Wing size and parity as markers of cohort demography for potential Anopheles (Culicidae: Diptera) malaria vectors in the Republic of Korea

Insights into the biology and insecticide susceptibility of the secondary malaria vector Anopheles parensis in an area with long-term use of insecticide-treated nets in northwestern Tanzania

BACKGROUND

The Anopheles funestus group includes at least 11 sibling species, with Anopheles funestus Giles being the most studied and significant malaria vector. Other species, like Anopheles parensis, are understudied despite their potential role in transmission. This article provides insights into the biology and insecticide susceptibility of An. parensis, with observations of its densities in northwestern Tanzania.

METHODS

Mosquitoes were collected in three villages in Misungwi district, northwestern Tanzania, using CDC light traps and battery-powered aspirators indoors and human-baited double net traps outdoors. Female Anopheles adults were morphologically sorted and identified by PCR, and a subset was tested by ELISA for vertebrate blood meal sources and Plasmodium sporozoite infections. Insecticide susceptibility was assessed using the WHO protocol (2nd edition, 2018). Unfed females were dissected to assess parity, gonotrophic status and insemination status, while blood-fed females were monitored for oviposition to estimate egg counts. The prevalence of An. parensis was generally < 24% across all sites, except in Ngaya village, where it unexpectedly constituted 84% of PCR-amplified An. funestus sensu lato. This species was present in both indoor and outdoor collections, yet the females exclusively fed on non-human vertebrates, with no human blood meals detected. Parity rates were approximately 49% for resting and 46% for host-seeking females, with slightly higher percentages of both parous and inseminated females in the dry season compared to the wet season. Most parous females had oviposited once or twice, with those in the dry season ovipositing significantly more eggs. The average wing length of female An. parensis was 2.93 mm, and there was no significant impact of body size on parity, fecundity or insemination. The An. parensis mosquitoes were fully susceptible to pyrethroids, carbamates, organophosphates and organochlorides.

RESULTS

The prevalence of An. parensis was generally < 24% across all sites, except in Ngaya village, where it unexpectedly constituted 84% of PCR-amplified An. funestus sensu lato. This species was present in both indoor and outdoor collections, yet the females exclusively fed on non-human vertebrates, with no human blood meals detected. Parity rates were approximately 49% for resting and 46% for host-seeking females, with slightly higher percentages of both parous and inseminated females in the dry season compared to the wet season. Most parous females had oviposited once or twice, with those in the dry season ovipositing significantly more eggs. The average wing length of female An. parensis was 2.93 mm, and there was no significant impact of body size on parity, fecundity or insemination. The An. parensis mosquitoes were fully susceptible to pyrethroids, carbamates, organophosphates and organochlorides..

CONCLUSION

This study offers insights into the behaviours and insecticide susceptibility of An. parensis. Primarily feeding on non-human hosts, An. parensis is less significant in malaria transmission than more anthropophilic vectors. Unlike the pyrethroid-resistant An. funestus sensu stricto, An. parensis remains fully susceptible to public health insecticides despite the use of insecticidal bed nets. These findings provide a foundation for future research and may inform control strategies targeting residual malaria transmission involving An. parensis.

Role of vector phenotypic plasticity in disease transmission as illustrated by the spread of dengue virus by Aedes albopictus

The incidence of vector-borne disease is on the rise globally, with burdens increasing in endemic countries and outbreaks occurring in new locations. Effective mitigation and intervention strategies require models that accurately predict both spatial and temporal changes in disease dynamics, but this remains challenging due to the complex and interactive relationships between environmental variation and the vector traits that govern the transmission of vector-borne diseases. Predictions of disease risk in the literature typically assume that vector traits vary instantaneously and independently of population density, and therefore do not capture the delayed response of these same traits to past biotic and abiotic environments. We argue here that to produce accurate predictions of disease risk it is necessary to account for environmentally driven and delayed instances of phenotypic plasticity. To show this, we develop a stage and phenotypically structured model for the invasive mosquito vector, Aedes albopictus, and dengue, the second most prevalent human vector-borne disease worldwide. We find that environmental variation drives a dynamic phenotypic structure in the mosquito population, which accurately predicts global patterns of mosquito trait-abundance dynamics. In turn, this interacts with disease transmission to capture historic dengue outbreaks. By comparing the model to a suite of simpler models, we reveal that it is the delayed phenotypic structure that is critical for accurate prediction. Consequently, the incorporation of vector trait relationships into transmission models is critical to improvement of early warning systems that inform mitigation and control strategies.

Comparison of the variability in mortality data generated by CDC bottle bioassay, WHO tube test, and topical application bioassay using Aedes aegypti mosquitoes

BACKGROUND

Insecticide resistance remains a major public health problem. Resistance surveillance is critical for effective vector control and resistance management planning. Commonly used insecticide susceptibility bioassays for mosquitoes are the CDC bottle bioassay and the WHO tube test. Less commonly used in the field but considered the gold standard for assessing insecticide susceptibility in the development of novel insecticides is the topical application bioassay. Each of these bioassays has critical differences in how they assess insecticide susceptibility that impacts their ability to differentiate between resistant and susceptible populations or determine different levels of resistance intensity.

METHODS

We compared the CDC bottle bioassay, the WHO tube test, and the topical application bioassay in establishing the dose-response against deltamethrin (DM) using the DM-resistant Aedes aegypti strain MC1. Mosquitoes were exposed to a range of insecticide concentrations to establish a dose-response curve and assess variation around model predictions. In addition, 10 replicates of 20-25 mosquitoes were exposed to a fixed dose with intermediate mortality to assess the degree of variation in mortality.

RESULTS

The topical application bioassay exhibited the lowest amount of variation in the dose-response data, followed by the WHO tube test. The CDC bottle bioassay had the highest level of variation. In the fixed-dose experiment, a higher variance was similarly found for the CDC bottle bioassay compared with the WHO tube test and topical application bioassay.

CONCLUSION

These data suggest that the CDC bottle bioassay has the lowest power and the topical application bioassay the highest power to differentiate between resistant and susceptible populations and assess changes over time and between populations. This observation has significant implications for the interpretation of surveillance results from different assays. Ultimately, it will be important to discuss optimal insecticide resistance surveillance tools in terms of the surveillance objective, practicality in the field, and accuracy of the tool to reach that objective.