Spatial distribution of insecticide resistant populations of Aedes aegypti and Ae. albopictus and first detection of V410L mutation in Ae. aegypti from Cameroon

Entomological surveys and insecticide susceptibility profile of Aedes aegypti during the dengue outbreak in Sao Tome and Principe in 2022

BACKGROUND

The first dengue outbreak in Sao Tome and Principe was reported in 2022. Entomological investigations were undertaken to establish the typology of Aedes larval habitats, the distribution of Ae. aegypti and Ae. albopictus, the related entomological risk and the susceptibility profile of Ae. aegypti to insecticides, to provide evidence to inform the outbreak response.

METHODOLOGY/PRINCIPAL FINDINGS

Entomological surveys were performed in all seven health districts of Sao Tome and Principe during the dry and rainy seasons in 2022. WHO tube and synergist assays using piperonyl butoxide (PBO) and diethyl maleate (DEM) were carried out, together with genotyping of F1534C/V1016I/V410L mutations in Ae. aegypti. Aedes aegypti and Ae. albopictus were found in all seven health districts of the country with high abundance of Ae. aegypti in the most urbanised district, Agua Grande. Both Aedes species bred mainly in used tyres, discarded tanks and water storage containers. In both survey periods, the Breteau (BI > 50), house (HI > 35%) and container (CI > 20%) indices were higher than the thresholds established by WHO to indicate high potential risk of dengue transmission. The Ae. aegypti sampled were susceptible to all insecticides tested except dichlorodiphenyltrichloroethane (DDT) (9.2% mortality, resistant), bendiocarb (61.4% mortality, resistant) and alpha-cypermethrin (97% mortality, probable resistant). A full recovery was observed in Ae. aegypti resistant to bendiocarb after pre-exposure to synergist PBO. Only one Ae. aegypti specimen was found carrying F1534C mutation.

CONCLUSIONS/SIGNIFICANCE

These findings revealed a high potential risk for dengue transmission throughout the year, with the bulk of larval breeding occurring in used tyres, water storage and discarded containers. Most of the insecticides tested remain effective to control Aedes vectors in Sao Tome, except DDT and bendiocarb. These data underline the importance of raising community awareness and implementing routine dengue vector control strategies to prevent further outbreaks in Sao Tome and Principe, and elsewhere in the subregion.

Aedes albopictus is present in the lowlands of southern Zambia

Identifying the current geographic range of disease vectors is a critical first step towards determining effective mechanisms for controlling and potentially eradicating them. This is particularly true given that historical vector ranges may expand due to changing climates and human activity. The Aedes subgenus Stegomyia contains over 100 species, and among them, Ae. aegypti and Ae. albopictus mosquitoes represent the largest concern for public health, spreading dengue, chikungunya, and zika viruses. While Ae. aegypti has been observed in the country of Zambia for decades, Ae. albopictus has not. In 2015 we sampled four urban and three rural areas in Zambia for Aedes species. Using DNA barcoding, we confirmed the presence of immature and adult Ae. albopictus at two sites: Siavonga and Livingstone. These genotypes seem most closely related to specimens previously collected in Mozambique based on mtDNA barcoding. We resampled Siavonga and Livingstone sites in 2019, again observing immature and adult Ae. albopictus at both sites. Relative Ae. albopictus frequencies were similar between sites, with the exception of immature life stages, which were higher in Siavonga than in Livingstone in 2019. While Ae. albopictus frequencies did not vary through time in Livingstone, both immature and adult frequencies increased through time in Siavonga. This report serves to document the presence of Ae. albopictus in Zambia, which will contribute to understanding the potential public health implications of this disease vector in southern Africa.

Leucine to tryptophane substitution in the pore helix IIP1 confer sodium channel resistance to pyrethroids and DDT

Aedes aegypti is responsible for transmitting a variety of arboviral infectious diseases such as dengue and chikungunya. Insecticides, particularly pyrethroids, are used widely for mosquito control. However, intensive used of pyrethroids has led to the selection of kdr mutations on sodium channels. L982W, locating in the PyR1 (Pyrethroid receptor site 1), was first reported in Ae. aegypti populations collected from Vietnam. Recently, the high frequency of L982W was detected in pyrethroid-resistant populations of Vietnam and Cambodia, and also concomitant mutations L982W + F1534C was detected in both countries. However, the role of L982W in pyrethroid resistance remains unclear. In this study, we examined the effects of L982W on gating properties and pyrethroid sensitivity in Xenopus oocytes. We found that mutations L982W and L982W + F1534C shifted the voltage dependence of activation in the depolarizing direction, however, neither mutations altered the voltage dependence of inactivation. L982W significantly reduced channel sensitivity to Type I pyrethroids, permethrin and bifenthrin, and Type II pyrethroids, deltamethrin and cypermethrin. No enhancement was observed when synergized with F1534C. In addition, L982W and L982W + F1534C mutations reduced the channel sensitivity to DDT. Our results illustrate the molecular basis of resistance mediates by L982W mutation, which will be helpful to understand the interacions of pyrethroids or DDT with sodium channels and develop molecular markers for monitoring pest resistance to pyrethroids and DDT.

The oral repellent - science fiction or common sense? Insects, vector-borne diseases, failing strategies, and a bold proposition

Over the last decades, unimaginable amounts of money have gone into research and development of vector control measures, repellents, treatment, and vaccines for vector borne diseases. Technological progress and scientific breakthroughs allowed for ever more sophisticated and futuristic strategies. Yet, each year, millions of people still die or suffer from potentially serious consequences of malaria or dengue to more recent infections, such as zika or chikungunya, or of debilitating consequences of neglected tropical diseases. This does not seem value for money. In addition, all current vector control strategies and personal protection methods have shortcomings, some serious, that are either destructive to non-target species or unsatisfactory in their effectiveness. On the other hand, the rapid decline in insect populations and their predators reflects decades-long aggressive and indiscriminate vector control. This major disruption of biodiversity has an impact on human life not anticipated by the well-meaning killing of invertebrates. The objective of this paper is to re-examine current control methods, their effectiveness, their impact on biodiversity, human and animal health, and to call for scientific courage in the pursuit of fresh ideas. This paper brings together topics that are usually presented in isolation, thereby missing important links that offer potential solutions to long-standing problems in global health. First, it serves as a reminder of the importance of insects to human life and discusses the few that play a role in transmitting disease. Next, it examines critically the many currently employed vector control strategies and personal protection methods. Finally, based on new insights into insect chemo-sensation and attractants, this perspective makes a case for revisiting a previously abandoned idea, the oral repellent, and its use via currently successful methods of mass-application. The call is out for focused research to provide a powerful tool for public health, tropical medicine, and travel medicine.