First large-scale assessment of pyrethroid resistance in Anopheles darlingi (Diptera: Culicidae) in Brazil (2021-2024): a crucial step in informing decision-making in malaria control.

Background: Malaria continues to pose a significant public health threat in northern Brazil. Current control strategies for Anopheles darlingi, the primary malaria vector in the Amazon region, depend on long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) with pyrethroid insecticides. Despite decades of insecticide use, there are very few records of pyrethroid resistance in this mosquito species in Brazil, likely due to a lack of investigations, underscoring the urgent need for further actions.

Objectives: To assess the susceptibility of An. darlingi from all malaria-endemic regions in Brazil to the pyrethroids used by the Malaria Prevention and Control Programme (NMCP) for vector control.

Methods: Adult females An. darlingi were collected from 28 locations in the states of Amapá, Acre, Amazonas, Pará, Rondônia, Roraima, Mato Grosso, Maranhão, and Tocantins. These locations were chosen because of their high malaria incidence in recent years. The collected mosquitoes were sent to the Laboratory of Biology, Control, and Surveillance of Insect Vectors to produce F1 progeny. Discriminating concentration (DC) WHO tube bioassays were performed on deltamethrin (0.05%), etofenprox (0.5%), and permethrin (0.75%). The intensity of resistance was evaluated by comparing the mortality rates of mosquitoes exposed to papers treated with 1 × and 5 × the DC of these insecticides.

Results: Of the 19 An. darlingi populations evaluated, only four were susceptible to deltamethrin (Tapauá, Jacareacanga, Cantá, and Caracaraí). For etofenprox, 13 populations were resistant, whereas five were susceptible (Tapauá, Porto Velho, Porto Grande, Cantá, and Caracaraí). With respect to permethrin, 18 populations were evaluated, of which 12 were classified as susceptible and 6 as resistant (Coari, Manaus, Barcelos, Guajará, Rodrigues Alves, and Cruzeiro do Sul). Resistance intensity tests indicated that all populations, except Barcelos, presented low resistance to pyrethroids according to the WHO classification.

Conclusions: The first large scale detection of pyrethroid resistance in An. darlingi in Brazil is concerning and calls for urgent action to prevent its spread in the Amazon region. This study represents a critical step toward establishing comprehensive resistance monitoring and management plans for malaria vectors in Brazil.