Increased pyrethroid resistance in malaria vectors and decreased bed net effectiveness, Burkina Faso

Abstract

Malaria control is dependent on insecticides. Increases in prevalence of insecticide resistance in malaria vectors across Africa are well-documented. However, few attempts have been made to quantify the strength of this resistance and link it to the effectiveness of control tools. Using quantitative bioassays, we show that in Burkina Faso pyrethroid resistance in Anopheles gambiae mosquitoes has increased in intensity in recent years and now exceeds 1,000-fold. In laboratory assays, this level of resistance renders insecticides used to impregnate bed nets ineffective. Thus, the level of personal and community protection afforded by long-lasting insecticide-treated net campaigns will probably be reduced. Standardized methods are needed to quantify resistance levels in malaria vectors and link these levels to failure of vector control methods.

Figure 1

Results of World Health Organization (WHO) susceptibility tests for Anopheles gambiae VK7 mosquitoes, Burkina Faso. Adult female mosquitos were exposed to the WHO diagnostic dose of insecticides for 1 h, and mortality rates were recorded 24 h later. Error bars indicate 95% binomial CIs for 3 consecutive years (2011–2013) of sampling.

Figure 2

Time-response curves for Anopheles gambiae VK7 mosquitoes, Burkina Faso, July–October 2011. Adult females were exposed to 0.05% deltamethrin according to World Health Organization standard protocols. Time-response curves were fitted to data by using a regression logistic model and R software (http://www.r-project.org/). Dotted line indicates 50% mortality rate. Error bars indicate 95% binomial CIs for each time point. The 50% lethality times were 1 h 38 min for July and 4 h 14 min for October, which indicates an October:July resistance ratio increase of 2.6-fold.

Figure 3

Dose-response curves for 3- to 5-day-old Anopheles gambiae VK7 female mosquitoes and Kisumu laboratory strain mosquitoes (insecticide-susceptible), Burkina Faso. Mosquitoes were exposed to different concentrations of deltamethrin in 250-mL glass bottles for 1 h. Dose-response curves were fitted to data by using a regression logistic model and R software (http://www.r-project.org/). Dotted line indicates 50% mortality rate. 50% lethality concentrations were 38.787 μg/mL (95% CI 32.993 μg/mL–46.062 μg/mL) in July 2013, 21.547 μg/mL (95% CI 15.771 μg/mL–31.223 μg/mL) in October 2013, and 0.021 μg/mL (95% CI 0.015 μ /mL–0.029 μg/mL) for the Kisumu strain.

Figure 4

Mean mortality rates 24 h after exposure to new and used long-lasting insecticide–treated bed nets for A) Anopheles gambiae Kisumu laboratory strain mosquitoes (insecticide-susceptible) and B) An. gambiae VK7 mosquitoes, Burkino Faso. Error bars indicate 95% binomial CIs for the average of net type. *Indicates significant variation between independent nets of each type (p<0.05), ‡Indicates a significant difference between new and currently long-lasting insecticide-treated bed nets (p<0.05).