Impact of pyrethroid resistance on operational malaria control in Malawi

Abstract

The impact of insecticide resistance on insect-borne disease programs is difficult to quantify. The possibility of eliminating malaria in high-transmission settings is heavily dependent on effective vector control reducing disease transmission rates. Pyrethroids are the dominant insecticides used for malaria control, with few options for their replacement. Their failure will adversely affect our ability to control malaria. Pyrethroid resistance has been selected in Malawi over the last 3 y in the two major malaria vectors Anopheles gambiae and Anopheles funestus, with a higher frequency of resistance in the latter. The resistance in An. funestus is metabolically based and involves the up-regulation of two duplicated P450s. The same genes confer resistance in Mozambican An. funestus, although the levels of up-regulation differ. The selection of resistance over 3 y has not increased malaria transmission, as judged by annual point prevalence surveys in 1- to 4-y-old children. This is true in areas with long-lasting insecticide-treated nets (LLINs) alone or LLINs plus pyrethroid-based insecticide residual spraying (IRS). However, in districts where IRS was scaled up, it did not produce the expected decrease in malaria prevalence. As resistance increases in frequency from this low initial level, there is the potential for vector population numbers to increase with a concomitant negative impact on control efficacy. This should be monitored carefully as part of the operational activities in country.

Fig. 1.

Map of Malawi showing the localization of the different collection sites.

Fig. 2.

Box plots of results from biochemical assays. The median activity of the An. funestus population of Chikwawa compared with the An. gambiae Kisumu reference strain is shown by a horizontal bar; the box denotes the upper and lower quartiles. The vertical lines show the full range of the data set. (A) Range of esterase activity with the substrate p-nitrophenyl acetate; (B) acetylcholinesterase inhibition ranges. (C) Range of GST activity. (D) Estimated levels of cytochrome P450s (representing monooxygenase activity).

Fig. 3.

(A) Schematic representation of haplotypes (H) of the 917-bp portion of the VGSC observed in the Chikwawa population. Only polymorphic sites are shown, and these are numbered from the beginning of the 917-bp sequence. Dots mean identity with the first sequence. A number has been given to each haplotype preceded by the letter R or S if it is unique to the resistant or susceptible sample, respectively. In case of a shared haplotype, the number is preceded by an asterisk. The column (N) indicates the number of individuals sharing the haplotype. Below the list of haplotypes, R or S indicates the positions that are polymorphic in the resistant or susceptible mosquitoes, respectively, whereas an asterisk marks a position polymorphic in both phenotypes. (B) Neighbor-joining tree of the VGSC haplotypes from Chikwawa. (C) Distribution of the sodium channel gene haplotypes between susceptible and resistant mosquitoes from Chikwawa.

Fig. 4.

Transcription profile of candidate genes in the Chikwawa population. (A) Comparison of the patterns of gene expression of CYP6P9a and CYP6P9b for each sample. (B) Expression ratio of CYP6P9a and CYP6P9b for sample. The normalized expression ratio of each gene against RSP7 gene is represented on the vertical axis. (C) Fold change of CYP6P9a and CYP6P9b between different comparisons. P, permethrin resistant; B, bendiocarb resistant; C, nonexposed mosquitoes; S, FANG susceptible strain.