Do avian malaria parasites reduce vector longevity?

Survival effects of antibiotic exposure during the larval and adult stages in the West Nile virus vector Culex pipiens

The ability of mosquitoes to transmit a pathogen is affected, among other factors, by their survival rate, which is partly modulated by their microbiota. Mosquito microbiota is acquired during the larval phase and modified during their development and adult feeding behavior, being highly dependent on environmental factors. Pharmaceutical residues including antibiotics are widespread pollutants potentially being present in mosquito breeding waters likely affecting their microbiota. Here, we used Culex pipiens mosquitoes to assess the impact of antibiotic exposure during the larval and adult stages on the survival rate of adult mosquitoes. Wild-collected larvae were randomly assigned to two treatments: larvae maintained in water supplemented with antibiotics and control larvae. Emerged adults were subsequently assigned to each of two treatments, fed with sugar solution with antibiotics and fed only with sugar solution (controls). Larval exposure to antibiotics significantly increased the survival rate of adult females that received a control diet. In addition, the effect of adult exposure to antibiotics on the survival rate of both male and female mosquitoes depended on the number of days that larvae fed ad libitum in the laboratory before emergence. In particular, shorter larval ad libitum feeding periods reduced the survival rate of antibiotic-treated adult mosquitoes compared with those that emerged after a longer larval feeding period. These differences were not found in control adult mosquitoes. Our results extend the current understanding of the impact of antibiotic exposure of mosquitoes on a key component of vectorial capacity, that is the vector survival rate.

The interplay between vector microbial community and pathogen transmission on the invasive Asian tiger mosquito, Aedes albopictus: current knowledge and future directions

The invasive Asian tiger mosquito Aedes albopictus is nowadays broadly distributed with established populations in all continents except Antarctica. In the invaded areas, this species represents an important nuisance for humans and, more relevant, it is involved in the local transmission of pathogens relevant under a public health perspective. Aedes albopictus is a competent vector of parasites such as Dirofilaria and viruses including dengue virus, Zika virus, and chikungunya virus, among others. The mosquito microbiota has been identified as one of the major drivers of vector competence, acting upon relevant vector functions as development or immunity. Here, we review the available literature on the interaction between Ae. albopictus microbiota and pathogen transmission and identify the knowledge gaps on the topic. Most studies are strictly focused on the interplay between pathogens and Wolbachia endosymbiont while studies screening whole microbiota are still scarce but increasing in recent years, supported on Next-generation sequencing tools. Most experimental trials use lab-reared mosquitoes or cell lines, exploring the molecular mechanisms of the microbiota-pathogen interaction. Yet, correlational studies on wild populations are underrepresented. Consequently, we still lack sufficient evidence to reveal whether the microbiota of introduced populations of Ae. albopictus differ from those of native populations, or how microbiota is shaped by different environmental and anthropic factors, but especially, how these changes affect the ability of Ae. albopictus to transmit pathogens and favor the occurrence of outbreaks in the colonized areas. Finally, we propose future research directions on this research topic.

Bird community effects on avian malaria infections

In community assembly processes, interspecific interactions play an important role in shaping community diversity, especially at the local scale. Changes in species richness or abundance can modify local infectious disease dynamics, either reducing or increasing the risk of transmission within the community. This study evaluates the effects of bird community on avian haemosporidians infections in a Neotropical region. Bird samples were collected from areas surrounding three dams, and molecular analysis were performed to identify blood-parasitic haemosporidia infecting the birds. Generalized linear models were used to analyze the relationships between the bird community and the prevalence, number of infections, and richness of avian haemosporidian lineages. Non-significant effects of bird community dominance and richness on the prevalence of avian parasites and the number of infections of Haemoproteus were found. However, there was evidence of an amplification effect. Host dominance was associated with the total number of infections, the number Plasmodium infections and the expected richness of Plasmodium lineages, while the expected richness of Haemoproteus lineages was associated with the richness of bird species. These findings highlight the role of host community dominance and richness in the dynamics of parasite infections, potentially influenced by the availability of competent hosts. This study contributes significantly to our understanding of blood parasite diversity in tropical birds within a relatively understudied region of South America.

First evidence for development of Plasmodium relictum (Grassi and Feletti, 1891) sporozoites in the salivary glands of Culex modestus Ficalbi, 1889

The competence of insect vectors to transmit diseases plays a key role in host-parasite interactions and in the dynamics of avian malaria and other haemosporidian infections (Apicomplexa, Haemosporida). However, the presence of parasite DNA in the body of blood-sucking insects does not always constitute evidence for their competence as vectors. In this study, we investigate the susceptibility of wild-caught mosquitoes (Culex spp.) to complete sporogony of Plasmodium relictum (cyt b lineage SGS1) isolated from great tits (Parus major L., 1758). Adult female mosquitoes were collected with a CO₂ bait trap overnight. A set of 50 mosquitoes was allowed to feed for 3 h at night on a single great tit infected with P. relictum. This trial was repeated on 6 different birds. The bloodfed mosquitoes that survived (n = 68) were dissected within 1-2 days (for ookinetes, n = 10) and 10-33 days post infection (for oocysts and sporozoites, n = 58) in order to confirm the respective parasite stages in their organs. The experiment confirmed the successful development of P. relictum (cyt b lineage SGS1) to the stage of sporozoites in Culex pipiens L., 1758 (n = 27) and in Culex modestus (n = 2). Our study provides the first evidence that C. modestus is a competent vector of P. relictum isolated from great tits, suggesting that this mosquito species could also play a role in the natural transmission of avian malaria.

Evaluation of sustainable susceptibility to Plasmodium vivax infection among colonized Anopheles darlingi and Anopheles deaneorum

BACKGROUND

The colonization of mosquitoes susceptible to Plasmodium vivax via direct membrane feeding assay (DMFA) has the potential to significantly advance our knowledge of P. vivax biology, vector-parasite interaction and transmission-blocking vaccine research. Anopheles darlingi and Anopheles deaneorum are important vectors of malaria in the Western Brazilian Amazon. Since 2018, well-established colonies of these species have been maintained in order to mass produce mosquitoes destined for P. vivax infection. Plasmodium susceptibility was confirmed when the colonies were established, but susceptibility needs to be maintained for these colonies to remain good models for pathogen transmission. Thus, the susceptibility was assessed of colonized mosquitoes to P. vivax isolates circulating in the Western Amazon.

METHODS

Laboratory-reared mosquitoes from F10-F25 generations were fed on P. vivax blood isolates via DMFA. Susceptibility was determined by prevalence and intensity of infection as represented by oocyst load seven days after blood feeding, and sporozoite load 14 days after blood feeding. The effect of infection on mosquito survival was evaluated from initial blood feeding until sporogonic development and survival rates were compared between mosquitoes fed on infected and uninfected blood. Correlation was calculated between gametocytaemia and prevalence/intensity of infection, and between oocyst and sporozoite load.

RESULTS

Significant differences were found in prevalence and intensity of infection between species. Anopheles darlingi showed a higher proportion of infected mosquitoes and higher oocyst and sporozoite intensity than An. deaneorum. Survival analysis showed that An. deaneorum survival decreased drastically until 14 days post infection (dpi). Plasmodium vivax infection decreased survival in both species relative to uninfected mosquitoes. No correlation was observed between gametocytaemia and prevalence/intensity of infection, but oocyst and sporozoite load had a moderate to strong correlation.

CONCLUSIONS

Colonized An. darlingi make excellent subjects for modelling pathogen transmission. On the other hand, An. deaneorum could serve as a model for immunity studies due the low susceptibility under current colonized conditions. In the application of DMFA, gametocyte density is not a reliable parameter for predicting mosquito infection by P. vivax, but oocyst intensity should be used to schedule sporozoite experiments.

The Impact of Temperature on the Sporogonic Development of the Tropical Avian Malaria Parasite Plasmodium relictum (Genetic Lineage pGRW4) in Culex pipiens Form molestus Mosquitoes

The avian malaria parasite Plasmodium relictum (genetic lineage pGRW4) is known to cause severe pathology in nonadapted vertebrate hosts. This parasite is prevalent in some bird species in Northern Europe, however the records obtained are only from adult long-distance migrant birds after their return from the wintering grounds. A recent experimental study showed that this parasite completes sporogonic development in the local European vector Culex pipiens at a controlled mean temperature of 19 °C. Thereby, temperature limits for the transmission of this parasite in Northern Europe remain unknown. In this study, we took a step further and tested the impact of different temperature conditions, including some extreme fluctuations between 23 °C down to 7 °C, on the sporogonic development of P. relictum (pGRW4) in the vector Culex pipiens form molestus. Mosquitoes were exposed to infection and kept under different air-temperature conditions: (i) constant warm temperature, (ii) natural outdoor temperatures and (iii) temporary exposure to low temperatures. Plasmodium relictum (pGRW4) completed sporogony in mosquitoes of all experimental groups, however different patterns of the sporogonic development depending on temperature conditions were observed. Based on these results, we conclude that the cool air temperature of Northern Europe in summer is not a limiting factor in successful development of the parasite. However, delayed sporogony caused by low summer temperatures may have a detrimental impact on the active transmission of this parasite in Northern Europe.

Culex quinquefasciatus (Diptera: Culicidae) survivorship following the ingestion of bird blood infected with Haemoproteus sp. parasites

Arthropod vectors are frequently exposed to a diverse assemblage of parasites, but the consequence of these infections on their biology and behavior are poorly understood. We experimentally evaluated whether the ingestion of a common protozoan parasite of avian hosts (Haemoproteus spp.; Haemosporida: Haemoproteidae) impacted the survivorship of Culex quinquefasciatus (Say) (Diptera: Culicidae). Blood was collected from wild northern cardinals (Cardinalis cardinalis) in College Station, Texas, and screened for the presence of Haemoproteus spp. parasites using microscopic and molecular methods. Experimental groups of Cx. quinquefasciatus mosquitoes were offered Haemoproteus-positive cardinal blood through an artificial feeding apparatus, while control groups received Haemoproteus-negative cardinal blood or domestic canary (Serinus canaria domestica) blood. Culex quinquefasciatus mosquitoes exposed to Haemoproteus infected cardinal blood survived significantly fewer days than mosquitoes that ingested Haemoproteus-negative cardinal blood. The survival of mosquitoes fed on positive cardinal blood had a median survival time of 18 days post-exposure and the survival of mosquitoes fed on negative cardinal blood exceeded 50% across the 30 day observation period. Additionally, mosquitoes that fed on canary controls survived significantly fewer days than cardinal negative controls, with canary control mosquitoes having a median survival time of 17 days. This study further supports prior observations that Haemoproteus parasites can be pathogenic to bird-biting mosquitoes, and suggests that Haemoproteus parasites may indirectly suppress the transmission of co-circulating vector-borne pathogens by modulating vector survivorship. Our results also suggest that even in the absence of parasite infection, bloodmeals from different bird species can influence mosquito survivorship.

Effects of Mosquito Microbiota on the Survival Cost and Development Success of Avian Plasmodium

Both intrinsic and extrinsic factors affect the capacity of mosquitoes for the transmission of vector-borne pathogens. Among them, mosquito microbiota may play a key role determining the development of pathogens in mosquitoes and the cost of infections. Here, we used a wild avian malaria-mosquito assemblage model to experimentally test the role of vector microbiota on the cost of infection and their consequences for parasite development. To do so, a cohort of Culex pipiens mosquitoes were treated with antibiotics, including gentamicin sulfate and penicillin-streptomycin, to alter their microbiota, and other cohort was treated with sterilized water as controls. Subsequently, both cohorts were allowed to feed on Plasmodium infected or uninfected house sparrows (Passer domesticus). The antibiotic treatment significantly increased the survival rate of mosquitoes fed on infected birds while this was not the case of mosquitoes fed on uninfected birds. Additionally, a higher prevalence of Plasmodium in the saliva of mosquitoes was found in antibiotic treated mosquitoes than in mosquitoes of the control group at 20 days post exposure (dpe). Analyses of the microbiota of a subsample of mosquitoes at 20 dpe suggest that although the microbiota diversity did not differ between individuals of the two treatments, microbiota in control mosquitoes had a higher number of unique features and enriched in biochemical pathways related to the immune system than antibiotic treated ones. In sum, this study provides support for the role of mosquito microbiota on mosquito survival and the presence of parasite DNA in their saliva.

Impact of field-realistic doses of glyphosate and nutritional stress on mosquito life history traits and susceptibility to malaria parasite infection

Glyphosate is the world's most widely used herbicide. The commercial success of this molecule is due to its nonselectivity and its action, which would supposedly target specific biosynthetic pathways found mainly in plants. Multiple studies have however provided evidence for high sensitivity of many nontarget species to glyphosate and/or to formulations (glyphosate mixed with surfactants). This herbicide, found at significant levels in aquatic systems through surface runoffs, impacts life history traits and immune parameters of several aquatic invertebrates' species, including disease-vector mosquitoes. Mosquitoes, from hatching to emergence, are exposed to aquatic chemical contaminants. In this study, we first compared the toxicity of pure glyphosate to the toxicity of glyphosate-based formulations for the main vector of avian malaria in Europe, Culex pipiens mosquito. Then we evaluated, for the first time, how field-realistic dose of glyphosate interacts with larval nutritional stress to alter mosquito life history traits and susceptibility to avian malaria parasite infection. Our results show that exposure of larvae to field-realistic doses of glyphosate, pure or in formulation, did not affect larval survival rate, adult size, and female fecundity. One of our two experimental blocks showed, however, that exposure to glyphosate decreased development time and reduced mosquito infection probability by malaria parasite. Interestingly, the effect on malaria infection was lost when the larvae were also subjected to a nutritional stress, probably due to a lower ingestion of glyphosate.

Plasmodium transmission differs between mosquito species and parasite lineages

Factors such as the particular combination of parasite-mosquito species, their co-evolutionary history and the host's parasite load greatly affect parasite transmission. However, the importance of these factors in the epidemiology of mosquito-borne parasites, such as avian malaria parasites, is largely unknown. Here, we assessed the competence of two mosquito species [Culex pipiens and Aedes (Ochlerotatus) caspius], for the transmission of four avian Plasmodium lineages (Plasmodium relictum SGS1 and GRW11 and Plasmodium cathemerium-related lineages COLL1 and PADOM01) naturally infecting wild house sparrows. We assessed the effects of parasite identity and parasite load on Plasmodium transmission risk through its effects on the transmission rate and mosquito survival. We found that Cx. pipiens was able to transmit the four Plasmodium lineages, while Ae. caspius was unable to transmit any of them. However, Cx. pipiens mosquitoes fed on birds infected by P. relictum showed a lower survival and transmission rate than those fed on birds infected by parasites related to P. cathemerium. Non-significant associations were found with the host-parasite load. Our results confirm the existence of inter- and intra-specific differences in the ability of Plasmodium lineages to develop in mosquito species and their effects on the survival of mosquitoes that result in important differences in the transmission risk of the different avian malaria parasite lineages studied.

Molecular characterization of six widespread avian haemoproteids, with description of three new Haemoproteus species

Species of Haemoproteus (Haemosporida, Haemoproteidae) are widespread and often prevalent blood parasites of birds all over the word. They are particularly diverse in tropical countries. Due to limited knowledge of life cycles, these pathogens usually have been considered relatively benign and were neglected in veterinary medicine and bird management. However, recent molecular studies provided evidence that Haemoproteus parasites might cause severe diseases if they infect non-adapted (wrong) avian hosts due to marked damage of organs by exo-erythrocytic stages (megalomeronts). Additionally, high Haemoproteus infections are lethal to blood-sucking insects. Molecular markers are essential for reliable detection and species identification both at tissue stages in vertebrates and sporogonic stages in arthropods however, remain insufficiently developed for wildlife haemosporidian parasites. This study combined PCR-based and microscopic approaches and reported cytochrome b gene (cytb) and apicoplast gene (clpc) markers for characterization of six widespread species of haemoproteids parasitizing common birds wintering in tropics and subtropics of the Old World. Three new Haemoproteus species were described using morphological and molecular markers. Molecular characterization of haemoproteids parasitizing falcons was developed. Morphological and phylogenetic characterization of Haemoproteus tinnunculi (cytb lineage hFALSUB01), H. brachiatus (hLK03), H. parabelopolskyi (hSYAT1), H. homogeneae n. sp. (hSYAT16), H. homopicae n. sp. (hGAGLA07) and H. homominutus n. sp. (hCUKI1) was performed and provides clues for infections diagnostics. This study adds three species to the group of morphologically readily distinct Haemoproteus parasites, which differ in few base pairs (< 1%) in their partial cytb sequences, indicating that low genetic difference in such sequences often show between-species divergence and should be carefully applied in taxonomic biodiversity studies of haemosporidian parasites. Bayesian phylogenetic analysis identified the position of detected lineages in regard of other Haemoproteus species, suggesting that all reported parasites belong to subgenus Parahaemoproteus and likely are transmitted by Culicoides biting midges. Importance of clpc gene sequences was specified in haemosporidian parasite taxonomy on species levels.

Experimental reduction of host Plasmodium infection load affects mosquito survival

Plasmodium transmission success depends upon the trade-off between the use of host resources to favour parasite reproduction and the negative effects on host health, which can be mediated by infection intensity. Despite its potential influence on parasite dynamics, the effects of infection intensity on both, birds and vectors, and on Plasmodium transmission success are still poorly understood. Here, we experimentally reduced the Plasmodium load in naturally infected wild house sparrows with the antimalarial primaquine to assess the effects of intensity of infection in the vertebrate hosts on Plasmodium transmission to and by mosquitoes. We monitored the survival of Culex pipiens mosquitoes throughout the development of the parasite and the infection status of the mosquitoes by analysing the head-thorax and saliva at 13 days post-exposure to birds. The proportion of mosquitoes infected by Plasmodium and the presence of Plasmodium in saliva were not associated with the medication treatment of birds. However, the experimental treatment affected vector survival with mosquitoes fed on medicated birds showing a higher survival rate than those fed on control individuals. These results provide strong experimental evidence of the impact of parasite load of vertebrate hosts on the survival probability of malaria vectors.