Diurnal Temperature Range and Chikungunya Virus Infection in Invasive Mosquito Vectors

Mosquito thermal tolerance is remarkably constrained across a large climatic range

How mosquitoes may respond to rapid climate warming remains unknown for most species, but will have major consequences for their future distributions, with cascading impacts on human well-being, biodiversity and ecosystem function. We investigated the adaptive potential of a wide-ranging mosquito species, Aedes sierrensis, across a large climatic gradient by conducting a common garden experiment measuring the thermal limits of mosquito life-history traits. Although field-collected populations originated from vastly different thermal environments that spanned over 1200 km, we found limited variation in upper thermal tolerance between populations. In particular, the upper thermal limits of all life-history traits varied by less than 3°C across the species range and, for most traits, did not differ significantly between populations. For one life-history trait-pupal development rate-we did detect significant variation in upper thermal limits between populations, and this variation was strongly correlated with source temperatures, providing evidence of local thermal adaptation for pupal development. However, we found that maximum environmental temperatures across most of the species' range already regularly exceed the highest upper thermal limits estimated under constant temperatures. This result suggests that strategies for coping with and/or avoiding thermal extremes are likely key components of current and future mosquito thermal tolerance.

Identifying Knowledge Gaps through the Systematic Review of Temperature-Driven Variability in the Competence of Aedes aegypti and Ae. albopictus for Chikungunya Virus

Temperature is a well-known effector of several transmission factors of mosquito-borne viruses, including within mosquito dynamics. These dynamics are often characterized by vector competence and the extrinsic incubation period (EIP). Vector competence is the intrinsic ability of a mosquito population to become infected with and transmit a virus, while EIP is the time it takes for the virus to reach the salivary glands and be expectorated following an infectious bloodmeal. Temperatures outside the optimal range act on life traits, decreasing transmission potential, while increasing temperature within the optimal range correlates to increasing vector competence and a decreased EIP. These relatively well-studied effects of other Aedes borne viruses (dengue and Zika) are used to make predictions about transmission efficiency, including the challenges presented by urban heat islands and climate change. However, the knowledge of temperature and chikungunya (CHIKV) dynamics within its two primary vectors-Ae. aegypti and Ae. albopictus-remains less characterized, even though CHIKV remains a virus of public-health importance. Here, we review the literature and summarize the state of the literature on CHIKV and temperature dependence of vector competence and EIP and use these data to demonstrate how the remaining knowledge gap might confound the ability to adequately predict and, thus, prepare for future outbreaks.

Mosquito thermal tolerance is remarkably constrained across a large climatic range

How mosquitoes may respond to rapid climate warming remains unknown for most species, but will have major consequences for their future distributions, with cascading impacts on human well-being, biodiversity, and ecosystem function. We investigated the adaptive potential of a wide-ranging mosquito species, Aedes sierrensis, across a large climatic gradient by conducting a common garden experiment measuring the thermal limits of mosquito life history traits. Although field-collected populations originated from vastly different thermal environments that spanned over 1,200 km, we found remarkably limited variation in upper thermal tolerance between populations, with the upper thermal limits of fitness varying by <1°C across the species range. For one life history trait-pupal development rate-we did detect significant variation in upper thermal limits between populations, and this variation was strongly correlated with source temperatures, providing evidence of local thermal adaptation for pupal development. However, we found environmental temperatures already regularly exceed our highest estimated upper thermal limits throughout most of the species range, suggesting limited potential for mosquito thermal tolerance to evolve on pace with warming. Strategies for avoiding high temperatures such as diapause, phenological shifts, and behavioral thermoregulation are likely important for mosquito persistence.

Population genomic analyses reveal population structure and major hubs of invasive Anopheles stephensi in the Horn of Africa

Anopheles stephensi invasion in the Horn of Africa (HoA) poses a substantial risk of increased malaria disease burden in the region. An understanding of the history of introduction(s), establishment(s) and potential A. stephensi sources in the HoA is needed to predict future expansions and establish where they may be effectively controlled. To this end, we take a landscape genomic approach to assess A. stephensi origins and spread throughout the HoA, information essential for vector control. Specifically, we assayed 2070 genome-wide single nucleotide polymorphisms across 214 samples spanning 13 populations of A. stephensi from Ethiopia and Somaliland collected in 2018 and 2020, respectively. Principal component and genetic ancestry analyses revealed clustering that followed an isolation-by-distance pattern, with genetic divergence among the Ethiopian samples significantly correlating with geographical distance. Additionally, genetic relatedness was observed between the northeastern and east central Ethiopian A. stephensi populations and the Somaliland A. stephensi populations. These results reveal population differentiation and genetic connectivity within HoA A. stephensi populations. Furthermore, based on genetic network analysis, we uncovered that Dire Dawa, the site of a spring 2022 malaria outbreak, was one of the major hubs from which sequential founder events occurred in the rest of the eastern Ethiopian region. These findings can be useful for the selection of sites for heightened control to prevent future malaria outbreaks. Finally, we did not detect significant genotype-environmental associations, potentially due to the recency of their colonization and/or other anthropogenic factors leading to the initial spread and establishment of A. stephensi. Our study highlights how coupling genomic data at landscape levels can shed light into even ongoing invasions.

Temperature and transmission of chikungunya, dengue, and Zika viruses: A systematic review of experimental studies on Aedes aegypti and Aedes albopictus

Mosquito-borne viruses are leading causes of morbidity and mortality in many parts of the world. In recent years, modelling studies have shown that climate change strongly influences vector-borne disease transmission, particularly rising temperatures. As a result, the risk of epidemics has increased, posing a significant public health risk. This review aims to summarize all published laboratory experimental studies carried out over the years to determine the impact of temperature on the transmission of arboviruses by the mosquito vector. Given their high public health importance, we focus on dengue, chikungunya, and Zika viruses, which are transmitted by the mosquitoes Aedes aegypti and Aedes albopictus. Following PRISMA guidelines, 34 papers were included in this systematic review. Most studies found that increasing temperatures result in higher rates of infection, dissemination, and transmission of these viruses in mosquitoes, although several studies had differing findings. Overall, the studies reviewed here suggest that rising temperatures due to climate change would alter the vector competence of mosquitoes to increase epidemic risk, but that some critical research gaps remain.

Estimating the Impact of Consecutive Blood Meals on Vector Competence of Aedes albopictus for Chikungunya Virus

The continuous expansion of Aedes albopictus in Europe and the increases in autochthonous arboviruses transmissions in the region urge a better understanding of the virus transmission dynamic. Recent work described enhanced chikungunya virus (CHIKV) dissemination in Aedes aegypti mosquitoes exposed to a virus-free blood meal three days after their infection with CHIKV. Our study investigated the impact of a second blood meal on the vector competence of Ae. albopictus from southern Switzerland infected with CHIKV. Seven-day-old Ae. albopictus females were exposed to CHIKV-spiked blood and incubated at constant (27 °C) and fluctuating (14-28 °C) temperatures. Four days post-infection (dpi), some of these females were re-fed with a non-infectious blood meal. Virus infectivity, dissemination, transmission rate, and efficiency were investigated at seven and ten dpi. No enhanced dissemination rate was observed among females fed a second time; however, re-fed females have shown higher transmission efficiency than those fed only once after seven days post-infection and incubated under a fluctuating temperature regime. Vector competence for CHIKV was confirmed in Ae. albopictus from southern Switzerland. We did not observe an increase in dissemination rates among mosquitoes fed a second time (second blood meal), regardless of the temperature regime.

Seasonality influences key physiological components contributing to Culex pipiens vector competence

Mosquitoes are the most important animal vector of disease on the planet, transmitting a variety of pathogens of both medical and veterinary importance. Mosquito-borne diseases display distinct seasonal patterns driven by both environmental and biological variables. However, an important, yet unexplored component of these patterns is the potential for seasonal influences on mosquito physiology that may ultimately influence vector competence. To address this question, we selected Culex pipiens, a primary vector of the West Nile virus (WNV) in the temperate United States, to examine the seasonal impacts on mosquito physiology by examining known immune and bacterial components implicated in mosquito arbovirus infection. Semi-field experiments were performed under spring, summer, and late-summer conditions, corresponding to historically low-, medium-, and high-intensity periods of WNV transmission, respectively. Through these experiments, we observed differences in the expression of immune genes and RNA interference (RNAi) pathway components, as well as changes in the distribution and abundance of Wolbachia in the mosquitoes across seasonal cohorts. Together, these findings support the conclusion that seasonal changes significantly influence mosquito physiology and components of the mosquito microbiome, suggesting that seasonality may impact mosquito susceptibility to pathogen infection, which could account for the temporal patterns in mosquito-borne disease transmission.

Deciphering the Tissue Tropism of the RNA Viromes Harbored by Field-Collected Anopheles sinensis and Culex quinquefasciatus

Arboviruses and insect-specific viruses (ISVs) are two major types of viruses harbored by mosquitoes that are distinguished by the involvement of vertebrate hosts in their transmission cycles. While intensive studies have focused on the transmission, tissue tropism, and evolution of arboviruses, these characteristics are poorly investigated in ISVs, which dominate the mosquito virome. Therefore, in this study, we collected two mosquito species, Anopheles sinensis and Culex quinquefasciatus, in the field and used a metatranscriptomics approach to characterize their RNA viromes in different tissues, such as the midgut, legs, salivary gland, eggs, and the remainder of the carcass. Blood-engorged individuals of these species were captured in 3 locations, and 60 mosquitoes were pooled from each species and location. A total of 40 viral species from diverse viral taxa associated with all viral RNA genome types were identified, among which 19 were newly identified in this study. According to the current viral taxonomy, some of these viruses, such as Yancheng Anopheles associated virus 2 (Narnaviridae) and Jiangsu Anopheles-related virus (Ghabrivirales), were novel. The two investigated mosquito species generally harbored distinct viromes. Nevertheless, the viruses were generally shared among different tissue types to various degrees. Specifically, the eggs possessed a viral community with significantly lower diversity and abundance than those in other tissues, whereas the legs and salivary glands exhibited higher viral abundance. The compositions and distributions of the viromes of different mosquito tissues were demonstrated for the first time in our study, providing important insight into the virome dynamics within individual mosquitoes. IMPORTANCE ISVs are considered to be ancestral to arboviruses. Because of their medical importance, arboviruses have been well studied from the aspects of their transmission mode, evolution of dual-host tropism, and genetic dynamics within mosquito vectors. However, the mode of ISV maintenance is poorly understood, even though many novel ISVs have been identified with the emergence of sequencing technology. In our study, in addition to the identification of a diverse virus community, the tissue tropism of RNA viromes harbored by two field-collected mosquito species was demonstrated for the first time. According to the results, the virus communities of different tissues, such as the salivary glands, midguts, legs, and eggs, can help us understand the evolution, transmission routes, and maintenance modes of mosquito-specific viruses in nature.

Emergence potential of mosquito-borne arboviruses from the Florida Everglades

The Greater Everglades Region of South Florida is one of the largest natural wetlands and the only subtropical ecosystem found in the continental United States. Mosquitoes are seasonally abundant in the Everglades where several potentially pathogenic mosquito-borne arboviruses are maintained in natural transmission cycles involving vector-competent mosquitoes and reservoir-competent vertebrate hosts. The fragile nature of this ecosystem is vulnerable to many sources of environmental change, including a wetlands restoration project, climate change, invasive species and residential development. In this study, we obtained baseline data on the distribution and abundance of both mosquitos and arboviruses occurring in the southern Everglades region during the summer months of 2013, when water levels were high, and in 2014, when water levels were low. A total of 367,060 mosquitoes were collected with CO₂-baited CDC light traps at 105 collection sites stratified among the major landscape features found in Everglades National Park, Big Cypress National Preserve, Fakahatchee State Park Preserve and Picayune State Forest, an area already undergoing restoration. A total of 2,010 pools of taxonomically identified mosquitoes were cultured for arbovirus isolation and identification. Seven vertebrate arboviruses were isolated: Everglades virus, Tensaw virus, Shark River virus, Gumbo Limbo virus, Mahogany Hammock virus, Keystone virus, and St. Louis encephalitis virus. Except for Tensaw virus, which was absent in 2013, the remaining viruses were found to be most prevalent in hardwood hammocks and in Fakahatchee, less prevalent in mangroves and pinelands, and absent in cypress and sawgrass. In contrast, in the summer of 2014 when water levels were lower, these arboviruses were far less prevalent and only found in hardwood hammocks, but Tensaw virus was present in cypress, sawgrass, pinelands, and a recently burned site. Major environmental changes are anticipated in the Everglades, many of which will result in increased water levels. How these might lead to the emergence of arboviruses potentially pathogenic to both humans and wildlife is discussed.

The Effect of Fluctuating Incubation Temperatures on West Nile Virus Infection in Culex Mosquitoes

Temperature plays a significant role in the vector competence, extrinsic incubation period, and intensity of infection of arboviruses within mosquito vectors. Most laboratory infection studies use static incubation temperatures that may not accurately reflect daily temperature ranges (DTR) to which mosquitoes are exposed. This could potentially compromise the application of results to real world scenarios. We evaluated the effect of fluctuating DTR versus static temperature treatments on the infection, dissemination, and transmission rates and viral titers of Culex tarsalis and Culex quinquefasciatus mosquitoes for West Nile virus. Two DTR regimens were tested including an 11 and 15 °C range, both fluctuating around an average temperature of 28 °C. Overall, no significant differences were found between DTR and static treatments for infection, dissemination, or transmission rates for either species. However, significant treatment differences were identified for both Cx. tarsalis and Cx. quinquefasciatus viral titers. These effects were species-specific and most prominent later in the infection. These results indicate that future studies on WNV infections in Culex mosquitoes should consider employing realistic DTRs to reflect interactions most accurately between the virus, vector, and environment.

Shifts in mosquito diversity and abundance along a gradient from oil palm plantations to conterminous forests in Borneo

Deforestation precipitates spillover of enzootic, vector-borne viruses into humans, but specific mechanisms for this effect have rarely been investigated. Expansion of oil palm cultivation is a major driver of deforestation. Here, we demonstrate that mosquito abundance decreased over ten stepwise distances from interior forest into conterminous palm plantations in Borneo. Diversity in interior plantation narrowed to one species, Aedes albopictus, a potential bridge vector for spillover of multiple viruses. A. albopictus was equally abundant across all distances in forests, forest-plantation edge, and plantations, while A. niveus, a known vector of sylvatic dengue virus, was found only in forests. A. albopictus collections were significantly female-biased in plantation but not in edge or forest. Our data reveal that the likelihood of encountering any mosquito is greater in interior forest and edge than plantation, while the likelihood of encountering A. albopictus is equivalent across the gradient sampled from interior plantation to interior forest.

A Method for Repeated, Longitudinal Sampling of Individual Aedes aegypti for Transmission Potential of Arboviruses

Mosquito-borne viruses are the cause of significant morbidity and mortality worldwide, especially in low- and middle-income countries. Assessing risk for viral transmission often involves characterization of the vector competence of vector-virus pairings. The most common determination of vector competence uses discreet, terminal time points, which cannot be used to investigate variation in transmission aspects, such as biting behavior, over time. Here, we present a novel method to longitudinally measure individual biting behavior and Zika virus (ZIKV) transmission. Individual mosquitoes were exposed to ZIKV, and from 9 to 24 days post-exposure, individuals were each offered a 180 μL bloodmeal every other day. Biting behavior was observed and characterized as either active probing, feeding, or no bite. The bloodmeal was then collected, spun down, serum collected, and tested for ZIKV RNA via qRT-PCR to determine individuals' vector competence over time. This included whether transmission to the bloodmeal was successful and the titer of expectorated virus. Additionally, serum was inoculated onto Vero cells in order to determine infectiousness of positive recovered sera. Results demonstrate heterogeneity in not only biting patterns but expectorated viral titers among individual mosquitoes over time. These findings demonstrate that the act of transmission is a complex process governed by mosquito behavior and mosquito-virus interaction, and herein we offer a method to investigate this phenomenon.

Dynamics of Transmission of Urban Arbovirus Dengue, Zika and Chikungunya in Southwestern Region of Bahia, Brazil

Arboviral diseases are disseminated all over the world. In Brazil, they remain neglected, alerting public authorities to possible outbreaks. Over here, we report the epidemiological indicators of Dengue from 2010 to 2015, Zika between 2015 and 2016, and Chikungunya from 2014 to 2016, within 19 municipalities of Southwestern Region of Bahia, Brazil. The data were collected from Brazilian national public information systems (SISFAD, SINAN, and IBGE) and by Endemic Control Agents. The analysis consisted of a description of vector characteristics, Home Infestation Index and characterization of human reported cases. The years 2011 and 2013 were recorded as having the highest frequencies of positive properties for the presence of the arboviruse vectors. Most municipalities presented high annual values of Home Infestation Index indicating an alert situation (62.28%). In the evaluated period, there were (i) 9,196 cases of Dengue, (ii) 636 cases of Zika and (iii) 224 cases of Chikungunya reported. This is the first report of the epidemiological characteristics of these arboviruses in the 19 municipalities of Bahia. It is believed that the data collected may contribute to public health policies aimed at controlling future epidemics of these arboviruses.

High-Throughput Method for Detection of Arbovirus Infection of Saliva in Mosquitoes Aedes aegypti and Ae. albopictus

Vector competence refers to the ability of a vector to acquire, maintain, and transmit a pathogen. Collecting mosquito saliva in medium-filled capillary tubes has become the standard for approximating arbovirus transmission. However, this method is time-consuming and labor-intensive. Here we compare the capillary tube method to an alternative high-throughput detection method the collection of saliva on paper cards saturated with honey, with (FTA card) and without (filter paper) reagents for the preservation of nucleic acid for Aedes aegypti and Aedes albopictus mosquitoes infected with two emerging genotypes of the chikungunya virus (CHIKV). Model results showed that the Asian genotype CHIKV dissemination in the harvested legs of both Ae. aegypti and Ae. albopictus increased the odds of females having a positive salivary infection and higher salivary viral titers, while for the IOL genotype the same effect was observed only for Ae. aegypti. Of the three tested detection methods, the FTA card was significantly more effective at detecting infected saliva of Ae. aegypti and Ae. albopictus females than the capillary tube and filter paper was as effective as the capillary tube for the Asian genotype. We did not find significant effects of the detection method in detecting higher viral titer for both Asian and IOL genotypes. Our results are discussed in light of the limitations of the different tested detection methods.

Age-structured vectorial capacity reveals timing, not magnitude of within-mosquito dynamics is critical for arbovirus fitness assessment

Background

Transmission dynamics of arboviruses like Zika virus are often evaluated by vector competence (the proportion of infectious vectors given exposure) and the extrinsic incubation period (EIP, the time it takes for a vector to become infectious), but vector age is another critical driver of transmission dynamics. Vectorial capacity (VC) is a measure of transmission potential of a vector-pathogen system, but how these three components, EIP, vector competence and vector age, affect VC in concert still needs study.

Methods

The interaction of vector competence, EIP, and mosquito age at the time of infection acquisition (Age_acquisition) was experimentally measured in an Aedes aegypti-ZIKV model system, as well as the age-dependence of probability of survival and the willingness to bite. An age-structured vectorial capacity framework (VC_age) was then developed using both EIP_Min and EIP_Max, defined as the time to first observed minimum proportion of transmitting mosquitoes and the time to observed maximum proportion of transmitting mosquitoes.

Results

The within-mosquito dynamics of vector competence/EIP were not significant among treatments where mosquitoes were exposed at different ages. However, VC_age revealed: (i) age-dependence in vector-virus interactions is important for transmission success; (ii) lower vector competence but at shorter EIPs was sufficient for transmission perpetuation; and (iii) R₀ may be overestimated by using non-age-structured VC.

Conclusions

The results indicate that ultimately the temporal component of the virus-vector dynamics is most critical, especially when exposure occurred at advanced mosquito age. While our study is limited to a single virus-vector system, and a multitude of other factors affect both vector competence and mosquito mortality, our methods can be extrapolated to these other scenarios. Results indicate that how ‘highly’ or ‘negligibly’ competent vectors are categorized may need adjustment.

Arbovirus vectors of epidemiological concern in the Americas: A scoping review of entomological studies on Zika, dengue and chikungunya virus vectors

Background

Three arthropod-borne viruses (arboviruses) causing human disease have been the focus of a large number of studies in the Americas since 2013 due to their global spread and epidemiological impacts: Zika, dengue, and chikungunya viruses. A large proportion of infections by these viruses are asymptomatic. However, all three viruses are associated with moderate to severe health consequences in a small proportion of cases. Two mosquito species, Aedes aegypti and Aedes albopictus, are among the world’s most prominent arboviral vectors, and are known vectors for all three viruses in the Americas.

Objectives

This review summarizes the state of the entomological literature surrounding the mosquito vectors of Zika, dengue and chikungunya viruses and factors affecting virus transmission. The rationale of the review was to identify and characterize entomological studies that have been conducted in the Americas since the introduction of chikungunya virus in 2013, encompassing a period of arbovirus co-circulation, and guide future research based on identified knowledge gaps.

Methods

The preliminary search for this review was conducted on PubMed (National Library of Health, Bethesda, MD, United States). The search included the terms ‘zika’ OR ‘dengue’ OR ‘chikungunya’ AND ‘vector’ OR ‘Aedes aegypti’ OR ‘Aedes albopictus’. The search was conducted on March 1^st of 2018, and included all studies since January 1^st of 2013.

Results

A total of 96 studies were included in the scoping review after initial screening and subsequent exclusion of out-of-scope studies, secondary data publications, and studies unavailable in English language.

Key findings

We observed a steady increase in number of publications, from 2013 to 2018, with half of all studies published from January 2017 to March 2018. Interestingly, information on Zika virus vector species composition was abundant, but sparse on Zika virus transmission dynamics. Few studies examined natural infection rates of Zika virus, vertical transmission, or co-infection with other viruses. This is in contrast to the wealth of research available on natural infection and co-infection for dengue and chikungunya viruses, although vertical transmission research was sparse for all three viruses.

Correlation of the basic reproduction number (R0) and eco-environmental variables in Colombian municipalities with chikungunya outbreaks during 2014-2016

Chikungunya virus (CHIKV) emerged in Colombia in 2014 into a population presumed fully susceptible. This resulted in a quick and intense spread across Colombia, resulting in an epidemic that affected an estimated 450,000 people. The reported Colombian cases accounted for over 49% of all the cases reported to the PAHO. Eco-environmental factors are known to be associated with the spread of arboviruses such as CHIKV, and likely contribute to the differences in transmission profiles that were observed across several municipalities. To determine the association of eco-environmental factors and CHIKV, the basic reproduction number (R₀) in 85 municipalities, which accounted for 65.6% of reported CHIKV cases in Colombia, was estimated. Estimates of R₀ ranged from 1 to 9, where over 76% of municipalities had R₀ values between 1 and 2. When we looked at the distribution of R₀, the cumulative proportions were 20% with R₀>2, 14% with R₀>3, and 9% with R₀>4. Next, we determined that there were different patterns of correlation between environmental and/or ecological variables and R₀ when we considered different R₀ lower-thresholds. Broadly, we found that temperature-related variables are significantly and positively correlated to R₀ regardless of the lower threshold, while other variables like duration of outbreak and size of the urban area are inversely related to R₀. Specifically, we conclude that high values of temperature-related variables where R₀ > 1 will result in a fast growth of cases in a shorter time period (with faster cessation of outbreak transmission) but will result overall in a fewer total cases compared to outbreak areas (R₀ > 1, but classified as lower). Thus, in the absence of vector control, a less explosive outbreak may be more advantageous for the virus in terms of transmission.

Chikungunya virus emerged in Colombia in 2014 into a presumed fully susceptible population and rapidly spread in the country. Numerous municipalities were differently affected by this virus across the country. The main purpose of this work was understanding why those differences were produced and, in turn, what are the variables addressing such differences. For this purpose, we estimated for 85 municipalities the basic reproduction number (R₀), a crucial parameter to understand epidemics that is expressed as the number of secondary cases produced by a primary case. Such parameter was correlated with numerous variables resulting evident a crucial role of temperature in the increase of R₀. Interestingly, other variables like size of the urban area and cases showed to be negatively correlated with R₀. Results shows that high temperatures produce high R₀, but those municipalities that showed high R₀ showed an explosive epidemic with faster increase of cases that ceased equally fast, so the duration of epidemic is short producing small amount of cases. In this way, more cases are expected with municipalities with lower values of R₀, which is suitably explained by the tortoise-hare model, where the less explosive outbreak results to be more advantageous for the virus.

A predominant dengue virus-1 endemic strain and the vector competence of Aedes albopictus from Guangzhou City, China

INTRODUCTION

Dengue has been a serious public health burden and dengue virus-1 (DENV-1) is the predominant strain in Guangdong province, China. Differences exist in the transmission dynamics amongAedes albopictus and DENV in different geographical regions. However, little is known about the vector competence of indigenous Aedes albopictus for the predominant dengue strain in Guangdong province, China.

METHODOLOGY

In this study, the field-derivedAedes albopictus collected from Guangzhou city, Guangdong province were infected with the predominant DENV endemic strain DENV-1 GZ201401 by feeding on serially diluted artificial infectious blood or infected suckling mice. DENV-infected mosquitoes were evaluated for viral load at five-time intervals in three tissues, the head, body and legs using reverse transcription-quantitative PCR (RT-qPCR). The vertical transmission of DENV in Ades albopictus was also analysed. Suckling mice were used to assess the transmission of DENV by Aedes albopictus.

RESULTS

There was no difference in infection rates between mosquitoes infected by infected suckling mice or by artificial infectious blood. The proportion of DENV-1 positive mosquitoes increased over time after an infectious blood meal, but there was no difference in the positive rate beyond 7days after the blood meal. The positive rate of DENV-1 infected mosquitoes increased with the DENV titer in the blood meal. Most of the infections the infected mosquitoes were disseminated more than 7 days after imbibing the artificial infectious blood. The median infective doses (MID50) at 7,14,21 and 28 days after artificial infectious blood meal [7, 14, 21 and 28 days post-infection (dpi)] were 7.86 × 10⁷, 1.57 × 10⁷, 6.39 × 10⁶ and 4.96 × 10⁶ TCID₅₀ (50% tissue culture infective dose)/ml, respectively. The mosquitoes can spread DENV-1 GZ201401 to hosts as early as 3 dpi. The vertical transmission of DENV-1 was documented with a cumulative rate of 17.61%.

CONCLUSION

Our results demonstrated that Aedes albopictus mosquitoes are competent vectors for DENV-1, and are capable of maintaining autochthonous dengue outbreaks in Guangdong province, China, which may have been promoted by vertical transmission.

The Role of Temperature in Transmission of Zoonotic Arboviruses

We reviewed the literature on the role of temperature in transmission of zoonotic arboviruses. Vector competence is affected by both direct and indirect effects of temperature, and generally increases with increasing temperature, but results may vary by vector species, population, and viral strain. Temperature additionally has a significant influence on life history traits of vectors at both immature and adult life stages, and for important behaviors such as blood-feeding and mating. Similar to vector competence, temperature effects on life history traits can vary by species and population. Vector, host, and viral distributions are all affected by temperature, and are generally expected to change with increased temperatures predicted under climate change. Arboviruses are generally expected to shift poleward and to higher elevations under climate change, yet significant variability on fine geographic scales is likely. Temperature effects are generally unimodal, with increases in abundance up to an optimum, and then decreases at high temperatures. Improved vector distribution information could facilitate future distribution modeling. A wide variety of approaches have been used to model viral distributions, although most research has focused on the West Nile virus. Direct temperature effects are frequently observed, as are indirect effects, such as through droughts, where temperature interacts with rainfall. Thermal biology approaches hold much promise for syntheses across viruses, vectors, and hosts, yet future studies must consider the specificity of interactions and the dynamic nature of evolving biological systems.

Temperature impacts on dengue emergence in the United States: Investigating the role of seasonality and climate change

Tropical mosquito-borne viruses have been expanding into more temperate regions in recent decades. This is partly due to the coupled effects of temperature on mosquito life history traits and viral infection dynamics and warming surface temperatures, resulting in more suitable conditions for vectors and virus transmission. In this study, we use a deterministic ordinary differential equations model to investigate how seasonal and diurnal temperature fluctuations affect the potential for dengue transmission in six U.S. cities. We specifically consider temperature-dependent mosquito larval development, adult mosquito mortality, and the extrinsic incubation period of the virus. We show that the ability of introductions to lead to outbreaks depends upon the relationship between a city's temperature profile and the time of year at which the initial case is introduced. We also investigate how the potential for outbreaks changes with predicted future increases in mean temperatures due to climate change. We find that climate change will likely lead to increases in suitability for dengue transmission and will increase the periods of the year in which introductions may lead to outbreaks, particularly in cities that typically have mild winters and warm summers, such as New Orleans, Louisiana, and El Paso, Texas. We discuss our results in the context of temperature heterogeneity within and across cities and how these differences may impact the potential for dengue emergence given present day and predicted future temperatures.

Do we know how mosquito disease vectors will respond to climate change?

Mosquito-borne diseases are on the rise globally. Besides invasion processes and the increasing connectivity between distant regions through the trade of goods and human mobility, climate change is seen as an important driver for changing the likelihood of occurrence of vectors and diseases, respectively. Ectothermic insects respond directly to thermal conditions and thus we can expect them to follow climatic trends. However, a variety of species and different stages in their life cycles need to be considered. Here, we review the current literature in this field and disentangle the state of knowledge and the challenges and open questions for future research. The integration of diurnal temperature ranges in prospective experimental studies will strongly improve the knowledge of mosquitoes’ ecology and mosquito-borne disease transmission for temperate regions in particular. In addition, invasive mosquitoes are known to rapidly adapt to the climatic conditions, but the underlying processes are not yet fully understood.

Vector Competence of Culicoides sonorensis (Diptera: Ceratopogonidae) for Epizootic Hemorrhagic Disease Virus Serotype 2 Strains from Canada and Florida

Epizootic hemorrhagic disease virus (EHDV), an Orbivirus transmitted by Culicoides spp. vectors, is represented by seven serotypes and numerous strains worldwide. While studies comparing vector competence between serotypes exist, studies between viral strains are lacking. In this study, we examined the rates of infection, dissemination, and transmission of two strains of EHDV-2 orally fed to the known vector, Culicoides sonorensis Wirth & Jones. Culicoides sonorensis cohorts were fed an infectious blood meal containing EHDV-2 strains from either Alberta, Canada (Can-Alberta) or Florida (5.5 log₁₀ PFUe/mL) and tested for the vector’s susceptibility to infection and dissemination. In addition, transmission rates of the virus were assessed and compared using capillary tube and honey card methods. Our results show that the Florida strain had higher infection and dissemination rates than the Can-Alberta strain in spite of the Florida strain having significantly lower viral titers in C. sonorensis bodies, legs, and saliva than the Can-Alberta strain. Overall transmission rates were not significantly different between the two strains but varied significantly between the methods used. These findings suggest that the consequences of EHDV infection in C. sonorensis vary between virus strains and have huge implications in future vector competence studies involving Culicoides species and Orbiviruses.

Daily temperature variation magnifies the toxicity of a mixture consisting of a chemical pesticide and a biopesticide in a vector mosquito

While many studies on the toxicity of pesticides looked at the effects of a higher mean temperature, effects of the realistic scenario of daily temperature variation are understudied. Moreover, despite the increasing interest for the toxicity of pesticide mixtures how this is influenced by temperature has been largely ignored. We tested whether daily temperature variation (DTV) magnifies the toxicity of two pesticides with a different mode of action, the organophosphate pesticide chlorpyrifos (CPF) and the biopesticide Bacillus thuringiensis var. israelensis (Bti), and of their mixture in the vector mosquito Culex pipiens. Single exposure to CPF and Bti increased mortality and reduced female development time, and exposure to CPF also increased female wing length. DTV was not lethal and did not change the toxicity of the individual pesticides. Yet, a key novel finding was that high DTV increased the mortality of the mixture by changing the interaction between both pesticides from additive to synergistic. Given that in nature daily temperature variation is omnipresent, this is important both for vector control and for ecological risk assessment. The higher toxicity of the mixture at high DTV compared to the typically used constant test temperatures in the laboratory urges caution when evaluating the environmental impact of pesticide mixtures.

Investigating the probability of establishment of Zika virus and detection through mosquito surveillance under different temperature conditions

Because of the increasing threat that Zika virus (ZIKV) poses to more sub-tropical area due to increased global travel, there is a need for better understanding of the effect(s) of temperature on the establishment potential of ZIKV within these subtropical, temperate, and/or seasonal Ae. aegypti populations. The first step to determining risk establishment of ZIKV in these regions is to assess ZIKV's ability to infect mosquitoes at less tropical temperatures, and thus be detected through common surveillance programs. To that end, the effect of two rearing temperatures (RT) and extrinsic incubation temperatures (EIT) on infection and dissemination rates was evaluated, as well as the interactions of such. Total, there were four combinations (RT24-EIT24, RT24-EIT28, RT28-EIT24, RT28-EIT28). Further, a stochastic SEIR framework was adapted to determine whether observed data could lead to differential success of establishment of ZIKV in naive mosquito populations. There was no consistent pattern in significant differences found across treatments for either infection or dissemination rates (p>0.05), where only a significant difference was found in infection rates between RT24-EIT24 (44%) and RT28-EIT24 (82.6%). Across all temperature conditions, the model predicted between a 76.4% and 95.4% chance of successful establishment of ZIKV in naive mosquito populations under model assumptions. We further show that excluding the maximum observed infection and dissemination rates likely overestimates the probability of local establishment of ZIKV. These results indicate that 1) there is no straightforward relationship between RT, EIT, and infection/dissemination rates, 2) in more temperate climates, ZIKV may still have the ability to establish in populations of Aedes aegypti, 3) despite an overall lack of significant differences in infection/dissemination rates, temperature may still alter the kinetics of ZIKV within the mosquito enough to affect the likelihood of infection establishment and detection within the context of mosquito surveillance programs, and 4) both the temporal and magnitude qualities of vector competence are necessary for parameterization of within-mosquito virus kinetics.

Chikungunya virus vector competency of Brazilian and Florida mosquito vectors

Chikungunya virus is a vector-borne alphavirus transmitted by the bites of infected female Ae. aegypti and Ae. albopictus. In Brazil between 2014 and 2016 almost 320 thousand autochthonous human cases were reported and in Florida numerous imported CHIKV viremic cases (> 3,800) demonstrate the potential high risk to establishment of local transmission. In the present study, we carried out a series of experiments to determine the viral dissemination and transmission rates of different Brazilian and Florida populations of Ae. aegypti and Ae. albopictus at 2, 5, and 13 days post-infection for the emergent Asian genotype of CHIKV. Our results show that all tested populations of Ae. aegypti and Ae. albopictus have a high proportion (> 0.80) of individuals with disseminated infection as early as 2 days-post exposure. We found no significant treatment effects of mosquito population origin effects on viral dissemination rates. Transmission rates had a heterogeneous pattern, with US Ae. aegypti and Brazilian Ae. albopictus having the highest proportion of individuals with successful infection (respectively 0.50 and 0.82 as early as 2 days-post infection). Model results found significant effects of population origin, population origin x species, population origin x days post-infection and population origin x species x days post infection.