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Background: Eave tube technology is a novel method of insecticide application that uses an electrostatic coating system to boost insecticide efficacy against resistant mosquitoes. A series of previous experiments showed encouraging insecticidal effects against malaria vectors. This study was undertaken to assess the effects of the eave tube approach on other Culicidae, in particular Culex quinquefasciatus, under laboratory and semi-field conditions. Methods: Larvae of Cx. quinquefasciatus from Bouaké were collected and reared to adult stage, and World Health Organization (WHO) cylinder tests were performed to determine their resistance status. WHO standard 3-min cone bioassays were conducted using PermaNet 2.0 netting versus eave tube-treated inserts. To assess the transient exposure effect on Cx. quinquefasciatus, eave tube assay utilizing smelly socks as attractant was performed with exposure time of 30 s, 1 min, and 2 min on 10% beta-cyfluthrin-treated inserts. Residual activity of these treated inserts was then monitored over 9 months. Field tests involving release–recapture of Cx. quinquefasciatus within enclosures around experimental huts fitted with windows and untreated or insecticide-treated eave tubes were conducted to determine house entry preference and the impact of tubes on the survival of this species. Results: Bouaké Cx. quinquefasciatus displayed high resistance to three out of four classes of insecticides currently used in public health. After 3 min of exposure in cone tests, 10% beta-cyfluthrin-treated inserts induced 100% mortality in Cx. quinquefasciatus, whereas the long-lasting insecticidal net (LLIN) only killed 4.5%. With reduced exposure time on the eave tube insert, mortality was still 100% after 2 min, 88% after 1 min, and 44% after 30 s. Mortality following 1 h exposure on 10% beta-cyfluthrin-treated insert was > 80% continuously up to 7 months post-treatment. Data suggest that Cx. quinquefasciatus have a stronger preference for entering

Most empirical and theoretical studies on reproductive senescence focus on observable attributes of offspring produced, such as size or postnatal survival. While harder to study, an important outcome of reproduction for a breeding individual is whether a viable offspring is produced at all. While prenatal mortality can sometimes be directly observed, this can also be indicated through an increase in the interval between offspring production. Both direct reproductive loss and presumed losses have been found to increase in older females across several species. Here, we study such reproductive loss (or “abortion”) in tsetse, a viviparous and relatively long-lived fly with high maternal allocation. We consider how age-dependent patterns of abortion depend on the developmental stage of offspring and find that, as per previous laboratory studies, older females have higher rates of abortion at the late-larval stage, while egg-stage abortions are high both for very young and older females. We track the reproductive output of individual females and find little evidence that experiencing an abortion is an adaptive strategy to improve future reproductive outcomes. After an abortion, females do not generally take less time to produce their next offspring, these offspring are not larger, and there is no sex bias towards females, the sex with presumed higher fitness returns (being slightly larger and longer-lived than males, and with high insemination rates). Abortion rates are higher for breeding females experiencing stress, measured as nutritional deprivation, which echoes previous work in tsetse and other viviparous species, i.e., humans and baboons. We discuss our results in the context of studies on reproductive loss across taxa and argue that this is an important yet often overlooked reproductive trait which can vary with maternal age and can also depend on environmental stressors.

There is growing empirical evidence that animal hosts actively control the density of their mutualistic symbionts according to their requirements. Such active regulation can be facilitated by compartmentalization of symbionts within host tissues, which confers a high degree of control of the symbiosis to the host. Here, we build a general theoretical framework to predict the underlying ecological drivers and evolutionary consequences of host-controlled endosymbiont density regulation for a mutually obligate association between a host and a compartmentalized, vertically transmitted symbiont. Building on the assumption that the costs and benefits of hosting a symbiont population increase with symbiont density, we use state-dependent dynamic programming to determine an optimal strategy for the host, i.e., that which maximizes host fitness, when regulating the density of symbionts. Simulations of active host-controlled regulation governed by the optimal strategy predict that the density of the symbiont should converge to a constant level during host development, and following perturbation. However, a similar trend also emerges from alternative strategies of symbiont regulation. The strategy which maximizes host fitness also promotes symbiont fitness compared to alternative strategies, suggesting that active host-controlled regulation of symbiont density could be adaptive for the symbiont as well as the host. Adaptation of the framework allowed the dynamics of symbiont density to be predicted for other host-symbiont ecologies, such as for non-essential symbionts, demonstrating the versatility of this modelling approach.

Background: Human African trypanosomiasis is a parasitic disease caused by trypanosomes among which Trypanosoma brucei gambiense is responsible for a chronic form (gHAT) in West and Central Africa. Its elimination as a public health problem (EPHP) was targeted for 2020. Côte d'Ivoire was one of the first countries to be validated by WHO in 2020 and this was particularly challenging as the country still reported around a hundred cases a year in the early 2000s. This article describes the strategies implemented including a mathematical model to evaluate the reporting results and infer progress towards sustainable elimination. Methods: The control methods used combined both exhaustive and targeted medical screening strategies including the follow-up of seropositive subjects– considered as potential asymptomatic carriers to diagnose and treat cases– as well as vector control to reduce the risk of transmission in the most at-risk areas. A mechanistic model was used to estimate the number of underlying infections and the probability of elimination of transmission (EoT) was met between 2000–2021 in two endemic and two hypo-endemic health districts. Results: Between 2015 and 2019, nine gHAT cases were detected in the two endemic health districts of Bouaflé and Sinfra in which the number of cases/10,000 inhabitants was far below 1, a necessary condition for validating EPHP. Modelling estimated a slow but steady decline in transmission across the health districts, bolstered in the two endemic health districts by the introduction of vector control. The decrease in underlying transmission in all health districts corresponds to a high probability that EoT has already occurred in Côte d'Ivoire. Conclusion: This success was achieved through a multi-stakeholder and multidisciplinary one health approach where research has played a major role in adapting tools and strategies to this large epidemiological transition to a very low prevalence. This integrated approach will need to conti

Iteroparous parents face a trade-off between allocating current resources to reproduction versus maximizing survival to produce further offspring. Parental allocation varies across age and follows a hump-shaped pattern across diverse taxa, including mammals, birds and invertebrates. This nonlinear allocation pattern lacks a general theoretical explanation, potentially because most studies focus on offspring number rather than quality and do not incorporate uncertainty or age-dependence in energy intake or costs. Here, we develop a life-history model of maternal allocation in iteroparous animals. We identify the optimal allocation strategy in response to stochasticity when energetic costs, feeding success, energy intake and environmentally driven mortality risk are age-dependent. As a case study, we use tsetse, a viviparous insect that produces one offspring per reproductive attempt and relies on an uncertain food supply of vertebrate blood. Diverse scenarios generate a hump-shaped allocation when energetic costs and energy intake increase with age and also when energy intake decreases and energetic costs increase or decrease. Feeding success and environmentally driven mortality risk have little influence on age-dependence in allocation. We conclude that ubiquitous evidence for age-dependence in these influential traits can explain the prevalence of nonlinear maternal allocation across diverse taxonomic groups.

Background: Social contact survey data forms a core component of modern epidemic models: however, there has been little assessment of the potential biases in such data. Methods: We conducted focus groups with university students who had (n = 13) and had never (n = 14) completed a social contact survey during the COVID-19 pandemic. Qualitative findings were explored quantitatively by analysing participation data. Results: The opportunity to contribute to COVID-19 research, to be heard and feel useful were frequently reported motivators for participating in the contact survey. Reductions in survey engagement following lifting of COVID-19 restrictions may have occurred because the research was perceived to be less critical and/or because the participants were busier and had more contacts. Having a high number of contacts to report, uncertainty around how to report each contact, and concerns around confidentiality were identified as factors leading to inaccurate reporting. Focus groups participants thought that financial incentives or provision of study results would encourage participation. Conclusions: Incentives could improve engagement with social contact surveys. Qualitative research can inform the format, timing, and wording of surveys to optimise completion and accuracy.

Many insects rely on intracellular bacterial symbionts to supplement their specialized diets with micronutrients. Using data from diverse and well-studied insect systems, we propose three lines of evidence suggesting that hosts have tight control over the density of their obligate, intracellular bacterial partners. First, empirical studies have demonstrated that the within-host symbiont density varies depending on the nutritional and developmental requirements of the host. Second, symbiont genomes are highly reduced and have limited capacity for self-replication or transcriptional regulation. Third, several mechanisms exist for hosts to tolerate, regulate and remove symbionts including physical compartmentalization and autophagy. We then consider whether such regulation is adaptive, by discussing the relationship between symbiont density and host fitness. We discuss current limitations of empirical studies for exploring fitness effects in host–symbiont relationships, and emphasize the potential for using mathematical models to formalize evolutionary hypotheses and to generate testable predictions for future work.

Many organisms show signs of deterioration with age in terms of survival and reproduction. We tested whether intraspecific variation in such senescence patterns can be driven by resource availability or reproductive history. We did this by manipulating nutritional stress and age at first reproduction and measuring age-dependent reproductive output in tsetse (Glossina morsitans morsitans), a viviparous fly with high maternal allocation. Across all treatments, offspring weight followed a bell-shaped curve with maternal age. Nutritionally stressed females had a higher probability of abortion and produced offspring with lower starvation tolerance. There was no evidence of an increased rate of reproductive senescence in nutritionally stressed females, or a reduced rate due to delayed mating, as measured by patterns of abortion, offspring weight or offspring starvation tolerance. Therefore, although we found evidence of reproductive senescence in tsetse, our results did not indicate that resource allocation trade-offs or costs of reproduction increase the rate of senescence.

Housing improvement such as blocking eaves and screening windows can help in reducing exposure to indoor biting mosquitoes. The impacts of physical barriers could potentially be boosted by the addition of a mechanism that kills mosquitoes as they attempt to enter the house. One example is to combine household screening with EaveTubes, which are insecticide-treated tubes inserted into closed eaves that attract and kill host-searching mosquitoes. The epidemiological impact of screening + EaveTubes is being evaluated in a large cluster randomized trial in Cote d'Ivoire. The study presented here is designed as a complement to this trial to help better understand the functional roles of screening and EaveTubes. We began by evaluating householder behaviour and household condition in the study villages. This work revealed that doors (and to some extent windows) were left open for large parts of the evening and morning, and that even houses modified to make them more 'mosquito proof' often had possible entry points for mosquitoes. We next built two realistic experimental houses in a village to enable us to explore how these aspects of behaviour and household quality affected the impact of screening and EaveTubes. We found that screening could have a substantial impact on indoor mosquito densities, even with realistic household condition and behaviour. By contrast, EaveTubes had no significant impact on indoor mosquito density, either as a stand-alone intervention or in combination with screening. However, there was evidence that mosquitoes recruited to the EaveTubes, and the resulting mortality could create a community benefit. These complementary modes of action of screening and EaveTubes support the rationale of combining the technologies to create a 'Lethal House Lure'.

While across the animal kingdom offspring are born smaller than their parents, notable exceptions exist. Several dipteran species belonging to the Hippoboscoidea superfamily can produce offspring larger than themselves. In this essay, the blood-feeding tsetse is focused on. It is suggested that the extreme reproductive strategy of this fly is enabled by feeding solely on highly nutritious blood, and producing larval offspring that are soft and malleable. This immense reproductive expenditure may have evolved to avoid competition with other biting flies. Tsetse also transmit blood-borne parasites that cause the fatal diseases called African trypanosomiases. It is discussed how tsetse life history and reproductive strategy profoundly influence the type of vector control interventions used to reduce fly populations. In closing, it is argued that the unusual life history of tsetse warrants their preservation in the areas where human and animal health is not threatened.

Background: There is growing interest in the potential to modify houses to target mosquitoes with insecticides or repellents as they search for human hosts. One version of this 'Lethal House Lure' approach is the In2Care® EaveTube, which consists of a section of polyvinyl chloride (PVC) pipe fitted into a closed eave, with an insert comprising electrostatic netting treated with insecticide powder placed inside the tube. Preliminary evidence suggests that when combined with screening of doors and windows, there is a reduction in entry of mosquitoes and an increase in mortality. However, the rate of overnight mortality remains unclear. The current study used a field enclosure built around experimental huts to investigate the mortality of cohorts of mosquitoes over multiple nights. Methods: Anopheles gambiae sensu lato mosquitoes were collected from the field as larvae and reared through to adult. Three-to-five days old adult females were released inside an enclosure housing two modified West African style experimental huts at a field site in M'be, Côte d'Ivoire. Huts were either equipped with insecticide-treated tubes at eave height and had closed windows (treatment) or had open windows and open tubes (controls). The number of host-seeking mosquitoes entering the huts and cumulative mortality were monitored over 2 or 4 days. Results: Very few (0–0.4%) mosquitoes were able to enter huts fitted with insecticide-treated tubes and closed windows. In contrast, mosquitoes continually entered the control huts, with a cumulative mean of 50–80% over 2 to 4 days. Baseline mortality with control huts was approximately 2–4% per day, but the addition of insecticide-treated tubes increased mortality to around 25% per day. Overall cumulative mortality was estimated to be up to 87% over 4 days when huts were fitted with tubes. Conclusion: Only 20–25% of mosquitoes contacted insecticide-treated tubes or entered control huts in a given night. However, mosquitoes continue to host search

Background: Eave tubes are a type of housing modification that provide a novel way of delivering insecticides to mosquitoes as they attempt to enter the house. The current study reports on a series of semi-field studies aimed at improving the understanding of how eave tubes might impact mosquito mortality and behaviour. Methods: Experiments were conducted using West African style experimental huts at a field site in M'be, Côte d'Ivoire. Huts were modified in various ways to determine: (i) whether mosquitoes in this field setting naturally recruit to eave tubes; (ii) whether eave tubes can reduce house entry even in the absence of screening; (iii) whether mosquitoes suffer mortality if they attempt to exit a house via treated eave tubes; and, (iv) whether screening and eave tubes might deflect mosquitoes into neighbouring houses without the intervention. Results: Ninety percent more mosquitoes (Anopheles gambiae sensu lato, and other species) entered huts through open eaves tubes compared to window slits. The addition of insecticide-treated eave tubes reduced mosquito entry by 60%, even when windows remained open. Those mosquitoes that managed to enter the huts exhibited a 64% reduction in blood feeding and a tendency for increased mortality, suggesting contact with insecticide-treated inserts prior to hut entry. When An. gambiae mosquitoes were deliberately introduced into huts with treated eave tubes, there was evidence of six times increase in overnight mortality, suggesting mosquitoes can contact treated eave tube inserts when trying to exit the hut. There was no evidence for deflection of mosquitoes from huts with screening, or screening plus eave tubes, to adjacent unmodified huts. Conclusions: Eave tubes are a potentially effective way to target Anopheles mosquitoes with insecticides. That treated eave tubes can reduce mosquito entry even when windows are open is a potentially important result as it suggests that eave tubes might not need to be combined with h

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

Background: Melanisation – the production and deposition of a layer of melanin that encapsulates many pathogens, including bacteria, filarial nematodes and malaria parasites is one of the main immune responses in mosquitoes. Can a high parasite load overload this immune response? If so, how is the melanisation response distributed among the individual parasites? Methods: We considered these questions with the mosquito Anopheles gambiae by inoculating individuals simultaneously with one, two or three negatively charged Sephadex beads, and estimating the melanisation as the darkness of the bead (which ranges from about 0 for unmelanised beads to 100 for the most melanised beads of our experiment). Results: As the number of beads increased, the average degree to which beads were melanised decreased from 71 to 50. While the darkness of the least melanised bead in a mosquito decreased from an average of 71 to 35, the darkness of the most strongly melanised one did not change with the number of beads. Conclusions: As the number of beads increased, the mosquito's immune response became overloaded. The mosquito's response was to prioritise the melanisation of one bead rather than distributing its response over all beads. Such immune overloading may be an important factor underlying the evolution of resistance against vector-borne diseases.

Le paludisme est une maladie infectieuse causée par des parasites protozoaires et transmise à l'homme par la piqûre de moustiques infectés. Avec plus d'un million de morts chaque année, c'est l'une des maladies infectieuses les plus mortelles au niveau mondial. Le changement climatique rend l'impact de cette maladie d'autant plus préoccupant. En effet, le réchauffement global risque d'augmenter la transmission du paludisme et de modifier les interactions entre le vecteur et le parasite. L'éco-immunologie des moustiques est un moyen de répondre à ces inquiétudes, car elle prend en compte les facteurs intrinsèques au moustique et les facteurs environnementaux qui peuvent influencer sa compétence vectorielle. En effet, cette approche d'écologie évolutive a pour but d'étudier les corrélations entre l'immunité du moustique et les autres traits d'histoire de vie essentiels à la détermination de la fitness du moustique - durée de vie, succès reproductif, taille. Pour améliorer notre compréhension de l'éco-immunologie des moustiques, nous avons décidé d'étudier la variabilité de la réponse immunitaire et l'influence des facteurs environnementaux sur la transmission du paludisme par Anopheles gambiae en laboratoire. Tout d'abord, nous avons étudié la variabilité de la résistance du moustique face aux parasites du paludisme, certains devenant infectés malgré la présence d'un système immunitaire fonctionnel. Nous nous sommes concentrés sur deux réponses immunitaires majeures: l'encapsulation des parasites avec de la mélanine et la production de peptides antimicrobiens qui vont tuer les parasites. La mélanisation et la réponse antimicrobienne sont coûteuses pour le moustique avec pour conséquence la réduction de la durée de vie. Nous avons également observé des corrélations génétiques négatives entre les coûts des deux réponses immunitaires et entre le coût et l'efficacité de la mélanisation. En outre, la mélanisation est une réponse qui peut être rapidement surchar, Malaria is a vector borne disease caused by protozoan parasites and transmitted to humans by mosquitoes. It is one of the most deadly infectious diseases in the world, with more than one million deaths each year. Malaria is even more worrying in the context of climate change, as global warming is predicted to increase the transmission to humans and to influence vector-parasite interactions. To answer these concerns, a better understanding of the ecological immunity of mosquitoes is needed, as it takes into account the intrinsic and environmental factors that may affect the mosquito vector competence. This evolutionary ecology approach relies on the correlations between the immune responses and other life history traits key to the fitness of the mosquito - eg. longevity, reproductive success, size. To improve our knowledge of the ecological immunity of mosquitoes, we decided to study the variability in immune responses and the influence of environmental factors on malaria transmission by Anopheles gambiae mosquitoes in the laboratory. We first investigated the variability in the resistance to malaria parasites, as some mosquitoes become infectious and transmit the parasite, despite an efficient immune system. We focused on two major immune effectors the encapsulation of parasites with melanin and their lysis by antimicrobial peptides. Both the melanisation response and the antimicrobial response were physiologically costly in A. gambiae, with a reduction of the lifespan after an immune challenge. Besides, we found also negative genetic correlations between the costs of the two immune responses and between the cost and the efficacy of the melanisation response. In addition, the melanisation response could be overloaded by an increasing immune challenge, with an unequal repartition of the melanisation effort. Mosquito immunity is made of costs, physiological limits and complex interactions between benefits and costs that will determine the evolution