Generation of specific immune memory by bacterial exposure in the major malaria vector Anopheles arabiensis (Diptera: Culicidae)

There is a growing body of evidence that invertebrates can generate improved secondary responses after a primary challenge. This immunological memory can be primed by a range of pathogens, including bacteria. The generation of immunological memory has been demonstrated in mosquitoes, with the memory primed by a range of initial stimuli. This study aimed to examine whether insecticide resistance affects the capacity to generate immunological memory. The primary hypothesis was tested by examining the capacity of genetically related laboratory-reared Anopheles arabiensis strains that differ by insecticide resistant phenotype to generate immunological memory. The competing hypothesis tested was that the bacterial virulence was the key determinant in generating immunological memory. Immune memory was generated in F1 females but not males. Immunological memory was demonstrated in both laboratory strains, but the efficacy differed by the insecticide resistant phenotype of the strain. An initial oral challenge provided by a blood meal resulted generated better memory than an oral challenge by sugar. The efficacy of memory generation between the two bacterial strains differed between the two mosquito strains. Regardless of the challenge, the two strains differed in their capacity to generate memory. This study therefore demonstrated that insecticide resistant phenotype affected the capacity of the two strains to generate immunological memory. Although this study needs to be replicated with wild mosquitoes, it does suggest that a potential role for insecticide resistance in the functioning of the immune system and memory generation of An. arabiensis.

Chromobacterium Biopesticide Exposure Does Not Select for Resistance in Aedes Mosquitoes

Developing effective tools to control mosquito populations is essential for reducing the incidence of diseases like malaria and dengue. Biopesticides of microbial origin are a rich, underexplored source of mosquitocidal compounds. We previously developed a biopesticide from the bacterium Chromobacterium sp. Panama that rapidly kills vector mosquito larvae, including Aedes aegypti and Anopheles gambiae. Here, we demonstrate that two independent Ae. aegypti colonies exposed to a sublethal dose of that biopesticide over consecutive generations persistently exhibited high mortality and developmental delays, indicating that resistance did not develop during the study period. Critically, the descendants of biopesticide-exposed mosquitoes experienced decreased longevity and did not display increased susceptibility to dengue virus or decreased susceptibility to common chemical insecticides. Through RNA sequencing, we observed no link between biopesticide exposure and the increased activity of xenobiotic metabolism and detoxification genes typically associated with insecticide resistance. These findings indicate that the Chromobacterium biopesticide is an exciting, emerging mosquito control tool. IMPORTANCE Vector control is an essential part of mitigating diseases caused by pathogens that mosquitoes spread. Modern vector control is highly reliant on using synthetic insecticides to eliminate mosquito populations before they can cause disease. However, many of these populations have become resistant to commonly used insecticides. There is a strong need to explore alternative vector control strategies that aim to mitigate disease burden. Biopesticides, insecticides of biological origin, can have unique mosquitocidal activities, meaning they can effectively kill mosquitoes that are already resistant to other insecticides. We previously developed a highly effective mosquito biopesticide from the bacterium Chromobacterium sp. Csp_P. Here, we investigate whether exposure to a sublethal dose of this Csp_P biopesticide over 9 to 10 generations causes resistance to arise in Aedes aegypti mosquitoes. We find no evidence of resistance at the physiological or molecular levels, confirming that the Csp_P biopesticide is a highly promising new tool for controlling mosquito populations.

Impact of insecticide resistance on malaria vector competence: a literature review

Since its first report in Anopheles mosquitoes in 1950s, insecticide resistance has spread very fast to most sub-Saharan African malaria-endemic countries, where it is predicted to seriously jeopardize the success of vector control efforts, leading to rebound of disease cases. Supported mainly by four mechanisms (metabolic resistance, target site resistance, cuticular resistance, and behavioural resistance), this phenomenon is associated with intrinsic changes in the resistant insect vectors that could influence development of invading Plasmodium parasites. A literature review was undertaken using Pubmed database to collect articles evaluating directly or indiretly the impact of insecticide resistance and the associated mechanisms on key determinants of malaria vector competence including sialome composition, anti-Plasmodium immunity, intestinal commensal microbiota, and mosquito longevity. Globally, the evidence gathered is contradictory even though the insecticide resistant vectors seem to be more permissive to Plasmodium infections. The actual body of knowledge on key factors to vectorial competence, such as the immunity and microbiota communities of the insecticide resistant vector is still very insufficient to definitively infer on the epidemiological importance of these vectors against the susceptible counterparts. More studies are needed to fill important knowledge gaps that could help predicting malaria epidemiology in a context where the selection and spread of insecticide resistant vectors is ongoing.

Outdoor malaria vector species profile in dryland ecosystems of Kenya

Outdoor biting by anopheline mosquitoes is one of the contributors to residual malaria transmission, but the profile of vectors driving this phenomenon is not well understood. Here, we studied the bionomics and genetically characterized populations of An. gambiae and An. funestus complexes trapped outdoors in three selected dryland areas including Kerio Valley, Nguruman and Rabai in Kenya. We observed a higher abundance of Anopheles funestus group members (n = 639, 90.6%) compared to those of the An. gambiae complex (n = 66, 9.4%) with An. longipalpis C as the dominant vector species with a Plasmodium falciparum sporozoite rate (Pfsp) of 5.2% (19/362). The known malaria vectors including An. funestus s.s. (8.7%, 2/23), An. gambiae (14.3%, 2/14), An. rivulorum (14.1%, 9/64), An. arabiensis (1.9%, 1/52) occurred in low densities and displayed high Pfsp rates, which varied with the site. Additionally, six cryptic species found associated with the An. funestus group harbored Pf sporozoites (cumulative Pfsp rate = 7.2%, 13/181). We detected low frequency of resistant 119F-GSTe2 alleles in An. funestus s.s. (15.6%) and An. longipalpis C (3.1%) in Kerio Valley only. Evidence of outdoor activity, emergence of novel and divergent vectors and detection of mutations conferring metabolic resistance to pyrethroid/DDT could contribute to residual malaria transmission posing a threat to effective malaria control.

Exploring the impact of glutathione S-transferase (GST)-based metabolic resistance to insecticide on vector competence of Anopheles funestus for Plasmodium falciparum

Background:Malaria control heavily relies on insecticide-based interventions against mosquito vectors. However, the increasing spread of insecticide resistance is a major threat. The extent to which such resistance, notably metabolic resistance, interferes with the development of thePlasmodiumparasite and its impact on overall malaria transmission remains poorly characterized. Here, we investigated whether glutathione S-transferase-based resistance could influencePlasmodium falciparumdevelopment inAnopheles funestus.Methods:Anopheles funestusfemales were infected withP. falciparumgametocytes and midguts were dissected at day 7 post infection for detection/quantification of oocysts. Infection parameters were compared between individual with different L119F-GSTe2 genotypes, and the polymorphism of the GSTe2 gene was analyzed in infected and uninfected mosquito groups.Results:Overall, 403 mosquitoes were dissected and genotyped. The frequency of the L119F-GSTe2 resistance allele was significantly higher in non-infected (55.88%) compared to infected (40.99%) mosquitoes (Fisher's exact test, P<0.0001). Prevalence of infection was significantly higher in heterozygous and homozygous susceptible genotypes (P<0.001). However, homozygous resistant and heterozygous mosquitoes exhibited significantly higher infection intensity (P<0.01). No association was observed between the GSTe2 polymorphism and the infection status of mosquitoes.Conclusion:Altogether, these results suggest that GSTe2-based metabolic resistance may affect the vectorial competence of resistantAn. funestusmosquitoes toP. falciparuminfection, by increasing its permissiveness toPlasmodiuminfection.