Malaria is the main cause of morbidity and mortality in Thailand with around 5,000 annual cases in 2019 treated in the public health system alone (BVBD 2019). Malaria control in Thailand relies mainly on vector control through the use of insecticide-treated nets, long-lasting insecticide nets (LLINs), and indoor residual spraying mostly in regions of perennial and seasonal transmission. The success of such interventions requires a good knowledge of vector populations particularly their susceptibility status to the main insecticides used for such control program in order to detect and monitor resistance to these insecticides. This requires the ability to test sufficient numbers of field mosquitoes for insecticide resistance assessment and residual efficacy tests.
In Thailand, seven Anopheline species (Anopheles dirus Peyton & Harrison, Anopheles minimus Theobald, Anopheles maculatus Theobald, Anopheles baimaii Sallum & Peyton, Anopheles sawadwongponi Rattanarithikul and Green, Anopheles aconitus Dönitz, and Anopheles psudowillmori Theobald) are considered vector species of malaria (Tainchum et al. 2014, Tananchai et al. 2019). Due to their higher abundance, An. dirus, An. minimus, and An. maculatus are involved in residual malaria transmission in forested areas where information on insecticide susceptibility for most vector species is limited (Edwards et al. 2019). Anopheles dirus has also been shown to be a highly efficient vector of artemisinin-resistant Plasmodium parasites, representing a significant challenge to the elimination of malaria in Southeast Asia and the prevention of spread of multidrug resistance (Laurent et al. 2015, WHO 2018).
A recent survey evaluating the current status of vector monitoring across the Asia-Pacific region confirmed resistance to pyrethroids in most countries with varying frequencies; however, the complete extent of resistance is unknown (Knox 2018). While 76 percent of 34 countries from four WHO regions—PAHO, AFRO, SEARO, and WPRO were monitoring insecticide resistance, only 32 percent did not collect data to determine adult resistance mechanisms (Burkot et al. 2018, Dahmash et al. 2021). Consequently, the impact of insecticide resistance on the effectiveness of vector control tools remains poorly understood, thus stalling decisions on switching to new insecticides, e.g., neonicotinoids, pyrethroid-piperonyl butoxide synergist bed nets, and other prequalified-listed products. The challenges in insecticide resistance monitoring are attributed to the wide diversity of malaria vectors in Asia-Pacific, insufficient number of adult female mosquitoes of each species as required by WHO guidelines (2019) and the general difficulty to induce oviposition from field blood-fed mosquitoes or gravid indoor resting females. Quite often, the egg production and density from field caught females is very low which limits the amount of bioassays that could be successfully carried out to assess insecticide susceptibility of primary and secondary malaria vectors or to carry out contact bioassays of LLINs and sprayed surfaces.
To address these challenges, we used a new method of forced-egg laying which has been very successful in generating abundant F1 adult mosquitoes (Morgan et al. 2010, Nepomichene et al. 2017) for an extensive assessment of the insecticide susceptibility status of Anopheles funestus Giles population against several insecticides (Morgan et al. 2010). The aim of this study was to demonstrate the feasibility of using the forced-egg laying method for producing F1 from seven Anopheles species, comprising nine test populations, encountered throughout the Greater Mekong Subregion.
Published in the Journal of Medical Entomology
Country: ThailandKeywords: Research | Malaria
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