38.96 and 46.55°S. Mosquitoes were reported at 35 of the 105 inspected sites. Five species from the genus
Culex
were identified, all of which were present in the steppe and the forest, while only
Cx. apicinus
and members of the
Cx. pipiens
complex were collected in the shrubland
.
Within the latter, a total of 150 specimens were molecularly identified by PCR amplification of Ace-2 and CQ11 loci. The first-to-date occurrence of bioform
pipiens
in South America is reported, along with the first records of
Cx. quinquefasciatus
signatures in Patagonia. In addition, the distribution of
Cx. acharistus
and
Cx. dolosus
as south as Santa Cruz province is expanded, and the first record of
Cx. eduardoi
in Río Negro province is provided. Immature specimens of
Cx. pipiens
were conspicuous in human-made aquatic habitats (both containers and in the ground), while
Cx. acharistus
was more prominent in artificial containers and
Cx. eduardoi
was mainly in ground habitats, either natural or human-made. These findings provide valuable insights into the distribution and ecological roles of these mosquito species in a region of extreme environmental conditions.
Journal Article