Temperature and age, individually and interactively, shape the size, weight, and body composition of adult female mosquitoes

Phenotypic and genetic variation of Aedes albopictus (Diptera: Culicidae) in Thailand and its global relationships: Insights from wing morphometric and mitochondrial COI gene analyses

Aedes albopictus (Diptera: Culicidae), commonly known as the Asian tiger mosquito, is an important vector transmitting dangerous arboviruses to humans. This study investigated the phenotypic and genetic variation of this species in Thailand through wing geometric morphometric (GM) and mitochondrial cytochrome c oxidase subunit I (COI) gene sequence analyses. A total of 236 Ae. albopictus specimens from 12 populations in Thailand and 89 specimens from invasive populations in Florida, Hawaii and Brazil underwent wing GM analysis. The centroid size (CS) of Ae. albopictus populations in Thailand ranged from 2.00 mm in Bangkok to 2.36 mm in Chanthaburi, while in invasive populations, CS varied from 2.25 mm in Brazil to 2.47 mm in Florida. Pairwise comparisons of wing shape revealed significant differences for most population pairs, with distances ranging from 1.63 to 10.02. The clustering tree indicated distant relationships in wing shape between native and invasive populations. Additionally, partial COI gene sequences were amplified from 108 specimens, revealing a mean haplotype diversity of 0.842 ± 0.025 and a mean nucleotide diversity of 0.002 ± 0.001. The results from neutral Tajima's D and Fu's Fs tests indicated negative and statistically significant values (-2.159 and -33.846, respectively), suggesting population expansion. Further examination of haplotype relationships between Thailand and other countries identified two distinct groups: a Southeast Asia group, with Thai haplotypes clustered exclusively within it, and a non-Southeast Asia group. These findings highlight the phenotypic and genetic variation of Ae. albopictus in Thailand, providing essential insights for disease control strategies and tracing the mosquito's origins across regions.

Cellular immune senescence in mosquitoes accelerates when the temperature is warmer

Mosquitoes are poikilotherms and ectotherms, so their body temperature is predicated by the temperature of their environment. As the temperature rises, development and metabolism quicken. At the same time, immune strength weakens with aging, a process called senescence. Aging can be characterized as a function of time (chronological age) or as a function of how well the body operates (physiological age), and we predict that warmer temperature decouples chronological and physiological aging, accelerating immune senescence. We evaluated how warmer temperature and aging interact to alter cellular immunity in the mosquito, Anopheles gambiae, by rearing them at three temperatures and quantifying the number of immune cells, called hemocytes, and their phagocytic activity at four ages. We discovered that the number of circulating hemocytes decreases with warmer temperature and aging, and that the aging-dependent decrease occurs faster when the temperature is warmer. However, the number of sessile hemocytes attached to the dorsal abdominal wall increases with infection and warmer temperature but decreases with aging. When a mosquito is infected, the aging-dependent decrease in the number of sessile hemocytes occurs faster when the temperature is warmer. Although the number of hemocytes decreases with aging, the phagocytic activity of individual hemocytes increases, with the aging-dependent increase occurring earlier when the temperature is warmer. Overall, warmer temperature accelerates senescence of the cellular immune response in mosquitoes, which has implications for how poikilotherms and ectotherms fight infections as they age in our warming world.

Resources Modulate Developmental Shifts but Not Infection Tolerance Upon Co-Infection in an Insect System

Energetic resources within organisms fuel both parasite growth and immune responses against them, but it is unclear whether energy allocation is sufficient to explain changes in infection outcomes under the threat of multiple parasites. We manipulated diet in flour beetles (Tribolium confusum) infected with two natural parasites and used a combination of transcriptomic and phenotypic assays to investigate the role of resources in shifting metabolic and immune responses after single and co-infection. Our results suggest that relatively benign, single-celled, eukaryotic gregarine parasites alter the within-host energetic environment and, by extension, juvenile development time, in a diet-dependent manner. While they do not affect host resistance to acute bacterial infection, the mRNA-seq results reveal that they stimulate the expression of an alternative set of immune genes and promote damage to the gut, ultimately contributing to reduced survival regardless of diet. Thus, energy allocation is not sufficient to explain the immunological contribution to co-infection outcomes, emphasising the importance of mechanistic insight for predicting the impact of co-infection across levels of biological organisation.

Reproductive Trade-Offs in Culex pipiens: Effects of CYV Infection and Delayed Mating

Arbovirus control strategies often target vector reproductive dynamics, with insect-specific viruses (ISVs) like Culex Y virus (CYV) offering potential as eco-friendly approaches by influencing mosquito reproduction without affecting humans or animals. This study investigated the interplay between autogeny, anautogeny, nutrient availability, and viral infection and their effect on reproductive success in Culex pipiens biotype molestus and Culex pipiens quinquefasciatus. CYV infection had a minimal impact on mosquito reproductive parameters, even after a five-day incubation period. Autogeny enabled Cx. pipiens biotype molestus to produce eggs without blood meals, yet older females (3-5 days post-emergence) showed reduced reproductive success unrelated to nutrient deficiency, as blood-feeding did not restore their egg production. These findings demonstrate that age affects reproductive success in Cx. pipiens biotype molestus but not in Cx. pipiens quinquefasciatus and suggest that CYV has negligible direct effects on mosquito reproduction. This work enhances our understanding of ISV biology and vector ecology, supporting the development of innovative, sustainable arbovirus-control strategies.

[The effects of climate change on the emergence of dengue]

In recent decades, dengue has become a global issue due to its rapid spread and significant public health impact. Climate change is recognized as a key factor in the geographical spread of dengue and its vectors. Climate change affects dengue transmission through changes in temperature and precipitation, which affect both vectors and arboviruses. Climate change can also disrupt human migration patterns facilitating the spread of the virus and the invasion of vectors into new regions. Understanding the impact of climate change on dengue and its vectors is essential for developing strategies to prevent and control the disease. Appropriate mosquito control strategies, enhanced epidemiological surveillance and tailored public health systems are needed to mitigate the increasing burden of dengue in the context of climate change.

Thermal variation influences the transcriptome of the major malaria vector Anopheles stephensi

The distribution and abundance of ectothermic mosquitoes are strongly affected by temperature, but mechanisms remain unexplored. We describe the effect of temperature on the transcriptome of Anopheles stephensi, an invasive vector of human malaria. Adult females were maintained across a range of mean temperatures (20 °C, 24 °C and 28 °C), with daily fluctuations of +5 °C and -4 °C at each mean temperature. Transcriptomes were described up to 19 days post-blood meal. Of the >3100 differentially expressed genes, we observed shared temporal expression profiles across all temperatures, suggesting their indispensability to mosquito life history. Tolerance to 20 and 28 ( + 5°C/-4°C) was associated with larger and more diverse transcriptomes compared to 24 ( + 5 °C/-4 °C). Finally, we identified two distinct trends in gene expression in response to blood meal ingestion, oxidative stress, and reproduction. Our work has implications for mosquitoes' response to thermal variation, mosquito immune-physiology, mosquito-malaria interactions and the development of vector control tools.

Interplay of climate change with physiological changes in adult Aedes albopictus

The effect of unprecedented climate change conditions on the environment has attracted the attention of experts from various disciplines who wish to predict its evolution. This is due to its interaction with population health and people's quality of life. The emergence of vector-borne diseases (VBD) in Europe, particularly caused by invasive mosquitoes, has resulted in autochthonous transmission of Dengue and Chikungunya virus cases. In this contribution, we delve into the significant decline in the population of tiger mosquitoes and the noticeable reduction in their size, related to the changes that occur often in years with low rainfall and high temperatures during the summer months. This change can be traced to the morphology of the species, which has been analysed in this work and related to the possible enhanced transmission efficiency and susceptibility of vectors to VBD dissemination. Due to the greater susceptibility to pathogen replication and subsequent transmission of the pathogens to a host during blood feeding, these morphologically distinct species relatively seem to be an efficient vector. The relationship between the sizes of studied mosquitoes and rainfall levels requires more research by mathematical epidemiologists.

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

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

Adaptation and carry over effects of extreme sporadic heat stress in Culex mosquitoes

Mosquitoes, as temperature-sensitive ectothermic vectors, exhibit temperature-dependence. This study investigates Culex pipiens pallens (Cx. pallens) responses to abrupt temperature increases and their implications on mosquito physiology. First instar larvae (24hr post hatching) and newly enclosed adults (24hr post emergence) were separately exposed to heat shock regimes of 33 °C, 37 °C, and 42 °C for 3 days alongside a control temperature of 27 °C. Results showed that mortality was triggered at 42 °C within a day. Adult male mosquitoes were less tolerant to all temperatures than larvae and adult females (p < 0.05). Exposing larvae to constant temperatures for 3 days significantly decreased larvae's development time, growth rate and adult emergence (p < 0.05). Reproductive fitness was significantly reduced (p < 0.05) in males emerging from larvae exposed to 37 °C. Life table parameters showed significant increased mortality rate, kill power and decreased life expectancy at the embryonic stage (p < 0.05). Furthermore, heatwaves deactivated the Transient receptor protein ankyrin 1 at 37 °C (p < 0.05) in larvae but not adults. Calmodium, Heat shock protein 90, and small heat shock protein expression were significantly decreased in larvae at 37 °C (p < 0.05) as compared to larvae raised at 33 °C and 27 °C. In conclusion, we classified the heat waves into three categories: adaptable (33 °C), critical (37 °C), and fatal (42 °C). Prolonged exposure of Culex pallens larvae to extreme heat affects the male reproductive output. These findings may serve as an important reference for forecasting vector and pest dynamics and used to tailor mosquito prevention and control measures.

Senescence of humoral antimicrobial immunity occurs in infected mosquitoes when the temperature is higher

Mosquitoes cannot use metabolism to regulate their body temperature and therefore climate warming is altering their physiology. Mosquitoes also experience a physiological decline with aging, a phenomenon called senescence. Because both high temperature and aging are detrimental to mosquitoes, we hypothesized that high temperatures accelerate senescence. Here, we investigated how temperature and aging, independently and interactively, shape the antimicrobial immune response of the mosquito Anopheles gambiae. Using a zone-of-inhibition assay that measures the antimicrobial activity of hemolymph, we found that antimicrobial activity increases following infection. Moreover, in infected mosquitoes, antimicrobial activity weakens as the temperature rises to 32°C, and antimicrobial activity increases from 1 to 5 days of age and stabilizes with further aging. Importantly, in E. coli-infected mosquitoes, higher temperature causes an aging-dependent decline in antimicrobial activity. Altogether, this study demonstrates that higher temperature can accelerate immune senescence in infected mosquitoes, thereby interactively shaping their ability to fight an infection.

Towards transforming malaria vector surveillance using VectorBrain: a novel convolutional neural network for mosquito species, sex, and abdomen status identifications

Malaria is a major public health concern, causing significant morbidity and mortality globally. Monitoring the local population density and diversity of the vectors transmitting malaria is critical to implementing targeted control strategies. However, the current manual identification of mosquitoes is a time-consuming and intensive task, posing challenges in low-resource areas like sub-Saharan Africa; in addition, existing automated identification methods lack scalability, mobile deployability, and field-test validity. To address these bottlenecks, a mosquito image database with fresh wild-caught specimens using basic smartphones is introduced, and we present a novel CNN-based architecture, VectorBrain, designed for identifying the species, sex, and abdomen status of a mosquito concurrently while being efficient and lightweight in computation and size. Overall, our proposed approach achieves 94.44±2% accuracy with a macro-averaged F1 score of 94.10±2% for the species classification, 97.66±1% accuracy with a macro-averaged F1 score of 96.17±1% for the sex classification, and 82.20±3.1% accuracy with a macro-averaged F1 score of 81.17±3% for the abdominal status classification. VectorBrain running on local mobile devices, paired with a low-cost handheld imaging tool, is promising in transforming the mosquito vector surveillance programs by reducing the burden of expertise required and facilitating timely response based on accurate monitoring.

Warmer environmental temperature accelerates aging in mosquitoes, decreasing longevity and worsening infection outcomes

BACKGROUND

Most insects are poikilotherms and ectotherms, so their body temperature is predicated by environmental temperature. With climate change, insect body temperature is rising, which affects how insects develop, survive, and respond to infection. Aging also affects insect physiology by deteriorating body condition and weakening immune proficiency via senescence. Aging is usually considered in terms of time, or chronological age, but it can also be conceptualized in terms of body function, or physiological age. We hypothesized that warmer temperature decouples chronological and physiological age in insects by accelerating senescence. To investigate this, we reared the African malaria mosquito, Anopheles gambiae, at 27 °C, 30 °C and 32 °C, and measured survival starting at 1-, 5-, 10- and 15-days of adulthood after no manipulation, injury, or a hemocoelic infection with Escherichia coli or Micrococcus luteus. Then, we measured the intensity of an E. coli infection to determine how the interaction between environmental temperature and aging shapes a mosquito's response to infection.

RESULTS

We demonstrate that longevity declines when a mosquito is infected with bacteria, mosquitoes have shorter lifespans when the temperature is warmer, older mosquitoes are more likely to die, and warmer temperature marginally accelerates the aging-dependent decline in survival. Furthermore, we discovered that E. coli infection intensity increases when the temperature is warmer and with aging, and that warmer temperature accelerates the aging-dependent increase in infection intensity. Finally, we uncovered that warmer temperature affects both bacterial and mosquito physiology.

CONCLUSIONS

Warmer environmental temperature accelerates aging in mosquitoes, negatively affecting both longevity and infection outcomes. These findings have implications for how insects will serve as pollinators, agricultural pests, and disease vectors in our warming world.

Evolutionary adaptation under climate change: Aedes sp. demonstrates potential to adapt to warming

Climate warming is expected to shift the distributions of mosquitoes and mosquito-borne diseases, facilitating expansions at cool range edges and contractions at warm range edges. However, whether mosquito populations could maintain their warm edges through evolutionary adaptation remains unknown. Here, we investigate the potential for thermal adaptation in Aedes sierrensis, a congener of the major disease vector species that experiences large thermal gradients in its native range, by assaying tolerance to prolonged and acute heat exposure, and its genetic basis in a diverse, field-derived population. We found pervasive evidence of heritable genetic variation in acute heat tolerance, which phenotypically trades off with tolerance to prolonged heat exposure. A simple evolutionary model based on our data shows that the estimated maximum rate of evolutionary adaptation in mosquito heat tolerance typically exceeds that of projected climate warming under idealized conditions. Our findings indicate that natural mosquito populations may have the potential to track projected warming via genetic adaptation. Prior climate-based projections may thus underestimate the range of mosquito and mosquito-borne disease distributions under future climate conditions.

The Impacts of Climate Change on the Emergence and Reemergence of Mosquito-Borne Diseases in Temperate Zones: An Umbrella Review Protocol

INTRODUCTION

Mosquito-borne diseases represent a global public health concern and are responsible for over 700 000 deaths globally every year. Additionally, many mosquito species have undergone a dramatic global expansion due to various factors, including climate change, and forecasts indicate that mosquito populations will persist in dispersing beyond their present geographic range, namely in temperate climates. The research literature on this topic has grown in recent years, including some systematic evidence synthesis. However, to provide a comprehensive overview of this growing literature needed for policy action, a summary of this evidence, including existing systematic reviews, is required. This study aims to undertake an umbrella review that explores the impacts of climate change on the emergence and reemergence of diseases transmitted by mosquitoes in temperate zones and the publication of the protocol is a fundamental step to ensure the credibility, transparency and reproducibility of this research.

METHODS AND ANALYSIS

Studies published in scientific journals indexed by PubMed, EMBASE, Cochrane Library, Epistemonikos, and Web of Science Core Collection to be included in this umbrella review will meet the following criteria: the topic of study (climate change and mosquito-borne diseases), regions (temperate zones), study designs (systematic reviews and meta-analysis), language (any) and date (since inception until December 31st, 2023). Titles and abstracts from selected articles will be evaluated by two authors independently and any discrepancy will be resolved through consensus or, if not possible, through a third author. The data will be extracted, and the risk of bias will be evaluated. The quality of the methodology of the included reviews will be assessed using AMSTAR 2. A narrative synthesis will examine the included systematic reviews. The quality of evidence for all outcomes will be judged using the Grading of Recommendations Assessment, Development and Evaluation working group methodology.

Geometric morphometrics to differentiate species and explore seasonal variation in three Mansonia species (Diptera: Culicidae) in central Thailand and their association with meteorological factors

Mansonia mosquito species are recognised as a significant vector of human pathogens, primarily transmitting the filarial nematode, Brugia malayi. In central Thailand, the three most prevalent Mansonia species are Mansonia annulifera, Mansonia indiana and Mansonia uniformis. This study explored the influence of seasonal changes on the phenotypic variation of these Mansonia species in central Thailand using the geometric morphometrics (GM). To ensure accurate species identification, we integrated GM techniques with DNA barcoding, examining distinctions in both phenotype and genotype among the species. The intraspecific genetic divergence ranged from 0.00% to 1.69%, whereas the interspecific genetic divergence ranged from 10.52% to 16.36%. The clear distinction between intra- and interspecific distances demonstrated the presence of a barcoding gap, confirming the successful differentiation of the three Mansonia mosquito species through DNA barcoding. Similarly, the interspecies GM assessment for classifying Mansonia species demonstrated a high degree of accuracy, with an overall performance of 98.12%. Exploring seasonal variation in the three Mansonia species revealed wing variations across different seasons, and pronounced variations appearing in the cool season. Regarding their association with meteorological factors, Ma. annulifera and Ma. uniformis showed significant positive correlations with temperature (p < 0.05), and Ma. uniformis also displayed a significant negative correlation with atmospheric pressure (p < 0.05). The insights from this study will deepen our understanding of the adaptive patterns of Mansonia mosquitoes in Thailand's central region, paving the way for enhanced disease surveillance related to these vectors.

Resources modulate developmental shifts but not infection tolerance upon coinfection in an insect system

Energetic resources fuel immune responses and parasite growth within organisms, but it is unclear whether energy allocation is sufficient to explain changes in infection outcomes under the threat of multiple parasites. We manipulated diet in flour beetles (Tribolium confusum) infected with two natural parasites to investigate the role of resources in shifting metabolic and immune responses after single and co-infection. Our results suggest that gregarine parasites alter the within-host energetic environment, and by extension juvenile development time, in a diet-dependent manner. Gregarines do not affect host resistance to acute bacterial infection but do stimulate the expression of an alternative set of immune genes and promote damage to the gut, ultimately contributing to reduced survival regardless of diet. Thus, energy allocation is not sufficient to explain the immunological contribution to coinfection outcomes, emphasizing the importance of mechanistic insight for predicting the impact of coinfection across levels of biological organization.

Higher temperature accelerates the aging-dependent weakening of the melanization immune response in mosquitoes

The body temperature of mosquitoes, like most insects, is dictated by the environmental temperature. Climate change is increasing the body temperature of insects and thereby altering physiological processes such as immune proficiency. Aging also alters insect physiology, resulting in the weakening of the immune system in a process called senescence. Although both temperature and aging independently affect the immune system, it is unknown whether temperature alters the rate of immune senescence. Here, we evaluated the independent and combined effects of temperature (27°C, 30°C and 32°C) and aging (1, 5, 10 and 15 days old) on the melanization immune response of the adult female mosquito, Anopheles gambiae. Using a spectrophotometric assay that measures phenoloxidase activity (a rate limiting enzyme) in hemolymph, and therefore, the melanization potential of the mosquito, we discovered that the strength of melanization decreases with higher temperature, aging, and infection. Moreover, when the temperature is higher, the aging-dependent decline in melanization begins at a younger age. Using an optical assay that measures melanin deposition on the abdominal wall and in the periostial regions of the heart, we found that melanin is deposited after infection, that this deposition decreases with aging, and that this aging-dependent decline is accelerated by higher temperature. This study demonstrates that higher temperature accelerates immune senescence in mosquitoes, with higher temperature uncoupling physiological age from chronological age. These findings highlight the importance of investigating the consequences of climate change on how disease transmission by mosquitoes is affected by aging.