Insecticide resistance and role in malaria transmission of Anopheles funestus populations from Zambia and Zimbabwe

Abstract

Background: Two mitochondrial DNA clades have been described in Anopheles funestus populations from southern Africa. Clade I is common across the continent while clade II is known only from Mozambique and Madagascar. The specific biological status of these clades is at present unknown. We investigated the possible role that each clade might play in the transmission of Plasmodium falciparum and the insecticide resistance status of An. funestus from Zimbabwe and Zambia.

Methods: Mosquitoes were collected inside houses from Nchelenge District, Zambia and Honde Valley, Zimbabwe in 2013 and 2014. WHO susceptibility tests, synergist assays and resistance intensity tests were conducted on wild females and progeny of wild females. ELISA was used to detect Plasmodium falciparum circumsporozoite protein. Specimens were identified to species and mtDNA clades using standard molecular methods.

Results: The Zimbabwean samples were all clade I while the Zambian population comprised 80% clade I and 20% clade II in both years of collection. ELISA tests gave an overall infection rate of 2.3% and 2.1% in 2013, and 3.5% and 9.2% in 2014 for Zimbabwe and Zambia respectively. No significant difference was observed between the clades. All populations were resistant to pyrethroids and carbamates but susceptible to organochlorines and organophosphates. Synergist assays indicated that pyrethroid resistance is mediated by cytochrome P450 mono-oxygenases. Resistance intensity tests showed high survival rates after 8-hrs continuous exposure to pyrethroids but exposure to bendiocarb gave the same results as the susceptible control.

Conclusions: This is the first record of An. funestus mtDNA clade II occurring in Zambia. No evidence was found to suggest that the clades are markers of biologically separate populations. The ability of An. funestus to withstand prolonged exposure to pyrethroids has serious implications for the use of these insecticides, either through LLINs or IRS, in southern Africa in general and resistance management strategies should be urgently implemented.

Figure 1

A very simple method for inducing egg-laying in wild Anopheles funestus that consistently results in >70% of the females ovipositing. Glass vials, 45 mm high x 25 mm diameter, with gauze lids are used. Small pieces of filter paper are placed at an angle in the bottom of the tube with approximately 1 ml water for egg laying. Females, resting inside the plastic lids, can be transferred to clean vials once they have laid eggs, facilitating blood-feeding for multiple egg batches.

Figure 2

Insecticide resistance intensity tests where susceptible (FANG) and Zimbabwe resistant An. funestus were exposed to 0.05% deltamethrin (FuZim-D) and lambda-cyhalothrin (FuZim-L) for 8 hours.

Figure 3

Insecticide resistance intensity tests where susceptible (FANG) and Zimbabwe resistant (FuZim-B) An. funestus were exposed to 0.1% bendiocarb for 8 hours.

Figure 4

Insecticide resistance intensity tests where susceptible (FANG) and Zambia resistant clade I (ZamF-C1-D) An. funestus were exposed to 0.05% deltamethrin for 8 hours.

Figure 5

Insecticide resistance intensity tests where susceptible (FANG) and Zambia resistant clade II (ZamF-C2-D) An. funestus were exposed to 0.05% deltamethrin for 8 hours.

Figure 6

Insecticide resistance intensity tests where susceptible (FANG) and Zambia resistant clade I (ZamF-C1-B) An. funestus were exposed to 0.1% bendiocarb for 8 hours.

Figure 7

Insecticide resistance intensity tests where susceptible (FANG) and Zambia resistant clade II (ZamF-C2-B) An. funestus were exposed to 0.1% bendiocarb for 8 hours.