Larval and adult environmental temperatures influence the adult reproductive traits of Anopheles gambiae s.s

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

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

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

Delineating dengue risk zones in Jaipur: An interdisciplinary approach to inform public health strategies

Dengue fever (DF) is a pervasive public health concern in tropical climates, with densely populated regions, such as India, disproportionately affected. Addressing this issue requires a multifaceted understanding of the environmental and sociocultural factors that contribute to the risk of dengue infection. This study aimed to identify high-risk zones for DF in Jaipur, Rajasthan, India, by integrating physical, demographic, and epidemiological data in a comprehensive risk analysis framework. We investigated environmental variables, such as soil type and plant cover, to characterize the potential habitats of Aedes aegypti, the primary dengue vector. Concurrently, demographic metrics were evaluated to assess the population's susceptibility to dengue outbreaks. High-risk areas were systematically identified through a comparative analysis that integrated population density and incidence rates per ward. The results revealed a significant correlation between high population density and an increased risk of dengue, predominantly facilitated by vertical transmission. Spatially, these high-risk zones are concentrated in the northern and southern sectors of Jaipur, with the northern and southwestern wards exhibiting the most acute risk profiles. This study underscores the importance of targeted public health interventions and vaccination campaigns in vulnerable areas. It further lays the groundwork for future research to evaluate the effectiveness of such interventions, thereby contributing to the development of robust evidence-based strategies for dengue risk mitigation.

Assessment of environmental and spatial factors influencing the establishment of Anopheles gambiae larval habitats in the malaria endemic province of Woleu-Ntem, northern Gabon

BACKGROUND

This study aimed to assess the spatial distribution of Anopheles mosquito larval habitats and the environmental factors associated with them, as a prerequisite for the implementation of larviciding.

METHODS

The study was conducted in December 2021, during the transition period between the end of the short rainy season (September-November) and the short dry season (December-February). Physical, biological, and land cover data were integrated with entomological observations to collect Anopheles larvae in three major towns: Mitzic, Oyem, and Bitam, using the "dipping" method during the transition from rainy to dry season. The collected larvae were then reared in a field laboratory established for the study period. After the Anopheles mosquitoes had emerged, their species were identified using appropriate morphological taxonomic keys. To determine the influence of environmental factors on the breeding of Anopheles mosquitoes, multiple-factor analysis (MFA) and a binomial generalized linear model were used.

RESULTS

According to the study, only 33.1% out of the 284 larval habitats examined were found to be positive for Anopheles larvae, which were primarily identified as belonging to the Anopheles gambiae complex. The findings of the research suggested that the presence of An. gambiae complex larvae in larval habitats was associated with various significant factors such as higher urbanization, the size and type of the larval habitats (pools and puddles), co-occurrence with Culex and Aedes larvae, hot spots in ambient temperature, moderate rainfall, and land use patterns.

CONCLUSIONS

The results of this research mark the initiation of a focused vector control plan that aims to eradicate or lessen the larval habitats of An. gambiae mosquitoes in Gabon's Woleu Ntem province. This approach deals with the root causes of malaria transmission through larvae and is consistent with the World Health Organization's (WHO) worldwide objective to decrease malaria prevalence in regions where it is endemic.

Chemical Exposure of Synthetic Pyrethroid on Deltamethrin Under the Selection Pressure over the Generations: A Reproductive Potential Study of Anopheles stephensi

Biochemical synthetic pyrethroids, deltamethrin are presently used insecticides for the control of mosquito vector-borne diseases in worldwide. Mosquito re-emergence with diseases becoming a serious problem due to development of insecticide resistance. The comprehensive knowledge on the underlying mechanisms of resistance against deltamethrin is required for implementation of an efficient vector control programme. The assessment of the biological fitness of a mosquito strain exposed to insecticide pressure is extremely vital because it provides information on the development of resistance. In the present study, the adult stage of malaria vector, Anopheles stephensi, was designated for the study of deltamethrin resistance (F40 generations). The non-blood-fed, laboratory-reared females to sub-lethal doses of deltamethrin (0.004%, 0.005%, 0.007%, or 0.01%) exposed to every generation for up to F40. The adult mosquito susceptibility was performed by WHO standard method for evaluation. After 24 h, mortality was recorded in both treated and control groups. Therefore, the biological fitness characteristics such as feeding, fecundity, hatchability, egg retention, immature duration, adult emergence, and adult life span were studied to assess the exposed deltamethrin under selection pressure as compared to the unexposed (control) population. The laboratory selection of An. stephensi exposed deltamethrin over the generations were diminished its biological fitness. Information on biological fitness including reproductive potential of mosquito strain under selection pressure against deltamethrin is incredibly necessary because it would facilitate in resistance management. Baseline information gives in this experiment will guide for future studies on the susceptibilities of wild malaria mosquito populations in India.

Spatial analyses of Plasmodium knowlesi vectors with reference to control interventions in Malaysia

BACKGROUND

Malaria parasites such as Plasmodium knowlesi, P. inui, and P. cynomolgi are spread from macaques to humans through the Leucosphyrus Group of Anopheles mosquitoes. It is crucial to know the distribution of these vectors to implement effective control measures for malaria elimination. Plasmodium knowlesi is the most predominant zoonotic malaria parasite infecting humans in Malaysia.

METHODS

Vector data from various sources were used to create distribution maps from 1957 to 2021. A predictive statistical model utilizing logistic regression was developed using significant environmental factors. Interpolation maps were created using the inverse distance weighted (IDW) method and overlaid with the corresponding environmental variables.

RESULTS

Based on the IDW analysis, high vector abundances were found in the southwestern part of Sarawak, the northern region of Pahang and the northwestern part of Sabah. However, most parts of Johor, Sabah, Perlis, Penang, Kelantan and Terengganu had low vector abundance. The accuracy test indicated that the model predicted sampling and non-sampling areas with 75.3% overall accuracy. The selected environmental variables were entered into the regression model based on their significant values. In addition to the presence of water bodies, elevation, temperature, forest loss and forest cover were included in the final model since these were significantly correlated. Anopheles mosquitoes were mainly distributed in Peninsular Malaysia (Titiwangsa range, central and northern parts), Sabah (Kudat, West Coast, Interior and Tawau division) and Sarawak (Kapit, Miri, and Limbang). The predicted Anopheles mosquito density was lower in the southern part of Peninsular Malaysia, the Sandakan Division of Sabah and the western region of Sarawak.

CONCLUSION

The study offers insight into the distribution of the Leucosphyrus Group of Anopheles mosquitoes in Malaysia. Additionally, the accompanying predictive vector map correlates well with cases of P. knowlesi malaria. This research is crucial in informing and supporting future efforts by healthcare professionals to develop effective malaria control interventions.

Impact of Human Activities on Disease-Spreading Mosquitoes in Urban Areas

Urbanization is one of the leading global trends of the twenty-first century that has a significant impact on health. Among health challenges caused by urbanization, the relationship of urbanization between emergence and the spread of mosquito-borne infectious diseases (MBIDs) is a great public health concern. Urbanization processes encompass social, economic, and environmental changes that directly impact the biology of mosquito species. In particular, urbanized areas experience higher temperatures and pollution levels than outlying areas but also favor the development of infrastructures and objects that are favorable to mosquito development. All these modifications may influence mosquito life history traits and their ability to transmit diseases. This review aimed to summarize the impact of urbanization on mosquito spreading in urban areas and the risk associated with the emergence of MBIDs. Moreover, mosquitoes are considered as holobionts, as evidenced by numerous studies highlighting the role of mosquito-microbiota interactions in mosquito biology. Taking into account this new paradigm, this review also represents an initial synthesis on how human-driven transformations impact microbial communities in larval habitats and further interfere with mosquito behavior and life cycle in urban areas.

Experimental evidence for stronger impacts of larval but not adult rearing temperature on female fertility and lifespan in a seed beetle

Temperature impacts behaviour, physiology and life-history of many life forms. In many ectotherms, phenotypic plasticity within reproductive traits could act as a buffer allowing adaptation to continued global warming within biological limits. But there could be costs involved, potentially affecting adult reproductive performance and population growth. Empirical data on the expression of reproductive plasticity when different life stages are exposed is still lacking. Plasticity in key components of fitness (e.g., reproduction) can impose life-history trade-offs. Ectotherms are sensitive to temperature variation and the resulting thermal stress is known to impact reproduction. So far, research on reproductive plasticity to temperature variation in this species has focused on males. Here, I explore how rearing temperature impacted female reproduction and lifespan in the bruchid beetle Callosobruchus maculatus by exposing them to four constant temperatures (17 °C, 25 °C, 27 °C and 33 °C) during larval or adult stages. In these experiments, larval rearing cohorts (exposed to 17 °C, 25 °C, 27 °C and 33 °C, from egg to adulthood) were tested in a common garden setting at 27 °C and adult rearing cohorts, after having developed entirely at 27 °C, were exposed to four constant rearing temperatures (17 °C, 25 °C, 27 °C and 33 °C). I found stage-specific plasticity in all the traits measured here: fecundity, egg morphological dimensions (length and width), lifespan and egg hatching success (female fertility). Under different larval rearing conditions, fecundity and fertility was drastically reduced (by 51% and 42%) at 17 °C compared to controls (27 °C). Female lifespan was longest at 17 °C across both larval and adult rearing: by 36% and 55% compared to controls. Collectively, these results indicate that larval rearing temperature had greater reproductive impacts. Integrating both larval and adult rearing effects, I present evidence that female fertility is more sensitive during larval development compared to adult rearing temperature in this system.

Larval Anopheles Species Composition and Diversity at Different Habitats and Seasons of Gondar Zuria District, Ethiopia

Anopheles species' larval habitats are diversified and season dependent. Anopheles larvae can be found at different habitats and their preference may vary seasonally. Knowledge of species diversity and distribution helps plan malaria control interventions. Anopheles larvae were sampled using the WHO standard 350 ml dipper from breeding habitats of irrigation, pond, sewage, stream, and swamp. The collected sample larvae were identified microscopically to species using morphological keys. Among Anopheles species recorded, Anopheles gambiae complex was the most abundant followed by Anopheles christyi, Anopheles cinereus, Anopheles demeilloni, and Anopheles pharoensis in descending order. Anopheles species occurred more in January than in other months of the study period and less in March and April. For any particular mosquito species, larval abundance did not significantly vary between the habitats; in other words, all habitats contributed equally. In this study, we confirmed that Anopheles mosquito larval population varied more with respect to species than to habitats and months. Interventions could be launched targeting each habitat; during the month, numbers were high.

Effects of Elevated Temperatures on the Growth and Development of Adult Anopheles gambiae (s.l.) (Diptera: Culicidae) Mosquitoes

Higher temperatures expected in a future warmer climate could adversely affect the growth and development of mosquitoes. This study investigated the effects of elevated temperatures on longevity, gonotrophic cycle length, biting rate, fecundity, and body size of Anopheles gambiae (s.l.) (Diptera: Culicidae) mosquitoes. Anopheles gambiae (s.l.) eggs obtained from laboratory established colonies were reared under eight temperature regimes (25, 28, 30, 32, 34, 36, 38, and 40°C), and 80 ± 10% RH. All adults were allowed to feed on a 10% sugar solution soaked in cotton wool; however, some mosquitoes were provided blood meal using guinea pig. Longevity was estimated for both blood-fed and non-blood-fed mosquitoes and analyzed using the Kaplan-Meier survival analysis. One-way ANOVA was used to test the effect of temperature on gonotrophic cycle length, biting rate, and fecundity. Adult measurement data were log-transformed and analyzed using ordinary least square regression with robust standard errors. Increasing temperature significantly decreased the longevity of both blood-fed (Log-rank test; X2(4) = 904.15, P < 0.001) and non-blood-fed (Log-rank test; X2(4) = 1163.60, P < 0.001) mosquitoes. In addition, the fecundity of mosquitoes decreased significantly (ANOVA; F(2,57) = 3.46, P = 0.038) with an increase in temperature. Body size (β = 0.14, 95% CI, 0.16, 0.12, P < 0.001) and proboscis length (β = 0.13, 95% CI, 0.17, 0.09, P < 0.001) significantly decreased with increasing temperature from 25 to 34°C. Increased temperatures expected in a future warmer climate could cause some unexpected effects on mosquitoes by directly influencing population dynamics and malaria transmission.

Differential Hatching, Development, Oviposition, and Longevity Patterns among Colombian Aedes aegypti Populations

Simple Summary

Aedes aegypti is a mosquito that transmits viruses responsible for several diseases in humans, such as dengue, Zika, and chikungunya. It is crucial to study mosquito populations from different countries and regions because control of disease transmission with insecticides can be more effective if adjusted to each population’s characteristics. For this reason, we determined several features of mosquitoes captured in different cities of Colombia: Neiva, Bello, Itagüí, and Riohacha. These included the length of their lifespan, the number of eggs they lay, and the stages in which they die. We found specific patterns for each population. This knowledge will help control programs determine the optimal times to apply insecticides and make surveillance, as well as the type of insecticide used.

Abstract

Dengue, Zika, and chikungunya are arboviral diseases for which there are no effective therapies or vaccines. The only way to avoid their transmission is by controlling the vector Aedes aegypti, but insecticide resistance limits this strategy. To generate relevant information for surveillance and control mechanisms, we determined life cycle parameters, including longevity, fecundity, and mortality, of Colombian Ae. aegypti populations from four different geographical regions: Neiva, Bello, Itagüí, and Riohacha. When reared at 28 °C, Bello had the shortest development time, and Riohacha had the longest. Each mosquito population had its own characteristic fecundity pattern during four gonotrophic cycles. The survival curves of each population were significantly different, with Riohacha having the longest survival in both males and females and Bello the shortest. High mortality was observed in mosquitoes from Neiva in the egg stage and for Bello in the pupae stage. Finally, when mosquitoes from Neiva and Bello were reared at 35 °C, development times and mortality were severely affected. In conclusion, each population has a unique development pattern with an innate trace in their biological characteristics that confers vulnerability in specific stages of development.

Effects of elevated temperatures on the development of immature stages of Anopheles gambiae (s.l.) mosquitoes

OBJECTIVE

This study investigated the effects of temperature on the development of the immature stages of An. gambiae (s.l.) mosquitoes.

METHODS

Mosquito eggs were obtained from laboratory established colonies and reared under eight temperature regimes (25, 28, 30, 32, 34, 36, 38 and 40 °C), and 80 ± 10% relative humidity. Larvae were checked daily for development to the next stage and for mortality. Pupation success, number of adults produced, and sex ratio of the newly emerged adults were recorded. Larval survival was monitored every 24 hours, and data were analyzed using Kaplan Meier survival analysis. Analysis of variance was used where data followed normal distribution, and a Kruskal-Wallis test where data were not normally distributed. Larval and pupal measurements were log-transformed and analyzed using ordinary least square regression with robust standard errors.

RESULTS

Increasing the temperature from 25 to 36 °C decreased the development time by 10.57 days. Larval survival (X² (6) = 5353.12, P < 0.001) and the number of adults produced (X² (5) = 28.16, P < 0.001) decreased with increasing temperature. Increasing temperatures also resulted in significantly smaller larvae and pupae (P < 0.001). At higher temperatures disproportionately more male than female mosquitoes were produced.

CONCLUSIONS

Increased temperature affected different developmental stages in the life cycle of An. gambiae (s.l.) mosquitoes, from larval to adult emergence. This study contributes to the knowledge on the relationship between temperature and Anopheles mosquitoes and provides useful information for modelling vector population dynamics in the light of climate change.

Bayesian spatio-temporal distributed lag modeling for delayed climatic effects on sparse malaria incidence data

Background

In many areas of the Greater Mekong Subregion (GMS), malaria endemic regions have shrunk to patches of predominantly low-transmission. With a regional goal of elimination by 2030, it is important to use appropriate methods to analyze and predict trends in incidence in these remaining transmission foci to inform planning efforts. Climatic variables have been associated with malaria incidence to varying degrees across the globe but the relationship is less clear in the GMS and standard methodologies may not be appropriate to account for the lag between climate and incidence and for locations with low numbers of cases.

Methods

In this study, a methodology was developed to estimate the spatio-temporal lag effect of climatic factors on malaria incidence in Thailand within a Bayesian framework. A simulation was conducted based on ground truth of lagged effect curves representing the delayed relation with sparse malaria cases as seen in our study population. A case study to estimate the delayed effect of environmental variables was used with malaria incidence at a fine geographic scale of sub-districts in a western province of Thailand.

Results

From the simulation study, the model assumptions which accommodated both delayed effects and excessive zeros appeared to have the best overall performance across evaluation metrics and scenarios. The case study demonstrated lagged climatic effect estimation of the proposed modeling with real data. The models appeared to be useful to estimate the shape of association with malaria incidence.

Conclusions

A new method to estimate the spatiotemporal effect of climate on malaria trends in low transmission settings is presented. The developed methodology has potential to improve understanding and estimation of past and future trends in malaria incidence. With further development, this could assist policy makers with decisions on how to more effectively distribute resources and plan strategies for malaria elimination.

Time-Varying Effects of Meteorological Variables on Malaria Epidemiology in the Context of Interrupted Control Efforts in the Amazon Rainforest, 2000-2017

Successful malaria control interventions, mostly based on the training of health workers, distribution of insecticide-treated nets, and spraying, decrease malaria incidence; however, when these interventions are interrupted, a resurgence may occur. In the Peruvian Amazon, after discontinuing the control activities implemented by the PAMAFRO project (2006–2010)-a Global Fund-sponsored project for the strengthening of malaria control and surveillance in multiple countries in Latin America– malaria cases re-emerged dramatically. In parallel, meteorological factors determine the conditions suitable for the development, reproduction, and survival of mosquito vectors and parasites. This study hypothesized that interruption of malaria interventions may have modified the meteorological-malaria relationships over time (i.e., temporal changes in the dose-response between meteorological variables and malaria incidence). In this panel data analysis, we assessed the extent that relationships between meteorological variables and malaria changed temporally using data of monthly malaria incidence due to Plasmodium vivax or P. falciparum in Loreto, Peru (2000–2017). Generalized additive models were used to explore how the effects of meteorological variables changed in magnitude before, during, and after the PAMAFRO intervention. We found that once the PAMAFRO intervention had been interrupted, the estimated effects (dose-response) of meteorological variables on incidence rates decreased for both malaria parasite species. However, these fitted effect estimates did not reach their baseline levels (before the PAMAFRO period); variations of time-varying slopes between 0.45 and 2.07 times were observed after the PAMAFRO intervention. We also reported significant heterogeneity in the geographical distributions of malaria, parasite species, and meteorological variables. High malaria transmission occurred consistently in the northwestern provinces of Loreto Department. Since the end of the PAMAFRO period, a higher effect of precipitation and actual evapotranspiration was described on P. falciparum compared to P. vivax. The effect of temperature on malaria was greater over a shorter time (1-month lag or less), compared with precipitation and actual evapotranspiration (12-month lag). These findings demonstrate the importance of sustained malaria control efforts since interruption may enhance the links between meteorological factors and malaria. Our results also emphasize the importance of considering the time-varying effect of meteorological factors on malaria incidence to tailor control interventions, especially to better manage the current and future climate change crisis.

A Systematic Review of the Effects of Temperature on Anopheles Mosquito Development and Survival: Implications for Malaria Control in a Future Warmer Climate

The rearing temperature of the immature stages can have a significant impact on the life-history traits and the ability of adult mosquitoes to transmit diseases. This review assessed published evidence of the effects of temperature on the immature stages, life-history traits, insecticide susceptibility, and expression of enzymes in the adult Anopheles mosquito. Original articles published through 31 March 2021 were systematically retrieved from Scopus, Google Scholar, Science Direct, PubMed, ProQuest, and Web of Science databases. After applying eligibility criteria, 29 studies were included. The review revealed that immature stages of An. arabiensis were more tolerant (in terms of survival) to a higher temperature than An. funestus and An. quadriannulatus. Higher temperatures resulted in smaller larval sizes and decreased hatching and pupation time. The development rate and survival of An. stephensi was significantly reduced at a higher temperature than a lower temperature. Increasing temperatures decreased the longevity, body size, length of the gonotrophic cycle, and fecundity of Anopheles mosquitoes. Higher rearing temperatures increased pyrethroid resistance in adults of the An. arabiensis SENN DDT strain, and increased pyrethroid tolerance in the An. arabiensis SENN strain. Increasing temperature also significantly increased Nitric Oxide Synthase (NOS) expression and decreased insecticide toxicity. Both extreme low and high temperatures affect Anopheles mosquito development and survival. Climate change could have diverse effects on Anopheles mosquitoes. The sensitivities of Anopeheles mosquitoes to temperature differ from species to species, even among the same complex. Notwithstanding, there seem to be limited studies on the effects of temperature on adult life-history traits of Anopheles mosquitoes, and more studies are needed to clarify this relationship.

Model building and assessment of the impact of covariates for disease prevalence mapping in low-resource settings: to explain and to predict

This paper provides statistical guidance on the development and application of model-based geostatistical methods for disease prevalence mapping. We illustrate the different stages of the analysis, from exploratory analysis to spatial prediction of prevalence, through a case study on malaria mapping in Tanzania. Throughout the paper, we distinguish between predictive modelling, whose main focus is on maximizing the predictive accuracy of the model, and explanatory modelling, where greater emphasis is placed on understanding the relationships between the health outcome and risk factors. We demonstrate that these two paradigms can result in different modelling choices. We also propose a simple approach for detecting over-fitting based on inspection of the correlation matrix of the estimators of the regression coefficients. To enhance the interpretability of geostatistical models, we introduce the concept of domain effects in order to assist variable selection and model validation. The statistical ideas and principles illustrated here in the specific context of disease prevalence mapping are more widely applicable to any regression model for the analysis of epidemiological outcomes but are particularly relevant to geostatistical models, for which the separation between fixed and random effects can be ambiguous.

Ten years of monitoring malaria trend and factors associated with malaria test positivity rates in Lower Moshi

Background

High altitude settings in Eastern Africa have been reported to experience increased malaria burden due to vector habitat expansion. This study explored possible associations between malaria test positivity rates and its predictors including malaria control measures and meteorological factors at a high-altitude, low malaria transmission setting, south of Mount Kilimanjaro.

Methods

Malaria cases reported at the Tanganyika Plantation Company (TPC) hospital’s malaria registers, meteorological data recorded at TPC sugar factory and data on bed nets distributed in Lower Moshi from 2009 to 2018 were studied. Correlation between bed nets distributed and malaria test positivity rates were explored by using Pearson correlation analysis and the associations between malaria test positivity rates and demographic and meteorological variables were determined by logistic regression and negative binomial regression analyses, respectively.

Results

Malaria cases reported at TPC hospital ranged between 0.48 and 2.26% per year and increased slightly at the introduction of malaria rapid diagnostic tests. The risk of testing positive for malaria were significantly highest among individuals aged between 6 and 15 years (OR = 1.65; 1.65 CI = 1.28–2.13; p = 0.001) and 16–30 years (OR = 1.49; CI = 1.17–1.89; p = 0.001) and when adjusted for age, the risk were significantly higher among male individuals when compared to female individuals (OR = 1.54; 1.00–1.31; p = 0.044). Malaria test positivity rates were positively associated with average monthly minimum temperatures and negatively associated with average monthly maximum temperatures (incidence rate ratio (IRR) = 1.37, 95% confidence interval (CI) = 1.05–1.78, p = 0.019 and IRR = 0.72, 95% CI = 0.58–0.91, p = 0.005, respectively). When analysed with one month lag for predictor variables, malaria test positivity rates were still significantly associated with average monthly minimum and maximum temperatures (IRR = 1.67, 95% CI = 1.28–2.19, p = 0.001 and IRR = 0.68, 95% CI = 0.54–0.85, p = 0.001, respectively). Average monthly rainfall and relative humidity with or without a one month lag was not associated with malaria test positivity rates in the adjusted models. Explopring possible associations between distribution of long-lasting insecticidal nets, (LLINs) and malaria test positivity rates showed no apparent correlation between numbers of LLINs distributed in a particular year and malaria test positivity rates.

Conclusion

In Lower Moshi, the risk of being tested positive for malaria was highest for older children and male individuals. Higher minimum and lower maximum temperatures were the strongest climatic predictors for malaria test positivity rates. In areas with extensive irrigation activity as in Lower Moshi, vector abundance and thus malaria transmission may be less dependent on rainfall patterns and humidity. Mass distribution of LLINs did not have an effect in this area with already very low malaria transmission.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03730-1.

Bulk-up synchronization of successive larval cohorts of Anopheles gambiae and Anopheles coluzzii through temperature reduction at early larval stages: effect on emergence rate, body size and mating success

Background

Malaria persists as a huge medical and economic burden. Although the number of cases and death rates have reduced in recent years, novel interventions are a necessity if such gains are to be maintained. Alternative methods to target mosquito vector populations that involve the release of large numbers genetically modified mosquitoes are in development. However, their successful introduction will require innovative strategies to bulk-up mosquito numbers and improve mass rearing protocols for Anopheles mosquitoes.

Methods

The relationship between mosquito aquatic stage development and temperature was exploited so that multiple cohorts of mosquitoes, from separate egg batches, could be synchronized to ‘bulk-up’ the number of mosquitoes released. First instar larvae were separated into two cohorts: the first, maintained under standard insectary conditions at 27^oC, the second subjected to an initial 5-day cooling period at 19^oC.

Results

Cooling of 1st instars slowed the mean emergence times of Anopheles coluzzii and Anopheles gambiae by 2.4 and 3.5 days, respectively, compared to their 27^oC counterparts. Pupation and emergence rates were good (> 85 %) in all conditions. Temperature adjustment had no effect on mosquito sex ratio and adult fitness parameters such as body size and mating success.

Conclusions

Bulk-up larval synchronization is a simple method allowing more operational flexibility in mosquito production towards mark-release-recapture studies and mass release interventions.

Age influences the thermal suitability of Plasmodium falciparum transmission in the Asian malaria vector Anopheles stephensi

Models predicting disease transmission are vital tools for long-term planning of malaria reduction efforts, particularly for mitigating impacts of climate change. We compared temperature-dependent malaria transmission models when mosquito life-history traits were estimated from a truncated portion of the lifespan (a common practice) versus traits measured across the full lifespan. We conducted an experiment on adult female Anopheles stephensi, the Asian urban malaria mosquito, to generate daily per capita values for mortality, egg production and biting rate at six constant temperatures. Both temperature and age significantly affected trait values. Further, we found quantitative and qualitative differences between temperature-trait relationships estimated from truncated data versus observed lifetime values. Incorporating these temperature-trait relationships into an expression governing the thermal suitability of transmission, relative R₀(T), resulted in minor differences in the breadth of suitable temperatures for Plasmodium falciparum transmission between the two models constructed from only An. stephensi trait data. However, we found a substantial increase in thermal niche breadth compared with a previously published model consisting of trait data from multiple Anopheles mosquito species. Overall, this work highlights the importance of considering how mosquito trait values vary with mosquito age and mosquito species when generating temperature-based suitability predictions of transmission.

Evidence for family-level variation of phenotypic traits in response to temperature of Brazilian Nyssorhynchus darlingi

BACKGROUND

Nyssorhynchus darlingi (also known as Anopheles darlingi) is the primary malaria vector in the Amazon River Basin. In Brazil, analysis of single nucleotide polymorphisms (SNPs) previously detected three major population clusters, and a common garden experiment in a laboratory setting revealed significant population variation in life history traits. Increasing temperatures and local level variation can affect life history traits, i.e. adult longevity, that alter vectorial capacity with implications for malaria transmission in Ny. darlingi.

METHODS

We investigated the population structure of Ny. darlingi from 7 localities across Brazil utilizing SNPs and compared them to a comprehensive Ny. darlingi catalog. To test the effects of local level variation on life history traits, we reared F1 progeny from the 7 localities at three constant temperatures (20, 24 and 28 °C), measuring key life history traits (larval development, food-starved adult lifespan, adult size and daily survival).

RESULTS

Using nextRAD genotyping-by-sequencing, 93 of the field-collected Ny. darlingi were genotyped at 33,759 loci. Results revealed three populations (K = 3), congruent with major biomes (Amazonia, Cerrado and Mata Atlântica), with greater FST values between biomes than within. In the life history experiments, increasing temperature reduced larval development time, adult lifespan, and wing length in all localities. The variation of family responses for all traits within four localities of the Amazonia biome was significant (ANOVA, P < 0.05). Individual families within localities revealed a range of responses as temperature increased, for larval development, adult lifespan, wing length and survival time.

CONCLUSIONS

SNP analysis of several Brazilian localities provided results in support of a previous study wherein populations of Ny. darlingi were clustered by three major Brazilian biomes. Our laboratory results of temperature effects demonstrated that population variation in life history traits of Ny. darlingi exists at the local level, supporting previous research demonstrating the high plasticity of this species. Understanding this plasticity and inherent variation between families of Ny. darlingi at the local level should be considered when deploying intervention strategies and may improve the likelihood of successful malaria elimination in South America.

Malaria predictions based on seasonal climate forecasts in South Africa: A time series distributed lag nonlinear model

Although there have been enormous demands and efforts to develop an early warning system for malaria, no sustainable system has remained. Well-organized malaria surveillance and high-quality climate forecasts are required to sustain a malaria early warning system in conjunction with an effective malaria prediction model. We aimed to develop a weather-based malaria prediction model using a weekly time-series data including temperature, precipitation, and malaria cases from 1998 to 2015 in Vhembe, Limpopo, South Africa and apply it to seasonal climate forecasts. The malaria prediction model performed well for short-term predictions (correlation coefficient, r > 0.8 for 1- and 2-week ahead forecasts). The prediction accuracy decreased as the lead time increased but retained fairly good performance (r > 0.7) up to the 16-week ahead prediction. The demonstration of the malaria prediction process based on the seasonal climate forecasts showed the short-term predictions coincided closely with the observed malaria cases. The weather-based malaria prediction model we developed could be applicable in practice together with skillful seasonal climate forecasts and existing malaria surveillance data. Establishing an automated operating system based on real-time data inputs will be beneficial for the malaria early warning system, and can be an instructive example for other malaria-endemic areas.

Regional variation in life history traits and plastic responses to temperature of the major malaria vector Nyssorhynchus darlingi in Brazil

The primary Brazilian malaria vector, Nyssorhynchus darlingi (formerly Anopheles darlingi), ranges from 0°S-23°S across three biomes (Amazonia, Cerrado, Mata Atlântica). Rising temperatures will increase mosquito developmental rates, and models predict future malaria transmission by Ny. darlingi in Brazil will shift southward. We reared F1 Ny. darlingi (progeny of field-collected females from 4 state populations across Brazil) at three temperatures (20, 24, 28 °C) and measured key life-history traits. Our results reveal geographic variation due to both genetic differences among localities and plastic responses to temperature differences. Temperature significantly altered all traits: faster larval development, shorter adult life and overall lifespan, and smaller body sizes were seen at 28 °C versus 20 °C. Low-latitude Amazonia mosquitoes had the fastest larval development at all temperatures, but at 28 °C, average development rate of high-latitude Mata Atlântica mosquitoes was accelerated and equivalent to low-latitude Amazonia. Body size of adult mosquitoes from the Mata Atlântica remained larger at all temperatures. We detected genetic variation in the plastic responses among mosquitoes from different localities, with implications for malaria transmission under climate change. Faster development combined with larger body size, without a tradeoff in adult longevity, suggests vectorial capacities of some Mata Atlântica populations may significantly increase under warming climates.

A comparison of thick-film microscopy, rapid diagnostic test, and polymerase chain reaction for accurate diagnosis of Plasmodium falciparum malaria

Background

Accurate diagnosis of malaria is important for effective disease management and control. In Cameroon, presumptive clinical diagnosis, thick-film microscopy (TFM), and rapid diagnostic tests (RDT) are commonly used to diagnose cases of Plasmodium falciparum malaria. However, these methods lack sensitivity to detect low parasitaemia. Polymerase chain reaction (PCR), on the other hand, enhances the detection of sub-microscopic parasitaemia making it a much-needed tool for epidemiological surveys, mass screening, and the assessment of interventions for malaria elimination. Therefore, this study sought to determine the frequency of cases missed by traditional methods that are detected by PCR.

Methods

Blood samples, collected from 551 febrile Cameroonian patients between February 2014 and February 2015, were tested for P. falciparum by microscopy, RDT and PCR. The hospital records of participants were reviewed to obtain data on the clinical diagnosis made by the health care worker.

Results

The prevalence of malaria by microscopy, RDT and PCR was 31%, 45%, and 54%, respectively. However, of the 92% of participants diagnosed as having clinical cases of malaria by the health care worker, 38% were malaria-negative by PCR. PCR detected 23% and 12% more malaria infections than microscopy and RDT, respectively. A total of 128 (23%) individuals had sub-microscopic infections in the study population. The sensitivity of microscopy, RDT, and clinical diagnosis was 57%, 78% and 100%; the specificity was 99%, 94%, and 17%; the positive predictive values were 99%, 94%, and 59%; the negative predictive values were 66%, 78%, and 100%, respectively. Thus, 41% of the participants clinically diagnosed as having malaria had fever caused by other pathogens.

Conclusions

Malaria diagnostic methods, such as TFM and RDT missed 12–23% of malaria cases detected by PCR. Therefore, traditional diagnostic approaches (TFM, RDT and clinical diagnosis) are not adequate when accurate epidemiological data are needed for monitoring malaria control and elimination interventions.

Environmental Factors Associated with Mosquito Vector Larvae in a Malaria-Endemic Area in Ratchaburi Province, Thailand

Malaria is transmitted by female mosquitoes in the genus Anopheles and is a major public health issue. Different species of Anopheles mosquitoes have different epidemiological characteristics, behaviors, and ecological requirements, and so an understanding of their biology and ecology in a particular area is critical for successful disease control. The aim of this study was to determine which environmental factors are associated with Anopheles larvae in a malaria-endemic area in Ratchaburi Province, Thailand, which shares a border with Myanmar. In October 2016, we collected mosquito larvae and measured six environmental factors at 10 study sites located along Lam Pachi River, which flows through Huay Nam Nak village in Ratchaburi Province. We found two species of Anopheles larvae (An. subpictus sensu lato (s.l.) Grassi and An. barbirostris s.l. van der Wulp) at 7 of the 10 study sites, the numbers of which significantly differed between sites (p < 0.05). Pearson correlation analysis showed that the numbers of larvae of both species were significantly positively correlated with the dissolved oxygen level (p < 0.01) and significantly negatively correlated with the width of the river (p < 0.05) and pH (p < 0.01). By contrast, turbidity, water depth, and water temperature were not associated with larval abundance. Mosquito species which belong to genus Anopheles are considered to be of public health and medical importance. Therefore, Anopheles mosquito surveillance and control in the study sites are essential. This information will facilitate vector-borne disease control and improve our understanding of the biology of Anopheles vectors in rivers located along international borders, further reducing the number of patients in this malaria-endemic area.

The relationship between size and longevity of the malaria vector Anopheles gambiae (s.s.) depends on the larval environment

BACKGROUND

Understanding the variation in vector-borne disease transmission intensity across time and space relies on a thorough understanding of the impact of environmental factors on vectorial capacity traits of mosquito populations. This is driven primarily by variation in larval development and growth, with carryover effects influencing adult traits such as longevity and adult body size. The relationship between body size and longevity strongly affects the evolution of life histories and the epidemiology of vector-borne diseases. This relationship ranges from positive to negative but the reasons for this variability are not clear. Both traits depend on a number of environmental factors, but primarily on temperature as well as availability of nutritional resources. We therefore asked how the larval environment of the mosquito Anopheles gambiae Giles (sensu stricto) (Diptera: Culicidae) affects the relationship between body size and longevity.

METHODS

We reared the larvae of An. gambiae individually at three temperatures (21, 25 and 29 °C) and two food levels (the standard and 50% of our laboratory diet) and measured adult size and longevity. We estimated the direct and indirect (via adult size) effects of food and temperature on longevity with a piecewise structural equation model (SEM).

RESULTS

We confirmed the direct effects of food and temperature during larval development on body size, as wing length decreased with increasing temperature and decreasing food levels. While the overall relationship between size and longevity was weak, we measured striking differences among environments. At 25 °C there was no clear relationship between size and longevity; at 29 °C the association was negative with standard food but positive with low food; whereas at 21 °C it was positive with standard food but negative with low food.

CONCLUSIONS

The larval environment influences the adult's fitness in complex ways with larger mosquitoes living longer in some environments but not in others. This confirmed our hypothesis that the relationship between size and longevity is not limited to a positive correlation. A better understanding of this relationship and its mechanisms may improve the modelling of the transmission of vector borne diseases, the evolution of life history traits, and the influence of vector control.

Modelling Allee effects in a transgenic mosquito population during range expansion

Mosquitoes are vectors for many diseases that cause significant mortality and morbidity. As mosquito populations expand their range, they may undergo mate-finding Allee effects such that their ability to successfully reproduce becomes difficult at low population density. With new technology, creating target specific gene modification may be a viable method for mosquito population control. We develop a mathematical model to investigate the effects of releasing transgenic mosquitoes into newly established, low-density mosquito populations. Our model consists of two life stages (aquatic and adults), which are divided into three genetically distinct groups: heterogeneous and homogeneous transgenic that cause female infertility and a homogeneous wild type. We perform analytical and numerical analyses on the equilibria to determine the level of saturation needed to eliminate mosquitoes in a given area. This model demonstrates the potential for a gene drive system to reduce the spread of invading mosquito populations.

Interactions between climatic changes and intervention effects on malaria spatio-temporal dynamics in Uganda

Background

Although malaria burden in Uganda has declined since 2009 following the scale-up of interventions, the disease is still the leading cause of hospitalization and death. Transmission remains high and is driven by suitable weather conditions. There is a real concern that intervention gains may be reversed by climatic changes in the country. In this study, we investigate the effects of climate on the spatio-temporal trends of malaria incidence in Uganda during 2013–2017.

Methods

Bayesian spatio-temporal negative binomial models were fitted on district-aggregated monthly malaria cases, reported by two age groups, defined by a cut-off age of 5 years. Weather data was obtained from remote sensing sources including rainfall, day land surface temperature (LSTD) and night land surface temperature (LSTN), Normalized Difference Vegetation Index (NDVI), altitude, land cover, and distance to water bodies. Spatial and temporal correlations were taken into account by assuming a conditional autoregressive and a first-order autoregressive process on district and monthly specific random effects, respectively. Fourier trigonometric functions modeled seasonal fluctuations in malaria transmission. The effects of climatic changes on the malaria incidence changes between 2013 and 2017 were estimated by modeling the difference in time varying climatic conditions at the two time points and adjusting for the effects of intervention coverage, socio-economic status and health seeking behavior.

Results

Malaria incidence declined steadily from 2013 to 2015 and then increased in 2016. The decrease was by over 38% and 20% in children <5 years and individuals ≥5 years, respectively. Temporal trends depict a strong bi-annual seasonal pattern with two peaks during April–June and October–December. The annual average of rainfall, LSTD and LSTN increased by 3.7 mm, 2.2 °C and 1.0 °C, respectively, between 2013 and 2017, whereas NDVI decreased by 6.8%. On the one hand, the increase in LSTD and decrease in NDVI were associated with a reduction in the incidence decline. On the other hand, malaria interventions and treatment seeking behavior had reverse effects, that were stronger compared to the effects of climatic changes. Important interactions between interventions with NDVI and LSTD suggest a varying impact of interventions on malaria burden in different climatic conditions.

Conclusion

Climatic changes in Uganda during the last five years contributed to a favorable environment for malaria transmission, and had a detrimental effect on malaria reduction gains achieved through interventions scale-up efforts. The NMCP should create synergies with the National Meteorological Authority with an ultimate goal of developing a Malaria Early Warning System to mitigate adverse climatic change effects on malaria risk in the country.

The effect of case management and vector-control interventions on space-time patterns of malaria incidence in Uganda

BACKGROUND

Electronic reporting of routine health facility data in Uganda began with the adoption of the District Health Information Software System version 2 (DHIS2) in 2011. This has improved health facility reporting and overall data quality. In this study, the effects of case management with artemisinin-based combination therapy (ACT) and vector control interventions on space-time patterns of disease incidence were determined using DHIS2 data reported during 2013-2016.

METHODS

Bayesian spatio-temporal negative binomial models were fitted on district-aggregated monthly malaria cases, reported by two age groups, defined by a cut-off age of 5 years. The effects of interventions were adjusted for socio-economic and climatic factors. Spatial and temporal correlations were taken into account by assuming a conditional autoregressive and a first-order autoregressive AR(1) process on district and monthly specific random effects, respectively. Fourier trigonometric functions were incorporated in the models to take into account seasonal fluctuations in malaria transmission.

RESULTS

The temporal variation in incidence was similar in both age groups and depicted a steady decline up to February 2014, followed by an increase from March 2015 onwards. The trends were characterized by a strong bi-annual seasonal pattern with two peaks during May-July and September-December. Average monthly incidence in children < 5 years declined from 74.7 cases (95% CI 72.4-77.1) in 2013 to 49.4 (95% CI 42.9-55.8) per 1000 in 2015 and followed by an increase in 2016 of up to 51.3 (95% CI 42.9-55.8). In individuals ≥ 5 years, a decline in incidence from 2013 to 2015 was followed by an increase in 2016. A 100% increase in insecticide-treated nets (ITN) coverage was associated with a decline in incidence by 44% (95% BCI 28-59%). Similarly, a 100% increase in ACT coverage reduces incidence by 28% (95% BCI 11-45%) and 25% (95% BCI 20-28%) in children < 5 years and individuals ≥ 5 years, respectively. The ITN effect was not statistically important in older individuals. The space-time patterns of malaria incidence in children < 5 are similar to those of parasitaemia risk predicted from the malaria indicator survey of 2014-15.

CONCLUSION

The decline in malaria incidence highlights the effectiveness of vector-control interventions and case management with ACT in Uganda. This calls for optimizing and sustaining interventions to achieve universal coverage and curb reverses in malaria decline.

Larval Age and Nutrition Affect the Susceptibility of Culex quinquefasciatus (Diptera: Culicidae) to Temephos

Larval age and nutrition significantly affected the insect's physiology. These influences are important when rearing a population of vectors that is used to monitor the resistance level, in which standardized conditions are crucial for a more harmonized result. Little information has been reported on the effects of larval age and nutrition on the susceptibility of insects to insecticides, and therefore, we studied the effects on the susceptibility of Culex quinquefasciatus Say's (Diptera: Culicidae) larvae to temephos by comparing the median lethal concentration (LC50) after 24 hr between the second and fourth instar larvae and between the larvae that fed on protein-based and carbohydrate-based larval diets. The susceptibility of the larvae was significantly affected by the larval diets, as the larvae that fed on protein-based beef food and milk food demonstrated significantly higher LC50 value compared with the larvae that fed on carbohydrate-based food: lab food and yeast food. The larval diet interacted significantly with the larval age: while the second instar larvae were susceptible to temephos when supplied with carbohydrate-based food, the second and fourth instar larvae had no significant effect when supplied with protein-based diets, implying that a protein-rich environment may cause the mosquito to be less susceptible to temephos. This study suggested the importance of standardizing nutrition when rearing a vector population in order to obtain more harmonized dosage-response results in an insecticide resistance monitoring program. Future research could focus on the biochemical mechanism between the nutrition and the enzymatic activities of the vector.

Quantifying the effects of temperature on mosquito and parasite traits that determine the transmission potential of human malaria

Malaria transmission is known to be strongly impacted by temperature. The current understanding of how temperature affects mosquito and parasite life history traits derives from a limited number of empirical studies. These studies, some dating back to the early part of last century, are often poorly controlled, have limited replication, explore a narrow range of temperatures, and use a mixture of parasite and mosquito species. Here, we use a single pairing of the Asian mosquito vector, An. stephensi and the human malaria parasite, P. falciparum to conduct a comprehensive evaluation of the thermal performance curves of a range of mosquito and parasite traits relevant to transmission. We show that biting rate, adult mortality rate, parasite development rate, and vector competence are temperature sensitive. Importantly, we find qualitative and quantitative differences to the assumed temperature-dependent relationships. To explore the overall implications of temperature for transmission, we first use a standard model of relative vectorial capacity. This approach suggests a temperature optimum for transmission of 29°C, with minimum and maximum temperatures of 12°C and 38°C, respectively. However, the robustness of the vectorial capacity approach is challenged by the fact that the empirical data violate several of the model’s simplifying assumptions. Accordingly, we present an alternative model of relative force of infection that better captures the observed biology of the vector–parasite interaction. This model suggests a temperature optimum for transmission of 26°C, with a minimum and maximum of 17°C and 35°C, respectively. The differences between the models lead to potentially divergent predictions for the potential impacts of current and future climate change on malaria transmission. The study provides a framework for more detailed, system-specific studies that are essential to develop an improved understanding on the effects of temperature on malaria transmission.

Many of the mosquito and parasite life history traits that combine to influence the transmission intensity of malaria (e.g., adult mosquito longevity, biting rate, the developmental period of the parasite within the mosquito, and the proportion of mosquitoes that become infectious) are strongly temperature sensitive. Yet, in spite of decades of research, the precise relationships between individual traits and temperature remain poorly characterized. As a consequence, the majority of studies exploring the influence of local environmental conditions, or prospective impacts of climate change, draw on a combination of studies that utilize different experimental methods and a range of mosquito and parasite species. Here, we use the Indian malaria mosquito, Anopheles stephensi, and the human malaria parasite, Plasmodium falciparum, to thoroughly characterize the influence of temperature on key transmission-related traits. The results reveal a number of novel insights and challenge some longstanding assumptions regarding the nature of mosquito and parasite thermal responses. This study provides an experimental blueprint for further system-specific studies necessary to more fully understand the implications of changing temperatures on malaria transmission.

Biology, Bionomics and Molecular Biology of Anopheles sinensis Wiedemann 1828 (Diptera: Culicidae), Main Malaria Vector in China

China has set a goal to eliminate all malaria in the country by 2020, but it is unclear if current understanding of malaria vectors and transmission is sufficient to achieve this objective. Anopheles sinensis is the most widespread malaria vector specie in China, which is also responsible for vivax malaria outbreak in central China. We reviewed literature from 1954 to 2016 on An. sinensis with emphasis on biology, bionomics, and molecular biology. A total of 538 references were relevant and included. An. sienesis occurs in 29 Chinese provinces. Temperature can affect most life-history parameters. Most An. sinensis are zoophilic, but sometimes they are facultatively anthropophilic. Sporozoite analysis demonstrated An. sinensis efficacy on Plasmodium vivax transmission. An. sinensis was not stringently refractory to P. falciparum under experimental conditions, however, sporozoite was not found in salivary glands of field collected An. sinensis. The literature on An. sienesis biology and bionomics was abundant, but molecular studies, such as gene functions and mechanisms, were limited. Only 12 molecules (genes, proteins or enzymes) have been studied. In addition, there were considerable untapped omics resources for potential vector control tools. Existing information on An. sienesis could serve as a baseline for advanced research on biology, bionomics and genetics relevant to vector control strategies.

Some like it hot: a differential response to changing temperatures by the malaria vectors Anopheles funestus and An. gambiae s.l

BACKGROUND

With the possible implications of global warming, the effect of temperature on the dynamics of malaria vectors in Africa has become a subject of increasing interest. Information from the field is, however, relatively sparse. We describe the effect of ambient temperature over a five-year period on the dynamics of An. funestus and An. gambiae s.l., collected from a single village in southern Mozambique where temperatures varied from a night-time minimum of 6 °C in the cool season to a daytime maximum of 35 °C in the hot season.

RESULTS

Mean daily air temperatures varied from 34 °C to 20 °C and soil temperatures varied from 26 °C to 12 °C. Diurnal variation was greatest in the cooler months of the year and were greater in air temperatures than soil temperatures. During the study 301, 705 female An. funestus were collected in 6,043 light-trap collections, 161, 466 in 7,397 exit collections and 16, 995 in 1,315 resting collections. The equivalent numbers for An. gambiae s.l. are 72,475 in light-traps, 33, 868 in exit collections and 5,333 from indoor resting collections. Numbers of mosquito were greatest in the warmer months. Numbers of An. gambiae s.l. went through a one hundredfold change (from a mean of 0.14 mosquitoes a night to 14) whereas numbers of An. funestus merely doubled (from a mean of 20 to 40 a night). The highest environmental correlations and mosquito numbers were between mean air temperature (r² = 0.52 for An. funestus and 0.77 for An. gambiae s.l.). Numbers of mosquito collected were not related to rainfall with lags of up to four weeks. Numbers of both gravid and unfed An. gambiae complex females in exit collections continued to increase at all temperatures recorded but gravid females of An. funestus decreased at temperatures above 28 °C. Overall the numbers of gravid and unfed An. funestus collected in exit collections were not correlated (p = 0.07). For an unknown reason the number of An. gambiae s.l. fell below monitoring thresholds during the study.

CONCLUSIONS

Mean air temperature was the most important environmental parameter affecting both vectors in this part of Mozambique. Numbers of An. gambiae s.l. increased at all temperatures recorded whilst An. funestus appeared to be adversely affected by temperatures of 28 °C and above. These differences may influence the distribution of the vectors as the planet warms.

The effect of elevated temperatures on the life history and insecticide resistance phenotype of the major malaria vector Anopheles arabiensis (Diptera: Culicidae)

BACKGROUND

Temperature plays a crucial role in the life history of insects. Recent climate change research has highlighted the importance of elevated temperature on malaria vector distribution. This study aims to examine the role of elevated temperatures on epidemiologically important life-history traits in the major malaria vector, Anopheles arabiensis. Specifically, the differential effects of temperature on insecticide-resistant and susceptible strains were examined.

METHODS

Two laboratory strains of A. arabiensis, the insecticide-susceptible SENN and the insecticide-resistant SENN DDT strains, were used to examine the effect of elevated temperatures on larval development and adult longevity. The effects of various elevated temperatures on insecticide resistance phenotypes were also examined and the biochemical basis of the changes in insecticide resistance phenotype was assessed.

RESULTS

SENN and SENN DDT larvae developed at similar rates at elevated temperatures. SENN DDT adult survivorship did not vary between control and elevated temperatures, while the longevity of SENN adults at constantly elevated temperatures was significantly reduced. SENN DDT adults lived significantly longer than SENN at constantly elevated temperatures. Elevated rearing temperatures, as well as a short-term exposure to 37 and 39 °C as adults, augmented pyrethroid resistance in adult SENN DDT, and increased pyrethroid tolerance in SENN. Detoxification enzyme activity was not implicated in this phenotypic effect. Quercertin-induced synergism of inducible heat shock proteins negated this temperature-mediated augmentation of pyrethroid resistance.

CONCLUSION

Insecticide-resistant A. arabiensis live longer than their susceptible counterparts at elevated temperatures. Exposure to heat shock augments pyrethroid resistance in both resistant and susceptible strains. This response is potentially mediated by inducible heat shock proteins.

Temperature-dependent effects on the replication and transmission of arthropod-borne viruses in their insect hosts

The transmissibility of vector borne viruses can be affected by a combination of factors, both extrinsic (climatic changes, temperature, urbanization, among others) and intrinsic (genetics, life span, immunity, among others). Temperature is of particular importance since the insect vectors of arthropod-borne viruses (arboviruses) are ectothermic and acutely susceptible to temperature changes. Modeling suggests that with increasing global temperature and urbanization, arboviral diseases will continue to emerge or reemerge. This review highlights current literature regarding temperature-dependent effects on virus-vector interactions and their potential to influence the transmission dynamics and epidemiology of arboviral diseases.

Effect of stable and fluctuating temperatures on the life history traits of Anopheles arabiensis and An. quadriannulatus under conditions of inter- and intra-specific competition

Background

Constant and fluctuating temperatures influence important life-history parameters of malaria vectors which has implications for community organization and the malaria disease burden. The effects of environmental temperature on the hatch rate, survivorship and development rate of Anopheles arabiensis and An. quadriannulatus under conditions of inter- and intra-specific competition are studied.

Methods

The eggs and larvae of laboratory established colonies were reared under controlled conditions at one constant (25 °C) and two fluctuating (20–30 °C and 18–35 °C) temperature treatments at a ratio of 1:0 or 1:1 (An. arabiensis: An. quadriannulatus). Monitoring of hatch rate, development rate and survival was done at three intervals, 6 to 8 h apart depending on developmental stage. Parametric ANOVAs were used where assumptions of equal variances and normality were met, and a Welch ANOVA where equal variance was violated (α = 0.05).

Results

Temperature significantly influenced the measured life-history traits and importantly, this was evident when these species co-occurred. A constant temperature resulted in a higher hatch rate in single species, larval treatments (P < 0.05). The treatment 18–35 °C generally reduced survivorship except for An. arabiensis in mixed, larval species treatments where it was similar to values reported for 25 °C. Survivorship of both species at 20–30 °C was not significantly impacted and the adult production was high across species treatments. The development rates at 25 °C and 20–30 °C were significantly different between species when reared alone and in mixed species from larvae and from eggs. The effect of temperature was more pronounced at 18–35 °C with An. arabiensis developing faster under both competitive scenarios and An. quadriannulatus slower, notably when in the presence of its competitor (P < 0.05).

Conclusions

The influence of temperature treatment on the development rate and survival from egg/larvae to adult differed across species treatments. Fluctuating temperatures incorporating the extremes influence the key life-history parameters measured here with An. arabiensis outcompeting An. quadriannulatus under these conditions. The quantification of the response variables measured here improve our knowledge of the link between temperature and species interactions and provide valuable information for modelling of vector population dynamics.