preprint · bioRxiv (Cold Spring Harbor Laboratory)
Natural populations of malaria-transmitting Anopheles gambiae mosquitoes in Burkina Faso were monitored between 2017 and 2023 across three districts following the cessation of indoor residual spraying campaigns. The research tracked mosquito susceptibility to pirimiphos-methyl and the frequency of the Ace-1 G119S resistance mutation over pre-spraying, active spraying, and post-spraying periods. Discontinuing indoor residual spraying led to a gradual restoration of susceptibility to pirimiphos-methyl alongside a sharp decline in the Ace-1 119S resistance allele, especially in Kampti and Solenzo. Mosquitoes with resistant genotypes showed a substantial survival advantage when exposed to the insecticide, and allele frequencies strongly correlated with mortality rates. The natural decline of resistance markers in the absence of insecticide pressure suggests significant biological fitness costs for resistant mosquitoes, providing field support for rotational or adaptive vector control strategies.
Insecticide resistance threatens global malaria control programmes by reducing the efficacy of chemical interventions. Demonstrating that mosquito populations naturally lose resistance once an insecticide is withdrawn indicates that resistance genes carry biological disadvantages. This knowledge helps public health organisations design rotational spraying programmes that sustainably preserve the effectiveness of vital vector control tools like organophosphates over the long term.
This applied field research informs public health authorities and vector control programme managers developing insecticide resistance management policies. The findings support structured rotation strategies for organophosphates, potentially guiding decision-making for national indoor residual spraying campaigns. While the biological evidence is field-tested, practical application depends on health policy adoption and operational coordination rather than immediate product commercialisation.
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Abstract Introduction Changes in vector control strategies alter the selection pressures exerted on natural populations of Anopheles gambiae s.l. and may influence the dynamics of insecticide resistance mechanisms. However, the consequences of discontinuing indoor residual spraying (IRS) campaigns on the evolution of Ace-1 -mediated resistance remain poorly documented under natural conditions. This study aimed to assess the spatiotemporal evolution of resistance to pirimiphos-methyl following the cessation of IRS and to investigate evidence consistent with the existence of a biological cost associated with Ace-1 -mediated resistance. Methods Natural populations of An. gambiae s.l. were collected between 2017 and 2023 in three districts in Burkina Faso that had undergone IRS campaigns (Kampti, Solenzo, and Kongoussi). Susceptibility tests with pirimiphos-methyl (0.25%) were conducted out following WHO protocols, and a subsample of exposed mosquitoes was genotyped to detect the Ace-1 G119S mutation. Spatiotemporal trends in mortality, allele frequencies and genotypes were analyzed according to the pre-IRS, IRS, and post-IRS periods. The association between the Ace-1 genotype and survival following exposure to pirimiphos-methyl was assessed using logistic regression, while the concordance between phenotypic and molecular indicators of resistance was examined using Spearman’s correlation. Results The susceptibility of An. gambiae s.l. populations to pirimiphos-methyl was gradually restored after the discontinuation of IRS at all sites. At the same time, the frequencies of the Ace-1 119S resistance allele declined sharply, particularly in Kampti and Solenzo, while they remained low in Kongoussi throughout the study period. Mosquitoes carrying resistant genotypes had a significantly higher probability of survival after exposure to pirimiphos-methyl than susceptible homozygotes, with resistant homozygotes (RR) exhibiting the greatest survival advantage (OR = 27.62; 95% CI: 6.91-110.45; p < 0.001). A significant negative correlation was observed between the frequency of the Ace-1 119S allele and phenotypic mortality (ρ = −0.48; p = 0.033), indicating a concordance between the two indicators of resistance. The progressive decline in allele frequencies, the decreasing prevalence of resistant genotypes, and the concomitant restoration of susceptibility are field observations consistent with the existence of biological costs associated with Ace-1 -mediated resistance. Conclusion This study provides field evidence consistent with the existence of biological costs associated with Ace-1 -mediated resistance in natural populations of An. gambiae s.l. These results underscore the value of an adaptive resistance management strategy based on alternating selection pressures and could guide future vector control strategies, particularly if indoor residual spraying campaigns or other interventions relying on organophosphates were reintroduced.
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DOI: 10.64898/2026.08.07.743444
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