Biology subjects

Murdock, C. C.

Publications and source records attributed to Murdock, C. C..

At least 19 recordsLinked to original sources

bioRxiv2026

Insecticide temephos alters thermal dependence of dengue vector

O_LIAlthough ecological research has long focused on the effects of temperature on population growth, arthropod pests are exposed to a wide variety of environmental factors that affect their performance, such as chemical pesticides targeted against them. Moreover, these environmental factors likely do not act in isolation. Identifying the extent to which abiotic factors interact to affect pest population dynamics can strengthen current and future pest management programs. C_LIO_LIHere, we investigated the extent to which temephos, a common pesticide applied to aquatic environments for mosquito control, influences the thermal performance of juvenile survival and development rate, as well as the intrinsic population growth rate, of the invasive mosquito pest, Aedes aegypti. We implemented a response surface experimental design to measure these traits across seven temperatures and five temephos concentrations and fit temperature- and insecticide-dependent performance curves to assess impacts on the overall performance and the thermal optimum, minimum, and maximum. C_LIO_LITemephos exposure profoundly altered the thermal performance of juvenile survival by reducing survival across all temperatures, shrinking the thermal breadth, and shifting the thermal optimum to warmer temperatures. Through this, temephos also altered the thermal performance of population growth primarily by reducing its thermal breadth. C_LIO_LISynthesis and applications: Our findings demonstrate that interactions between temperature and insecticide exposure can fundamentally reshape pest population dynamics, rather than acting as independent stressors. By quantifying this interaction, we showed that temphos is most effective below the pests thermal optimum, suggesting that larvicides may yield the greatest population suppression in cooler regions or during cooler periods of the year. Incorporating such temperature-dependent efficacy into pest management strategies could improve the timing and spatial targeting of control efforts. More broadly, these results highlight the need to integrate anthropogenic stressors with climatic drivers when predicting pest risk and optimizing management under ongoing environmental change. C_LI

ecology↗

bioRxiv2026

Humidity shapes the thermal niche of Anopheles stephensi,an invasive malaria vector

Vector-borne pathogens cause 17% of all human infectious diseases, and rising global temperatures are shifting the distribution and abundance of mosquito vectors. Because mosquitoes are ectotherms, temperature strongly governs biological rates and physiology; however, mosquitoes also experience other environmental factors that may interact with temperature to shape the thermal performance of traits driving population dynamics. Here, we use a factorial life-table experiment spanning five relative humidities (30-90%) and seven temperatures (16-38{whitebullet}C) to show that humidity modifies the thermal performance of key fitness traits in adult Anopheles stephensi, an invasive urban malaria vector. When integrated into a demographic model, humidity markedly reshapes projections of population fitness relative to temperatureonly models, suppressing growth and contracting year-round suitability in hot, arid regions while enhancing fitness in more humid or high-elevation climates characteristic of South Asia and Africa. Together, these results highlight the need to integrate multiple environmental drivers into projections of climatic suitability, as temperature-only approaches may mischaracterize both the magnitude and spatial structure of mosquito population fitness. More broadly, our findings demonstrate how moisture availability reshapes thermal niches, population fitness, and climate-driven projections of vector distributions.

ecology↗

bioRxiv2025

Beyond temperature: Relative humidity systematically shifts the temperature dependence of population growth in a malariavector

Understanding ectotherm responses to environmental change is central to coping with many of humanitys current and future challenges in public health, biodiversity conservation, and food security. Complex relationships between abiotic and biotic factors can influence ectotherm abundance and distribution patterns by introducing stage-specific variation in fitness trait responses. Variation in temperature, rainfall, competition, and habitat have all been considered in previous attempts to understand how environmental factors can interact and vary in their relative influence on species maximal population growth rates, rm. However, the combined effects of temperature and humidity on this fundamental metric are poorly understood. We show that variation in relative humidity can influence juvenile trait responses and rms temperature dependence in Anopheles stephensi, an important malaria vector. Our climate suitability maps show that the interactive effects of temperature x humidity on juvenile traits have important implications for predicting how environmental change will influence arthropod-mediated systems.

ecology↗

bioRxiv2025

Field-validation of multiple species distribution models shows variation in performance for predicting Aedes albopictus distributions at the invasion edge

Climate and land use changes have resulted in range expansion of many species. In this shifting disease landscape, it is important to leverage tools that can predict the potential distributions of invading vectors to target surveillance and control efforts and identify at risk populations. Species Distribution Models (SDMs) are widely used to predict ranges of invasive species; however, invasive species often violate assumptions of equilibrium and niche conservatism. Moreover, these studies are rarely validated using independent data. Here, we use long-term mosquito surveillance data for Aedes albopictus, a highly invasive mosquito capable of transmitting several arboviruses, at its range-edge to evaluate a variety of SDMs (MaxEnt, GAM, Random Forest, Boosted Regression Tree) in predicting Ae. albopictus range. We identify key environmental drivers of distributions and areas where models tended to disagree in predicting occurrence. At sites where models disagree, we sampled for Ae. albopictus to generate an independent dataset for field-validation of models in addition to the common practice of cross-validation. Finally, we determine if models based on early invasion data can predict later stage invasion ranges. We found that landscape and climatic variables are important drivers of population distributions. SDM methods varied in predictive accuracy between models and across validation methods (i.e. cross-vs. field-validation). GAM and MaxEnt best predicted later-stage invasion distributions, requiring fewer years of training data. Our work shows that SDMs can be useful tools to predict the ranges of invasive species and highlights the importance of comparing predictions of invasive species range. Author SummaryMosquitoes are greatly impacted by their surrounding environment. Environmental changes such as those driven by climate change or changes in land use (i.e. urbanization, etc.) can have profound impacts on mosquito ranges. Species distribution models (SDMs), which use the occurrence data of a species in combination with environmental data (e.g., landscape characteristics, temperature, etc.) to estimate suitable habitats for the species, are helpful tools to understand how species ranges can change with the environment through time and space. There are many different species distribution models to choose from - each with different methods for estimating suitable habitat - making it important to compare the performance of different models. Furthermore, because invasive species often violate assumptions of SDMs, it is important to rigorously explore how different methods perform when predicting invasive species ranges in newly invaded areas. In this study, we tested the predictive accuracy of different species distribution models, and we found it varied across modeling and validation method. We also tested whether models could use early-stage invasion data to predict the late-stage invasion distributions of Ae. albopictus. We found that some modeling methods performed well while others needed more data to improve accuracy of late-stage invasion range predictions. In summary, when modeling species ranges using SDMs, it is important to use multiple methods and compare results because methods will likely disagree, and further sampling may be required to determine which model is most accurate.

ecology↗

bioRxiv2024

Mean daily temperatures can predict the thermal limits of malaria transmission better than rate summation

Temperature shapes the distribution, seasonality, and magnitude of mosquito-borne disease outbreaks. Mechanistic models predicting transmission often use mosquito and pathogen thermal responses from constant temperature experiments. However, mosquitoes live in fluctuating environments. Rate summation (nonlinear averaging) is a common approach to infer performance in fluctuating environments, but its accuracy is rarely validated. We measured three mosquito traits that impact transmission (bite rate, survival, fecundity) in a malaria mosquito (Anopheles stephensi) across temperature gradients with three diurnal temperature ranges (0, 9 and 12{degrees}C). We compared thermal suitability models with temperature-trait relationships observed under constant temperatures, fluctuating temperatures, and those predicted by rate summation. We mapped results across An. stephenesis native Asian and invasive African ranges. We found: 1) daily temperature fluctuation significantly altered trait thermal responses; 2) rate summation partially captured decreases in performance near thermal optima, but also incorrectly predicted increases near thermal limits; and 3) while thermal suitability characterized across constant temperatures did not perfectly capture suitability in fluctuating environments, it was more accurate for estimating and mapping thermal limits than predictions from rate summation. Our study provides insight into methods for predicting mosquito-borne disease risk and emphasizes the need to improve understanding of organismal performance under fluctuating conditions.

ecology↗

bioRxiv2024

Mosquito population dynamics is shaped by the interaction among larval density, season, and land use

ABSTRACT (English)Understanding how variation in key abiotic and biotic factors interact at ecologically relevant spatial scales is crucial for predicting population dynamics, distributions, and abundances. This is especially true for vectors that transmit human pathogens. However, studies investigating the effects of environmental variation on vectors have typically investigated environmental factors in isolation or in laboratory experiments that examine constant environmental conditions that often do not occur in the field. To address these limitations, we conducted a semi-field experiment in Athens, Georgia using the invasive Asian tiger mosquito (Aedes albopictus). We selected nine sites that varied in impervious surface and vegetation cover to explore effects of natural variation in microclimate (specifically, temperature and relative humidity) on mosquitoes. We manipulated conspecific larval density at each site and repeated the experiment in the summer and fall to further increase the variability in temperature and relative humidity. We then evaluated the original design features (land cover, larval density, and season), and their interactions, on the mean proportion of females emerging, juvenile development time, size upon emergence, and estimated per capita population growth (i.e., fitness). We found significant effects of larval density, land cover, and season on all response variables, including a non-intuitive decrease in development time with increasing larval density in the fall. We repeated these analyses using the hypothesized microclimate drivers of these effects: temperature and relative humidity. In general, the model using the microclimate variables outperformed the model using land use and season, highlighting the roles of temperature and relative humidity and their interactions with density on mosquito traits and dynamics. Our study demonstrates that ignoring the interaction between variation in biotic (e.g., intraspecific competition) and abiotic (e.g., temperature and relative humidity) variables could reduce the accuracy and precision of models used to predict mosquito population and pathogen transmission dynamics, especially those inferring dynamics at finer-spatial scales across which transmission and control occur. ABSTRACT (Spanish)Es crucial comprender como los factores abioticos y bioticos interactuan a escalas ecologicamente relevantes para predecir la dinamica, distribucion, y abundancia de poblaciones. Esto es especialmente cierto para vectores que transmiten patogenos humanos. Sin embargo, la mayoria de estudios han considerado factores ambientales de forma aislada o en condiciones de laboratorio poco realistas. Para superar estas limitaciones, realizamos un experimento de semi-campo en Athens, Georgia, con el mosquito invasor Aedes albopictus. Manipulamos la densidad larval en nueve sitios que variaban en cobertura vegetal y superficie impermeable, y repetimos el experimento en el verano y otono. Luego, evaluamos los efectos de la cobertura del suelo, la densidad larval, y la estacion en la proporcion de hembras adultas emergidas, el tiempo de desarrollo, el tamano al emerger, y el crecimiento poblacional per capita. Encontramos efectos significativos de todas las variables, asi como fuertes interacciones entre estacion y densidad de mosquitos, incluyendo una disminucion no intuitiva del tiempo de desarrollo en el otono con mayor densidad larval. Repetimos los analisis usando factores microclimaticas hipotetizados como impulsores de estos efectos: temperatura y humedad. En general, el modelo que incorpora variables microclimaticas supero al modelo basado en el uso de suelo y estacion, evidenciando la importancia de la temperatura y la humedad, asi como sus interacciones con la densidad larval, en las caracteristicas biologicas y la dinamica poblacional de los mosquitos. Nuestros resultados demuestran que la interaccion entre variacion biotica (competencia intraespecifica) y abiotica (temperatura y humedad), es fundamental para mejorar la precision de los modelos que predicen la dinamica poblacional de mosquitos y la transmision de patogenos a escalas locales de transmision y control.

ecology↗

bioRxiv2024

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

The abundance and distribution of ectotherms is being shaped at unprecedented scales by human-induced climate change. Unraveling molecular adaptations to thermal variation is prescient. Vector-borne diseases are of particular concern and although mosquitoes are responsible for most of the 621 million cases annually, their response to temperature is not well characterized. We describe the transcriptome of the major vector for human malaria, Anopheles stephensi maintained under a diurnal temperature regime (DTR) totaling 9{degrees}C and daily means of 20{degrees}C, 24{degrees}C and 28{degrees}C respectively. Gene expression profiles were examined in the midguts and carcasses of adult mosquitoes from one to 19 days post-blood meal. Differences in temporal expression profiles at each temperature revealed a total of 3,106 and 3,590 genes in the carcasses and midguts respectively and analyzed further to make three inferences. First, we identified genes with shared temporal expression profiles across all three temperatures, suggesting their indispensability to mosquito life history. Second, tolerance to 20 DTR 9{degrees}C and 28 DTR 9{degrees}C was associated with a larger and more diverse repertoire of gene products compared to 24 DTR 9{degrees}C; assuming physiological costs scale accordingly, this finding could explain the fitness trade-offs underlying the unimodal effect of temperature. Third, the long duration of our study revealed two general trends in gene expression, representative of a coordinated, tissue- and temperature-specific response to blood meal digestion, managing oxidative stress, and reproduction. Our results suggest how temperature could regulate the mosquitos capacity to transmit Plasmodium parasites and offer a reference point for understanding ectotherm adaptation.

molecular biology↗

bioRxiv2023

Catching a wave: on the suitability of traveling-wave solutions in epidemiological modeling

Ordinary differential equation models such as the classical SIR model are widely used in epidemiology to study and predict infectious disease dynamics. However, these models typically assume that populations are homogeneously mixed, ignoring possible variations in disease prevalence due to spatial heterogeneity. To address this issue, reaction-diffusion models have been proposed as an alternative approach to modeling spatially continuous populations in which individuals move in a diffusive manner. In this study, we explore the conditions under which such spatial structure must be explicitly considered to accurately predict disease spread, and when the assumption of homogeneous mixing remains adequate. In particular, we derive a critical threshold for the diffusion coefficient below which disease transmission dynamics exhibit spatial heterogeneity. We validate our analytical results with individual-based simulations of disease transmission across a two-dimensional continuous landscape. Using this framework, we further explore how key epidemiological parameters such as the probability of disease establishment, its maximum incidence, and its final epidemic size are affected by incorporating spatial structure into SI, SIS, and SIR models. We discuss the implications of our findings for epidemiological modeling and identify design considerations and limitations for spatial simulation models of disease dynamics.

bioinformatics↗

bioRxiv2022

Mapping current and future thermal limits to suitability for malaria transmission by the invasive mosquito Anopheles stephensi

BackgroundAnopheles stephensi is a malaria-transmitting mosquito that has recently expanded from its primary range in Asia and the Middle East, to locations in Africa. This species is a competent vector of both P. falciparum (PF) and P. vivax (PV) malaria. Perhaps most alarming, the characteristics of An. stephensi, such as container breeding and anthropophily, make it particularly adept at exploiting built environments in areas with no prior history of malaria risk. MethodsIn this paper we created global maps of thermal transmission suitability and people at risk (PAR) for malaria transmission by An. stephensi, under current and future climate. Temperature-dependent transmission suitability thresholds derived from recently published species-specific thermal curves were used to threshold gridded, monthly mean temperatures under current and future climatic conditions. These temperature driven transmission models were coupled with gridded population data for 2020 and 2050, under climate-matched scenarios for future outcomes, to compare with baseline predictions for 2020 populations. ResultsUsing the Global Burden of Disease regions approach, we found that heterogenous regional increases and decreases in risk did not mask the overall pattern of massive increases of PAR for malaria transmission suitability with An. stephensi presence. General patterns of poleward expansion for thermal suitability were seen for both PF and PV transmission potential. ConclusionsUnderstanding the potential suitability for An. stephensi transmission in a changing climate provides a key tool for planning, given an ongoing invasion and expansion of the vector. Anticipating the potential impact of onward expansion to transmission suitable areas, and the size of population at risk under future climate scenarios, and where they occur, can serve as a large-scale call for attention, planning, and monitoring.

ecology↗

bioRxiv2021

An integrative approach to mosquito dynamics reveals differences in people's everyday experiences of mosquitoes

O_LIUrban environments are heterogeneous landscapes of social and environmental features, with important consequences for human-nature entanglements, such as that of mosquito-borne disease. C_LIO_LIInvestigations into this intra-urban heterogeneity in mosquito dynamics find conflicting results, likely due to the complex socio-ecological interactions and the importance of place-based context. Integrative research, which synthesizes multiple disciplines and epistemologies, can help place ecological results into their social context to explore these place-based differences. C_LIO_LIHere, we develop an integrative approach to understanding spatial patterns of mosquito burdens in urban systems by combining entomological surveys, semi-structured interviews, and sketch maps. C_LIO_LIAlthough we found no evidence for a difference in mosquito abundance across an urban gradient, there were differences in individuals everyday experiences with mosquitoes. These differences were mediated by how individuals moved through public space and their vulnerability to hazards in these spaces. C_LIO_LIThis example of integrative research illustrates what can be gained from the inclusion of multiple epistemologies, particularly for research in socio-ecological systems. C_LI

ecology↗

bioRxiv2021

Cool temperatures limit Zika virus genome replication

Zika virus is a mosquito-borne flavivirus known to cause severe birth defects and neuroimmunological disorders. We have previously demonstrated that mosquito transmission of Zika virus decreases with temperature. While transmission was optimized at 29{degrees}C, it was limited at cool temperatures (< 22{degrees}C) due to poor virus establishment in the mosquitoes. Temperature is one of the strongest drivers of vector-borne disease transmission due to its profound effect on ectothermic mosquito vectors, viruses, and their interaction. Although there is substantial evidence of temperature effects on arbovirus replication and dissemination inside mosquitoes, little is known about whether temperature affects virus replication directly or indirectly through mosquito physiology. In order to determine the mechanisms behind temperature-induced changes in Zika virus transmission potential, we investigated different steps of the virus replication cycle in mosquito cells (C6/36) at optimal (28{degrees}C) and cool (20{degrees}C) temperatures. We found that cool temperature did not alter Zika virus entry or translation but reduced the amount of double-stranded RNA replication intermediates. If replication complexes were first formed at 28{degrees}C and the cells were subsequently shifted to 20{degrees}C, the late steps in the virus replication cycle were efficiently completed. These data suggest that cool temperature decreases the efficiency of Zika virus genome replication in mosquito cells. This phenotype was observed in the Asian-lineage of Zika virus, while the African-lineage Zika virus was less restrictive at 20{degrees}C. ImportanceWith half of the human population at risk, arboviral diseases represent a substantial global health burden. Zika virus, previously known to cause sporadic infections in humans, emerged in the Americas in 2015 and quickly spread worldwide. There was an urgent need to better understand the disease pathogenesis, develop therapeutics and vaccines, as well as to understand, predict, and control virus transmission. In order to efficiently predict the seasonality and geography for Zika virus transmission, we need a deeper understanding of the host-pathogen interactions and how they can be altered by environmental factors such as temperature. Identifying the step in the virus replication cycle that is inhibited in cool conditions can have implications in modeling the temperature suitability for arbovirus transmission as global environmental patterns change. Understanding the link between pathogen replication and environmental conditions can potentially be exploited to develop new vector control strategies in the future.

microbiology↗

bioRxiv2020

Sexual selection theory meets disease vector control: Testing harmonic convergence as a "good genes" signal in Aedes aegypti mosquitoes

The mosquito Aedes aegypti is a medically important, globally distributed vector of the viruses that cause dengue, yellow fever, chikungunya, and Zika. Although reproduction and mate choice are key components of vector population dynamics and control, our understanding of the mechanisms of sexual selection in mosquitoes remains poor. In "good genes" models of sexual selection, females use male cues as an indicator of both mate and offspring inherent genetic quality. Recent studies in Ae. aegypti provide evidence that male wingbeat acoustics signal aspects of offspring quality and performance during mate selection in a process known as harmonic convergence. However, the extent to which harmonic convergence may signal overall inherent quality of mates and their offspring remains unknown. To examine this, we measured the potential acoustic signaling and heritability of a broad panel of parent and offspring fitness traits in two generations of field-derived Ae. aegypti originating from dengue endemic field sites in Thailand. Our data show that in this population of mosquitoes, harmonic convergence does not signal male fertility, female fecundity, and male flight performance traits, which despite displaying robust variability in both parents and their offspring were only weakly heritable. Together with previous work, our findings suggest that vector reproductive control programs should treat harmonic convergence as an indicator of some, but not all aspects of inherent quality, and that multiple forms of sexual selection are likely at play in Ae. aegypti in a trait, population, and environment dependent manner.

ecology↗

bioRxiv2020

Sex, Age, and Parental Harmonic Convergence Behavior Affect the Immune Performance of Aedes aegypti Offspring

Aedes aegypti is an important vector of several pathogenic arboviruses including dengue, chikungunya and Zika. Innovative approaches to control Aedes populations, involving synthetic transgenic modifications as well as Wolbachia bacteria, appear promising. For the various techniques requiring offspring inheritance of a trait, released males must successfully compete for mating partners against wildtype males. However, very little is known about mechanisms of mate selection in mosquitoes in general and in particular about potential correlations between mating success and offspring immune performance. Harmonic convergence signals have been proposed as a cue for females to predict male quality. We investigated whether offspring of converging parental pairs showed differences in immune competence compared to offspring derived from non-converging parental pairs using three different types of immune assays. We found that offspring immune responses (melanization response and response to a bacterial challenge) differed between offspring from converging and non-converging parents. However, immune responses were shaped by several interacting factors such as sex, age, reproductive status, and parental mating behavior. Parental mating behavior had a stronger effect on the immune response of male offspring than on female offspring. Further, a population of female offspring derived from converging parental pairs reached their peak dengue virus dissemination rate earlier compared to a population of offspring derived from non-converging parental pairs. Our results provide insight into a wide range of selective pressures shaping mosquito immune function. Evolutionary trade-offs between naturally and sexually selected traits can have important implications for disease transmission and control and should be considered in the development of reproductive control strategies.

ecology↗

bioRxiv2020

Effects of seasonality and land use on the abundance and distribution of mosquitoes on St. Kitts, West Indies

BackgroundHigh quality mosquito surveys that collect fine resolution local data on mosquito species abundances provide baseline data to help us understand potential host-pathogen-mosquito relationships, accurately predict disease transmission, and target mosquito control efforts in areas at risk of mosquito borne diseases. MethodsAs part of an investigation into arboviral sylvatic cycles on the Caribbean island of St. Kitts, we carried out an island wide mosquito survey from November 2017 to March 2019. Using Biogents Sentinel 2 and miniature CDC light traps that were set monthly and run for 48 hour intervals, we collected mosquitoes from a total of 30 sites distributed across the five common land covers on the island (agricultural, mangrove, rainforest, scrub, and urban). We developed a mixed effects negative binomial regression model to predict the effects of land cover, seasonality, and precipitation on observed counts of the most abundant mosquito species we found. ResultsWe captured 10 of the 14 mosquito species reported on the island, the four most abundant being Aedes taeniorhynchus, Culex quinquefasciatus, Aedes aegpyti, and Deinocerites magnus. Sampling in the mangroves yielded the most mosquitoes, with Ae. taeniorhynchus, Cx. quinquefasciatus, and De. magnus predominating. Aedes aegypti was recovered primarily from urban and agricultural habitats, but also at lower frequency in other land covers. Psorophora pygmaea and Toxorhynchites guadeloupensis were only captured in scrub habitat. Capture rates in rainforests were low. Our models indicated the relative abundance of the four most common species varied seasonally and with land cover. They also suggested that the extent to which monthly average precipitation influenced counts varied according to species. ConclusionsThis study demonstrates there is high seasonality in mosquito abundances and that land cover influenced the distribution and abundance of mosquito species on St. Kitts. Further, human-adapted mosquito species (e.g. Ae. aegypti and Cx. quinquefasciatus) that are known vectors for many human relevant pathogens are the most wide-spread (across land covers) and the least responsive to seasonal variation in precipitation.

ecology↗

bioRxiv2019

Effects of temperature on Zika dynamics and control

ABSTRACTWhen a formerly rare pathogen emerges to cause a pandemic, it is critical to understand the ecology of the disease dynamics and its potential effects on disease control. Here, we take advantage of newly available experimental data to parameterize a temperature-dependent dynamical model of Zika virus (ZIKV) transmission, and analyze the effects of temperature variability and the parameters related to control strategies on ZIKV R0 and the final epidemic size (i.e., total number of human cases). Sensitivity analyses identified that R0 and the final epidemic size were largely driven by different parameters, with the exception of temperature, which is the dominant driver of epidemic dynamics in the models. Our estimate of R0 had a single optimum temperature (≈ 30° C), comparable to recently published results (≈ 29°)1. However, the total number of human cases (“final epidemic size”) is maximized across a wider temperature range, from 24 to 36°C. The models indicate that the disease is highly sensitive to seasonal temperature variation. For example, although the model predicts that Zika transmission cannot occur at a constant temperature of 22°C, with seasonal variation of 5°C around a mean of 22°C, the model predicts a larger epidemic than what would occur at a constant 30°C, the temperature predicted to maximize R0. This suggests that the potential geographic range of Zika is wider than indicated from static R0 models, underscoring the importance of climate dynamics and variation on emerging infectious diseases.Competing Interest StatementThe authors have declared no competing interest.View Full Text

ecology↗

bioRxiv2019

Field relevant variation in ambient temperature modifies the density-dependent establishment of Plasmodium falciparum in mosquitoes: implications for the infectious reservoir and beyond?

The relationship between Plasmodium falciparum gametocyte density and infections in mosquitoes is central to understanding the rates of transmission with important implications for control. Here, we determined whether field relevant variation in environmental temperature could also modulate this relationship. Anopheles stephensi were challenged with three densities of P. falciparum gametocytes spanning a [~]10-fold gradient, and housed under diurnal/daily temperature range (\"DTR\") of 9{degrees}C around means of 20{degrees}C, 24{degrees}C and 28{degrees}C. At the peak stages of infection for each temperature, the proportion of mosquitoes infected with oocysts in the midguts or infectious with sporozoites in the salivary glands were measured (referred to collectively as vector competence hereon), in addition to oocyst intensities from infected midguts. While vector competence was similar at 20 DTR 9{degrees}C and 24 DTR 9{degrees}C, the proportion of mosquitoes infected and subsequently infectious were also comparable, with evidence, surprisingly, for higher vector competence in mosquitoes challenged with intermediate gametocyte densities. For the same gametocyte densities however, severe reduction in the proportion of infectious mosquitoes was accompanied by a significant decline in vector competence at 28 DTR 9{degrees}C, although density per se showed a positive and linear effect at this temperature. Unlike vector competence, oocyst intensities decreased with increasing temperatures with a predominantly positive and linear association with gametocyte density, especially at 28 DTR 9{degrees}C. Oocyst intensities across individual infected midguts suggested temperature-specific differences in mosquito susceptibility/resistance: at 20 DTR 9{degrees}C and 24 DTR 9{degrees}C, dispersion (aggregation) increased in a density-dependent manner but not at 28 DTR 9{degrees}C where the distributions were consistently random. Limitations notwithstanding, our results have manifold implications in, for instance, how variation in temperature could modify seasonal dynamics of infectious reservoirs and transmission and the contribution of high-/patent- and low-density/sub-patent carriers, to suggestions for design and deployment of transmission-blocking vaccines/drugs, but with a cautionary note suggesting how low efficacy could lead to transmission enhancement in certain environments.

ecology↗

bioRxiv2019

Microclimate and larval habitat density predict adult Aedes albopictus abundance in urban areas

The Asian tiger mosquito, Aedes albopictus, transmits several arboviruses of public health importance, including chikungunya and Zika. Since its introduction to the United States in 1985, the species has invaded over forty states, including temperate areas not previously at risk of Aedes-transmitted arboviruses. Mathematical models incorporate climatic variables in predictions of site-specific Ae. albopictus abundances to identify human populations at risk of disease. However, these models rely on coarse resolutions of environmental data that may not accurately represent the climatic profile experienced by mosquitoes in the field, particularly in climatically-heterogeneous urban areas. In this study, we pair field surveys of larval and adult Ae. albopictus mosquitoes with site-specific microclimate data across a range of land use types to investigate the relationships between microclimate, density of larval habitat, and adult mosquito abundance and determine whether these relationships change across an urban gradient. We find no evidence for a difference in larval habitat density or adult abundance between rural, suburban, and urban land classes. Adult abundance increases with increasing larval habitat density, which itself is dependent on microclimate. Adult abundance is strongly explained by microclimate variables, demonstrating that theoretically derived, lab-parameterized relationships in ectotherm physiology apply to the field. Our results provide support for the continued use of temperature-dependent models to predict Ae. albopictus abundance in urban areas.

ecology↗

bioRxiv2018

Impacts of temperature on Zika virus transmission potential: combining empirical and mechanistic modeling approaches

Temperature is a strong driver of vector-borne disease transmission. Yet, for emerging arboviruses we lack fundamental knowledge on the relationship between transmission and temperature. Current models rely on the untested assumption that Zika virus responds similarly to dengue virus, potentially limiting our ability to accurately predict the spread of Zika. We conducted experiments to estimate the thermal performance of Zika virus (ZIKV) in field-derived Aedes aegypti across eight constant temperatures. We observed strong, unimodal effects of temperature on vector competence, extrinsic incubation period, and mosquito survival. We used thermal responses of these traits to update an existing temperature-dependent model to infer temperature effects on ZIKV transmission. ZIKV transmission was optimized at 29{degrees}C, and had a thermal range of 22.7{degrees}C - 34.7{degrees}C. Thus, as temperatures move toward the predicted thermal optimum (29{degrees}C) due to climate change, urbanization, or seasonally, Zika could expand north and into longer seasons. In contrast, areas that are near the thermal optimum were predicted to experience a decrease in overall environmental suitability. We also demonstrate that the predicted thermal minimum for Zika transmission is 5{degrees}C warmer than that of dengue, and current global estimates on the environmental suitability for Zika are greatly over-predicting its possible range.

ecology↗