Rethinking vector immunology: the role of environmental temperature in shaping resistance

Microbial control of arthropod-borne disease

Arthropods harbor a diverse array of microbes that profoundly influence many aspects of host biology, including vector competence. Additionally, symbionts can be engineered to produce molecules that inhibit pathogens. Due to their intimate association with the host, microbes have developed strategies that facilitate their transmission, either horizontally or vertically, to conspecifics. These attributes make microbes attractive agents for applied strategies to control arthropod-borne disease. Here we discuss the recent advances in microbial control approaches to reduce the burden of pathogens such as Zika, Dengue and Chikungunya viruses, and Trypanosome and Plasmodium parasites. We also highlight where further investigation is warranted.

Bacterial Communities Associated with Houseflies (Musca domestica L.) Sampled within and between Farms

The housefly feeds and reproduces in animal manure and decaying organic substances and thus lives in intimate association with various microorganisms including human pathogens. In order to understand the variation and association between bacteria and the housefly, we used 16S rRNA gene amplicon sequencing to describe bacterial communities of 90 individual houseflies collected within and between ten dairy farms in Denmark. Analysis of gene sequences showed that the most abundant classes of bacteria found across all sites included Bacilli, Clostridia, Actinobacteria, Flavobacteria, and all classes of Proteobacteria and at the genus level the most abundant genera included Corynebacterium, Lactobacillus, Staphylococcus, Vagococcus, Weissella, Lactococcus, and Aerococcus. Comparison of the microbiota of houseflies revealed a highly diverse microbiota compared to other insect species and with most variation in species richness and diversity found between individuals, but not locations. Our study is the first in-depth amplicon sequencing study of the housefly microbiota, and collectively shows that the microbiota of single houseflies is highly diverse and differs between individuals likely to reflect the lifestyle of the housefly. We suggest that these results should be taken into account when addressing the transmission of pathogens by the housefly and assessing the vector competence variation under natural conditions.

Engineered Aedes aegypti JAK/STAT Pathway-Mediated Immunity to Dengue Virus

We have developed genetically modified Ae. aegypti mosquitoes that activate the conserved antiviral JAK/STAT pathway in the fat body tissue, by overexpressing either the receptor Dome or the Janus kinase Hop by the blood feeding-induced vitellogenin (Vg) promoter. Transgene expression inhibits infection with several dengue virus (DENV) serotypes in the midgut as well as systemically and in the salivary glands. The impact of the transgenes Dome and Hop on mosquito longevity was minimal, but it resulted in a compromised fecundity when compared to wild-type mosquitoes. Overexpression of Dome and Hop resulted in profound transcriptome regulation in the fat body tissue as well as the midgut tissue, pinpointing several expression signatures that reflect mechanisms of DENV restriction. Our transcriptome studies and reverse genetic analyses suggested that enrichment of DENV restriction factor and depletion of DENV host factor transcripts likely accounts for the DENV inhibition, and they allowed us to identify novel factors that modulate infection. Interestingly, the fat body-specific activation of the JAK/STAT pathway did not result in any enhanced resistance to Zika virus (ZIKV) or chikungunya virus (CHIKV) infection, thereby indicating a possible specialization of the pathway’s antiviral role.

Dengue has represented a significant public health burden for a number of decades, and given the lack of dengue-specific drugs and limited availability of licensed vaccine, new methods for prevention and control are urgently needed. Here, we investigated whether genetic manipulation of the mosquitoes’ native JAK/STAT pathway-mediated anti-DENV defense system could be used to render mosquitoes more resistant to infection. We generated Ae. aegypti mosquitoes overexpressing the JAK/STAT pathway components Dome and Hop under the control of a bloodmeal-inducible, fat body-specific vitellogenin (Vg) promoter. These genetically modified mosquitoes showed an increased resistance to DENV infection, likely because of higher expression of DENV restriction factors and lower expression of DENV host factors, as indicated by transcriptome analyses. Expression of the transgenes had a minimal impact on mosquito longevity; however, it significantly impaired the mosquitoes’ fecundity. Interestingly, bloodmeal-inducible fat body-specific overexpression of either Hop or Dome did not affect mosquito permissiveness to either ZIKV or CHIKV infection, suggesting a possible specialization of JAK/STAT pathway antiviral defenses. Thus, our study is the first to provide a proof-of-concept that genetic engineering of the mosquitoes’ JAK/STAT immune pathway can be used to render this host more resistant to DENV infection.

Do mosquitoes transmit the avian malaria-like parasite Haemoproteus? An experimental test of vector competence using mosquito saliva

Background

The life-cycle of many vector-borne pathogens includes an asexual replication phase in the vertebrate host and sexual reproduction in the insect vector. However, as only a small array of parasites can successfully develop infective phases inside an insect, few insect species are competent vectors for these pathogens. Molecular approaches have identified the potential insect vectors of blood parasites under natural conditions. However, the effectiveness of this methodology for verifying mosquito competence in the transmission of avian malaria parasites and related haemosporidians is still under debate. This is mainly because positive amplifications of parasite DNA in mosquitoes can be obtained not only from sporozoites, the infective phase of the malaria parasites that migrate to salivary glands, but also from different non-infective parasite forms in the body of the vector. Here, we assessed the vectorial capacity of the common mosquito Culex pipiens in the transmission of two parasite genera.

Methods

A total of 1,560 mosquitoes were allowed to feed on five house sparrows Passer domesticus naturally infected by Haemoproteus or co-infected by Haemoproteus/Plasmodium. A saliva sample of the mosquitoes that survived after 13 days post-exposure was taken to determine the presence of parasite DNA by PCR.

Results

Overall, 31.2% mosquito’s head-thorax and 5.8% saliva samples analysed showed positive amplifications for avian malaria parasites. In contrast to Haemoproteus DNA, which was not found in either the body parts or the saliva, Plasmodium DNA was detected in both the head-thorax and the saliva of mosquitoes. Parasites isolated from mosquitoes feeding on the same bird corresponded to the same Plasmodium lineage.

Conclusions

Our experiment provides good evidence for the competence of Cx. pipiens in the transmission of Plasmodium but not of Haemoproteus. Molecular analyses of saliva are an effective method for testing the vector competence of mosquitoes and other insects in the transmission of vector-borne pathogens.

Prediction and Prevention of Parasitic Diseases Using a Landscape Genomics Framework

Substantial heterogeneity exists in the dispersal, distribution and transmission of parasitic species. Understanding and predicting how such features are governed by the ecological variation of landscape they inhabit is the central goal of spatial epidemiology. Genetic data can further inform functional connectivity among parasite, host and vector populations in a landscape. Gene flow correlates with the spread of epidemiologically relevant phenotypes among parasite and vector populations (e.g., virulence, drug and pesticide resistance), as well as invasion and re-invasion risk where parasite transmission is absent due to current or past intervention measures. However, the formal integration of spatial and genetic data ('landscape genetics') is scarcely ever applied to parasites. Here, we discuss the specific challenges and practical prospects for the use of landscape genetics and genomics to understand the biology and control of parasitic disease and present a practical framework for doing so.

Genome Investigations of Vector Competence in Aedes aegypti to Inform Novel Arbovirus Disease Control Approaches

Dengue (DENV), yellow fever, chikungunya, and Zika virus transmission to humans by a mosquito host is confounded by both intrinsic and extrinsic variables. Besides virulence factors of the individual arboviruses, likelihood of virus transmission is subject to variability in the genome of the primary mosquito vector, Aedes aegypti. The “vectorial capacity” of A. aegypti varies depending upon its density, biting rate, and survival rate, as well as its intrinsic ability to acquire, host and transmit a given arbovirus. This intrinsic ability is known as “vector competence”. Based on whole transcriptome analysis, several genes and pathways have been predicated to have an association with a susceptible or refractory response in A. aegypti to DENV infection. However, the functional genomics of vector competence of A. aegypti is not well understood, primarily due to lack of integrative approaches in genomic or transcriptomic studies. In this review, we focus on the present status of genomics studies of DENV vector competence in A. aegypti as limited information is available relative to the other arboviruses. We propose future areas of research needed to facilitate the integration of vector and virus genomics and environmental factors to work towards better understanding of vector competence and vectorial capacity in natural conditions.

Vector competence of Aedes aegypti in transmitting Chikungunya virus: effects and implications of extrinsic incubation temperature on dissemination and infection rates

Background

Aedes aegypti is a competent arthropod vector of chikungunya virus (CHIKV). The rate at which the virus disseminate in the vector is limited by temperature of their environment which can be an important determinant of geographical and seasonal limits to transmission by the arthropods in the tropics. This study investigated the vector competence of Ae. aegypti for CHIKV at ambient temperature of 32 and 26 °C (Coastal and Western Kenya respectively) reared at Extrinsic Incubation Temperature (EIT) of 32 and 26 °C that resembles those in the two regions.

Methods

Ae. aegypti eggs were collected from coastal and Western Kenya, hatched in the insectary and reared to F₁ generation. Four-day old mosquitoes were exposed to CHIKV through a membrane feeding. They were then incubated in temperatures mimicking the mean annual temperatures for Trans-Nzoia (26 °C) and Lamu (32 °C). After every 7, 10 and 13 days post infection (DPI); one third of exposed mosquitoes were sampled and assayed for virus infection and dissemination.

Results

The midgut infection rates (MIR) of Ae. aegypti sampled from Coastal Region was significantly (p < 0.05) higher than those sampled from Western Kenya, with no statistical differences observed for the coastal Ae. aegypti at EIT 26 and at 32 °C. The MIR of Ae. aegypti from the Western Region was significantly (p < 0.05) affected by the EIT, with mosquito reared at EIT 32 °C exhibiting higher MIR than those reared at EIT 26 °C. There was a significant (p < 0.05) interactive effects of the region, EIT and DPI on MIR. The disseminated infection rates for the CHIKV in Ae. aegypti in the legs (DIR-L) was higher in mosquitoes sampled from Coast regardless of the EIT while those from Western Kenya, dissemination rates were significantly higher at higher EIT of 32 °C.

Conclusions

Vector competence was higher in mosquito populations reared under high temperatures which weakens the midgut infection barrier. Hence, suggesting Lamu population is more susceptible to CHIKV therefore having a weaker mid gut infection barrier than the Trans Nzoia population. These underscores importance of examining the course of infection at various ambient temperatures and EIT between regions mosquito populations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-016-0566-7) contains supplementary material, which is available to authorized users.

Larval environment influences vector competence of the malaria mosquito Anopheles gambiae

Background

While environmental factors such as temperature can influence the vector competence of mosquitoes directly, for example by affecting the longevity of the mosquito and the development of the malaria parasite they may also have an indirect impact on the parasite's transmission. By influencing larval development, they may affect the adult traits that are important for the parasite's development and transmission. We studied the influence of two larval environmental factors, food availability and temperature, on the probability that mosquitoes infected with the malaria parasite survived to harbour sporozoites in their salivary glands.

Materials and methods

Anopheles gambiae larvae were reared at 21ºC, 25ºC or 29ºC, and fed either a standard larval diet or half of it. Adults could blood feed on mice harbouring the infectious gametocytic stage of Plasmodium berghei ANKA transformed with green fluorescent protein (GFP). Survival was assessed every 24 hrs up to 21 days post infection, when surviving mosquitoes were dissected to check the salivary glands for sporozoites with a fluorescent microscope sensitive to GFP. Using a binomial GLM we analysed 'vector competence', i.e. if mosquitoes survived until dissection and harboured sporozoites in their salivary glands.

Results

Vector competence dropped by about a third if we fed larvae half the standard food regime. The effect of temperature during the larval period depended strongly on the food regime. At low food, increasing temperature from 21ºC to 29ºC increased vector competence from about 0.18 to 0.48, whereas at standard food, vector competence dropped from about 0.67 at 21ºC to 0.56 at 29ºC.

Conclusions

Thus, perceptions and models about the role of environmental change on the transmission of malaria should include how the environment changes adult life-history by influencing larval development.

Description, molecular characterisation, diagnostics and life cycle of Plasmodium elongatum (lineage pERIRUB01), the virulent avian malaria parasite

Plasmodium elongatum causes severe avian malaria and is distributed worldwide. This parasite is of particular importance due to its ability to develop and cause lethal malaria not only in natural hosts, but also in non-adapted endemic birds such as the brown kiwi and different species of penguins. Information on vectors of this infection is available but is contradictory. PCR-based analysis indicated the possible existence of a cluster of closely related P. elongatum lineages which might differ in their ability to develop in certain mosquitoes and birds. This experimental study provides information about molecular and morphological characterisation of a virulent P. elongatum strain (lineage pERIRUB01) isolated from a naturally infected European robin, Erithacus rubecula. Phylogenetic analysis based on partial cytochrome b gene sequences showed that this parasite lineage is closely related to P. elongatum (lineage pGRW6). Blood stages of both parasite lineages are indistinguishable, indicating that they belong to the same species. Both pathogens develop in experimentally infected canaries, Serinus canaria, causing death of the hosts. In both these lineages, trophozoites and erythrocytic meronts develop in polychromatic erythrocytes and erythroblasts, gametocytes parasitize mature erythrocytes, exoerythrocytic stages develop in cells of the erythrocytic series in bone marrow and are occasionally reported in spleen and liver. Massive infestation of bone marrow cells is the main reason for bird mortality. We report here on syncytium-like remnants of tissue meronts, which slip out of the bone marrow into the peripheral circulation, providing evidence that the syncytia can be a template for PCR amplification. This finding contributes to better understanding positive PCR amplifications in birds when parasitemia is invisible and improved diagnostics of abortive haemosporidian infections. Sporogony of P. elongatum (pERIRUB01) completes the cycle and sporozoites develop in widespread Culex quinquefasciatus and Culex pipiens pipiens form molestus mosquitoes. This experimental study provides information on virulence and within species lineage diversity in a single pathogenic species of haemosporidian parasite.

Biological Impacts of Thermal Extremes: Mechanisms and Costs of Functional Responses Matter

Thermal performance curves enable physiological constraints to be incorporated in predictions of biological responses to shifts in mean temperature. But do thermal performance curves adequately capture the biological impacts of thermal extremes? Organisms incur physiological damage during exposure to extremes, and also mount active compensatory responses leading to acclimatization, both of which alter thermal performance curves and determine the impact that current and future extremes have on organismal performance and fitness. Thus, these sub-lethal responses to extreme temperatures potentially shape evolution of thermal performance curves. We applied a quantitative genetic model and found that beneficial acclimatization and cumulative damage alter the extent to which thermal performance curves evolve in response to thermal extremes. The impacts of extremes on the evolution of thermal performance curves are reduced if extremes cause substantial mortality or otherwise reduce fitness differences among individuals. Further empirical research will be required to understand how responses to extremes aggregate through time and vary across life stages and processes. Such research will enable incorporating passive and active responses to sub-lethal stress when predicting the impacts of thermal extremes.

Insect immunology and hematopoiesis

Insects combat infection by mounting powerful immune responses that are mediated by hemocytes, the fat body, the midgut, the salivary glands and other tissues. Foreign organisms that have entered the body of an insect are recognized by the immune system when pathogen-associated molecular patterns bind host-derived pattern recognition receptors. This, in turn, activates immune signaling pathways that amplify the immune response, induce the production of factors with antimicrobial activity, and activate effector pathways. Among the immune signaling pathways are the Toll, Imd, Jak/Stat, JNK, and insulin pathways. Activation of these and other pathways leads to pathogen killing via phagocytosis, melanization, cellular encapsulation, nodulation, lysis, RNAi-mediated virus destruction, autophagy and apoptosis. This review details these and other aspects of immunity in insects, and discusses how the immune and circulatory systems have co-adapted to combat infection, how hemocyte replication and differentiation takes place (hematopoiesis), how an infection prepares an insect for a subsequent infection (immune priming), how environmental factors such as temperature and the age of the insect impact the immune response, and how social immunity protects entire groups. Finally, this review highlights some underexplored areas in the field of insect immunobiology.

Temperature and population density: interactional effects of environmental factors on phenotypic plasticity, immune defenses, and disease resistance in an insect pest

Temperature and crowding are key environmental factors mediating the transmission and epizooty of infectious disease in ectotherm animals. The host physiology may be altered in a temperature‐dependent manner and thus affects the pathogen development and course of diseases within an individual and host population, or the transmission rates (or infectivity) of pathogens shift linearly with the host population density. To our understanding, the knowledge of interactive and synergistic effects of temperature and population density on the host–pathogen system is limited. Here, we tested the interactional effects of these environmental factors on phenotypic plasticity, immune defenses, and disease resistance in the velvetbean caterpillar Anticarsia gemmatalis. Upon egg hatching, caterpillars were reared in thermostat‐controlled chambers in a 2 × 4 factorial design: density (1 or 8 caterpillars/pot) and temperature (20, 24, 28, or 32°C). Of the immune defenses assessed, encapsulation response was directly affected by none of the environmental factors; capsule melanization increased with temperature in both lone‐ and group‐reared caterpillars, although the lone‐reared ones presented the most evident response, and hemocyte numbers decreased with temperature regardless of the population density. Temperature, but not population density, affected considerably the time from inoculation to death of velvetbean caterpillar. Thus, velvetbean caterpillars succumbed to Anticarsia gemmatalis multiple nucleopolyhedrovirus (AgMNPV) more quickly at higher temperatures than at lower temperatures. As hypothesized, temperature likely affected caterpillars' movement rates, and thus the contact between conspecifics, which in turn affected the phenotypic expression of group‐reared caterpillars. Our results suggest that environmental factors, mainly temperature, strongly affect both the course of disease in velvetbean caterpillar population and its defenses against pathogens. As a soybean pest, velvetbean caterpillar may increase its damage on soybean fields under a scenario of global warming as caterpillars may reach the developmental resistance faster, and thus decrease their susceptibility to biological control by AgMNPV.

Analysis of a temperature- and rainfall-dependent model for malaria transmission dynamics

A new non-autonomous model is designed and used to assess the impact of variability in temperature and rainfall on the transmission dynamics of malaria in a population. In addition to adding age-structure in the host population and the dynamics of immature malaria mosquitoes, a notable feature of the new model is that recovered individuals do not revert to wholly-susceptible class (that is, recovered individuals enjoy reduced susceptibility to new malaria infection). In the absence of disease-induced mortality, the disease-free solution of the model is shown to be globally-asymptotically stable when the associated reproduction ratio is less than unity. The model has at least one positive periodic solution when the reproduction ratio exceeds unity (and the disease persists in the community in this case). Detailed uncertainty and sensitivity analysis, using mean monthly temperature and rainfall data from KwaZulu-Natal province of South Africa, shows that the top three parameters of the model that have the most influence on the disease transmission dynamics are the mosquito carrying capacity, transmission probability per contact for susceptible mosquitoes and human recovery rate. Numerical simulations of the model show that, for the KwaZulu-Natal province, malaria burden increases with increasing mean monthly temperature and rainfall in the ranges ([17-25]°C and [32-110] mm), respectively (and decreases with decreasing mean monthly temperature and rainfall values). In particular, transmission is maximized for mean monthly temperature and rainfall in the ranges [21-25]°C and [95-125] mm. This occurs for a six-month period in KwaZulu-Natal (hence, this study suggests that anti-malaria control efforts should be intensified during this period). It is shown, for the fixed mean monthly temperature of KwaZulu-Natal, that malaria burden decreases whenever the amount of rainfall exceeds a certain threshold value. It is further shown (through sensitivity analysis and numerical simulations) that incorporating host age-structure and reduced susceptibility due to prior malaria infection has marginal effect on the transmission dynamics of the disease.

The Microbiome of Animals: Implications for Conservation Biology

In recent years the human microbiome has become a growing area of research and it is becoming clear that the microbiome of humans plays an important role for human health. Extensive research is now going into cataloging and annotating the functional role of the human microbiome. The ability to explore and describe the microbiome of any species has become possible due to new methods for sequencing. These techniques allow comprehensive surveys of the composition of the microbiome of nonmodel organisms of which relatively little is known. Some attention has been paid to the microbiome of insect species including important vectors of pathogens of human and veterinary importance, agricultural pests, and model species. Together these studies suggest that the microbiome of insects is highly dependent on the environment, species, and populations and affects the fitness of species. These fitness effects can have important implications for the conservation and management of species and populations. Further, these results are important for our understanding of invasion of nonnative species, responses to pathogens, and responses to chemicals and global climate change in the present and future.

Disease Dynamics in Ants: A Critical Review of the Ecological Relevance of Using Generalist Fungi to Study Infections in Insect Societies

It is assumed that social life can lead to the rapid spread of infectious diseases and outbreaks. In ants, disease outbreaks are rare and the expression of collective behaviors is invoked to explain the absence of epidemics in natural populations. Here, we address the ecological approach employed by many studies that have notably focused (89% of the studies) on two genera of generalist fungal parasites (Beauveria and Metarhizium). We ask whether these are the most representative models to study the evolutionary ecology of ant-fungal parasite interactions. To assess this, we critically examine the literature on ants and their interactions with fungal parasites from the past 114years (1900-2014). We discuss how current evolutionary ecology approaches emerged from studies focused on the biological control of pest ants. We also analyzed the ecological relevance of the laboratory protocols used in evolutionary ecology studies employing generalist parasites, as well as the rare natural occurrence of these parasites on ants. After a detailed consideration of all the publications, we suggest that using generalist pathogens such as Beauveria and Metarhizium is not an optimal approach if the goal is to study the evolutionary ecology of disease in ants. We conclude by advocating for approaches that incorporate greater realism.

The Effect of Temperature on Wolbachia-Mediated Dengue Virus Blocking in Aedes aegypti

Dengue fever, caused by dengue virus (DENV), is endemic in more than 100 countries. The lack of effective treatment of patients and the suboptimal efficacies of the tetravalent vaccine in trials highlight the urgent need to develop alternative strategies to lessen the burden of dengue fever.Wolbachia pipientis, an obligate intracellular bacterium, is being developed as a biocontrol strategy against dengue because it limits the replication of the DENV in the mosquito vector,Aedes aegypti However, several recent studies have demonstrated the sensitivity of pathogens, vectors, and their symbionts to temperature. To understand how the tripartite interactions between the mosquito, DENV, and Wolbachia may change under different temperature regimes, we assessed the vector competence and transmission potential of DENV-infected mosquitoes reared at a common laboratory setting of a constant 25°C and at two diurnal temperature settings with mean of 25°C and 28°C and a fluctuating range of 8°C (±4°C). Temperature significantly affected DENV infection rate in the mosquitoes. Furthermore, temperature significantly influenced the proportion of mosquitoes that achieved transmission potential as measured by the presence of virus in the saliva. Regardless of the temperature regimes,Wolbachia significantly and efficiently reduced the proportion of mosquitoes achieving infection and transmission potential across all the temperature regimes studied. This work reinforces the robustness of the Wolbachia biocontrol strategy to field conditions in Cairns, Australia, and suggests that similar studies are required for local mosquito genotypes and field relevant temperatures for emerging field release sites globally.

Spatio-temporal dynamics of mosquitoes in stream pools of a biosphere reserve of Southern Western Ghats, India

The spatial and temporal dynamics of mosquitoes in stream pools were examined in a biosphere reserve of the Southern Western Ghats, India. The immature mosquitoes in stream pools were collected from stream substrates of bedrock pool, boulder cavity and sand puddle. The collected larvae and pupae were reared and identified. In total, 16 species from four genera of mosquitoes were collected. The mosquito species from Culex and Anopheles were predominantly occurred. The bedrock pool had the highest diversity and abundance of mosquitoes. The statistical analyses showed that the substrate specificity and the seasons were positively related to the distribution of mosquitoes rather than spatial pattern. This study described the spatial and temporal pattern of mosquitoes in stream pools of the Southern Western Ghats. This information would be helpful to National Vector borne disease control program for surveillance and control.

Climate influences on the cost-effectiveness of vector-based interventions against malaria in elimination scenarios

Despite the dependence of mosquito population dynamics on environmental conditions, the associated impact of climate and climate change on present and future malaria remains an area of ongoing debate and uncertainty. Here, we develop a novel integration of mosquito, transmission and economic modelling to assess whether the cost-effectiveness of indoor residual spraying (IRS) and long-lasting insecticidal nets (LLINs) against Plasmodium falciparum transmission by Anopheles gambiae s.s. mosquitoes depends on climatic conditions in low endemicity scenarios. We find that although temperature and rainfall affect the cost-effectiveness of IRS and/or LLIN scale-up, whether this is sufficient to influence policy depends on local endemicity, existing interventions, host immune response to infection and the emergence rate of insecticide resistance. For the scenarios considered, IRS is found to be more cost-effective than LLINs for the same level of scale-up, and both are more cost-effective at lower mean precipitation and higher variability in precipitation and temperature. We also find that the dependence of peak transmission on mean temperature translates into optimal temperatures for vector-based intervention cost-effectiveness. Further cost-effectiveness analysis that accounts for country-specific epidemiological and environmental heterogeneities is required to assess optimal intervention scale-up for elimination and better understand future transmission trends under climate change.

Climate, environmental and socio-economic change: weighing up the balance in vector-borne disease transmission

Arguably one of the most important effects of climate change is the potential impact on human health. While this is likely to take many forms, the implications for future transmission of vector-borne diseases (VBDs), given their ongoing contribution to global disease burden, are both extremely important and highly uncertain. In part, this is owing not only to data limitations and methodological challenges when integrating climate-driven VBD models and climate change projections, but also, perhaps most crucially, to the multitude of epidemiological, ecological and socio-economic factors that drive VBD transmission, and this complexity has generated considerable debate over the past 10-15 years. In this review, we seek to elucidate current knowledge around this topic, identify key themes and uncertainties, evaluate ongoing challenges and open research questions and, crucially, offer some solutions for the field. Although many of these challenges are ubiquitous across multiple VBDs, more specific issues also arise in different vector-pathogen systems.

The potential for fungal biopesticides to reduce malaria transmission under diverse environmental conditions

The effectiveness of conventional malaria vector control is being threatened by the spread of insecticide resistance. One promising alternative to chemicals is the use of naturally-occurring insect-killing fungi. Numerous laboratory studies have shown that isolates of fungal pathogens such as Beauveria bassiana can infect and kill adult mosquitoes, including those resistant to chemical insecticides.Unlike chemical insecticides, fungi may take up to a week or more to kill mosquitoes following exposure. This slow kill speed can still reduce malaria transmission because the malaria parasite itself takes at least eight days to complete its development within the mosquito. However, both fungal virulence and parasite development rate are strongly temperature-dependent, so it is possible that biopesticide efficacy could vary across different transmission environments.We examined the virulence of a candidate fungal isolate against two key malaria vectors at temperatures from 10-34 °C. Regardless of temperature, the fungus killed more than 90% of exposed mosquitoes within the predicted duration of the malarial extrinsic incubation period, a result that was robust to realistic diurnal temperature variation.We then incorporated temperature sensitivities of a suite of mosquito, parasite and fungus life-history traits that are important determinants of malaria transmission into a stage-structured malaria transmission model. The model predicted that, at achievable daily fungal infection rates, fungal biopesticides have the potential to deliver substantial reductions in the density of malaria-infectious mosquitoes across all temperatures representative of malaria transmission environments.Synthesis and applications. Our study combines empirical data and theoretical modelling to prospectively evaluate the potential of fungal biopesticides to control adult malaria vectors. Our results suggest that Beauveria bassiana could be a potent tool for malaria control and support further development of fungal biopesticides to manage infectious disease vectors.

The potential for fungal biopesticides to reduce malaria transmission under diverse environmental conditions

The effectiveness of conventional malaria vector control is being threatened by the spread of insecticide resistance. One promising alternative to chemicals is the use of naturally-occurring insect-killing fungi. Numerous laboratory studies have shown that isolates of fungal pathogens such as Beauveria bassiana can infect and kill adult mosquitoes, including those resistant to chemical insecticides.Unlike chemical insecticides, fungi may take up to a week or more to kill mosquitoes following exposure. This slow kill speed can still reduce malaria transmission because the malaria parasite itself takes at least eight days to complete its development within the mosquito. However, both fungal virulence and parasite development rate are strongly temperature-dependent, so it is possible that biopesticide efficacy could vary across different transmission environments.We examined the virulence of a candidate fungal isolate against two key malaria vectors at temperatures from 10-34 °C. Regardless of temperature, the fungus killed more than 90% of exposed mosquitoes within the predicted duration of the malarial extrinsic incubation period, a result that was robust to realistic diurnal temperature variation.We then incorporated temperature sensitivities of a suite of mosquito, parasite and fungus life-history traits that are important determinants of malaria transmission into a stage-structured malaria transmission model. The model predicted that, at achievable daily fungal infection rates, fungal biopesticides have the potential to deliver substantial reductions in the density of malaria-infectious mosquitoes across all temperatures representative of malaria transmission environments.Synthesis and applications. Our study combines empirical data and theoretical modelling to prospectively evaluate the potential of fungal biopesticides to control adult malaria vectors. Our results suggest that Beauveria bassiana could be a potent tool for malaria control and support further development of fungal biopesticides to manage infectious disease vectors.

Simulated Seasonal Photoperiods and Fluctuating Temperatures Have Limited Effects on Blood Feeding and Life History in Aedes triseriatus (Diptera: Culicidae)

Biotic and abiotic factors change seasonally and impact life history in temperate-zone ectotherms. Temperature and photoperiod are factors that change in predictable ways. Most studies testing for effects of temperature on vectors use constant temperatures and ignore potential correlated effects of photoperiod. In two experiments, we tested for effects of larval rearing environments creating ecologically relevant temperatures and photoperiods simulating early and late season conditions (June and August), or constant temperatures (cool and warm) with the June or August photoperiods, respectively. We determined effects on survivorship, development, size, and a composite performance index in a temperate-zone population of Aedes triseriatus (Say). We followed cohorts of resulting females, all held under the same environmental conditions, to assess carry-over effects of rearing conditions for larvae on longevity, blood feeding, and egg production. Larval survivorship was affected by treatment in one experiment. Development time was greater in the June and cool treatments, but the constant and fluctuating temperatures did not differ. Significantly larger mosquitoes were produced in fluctuating versus constant temperature treatments. There were no significant treatment effects on the composite performance index. Adult female longevity was lower after rearing at constant versus fluctuating temperature, but there was no difference between June and August, nor did size affect longevity. There was no effect of treatments on blood feeding and a limited effect on egg production. We conclude that seasonal temperatures and photoperiods during development have limited effects on this population of A. triseriatus and find little evidence of strong effects of fluctuating versus constant temperatures.

Connecting genotypes to medically relevant phenotypes in major vector mosquitoes

Transmission of mosquito-borne human disease relies on vectors maintaining strong human host preference and continued susceptibility to disease-causing pathogens or parasites. These traits are affected by the genetics and the environments of all involved organisms, and genotypic interactions are common between parasite and vector, and between virus and vector. A recent study on Aedes host preference has exploited natural genetic variation to make great progress. Here I review our current understanding of the genetic basis of transmission-relevant traits in Anopheles and Aedes, highlighting additional research areas that would benefit from the integration of natural genetic variation.

Three-way interactions between mosquito population, viral strain and temperature underlying chikungunya virus transmission potential

Interactions between pathogens and their insect vectors in nature are under the control of both genetic and non-genetic factors, yet most studies on mosquito vector competence for human pathogens are conducted in laboratory systems that do not consider genetic and/or environmental variability. Evaluating the risk of emergence of arthropod-borne viruses (arboviruses) of public health importance such as chikungunya virus (CHIKV) requires a more realistic appraisal of genetic and environmental contributions to vector competence. In particular, sources of variation do not necessarily act independently and may combine in the form of interactions. Here, we measured CHIKV transmission potential by the mosquito Aedes albopictus in all combinations of six worldwide vector populations, two virus strains and two ambient temperatures (20°C and 28°C). Overall, CHIKV transmission potential by Ae. albopictus strongly depended on the three-way combination of mosquito population, virus strain and temperature. Such genotype-by-genotype-by-environment (G × G × E) interactions question the relevance of vector competence studies conducted with a simpler set of conditions. Our results highlight the need to account for the complex interplay between vectors, pathogens and environmental factors to accurately assess the potential of vector-borne diseases to emerge.

Interactions between a fungal entomopathogen and malaria parasites within a mosquito vector

BACKGROUND

Mosquitoes are becoming increasingly resistant to the chemical insecticides currently available for malaria vector control, spurring interest in alternative management tools. One promising technology is the use of fungal entomopathogens. Fungi have been shown to impact the potential for mosquitoes to transmit malaria by reducing mosquito longevity and altering behaviour associated with flight and host location. Additionally, fungi could impact the development of malaria parasites within the mosquito via competition for resources or effects on the mosquito immune system. This study evaluated whether co-infection or superinfection with the fungal entomopathogen Beauveria bassiana affected malaria infection progress in Anopheles stephensi mosquitoes.

METHODS

The study used two parasite species to examine possible effects of fungal infection at different parasite development stages. First, the rodent malaria model Plasmodium yoelii was used to explore interactions at the oocyst stage. Plasmodium yoelii produces high oocyst densities in infected mosquitoes and thus was expected to maximize host immunological and resource demands. Second, fungal interactions with mature sporozoites were evaluated by infecting mosquitoes with the human malaria species Plasmodium falciparum, which is highly efficient at invading mosquito salivary glands.

RESULTS

With P. yoelii, there was no evidence that fungal co-infection (on the same day as the blood meal) or superinfection (during a subsequent gonotrophic cycle after parasite infection) affected the proportion of mosquitoes with oocysts, the number of oocysts per infected mosquito or the number of sporozoites per oocyst. Similarly, for P. falciparum, there was no evidence that fungal infection affected sporozoite prevalence. Furthermore, there was no impact of infection with either malaria species on fungal virulence as measured by mosquito survival time.

CONCLUSIONS

These results suggest that the impact of fungus on malaria control potential is limited to the well-established effects on mosquito survival and transmission behaviour. Direct or indirect interactions between fungus and malaria parasites within mosquitoes appear to have little additional influence.

Seasonal drivers of the epidemiology of arthropod-borne viruses in Australia

Arthropod-borne viruses are a major cause of emerging disease with significant public health and economic impacts. However, the factors that determine their activity and seasonality are not well understood. In Australia, a network of sentinel cattle herds is used to monitor the distribution of several such viruses and to define virus-free regions. Herein, we utilize these serological data to describe the seasonality, and its drivers, of three economically important animal arboviruses: bluetongue virus, Akabane virus and bovine ephemeral fever virus. Through epidemiological time-series analyses of sero-surveillance data of 180 sentinel herds between 2004–2012, we compared seasonal parameters across latitudes, ranging from the tropical north (−10°S) to the more temperate south (−40°S). This analysis revealed marked differences in seasonality between distinct geographic regions and climates: seasonality was most pronounced in southern regions and gradually decreased as latitude decreased toward the Equator. Further, we show that both the timing of epidemics and the average number of seroconversions have a strong geographical component, which likely reflect patterns of vector abundance through co-varying climatic factors, especially temperature and rainfall. Notably, despite their differences in biology, including insect vector species, all three viruses exhibited very similar seasonality. By revealing the factors that shape spatial and temporal distributions, our study provides a more complete understanding of arbovirus seasonality that will enable better risk predictions.

Arthropod-borne viruses (arboviruses) are a group of viruses that can have major impacts on public health, animal health and agricultural trade, and appear to be increasing in both number and prevalence worldwide. Despite their importance as emerging pathogens, the spatial patterns, long-term seasonal characteristics and drivers of seasonality in many arboviruses are poorly understood. The island continent of Australia provides an ideal case study for the spatial analysis of emerging arboviruses, harboring diverse climatic conditions across a wide range of latitudes. Herein we utilize long-term serological data from a nationwide network of sentinel herds in Australia to describe the seasonality of three economically important animal arboviruses: bluetongue virus, Akabane virus and bovine ephemeral fever virus. Using epidemiological time series analysis, we demonstrate that these viruses exhibit a distinct spatial pattern in both the peak timing and intensity of annual epidemic cycles, with the strongest seasonality observed in southerly geographic regions. In addition, we reveal the climatic factors that drive patterns of arbovirus distribution and, by doing so, provide a more complete understanding of arbovirus seasonality, which in turn will improve the risk assessment of these viruses.

The insect microbiome modulates vector competence for arboviruses

Diseases caused by arthropod-borne viruses (arboviruses), such as Dengue, West Nile, and Chikungunya, constitute a major global health burden and are increasing in incidence and geographic range. The natural microbiota of insect vectors influences various aspects of host biology, such as nutrition, reproduction, metabolism, and immunity, and recent studies have highlighted the ability of insect-associated bacteria to reduce vector competence for arboviruses and other pathogens. This reduction can occur through mechanisms, such as immune response activation, resource competition, or the production of anti-viral molecules. Studying the interactions between insect vectors and their microbiota is an important step toward developing alternative strategies for arbovirus transmission control.

Ambient temperature and dietary supplementation interact to shape mosquito vector competence for malaria

The extent to which environmental factors influence the ability of Anopheles mosquitoes to transmit malaria parasites remains poorly explored. Environmental variation, such as change in ambient temperature, will not necessarily influence the rates of host and parasite processes equivalently, potentially resulting in complex effects on infection outcomes. As proof of principle, we used Anopheles stephensi and the rodent malaria parasite, Plasmodium yoelii, to examine the effects of a range of constant temperatures on one aspect of host defense (detected as alterations in expression of nitric oxide synthase gene - NOS) to parasite infection. We experimentally boosted mosquito midgut immunity to infection through dietary supplementation with the essential amino acid l-Arginine (l-Arg), which increases midgut nitric oxide (NO) levels by infection-induced NOS catalysis in A. stephensi. At intermediate temperatures, supplementation reduced oocyst prevalence, oocyst intensity, and sporozoite prevalence suggesting that the outcome of parasite infection was potentially dependent upon the rate of NOS-mediated midgut immunity. At low and high temperature extremes, however, infection was severely constrained irrespective of supplementation. The effects of l-Arg appeared to be mediated by NO-dependent negative feedback on NOS expression, as evidenced by depressed NOS expression in l-Arg treated groups at temperatures where supplementation decreased parasite infection. These results suggest the need to consider the direct (e.g. effects of mosquito body temperature on parasite physiology) and indirect effects (e.g. mediated through changes in mosquito physiology/immunity) of environmental factors on mosquito-malaria interactions in order to understand natural variation in vector competence.

Temporal association of ambient temperature and relative humidity with Spirocerca lupi infection of Onthophagus sellatus: a 14-year longitudinal study

The parasitic nematode Spirocerca lupi causes major morbidity and mortality in dogs. The scarab beetle Onthophagus sellatus is its major intermediate host in Israel. We investigated the prevalence of beetle infection by S. lupi in different years between 1994 and 2008. The average monthly maximum and minimum relative humidity (RH) and ambient temperature (AT) throughout the study period were calculated based on daily meteorological data. The infection prevalence decreased over the study period, possibly due to a chronological change resulting from increased preventive treatment of dogs against S. lupi, or climate change. Multivariate analysis was performed for these two hypotheses. Under the first hypothesis, chronological change was forced into the model, and environmental variables were inserted stepwise. The final model included beetle-collection date, minimum RH (RH min) during the month preceding beetle collection, its interaction with maximal AT (AT max) during that same month, and the interaction of maximal RH (RH max) and AT max, during the month of beetle collection. Under the second hypothesis, chronological change was not forced. The final model included RH max during the month of beetle collection, average RH (RHave) during the month preceding beetle collection, and its interaction with AT max during the latter month. The results suggest that under both hypotheses, RH and AT during the month preceding beetle collection influence S. lupi's ability to develop and survive in O. sellatus, and may be used to predict the risk to dogs of S. lupi infection.

Integrating temperature and nutrition--environmental impacts on an insect immune system

Globally increasing temperatures may strongly affect insect herbivore performance. In contrast to direct effects of temperature on herbivores, indirect effects mediated via thermal effects on host-plant quality are only poorly understood, despite having the potential to substantially impact the herbivores' performance. Part of this performance is the organisms' immune system which may be of pivotal importance for local survival. We here use a full-factorial design to explore the direct (larvae were reared at 17°C or 25°C) and indirect effects (host plants were reared at 17°C or 25°C) of temperature on immune function of the temperate-zone butterfly Pieris napi. At the higher rearing temperature haemocyte numbers and prophenoloxidase activity were reduced. Plant temperature, in contrast, did not affect immune competence despite clear effects on insect growth patterns. Overall, thermal and dietary impacts on the insects' immune responses were weak and trait-specific.

The effects of temperature and innate immunity on transmission of Campylobacter jejuni (Campylobacterales: Campylobacteraceae) between life stages of Musca domestica (Diptera: Muscidae)

The house fly (Musca domestica L.) is a well-established vector of human pathogens, including Campylobacter spp., which can cause infection of broiler chicken flocks, and through contaminated broiler meat can cause outbreaks of campylobacteriosis in humans. We investigated whether Campylobacter jejuni (Jones) could be transferred between life stages of M. domestica (larvae-pupae-adults) and determined bacterial counts of C. jejuni at different time points after bacterial exposure. C. jejuni was transmitted from infected larvae to pupae, but not to the adult stage. Infected larvae maintained at 25 degrees C had mean bacterial numbers of 6.5 +/- 0.2 SE log10 (colony forming units [CFU]/g) that subsequently dropped to 3.6 +/- 0.3 SE log10 (CFU/g) 8 h after infection. Pupae originating from infected larvae contained mean bacterial numbers of 5.3 +/- 0.1 SE log10 (CFU/g), and these numbers dropped to 4.8 +/- 0.1 SE log10 (CFU/g) 24 h after pupation. The decline in C. jejuni numbers during pupal development coincided with increased expression of antimicrobial peptides, including cecropin, diptericin, attacin, and defensin, in the larva-pupa transition stage and a later second peak in older pupae (4 or 48 h). Conversely, there was a reduced expression of the digestive enzyme, lysozyme, in pupae and adults compared with larvae.

Plant immune response to pathogens differs with changing temperatures

Temperature fluctuation is a key determinant for microbial invasion and host evasion. In contrast to mammals that maintain constant body temperature, plant temperature oscillates on a daily basis. It remains elusive how plants operate inducible defenses in response to temperature fluctuation. Here we report that ambient temperature changes lead to pronounced shifts of the following two distinct plant immune responses: pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). Plants preferentially activate ETI signaling at relatively low temperatures (10-23 °C), whereas they switch to PTI signaling at moderately elevated temperatures (23-32 °C). The Arabidopsis arp6 and hta9hta11 mutants, phenocopying plants grown at elevated temperatures, exhibit enhanced PTI and yet reduced ETI responses. As the secretion of bacterial effectors favours low temperatures, whereas bacteria multiply vigorously at elevated temperatures accompanied with increased microbe-associated molecular pattern production, our findings suggest that temperature oscillation might have driven dynamic co-evolution of distinct plant immune signaling responding to pathogen physiological changes.

Temperature alters Plasmodium blocking by Wolbachia

Very recently, the Asian malaria vector (Anopheles stephensi) was stably transinfected with the wAlbB strain of Wolbachia, inducing refractoriness to the human malaria parasite Plasmodium falciparum. However, conditions in the field can differ substantially from those in the laboratory. We use the rodent malaria P. yoelii, and somatically transinfected An. stephensi as a model system to investigate whether the transmission blocking potential of wAlbB is likely to be robust across different thermal environments. wAlbB reduced malaria parasite prevalence and oocyst intensity at 28°C. At 24°C there was no effect on prevalence but a marked increase in oocyst intensity. At 20°C, wAlbB had no effect on prevalence or intensity. Additionally, we identified a novel effect of wAlbB that resulted in reduced sporozoite development across temperatures, counterbalancing the oocyst enhancement at 24°C. Our results demonstrate complex effects of temperature on the Wolbachia-malaria interaction, and suggest the impacts of transinfection might vary across diverse environments.

Natural malaria infection reduces starvation resistance of nutritionally stressed mosquitoes

In disease ecology, there is growing evidence that environmental quality interacts with parasite and host to determine host susceptibility to an infection. Most studies of malaria parasites have focused on the infection costs incurred by the hosts, and few have investigated the costs on mosquito vectors. The interplay between the environment, the vector and the parasite has therefore mostly been ignored and often relied on unnatural or allopatric Plasmodium/vector associations. Here, we investigated the effects of natural avian malaria infection on both fecundity and survival of field-caught female Culex pipiens mosquitoes, individually maintained in laboratory conditions. We manipulated environmental quality by providing mosquitoes with different concentrations of glucose-feeding solution prior to submitting them to a starvation challenge. We used molecular-based methods to assess mosquitoes' infection status. We found that mosquitoes infected with Plasmodium had lower starvation resistance than uninfected ones only under low nutritional conditions. The effect of nutritional stress varied with time, with the difference of starvation resistance between optimally and suboptimally fed mosquitoes increasing from spring to summer, as shown by a significant interaction between diet treatment and months of capture. Infected and uninfected mosquitoes had similar clutch size, indicating no effect of infection on fecundity. Overall, this study suggests that avian malaria vectors may suffer Plasmodium infection costs in their natural habitat, under certain environmental conditions. This may have major implications for disease transmission in the wild.

Finding the appropriate variables to model the distribution of vector-borne parasites with different environmental preferences: climate is not enough

Understanding how environmental variation influences the distribution of parasite diversity is critical if we are to anticipate disease emergence risks associated with global change. However, choosing the relevant variables for modelling current and future parasite distributions may be difficult: candidate predictors are many, and they seldom are statistically independent. This problem often leads to simplistic models of current and projected future parasite distributions, with climatic variables prioritized over potentially important landscape features or host population attributes. We studied avian blood parasites of the genera Plasmodium, Haemoproteus and Leucocytozoon (which are viewed as potential emergent pathogens) in 37 Iberian blackcap Sylvia atricapilla populations. We used Partial Least Squares regression to assess the relative importance of a wide array of putative determinants of variation in the diversity of these parasites, including climate, landscape features and host population migration. Both prevalence and richness of parasites were predominantly related to climate (an effect which was primarily, but not exclusively driven by variation in temperature), but landscape features and host migration also explained variation in parasite diversity. Remarkably, different models emerged for each parasite genus, although all parasites were studied in the same host species. Our results show that parasite distribution models, which are usually based on climatic variables alone, improve by including other types of predictors. Moreover, closely related parasites may show different relationships to the same environmental influences (both in magnitude and direction). Thus, a model used to develop one parasite distribution can probably not be applied identically even to the most similar host-parasite systems.

Exploitation of natural genetic diversity to study plant-virus interactions: what can we learn from Arabidopsis thaliana?

The development and use of cultivars that are genetically resistant to viruses is an efficient strategy to tackle the problems of virus diseases. Over the past two decades, the model plant Arabidopsis thaliana has been documented as a host for a broad range of viral species, providing access to a large panel of resources and tools for the study of viral infection processes and resistance mechanisms. Exploration of its natural genetic diversity has revealed a wide range of genes conferring virus resistance. The molecular characterization of some of these genes has unveiled resistance mechanisms distinct from those described in crops. In these respects, Arabidopsis represents a rich and largely untapped source of new genes and mechanisms involved in virus resistance. Here, we review the current status of our knowledge concerning natural virus resistance in Arabidopsis. We also address the impact of environmental conditions on Arabidopsis-virus interactions and resistance mechanisms, and discuss the potential of applying the knowledge gained from the study of Arabidopsis natural diversity for crop improvement.

Complex environmental drivers of immunity and resistance in malaria mosquitoes

Considerable research effort has been directed at understanding the genetic and molecular basis of mosquito innate immune mechanisms. Whether environmental factors interact with these mechanisms to shape overall resistance remains largely unexplored. Here, we examine how changes in mean ambient temperature, diurnal temperature fluctuation and time of day of infection affected the immunity and resistance of Anopheles stephensi to infection with Escherichia coli. We used quantitative PCR to estimate the gene expression of three immune genes in response to challenge with heat-killed E. coli. We also infected mosquitoes with live E. coli and ran bacterial growth assays to quantify host resistance. Both mosquito immune parameters and resistance were directly affected by mean temperature, diurnal temperature fluctuation and time of day of infection. Furthermore, there was a suite of complex two- and three-way interactions yielding idiosyncratic phenotypic variation under different environmental conditions. The results demonstrate mosquito immunity and resistance to be strongly influenced by a complex interplay of environmental variables, challenging the interpretation of the very many mosquito immune studies conducted under standard laboratory conditions.

Assessment of survival and body size variation of Culicoides imicola (Diptera: Ceratopogonidae) as functions of "Candidatus Cardinium" (Bacteroidetes) infection status

"Candidatus Cardinium hertigii" (Bacteroidetes) is a maternally inherited endosymbiont known from several arthropods. Its mechanisms for persistence in host populations are mostly reproductive manipulation, though it has been occasionally reported to improve fitness parameters in several hosts. In Culicoides (Diptera: Ceratopogonidae) biting midges, the prevalence of "Candidatus Cardinium" infection was documented as moderate, with no detectable sex bias. We therefore investigated whether "Candidatus Cardinium" affects important fitness parameters, such as survival and body size, in Culicoides imicola, a dominant vector species. Field-collected midges were trapped and analyzed for survival under different environmental conditions and antibiotic treatment, taking into account "Candidatus Cardinium" infection status and parity status (i.e., parous or nulliparous). Additionally, wing lengths were measured as a proxy parameter for body size and analyzed together with "Candidatus Cardinium" infection data. The findings revealed no difference in survival of Culicoides infected with "Candidatus Cardinium" and that of uninfected midges in both parity states and under all tested conditions: optimal, starvation, heat, and antibiotic treatment. Beyond survival, no wing length difference was found for "Candidatus Cardinium"-infected versus uninfected midges. In aggregate, these findings support our conclusion that "Candidatus Cardinium" does not have an overt effect on the survival and size of adult C. imicola midges. "Candidatus Cardinium" may affect immature stages or may alter adult reproductive performance.

The effects of climate change and globalization on mosquito vectors: evidence from Jeju Island, South Korea on the potential for Asian tiger mosquito (Aedes albopictus) influxes and survival from Vietnam rather than Japan

Background

Climate change affects the survival and transmission of arthropod vectors as well as the development rates of vector-borne pathogens. Increased international travel is also an important factor in the spread of vector-borne diseases (VBDs) such as dengue, West Nile, yellow fever, chikungunya, and malaria. Dengue is the most important vector-borne viral disease. An estimated 2.5 billion people are at risk of infection in the world and there are approximately 50 million dengue infections and an estimated 500,000 individuals are hospitalized with dengue haemorrhagic fever annually. The Asian tiger mosquito (Aedes albopictus) is one of the vectors of dengue virus, and populations already exist on Jeju Island, South Korea. Currently, colder winter temperatures kill off Asian tiger mosquito populations and there is no evidence of the mosquitos being vectors for the dengue virus in this location. However, dengue virus-bearing mosquito vectors can inflow to Jeju Island from endemic area such as Vietnam by increased international travel, and this mosquito vector's survival during colder winter months will likely occur due to the effects of climate change.

Methods and Results

In this section, we show the geographical distribution of medically important mosquito vectors such as Ae. albopictus, a vector of both dengue and chikungunya viruses; Culex pipiens, a vector of West Nile virus; and Anopheles sinensis, a vector of Plasmodium vivax, within Jeju Island, South Korea. We found a significant association between the mean temperature, amount of precipitation, and density of mosquitoes. The phylogenetic analyses show that an Ae. albopictus, collected in southern area of Jeju Island, was identical to specimens found in Ho Chi Minh, Vietnam, and not Nagasaki, Japan.

Conclusion

Our results suggest that mosquito vectors or virus-bearing vectors can transmit from epidemic regions of Southeast Asia to Jeju Island and can survive during colder winter months. Therefore, Jeju Island is no longer safe from vector borne diseases (VBDs) due to the effects of globalization and climate change, and we should immediately monitor regional climate change to identify newly emerging VBDs.

Non-genetic determinants of mosquito competence for malaria parasites

Understanding how mosquito vectors and malaria parasites interact is of fundamental interest, and it also offers novel perspectives for disease control. Both the genetic and environmental contexts are known to affect the ability of mosquitoes to support malaria development and transmission, i.e., vector competence. Although the role of environment has long been recognized, much work has focused on host and parasite genetic effects. However, the last few years have seen a surge of studies revealing a great diversity of ways in which non-genetic factors can interfere with mosquito-Plasmodium interactions. Here, we review the current evidence for such environmentally mediated effects, including ambient temperature, mosquito diet, microbial gut flora, and infection history, and we identify additional factors previously overlooked in mosquito-Plasmodium interactions. We also discuss epidemiological implications, and the evolutionary consequences for vector immunity and parasite transmission strategies. Finally, we propose directions for further research and argue that an improved knowledge of non-genetic influences on mosquito-Plasmodium interactions could aid in implementing conventional malaria control measures and contribute to the design of novel strategies.

Social interactions modulate the virulence of avian malaria infection

There is an increasing understanding of the context-dependent nature of parasite virulence. Variation in parasite virulence can occur when infected individuals compete with conspecifics that vary in infection status; virulence may be higher when competing with uninfected competitors. In vertebrates with social hierarchies, we propose that these competition-mediated costs of infection may also vary with social status. Dominant individuals have greater competitive ability than competing subordinates, and consequently may pay a lower prevalence-mediated cost of infection. In this study we investigated whether costs of malarial infection were affected by the occurrence of the parasite in competitors and social status in domestic canaries (Serinus canaria). We predicted that infected subordinates competing with non-infected dominants would pay higher costs than infected subordinates competing with infected dominants. We also predicted that these occurrence-mediated costs of infection would be ameliorated in infected dominant birds. We found that social status and the occurrence of parasites in competitors significantly interacted to change haematocrit in infected birds. Namely, subordinate and dominant infected birds differed in haematocrit depending on the infection status of their competitors. However, in contrast to our prediction, dominants fared better with infected subordinates, whereas subordinates fared better with uninfected dominants. Moreover, we found additional effects of parasite occurrence on mortality in canaries. Ultimately, we provide evidence for costs of parasitism mediated by social rank and the occurrence of parasites in competitors in a vertebrate species. This has important implications for our understanding of the evolutionary processes that shape parasite virulence and group living.

Using physiology to understand climate-driven changes in disease and their implications for conservation

Controversy persists regarding the contributions of climate change to biodiversity losses, through its effects on the spread and emergence of infectious diseases. One of the reasons for this controversy is that there are few mechanistic studies that explore the links among climate change, infectious disease, and declines of host populations. Given that host-parasite interactions are generally mediated by physiological responses, we submit that physiological models could facilitate the prediction of how host-parasite interactions will respond to climate change, and might offer theoretical and terminological cohesion that has been lacking in the climate change-disease literature. We stress that much of the work on how climate influences host-parasite interactions has emphasized changes in climatic means, despite a hallmark of climate change being changes in climatic variability and extremes. Owing to this gap, we highlight how temporal variability in weather, coupled with non-linearities in responses to mean climate, can be used to predict the effects of climate on host-parasite interactions. We also discuss the climate variability hypothesis for disease-related declines, which posits that increased unpredictable temperature variability might provide a temporary advantage to pathogens because they are smaller and have faster metabolisms than their hosts, allowing more rapid acclimatization following a temperature shift. In support of these hypotheses, we provide case studies on the role of climatic variability in host population declines associated with the emergence of the infectious diseases chytridiomycosis, withering syndrome, and malaria. Finally, we present a mathematical model that provides the scaffolding to integrate metabolic theory, physiological mechanisms, and large-scale spatiotemporal processes to predict how simultaneous changes in climatic means, variances, and extremes will affect host-parasite interactions. However, several outstanding questions remain to be answered before investigators can accurately predict how changes in climatic means and variances will affect infectious diseases and the conservation status of host populations.