Preliminary Report of Pyrethroid Resistance in Anopheles vagus, an Important Malaria Vector in Bangladesh

Anopheles vagus (Donitz), has a wide distribution across Asia including the Indian subcontinent, the Greater Mekong Sub-region, and China.¹ Anopheles vagus has been described as one of the most prevalent and widespread of the 36 Anopheles species documented in Bangladesh.^2–5 This species was first documented as a malaria vector during an outbreak in 1993 (Kishoreganj district, Bangladesh).⁶ Anopheles vagus is believed to be the primary vector in Bangladesh as it is continuously documented in large numbers with high rates of Plasmodium infections, with a concurrent lower number, or lack of other primary vectors.^2–4 This species has diverse oviposition sites that overlap with human habitation, including artificial containers—unusual for Anopheles mosquitoes.⁷ Other bionomic characteristics include primary exophily along with endophily, with anthropophilic as well as zoophilic feeding behavior.^6,8 Overall, An. vagus is an efficient vector of malaria in Bangladesh.

Both government and private agencies rely primarily on insecticides to control Anopheles vectors. The National Malaria Elimination Program (NMEP) in Bangladesh has distributed deltamethrin-impregnated long-lasting insecticidal nets (LLINs) since 2008, with deltamethrin-based indoor residual spraying (IRS) being implemented in hotspots during outbreaks.⁹ However, this approach may be threatened by the rise and spread of insecticide resistance.¹⁰

Resistance of An. vagus to dichlorodiphenyltrichloroethane (DDT) in Bangladesh has been reported since the 1980s.¹¹ A limited NMEP study (between 2010 and 2015) suggested resistance to deltamethrin in An. vagus in Bandarban and Rangamati districts.⁹ Other Southeast Asian countries have reported insecticide resistance in this species to several pyrethroids including deltamethrin, permethrin, alpha-cypermethrin, and lambda-cyhalothrin.¹¹ Whereas resistance has been indicated with organophosphate insecticides including malathion, susceptibility was seen with fenitrothion.¹⁰ Studies in Bangladesh have not conclusively demonstrated pyrethroid resistance in this vector. Major shortcomings in the NMEP study include the use of field-caught adult mosquitoes of unknown age, the recommended numbers of specimens tested remaining unknown, and the lack of susceptible control mosquitoes. Toward addressing this knowledge gap, we investigated insecticide resistance against two major pyrethroids—deltamethrin and permethrin, using An. vagus specimens from the highest malarious district in Bangladesh, that is, Bandarban (22.25°N, 92.19°E), using CDC bottle bioassays¹² along with insecticide-susceptible colony controls. This study was approved by the Research Review Committee and Ethical Review Committee of International Centre for Diarrhoeal Disease Research, Bangladesh (icddr,b).

Larvae were collected from three different areas in Bandarban toward a district-wide representative Anopheles sample. Collections were performed following WHO protocols,¹³ using a dipper in rice fields—the documented preferred larval site for An. vagus.⁷ Larvae, fed on ground fish food and dog biscuits, were reared to adulthood in the field insectary (icddr,b laboratory, Bandarban District Hospital). Sugar-fed 3- to 5-day-old adult Anopheles, irrespective of species, were bioassayed for insecticide resistance. Mosquitoes were morphologically identified¹⁴ after insecticide resistance tests, and data from An. vagus specimens were analysed for resistance. Low numbers of other Anopheles species rendered analyses of insecticide for these species unfeasible.

Bioassays were conducted using marked pre-coated bottles (Wheaton 250 mL, Wheaton^®, Millville, NJ).¹² Recommended insecticide concentrations used were as follows: 21.5 µg/bottle for permethrin and 12.5 µg/bottle for deltamethrin.^12,15

Mortality data was recorded at 0, 15, and 30 minutes of exposure to each insecticide. Mosquitoes unable to stand were considered dead. According to WHO recommendations,¹⁵ data from bioassay tests were interpreted as follows: ≥ 98% mortality, susceptible; 90–97% mortality, developing resistance; and < 90% mortality, resistant. High levels of resistance to permethrin (29% mortality, n = 131) and deltamethrin (55% mortality, n = 138) were observed for An. vagus collected from larval habitats across Bandarban district.

Pyrethroid-based insecticides have been extensively used in Bangladesh. Permethrin-impregnated nets—distributed in bed net campaigns across Bangladesh from 2008 to 2013, were replaced with deltamethrin-impregnated LLINs over the last 10 years (NMEP, personal communication). Deltamethrin is also used for IRS in hotspots of malaria transmission. In addition, permethrin is widely used by local government authorities for mosquito control (primarily Aedes and Culex) in urban areas across the country. High levels of resistance in An. vagus to both pyrethroids in the most malarious district are consistant with the historic use of this insecticide class and has important implications for malaria elimination in Bangladesh.

Limitations of this study include the small sample size and the need for more extensive collections across other regions of Bangladesh with malaria. The sampling of larval sites from across Bandarban enabled a regional evaluation of resistance to pyrethroids in the district that contributed to more than half the country’s malaria burden in 2018. The inclusion of other insecticide classes and other Anopheles species would also be beneficial to decision-making. However, this study focused on the only insecticides (pyrethroids) in use and the primary vector in the region.

Bandarban district has consistently contributed to most of the cases in Bangladesh—ranging from 40% of reported malaria in 2015 to a high of 60% and 58% in 2017 and 2018, respectively (NMEP, personal communication). The documentation of the primary vector—An. vagus, demonstrating resistance to the insecticide being used across a region with 58% of the country’s malaria—signals a reduction in intervention efficacy that may have contributed to the doubling of reported human malaria infections between 2018 and 2019 (NMEP 2020, unpublished data). Although this small sample size should usually be inadequate to guide national policy, implications of the loss of insecticide efficacy in this district alone are consequential to the malaria burden in the entire country—demonstrating the requirement for more data across malaria-endemic regions. The evaluation of a national strategy toward malaria elimination requires the documentation of gaps in protection and their relationship to disease epidemiology. Systematic surveillance for insecticide resistance is a required part of these processes. As evident from other countries, evaluation of LLINs with synergists,¹⁶ IRS with other insecticides such as pirimiphos-methyl¹⁷ or carbamate and organophosphate,¹⁸ and alternative intervention strategies should be conducted toward combating characterized gaps in protection—including insecticide resistance.