Arbovirus impact on mosquito behavior: the jury is still out

Parasites can manipulate host behavior to enhance transmission, but our understanding of arbovirus-induced changes in mosquito behavior is limited. Here, we explore current knowledge on such behavioral alterations in mosquito vectors, focusing on host-seeking and blood-feeding behaviors. Reviewing studies on dengue, Zika, La Crosse, Sindbis, and West Nile viruses in Aedes or Culex mosquitoes reveals subtle yet potentially significant effects. However, assay heterogeneity and limited sample sizes challenge definitive conclusions. To enhance robustness, we propose using deep-learning tools for automated behavior quantification and stress the need for standardized assays. Additionally, conducting longitudinal studies across the extrinsic incubation period and integrating diverse traits into modeling frameworks are crucial for understanding the nuanced implications of arbovirus-induced behavioral changes for virus transmission dynamics.

Carbon dioxide shapes parasite-host interactions in a human-infective nematode

Skin-penetrating nematodes infect nearly one billion people worldwide. The developmentally arrested infective larvae (iL3s) seek out hosts, invade hosts via skin penetration, and resume development inside the host in a process called activation. Activated infective larvae (iL3as) traverse the host body, ending up as parasitic adults in the small intestine. Skin-penetrating nematodes respond to many chemosensory cues, but how chemosensation contributes to host seeking, intra-host development, and intra-host navigation - three crucial steps of the parasite-host interaction - remains poorly understood. Here, we investigate the role of carbon dioxide (CO2) in promoting parasite-host interactions in the human-infective threadworm Strongyloides stercoralis. We show that S. stercoralis exhibits life-stage-specific preferences for CO2: iL3s are repelled, non-infective larvae and adults are neutral, and iL3as are attracted. CO2 repulsion in iL3s may prime them for host seeking by stimulating dispersal from host feces, while CO2 attraction in iL3as may direct worms toward high-CO2 areas of the body such as the lungs and intestine. We also identify sensory neurons that detect CO2; these neurons are depolarized by CO2 in iL3s and iL3as. In addition, we demonstrate that the receptor guanylate cyclase Ss-GCY-9 is expressed specifically in CO2-sensing neurons and is required for CO2-evoked behavior. Ss-GCY-9 also promotes activation, indicating that a single receptor can mediate both behavioral and physiological responses to CO2. Our results illuminate chemosensory mechanisms that shape the interaction between parasitic nematodes and their human hosts and may aid in the design of novel anthelmintics that target the CO2-sensing pathway.

Anatomical, phenological and genetic aspects of the host-parasite relationship between Andrena vaga (Hymenoptera) and Stylops ater (Strepsiptera)

Stylops ater is an endoparasite of the mining bee Andrena vaga with extreme sexual dimorphism and hypermetamorphosis. Its population structure, parasitization mode, genetic diversity and impact on host morphology were examined in nesting sites in Germany to better understand this highly specialized host–parasite interaction. The shift in host emergence due to stylopization was proven to be especially strong in A. vaga. Around 10% of bees hosted more than 1 Stylops, with at maximum 4. A trend in Stylops' preference for hosts of their own sex and a sex-specific position of extrusion from the host abdomen was found. Invasion of Andrena eggs by Stylops primary larvae was depicted for the first time. Cephalothoraces of female Stylops were smaller in male and pluristylopized hosts, likely due to lower nutrient supply. The genes H3, 18S and cytochrome c oxidase subunit 1 were highly conserved, revealing near-absence of local variation within Stylops. Ovaries of hosts with male Stylops contained poorly developed eggs while those of hosts with female Stylops were devoid of visible eggs, which might be due to a higher protein demand of female Stylops. Male Stylops, which might have a more energy-consuming development, led to a reduction in head width of their hosts. Host masculinization was present in the leaner shape of the metabasitarsus of stylopized females and is interpreted as a by-product of manipulation of the host's endocrine system to shift its emergence. Stylopization intensified tergal hairiness, most strongly in hosts with female Stylops, near the point of parasite extrusion, hinting towards substance-induced host manipulation.

Why are biting flies attracted to blue objects?

Diurnal biting flies are strongly attracted to blue objects. This behaviour is widely exploited for fly control, but its functional significance is debated. It is hypothesized that blue objects resemble animal hosts; blue surfaces resemble shaded resting places; and blue attraction is a by-product of attraction to polarized light. We computed the fly photoreceptor signals elicited by a large sample of leaf and animal integument reflectance spectra, viewed under open/cloudy illumination and under woodland shade. We then trained artificial neural networks (ANNs) to distinguish animals from leaf backgrounds, and shaded from unshaded surfaces, in order to find the optimal means of doing so based upon the sensory information available to a fly. After training, we challenged ANNs to classify blue objects used in fly control. Trained ANNs could make both discriminations with high accuracy. They discriminated animals from leaves based upon blue-green photoreceptor opponency and commonly misclassified blue objects as animals. Meanwhile, they discriminated shaded from unshaded stimuli using achromatic cues and never misclassified blue objects as shaded. We conclude that blue-green opponency is the most effective means of discriminating animals from leaf backgrounds using a fly's sensory information, and that blue objects resemble animal hosts through such mechanisms.

Humidity sensors that alert mosquitoes to nearby hosts and egg-laying sites

To reproduce and to transmit disease, female mosquitoes must obtain blood meals and locate appropriate sites for egg laying (oviposition). While distinct sensory cues drive each behavior, humidity contributes to both. Here, we identify the mosquito's humidity sensors (hygrosensors). Using generalizable approaches designed to simplify genetic analysis in non-traditional model organisms, we demonstrate that the ionotropic receptor Ir93a mediates mosquito hygrosensation as well as thermosensation. We further show that Ir93a-dependent sensors drive human host proximity detection and blood-feeding behavior, consistent with the overlapping short-range heat and humidity gradients these targets generate. After blood feeding, gravid females require Ir93a to seek high humidity associated with preferred egg-laying sites. Reliance on Ir93a-dependent sensors to promote blood feeding and locate potential oviposition sites is shared between the malaria vector Anopheles gambiae and arbovirus vector Aedes aegypti. These Ir93a-dependent systems represent potential targets for efforts to control these human disease vectors.

Salivary AsHPX12 influences pre-blood meal associated behavioral properties in Anopheles stephensi

BACKGROUND & OBJECTIVES

A successful blood meal acquisition process by an adult female mosquito is accomplished through salivary glands, which releases a cocktail of proteins to counteract the vertebrate host's immune homeostasis. Here, we characterize a salivary-specific Heme peroxidase family member HPX12, originally identified from Plasmodium vivax infected salivary RNAseq data of the mosquito Anopheles stephensi.

METHODS

To demonstrate we utilized a comprehensive in silico and functional genomics approach.

RESULTS

Our dsRNA-mediated silencing experiments demonstrate that salivary AsHPX12 may regulate pre-blood meal-associated behavioral properties such as probing time, probing propensity, and host attraction. Altered expression of the salivary secretory and antennal proteins expression may have accounted for salivary homeostasis disruption resulting in the unusual fast release of salivary cocktail proteins and delayed acquisition of blood meal in the AsHPX12 knockdown mosquitoes. We also observed a significant parallel transcriptional modulation in response to blood feeding and P. vivax infection.

INTERPRETATION & CONCLUSION

With this work, we establish a possible functional correlation of AsHPX12 role in the maintenance of salivary physiological-homeostasis, and Plasmodium sporozoites survival/transmission, though the mechanism is yet to unravel.

Elimination of vision-guided target attraction in Aedes aegypti using CRISPR

Blood-feeding insects, such as the mosquito, Aedes (Ae.) aegypti, use multiple senses to seek out and bite humans.^1,2 Upon exposure to the odor of CO₂, the attention of female mosquitoes to potential targets is greatly increased. Female mosquitoes are attracted to high-contrast visual cues and use skin olfactory cues to assist them in homing in on targets several meters away.^3-9 Within close range, convective heat from skin and additional skin odors further assist the mosquitoes' evaluation as to whether the object of interest might be a host.^10,11 Here, using CRISPR-Cas9, we mutated the gene encoding Op1, which is the most abundant of the five rhodopsins expressed in the eyes of Ae. aegypti. Using cage and wind-tunnel assays, we found that elimination of op1 did not impair CO₂-induced target seeking. We then mutated op2, which encodes the rhodopsin most similar to Op1, and also found that there was no impact on this behavior. Rather, mutation of both op1 and op2 was required for abolishing vision-guided target attraction. In contrast, the double mutants exhibited normal phototaxis and odor-tracking responses. By measuring the walking optomotor response, we found that the double mutants still perceived optic flow. In further support of the conclusion that the double mutant is not blind, the animals retained an electrophysiological response to light, although it was diminished. This represents the first genetic perturbation of vision in mosquitoes and indicates that vision-guided target attraction by Ae. aegypti depends on two highly related rhodopsins.

Characterizing the Role of Orco Gene in Detecting Aggregation Pheromone and Food Resources in Protaetia brevitarsis Leiws (Coleoptera: Scarabaeidae)

An accurate olfactory system for recognizing semiochemicals and environmental chemical signals plays crucial roles in survival and reproduction of insects. Among all olfaction-related proteins, olfactory receptors (ORs) contribute to the conversion of chemical stimuli to electric signals and thereby are vital in odorant recognition. Olfactory receptor co-receptor (Orco), one of the most conserved ORs, is extremely essential in recognizing odorants through forming a ligand-gated ion channel complex with conventional ligand-binding odorant receptors. We have previously identified aggregation pheromone in Protaetia brevitarsis (Coleoptera: Scarabaeidae), a native agricultural and horticultural pest in East-Asia. However, to our best knowledge, its olfaction recognition mechanisms are still veiled. To illustrate how P. brevitarsis recognize aggregation pheromone and host plants, in the present study we cloned and sequenced the full-length Orco gene from P. brevitarsis antennae (named PbreOrco) and found that PbreOrco is highly conserved and similar to Orcos from other Coleoptera insects. Our real-time quantitative PCR (qRT-PCR) results showed that PbreOrco is mainly expressed in antenna. We also demonstrated that silencing PbreOrco using RNA interference through injecting dsOrco fragment significantly inhibited PbreOrco expression in comparison with injecting control dsGFP and subsequently revealed using electroantennogram and behavioral bioassays that decreasing PbreOrco transcript abundance significantly impaired the responses of P. brevitarsis to intraspecific aggregation pheromone and prolonged the time of P. brevitarsis spending on food seeking. Overall, our results demonstrated that PbreOrco is crucial in mediating odorant perception in P. brevitarsis.

Mosquito Host Seeking in 3D Using a Versatile Climate-Controlled Wind Tunnel System

Future anthropogenic climate change is predicted to impact sensory-driven behaviors. Building on recent improvements in computational power and tracking technology, we have developed a versatile climate-controlled wind tunnel system, in which to study the effect of climate parameters, including temperature, precipitation, and elevated greenhouse gas levels, on odor-mediated behaviors in insects. To establish a baseline for future studies, we here analyzed the host-seeking behavior of the major malaria vector mosquito, Anopheles gambiae sensu strico, to human odor and carbon dioxide (CO₂), under tightly controlled climatic conditions, and isolated from potential background contamination by the presence of an experimenter. When presented with a combination of human foot odor and CO₂ (case study I), mosquitoes engaged in faster crosswind flight, spent more time in the filamentous odor plume and targeted the odor source more successfully. In contrast, female An. gambiae s. s. presented with different concentrations of CO₂ alone, did not display host-seeking behavior (case study II). These observations support previous findings on the role of human host-associated cues in host seeking and confirm the role of CO₂ as a synergist, but not a host-seeking cue on its own. Future studies are aimed at investigating the effect of climate change on odor-mediated behavior in mosquitoes and other insects. Moreover, the system will be used to investigate detection and processing of olfactory information in various behavioral contexts, by providing a fine-scale analysis of flight behavior.

Pathogens Manipulating Tick Behavior-Through a Glass, Darkly

Pathogens can manipulate the phenotypic traits of their hosts and vectors, maximizing their own fitness. Among the phenotypic traits that can be modified, manipulating vector behavior represents one of the most fascinating facets. How pathogens infection affects behavioral traits of key insect vectors has been extensively investigated. Major examples include Plasmodium, Leishmania and Trypanosoma spp. manipulating the behavior of mosquitoes, sand flies and kissing bugs, respectively. However, research on how pathogens can modify tick behavior is patchy. This review focuses on current knowledge about the behavioral changes triggered by Anaplasma, Borrelia, Babesia, Bartonella, Rickettsia and tick-borne encephalitis virus (TBEV) infection in tick vectors, analyzing their potential adaptive significance. As a general trend, being infected by Borrelia and TBEV boosts tick mobility (both questing and walking activity). Borrelia and Anaplasma infection magnifies Ixodes desiccation resistance, triggering physiological changes (Borrelia: higher fat reserves; Anaplasma: synthesis of heat shock proteins). Anaplasma infection also improves cold resistance in infected ticks through synthesis of an antifreeze glycoprotein. Being infected by Anaplasma, Borrelia and Babesia leads to increased tick survival. Borrelia, Babesia and Bartonella infection facilitates blood engorgement. In the last section, current challenges for future studies are outlined.

Chemosensory mechanisms of host seeking and infectivity in skin-penetrating nematodes

Approximately 800 million people worldwide are infected with one or more species of skin-penetrating nematodes. These parasites persist in the environment as developmentally arrested third-stage infective larvae (iL3s) that navigate toward host-emitted cues, contact host skin, and penetrate the skin. iL3s then reinitiate development inside the host in response to sensory cues, a process called activation. Here, we investigate how chemosensation drives host seeking and activation in skin-penetrating nematodes. We show that the olfactory preferences of iL3s are categorically different from those of free-living adults, which may restrict host seeking to iL3s. The human-parasitic threadworm Strongyloides stercoralis and hookworm Ancylostoma ceylanicum have highly dissimilar olfactory preferences, suggesting that these two species may use distinct strategies to target humans. CRISPR/Cas9-mediated mutagenesis of the S. stercoralis tax-4 gene abolishes iL3 attraction to a host-emitted odorant and prevents activation. Our results suggest an important role for chemosensation in iL3 host seeking and infectivity and provide insight into the molecular mechanisms that underlie these processes.

General Visual and Contingent Thermal Cues Interact to Elicit Attraction in Female Aedes aegypti Mosquitoes

Female Aedes aegypti mosquitoes use multiple sensory modalities to hunt human hosts and obtain a blood meal for egg production. Attractive cues include carbon dioxide (CO₂), a major component of exhaled breath [1, 2]; heat elevated above ambient temperature, signifying warm-blooded skin [3, 4]; and dark visual contrast [5, 6], proposed to bridge long-range olfactory and short-range thermal cues [7]. Any of these sensory cues in isolation is an incomplete signal of a human host, and so a mosquito must integrate multimodal sensory information before committing to approaching and biting a person [8]. Here, we study the interaction of visual cues, heat, and CO₂ to investigate the contributions of human-associated stimuli to host-seeking decisions. We show that tethered flying mosquitoes strongly orient toward dark visual contrast, regardless of CO₂ stimulation and internal host-seeking status. This suggests that attraction to visual contrast is general and not contingent on other host cues. In free-flight experiments with CO₂, adding a dark contrasting visual cue to a warmed surface enhanced attraction. Moderate warmth became more attractive to mosquitoes, and mosquitoes aggregated on the cue at all non-noxious temperatures. Gr3 mutants, unable to detect CO₂, were lured to the visual cue at ambient temperatures but fled and did not return when the surface was warmed to host-like temperatures. This suggests that attraction to thermal cues is contingent on the presence of the additional sensory cue CO₂. Our results illustrate that mosquitoes integrate general attractive visual stimuli with context-dependent thermal stimuli to seek promising sites for blood feeding.

Aedes aegypti Mosquitoes Detect Acidic Volatiles Found in Human Odor Using the IR8a Pathway

Mosquitoes use olfaction as a primary means of detecting their hosts. Previously, the functional ablation of a family of Aedes aegypti olfactory receptors, the odorant receptors (ORs), was not sufficient to reduce host seeking in the presence of carbon dioxide (CO₂). This suggests the olfactory receptors that remain, such as the ionotropic receptors (IRs), could play a significant role in host detection. To test this, we disrupted the Ir8a co-receptor in Ae. aegypti using CRISPR/Cas9. We found that Ir8a mutant female mosquitoes are not attracted to lactic acid, a behaviorally active component of human sweat, and they lack odor-evoked responses to acidic volatiles. The loss of Ir8a reduces mosquito attraction to humans and their odor. We show that the CO₂-detection pathway is necessary but not sufficient for IR8a to detect human odor. Our study reveals that the IR8a pathway is crucial for an anthropophilic vector mosquito to effectively seek hosts. VIDEO ABSTRACT.

Mosquitoes on a Diet Reduce Those Pesky Bites

Targeting the internal regulation of mosquito's human-seeking capacity provides a novel means for vector control. Mosquitoes bite us to obtain blood in order to develop their eggs. Professor Vosshall and colleagues (Cell 2019;176:687-701) have exploited the modulatory pathway to control this hunger for blood by designing drugs based on human diet-suppression pharmaceuticals.

Host Respiration Rate and Injury-Derived Cues Drive Host Preference by an Ectoparasite of Fruit Flies

Host bioenergetics and energy fluxes can be applied to measure the ecological and physiological effects of parasitism. By measuring changes in host metabolic rate, one can estimate the physiological costs of infection. Additionally, metabolic rate dictates the rate of resource conversion within a host and, by extension, the resources available to a parasite. We hypothesize that parasites are selected to respond to cues that indicate high resource availability, that is, host metabolic state. We investigated whether an ectoparasite mite (Macrocheles subbadius) can differentiate between potential hosts (Drosophilia nigrospiracula) on the basis of relative carbon dioxide output as measured by respirometry. In pairwise choice tests, mites were allowed to choose between two size-matched fruit flies with differing metabolic rates or levels of CO₂ output. Our results showed that mites preferentially infect flies with relatively higher respiration rates. Accordingly, we investigated whether fly respiratory rate (measured by CO₂ production) changes in response to injury, potentially explaining a previously reported preference for injured flies. We also tested whether chemical cues released during injury influence preference for injured hosts. We determined that fly exudate (mostly consisting of hemolymph) applied to an uninjured fly released at the site of injury significantly increased the likelihood of infection, but injury did not significantly change the CO₂ output of the flies. Our results suggest that parasites are relying on chemical cues not only for host finding but also to discriminate between hosts on the basis of the rate of respiration, with potentially important implications for the metabolic theory of ecology.

Short-Range Responses of the Kissing Bug Triatoma rubida (Hemiptera: Reduviidae) to Carbon Dioxide, Moisture, and Artificial Light

The hematophagous bug Triatoma rubida is a species of kissing bug that has been marked as a potential vector for the transmission of Chagas disease in the Southern United States and Northern Mexico. However, information on the distribution of T. rubida in these areas is limited. Vector monitoring is crucial to assess disease risk, so effective trapping systems are required. Kissing bugs utilize extrinsic cues to guide host-seeking, aggregation, and dispersal behaviors. These cues have been recognized as high-value targets for exploitation by trapping systems. A modern video-tracking system was used with a four-port olfactometer system to quantitatively assess the behavioral response of T. rubida to cues of known significance. Also, response of T. rubida adults to seven wavelengths of light-emitting diodes (LED) in paired-choice pitfall was evaluated. Behavioral data gathered from these experiments indicate that T. rubida nymphs orient preferentially to airstreams at either 1600 or 3200 ppm carbon dioxide and prefer relative humidity levels of about 30%, while adults are most attracted to 470 nm light. These data may serve to help design an effective trapping system for T. rubida monitoring. Investigations described here also demonstrate the experimental power of combining an olfactometer with a video-tracking system for studying insect behavior.

Blood-Feeding Behaviors of Anopheles stephensi but not Phlebotomus papatasi are Influenced by Actively Warming Guinea Pigs (Cavia porcellus) Under General Anesthesia

Animal models are often used to study hematophagous insect feeding behavior and evaluate products such as topical repellents. However, when these models are used the study animals often experience significant drops in core body temperature because of the effects of anesthesia. This study used a guinea pig model to investigate whether maintaining a normothermic core body temperature during anesthesia influenced the rate of Anopheles stephensi and Phlebotomus papatasi blood feeding. Experiments were conducted with anesthetized animals that had their body temperatures either maintained with a warming device or were allowed to drop naturally. Results showed that when guinea pigs were actively warmed by a heating device, An. stephensi feeding behavior was similar at the beginning and end of anesthesia. However, when a warming device was not used, fewer An. stephensi took a blood meal after the animals' temperatures had dropped. Phlebotomus papatasi were not as sensitive to changes in temperature and feeding rates were similar whether a warming device was used or not. These results are discussed and it is recommended that warming devices are used when conducting feeding experiments with insects sensitive to changes in host body temperature, such as An. stephensi.

Chemosensory behaviors of parasites

Many multicellular parasites seek out hosts by following trails of host-emitted chemicals. Host seeking is a characteristic of endoparasites such as parasitic worms as well as of ectoparasites such as mosquitoes and ticks. For host location, many of these parasites use CO(2), a respiration byproduct, in combination with host-specific chemicals. Recent work has begun to elucidate the behavioral responses of parasites to CO(2) and other host chemicals, and to unravel the mechanisms of these responses. Here we discuss recent findings that have greatly advanced our understanding of the chemosensory behaviors of host-seeking parasites. We focus primarily on well-studied parasites such as nematodes and insects, but also note broadly relevant findings in a few less well studied parasites.