Insights into factors sustaining persistence of high malaria transmission in forested areas of sub-Saharan Africa: the case of Mvoua, South Cameroon
Authors: Mieguim Ngninpogni Dominique , Ndo Cyrille , Ntonga Akono Patrick , Nguemo Anicet , Nguepi Amine , Metitsi Danale Rosine , Tombi Jeannette , Awono-Ambene Parfait , Bilong Bilong Charles Félix 
Published: January 2021

Background

In Mvoua, a village situated in a forested area of Cameroon, recent studies have reported high prevalence ofPlasmodium falciparuminfection among the population. In order to understand factors that can sustain such a high malaria transmission, we investigated the biology ofAnophelesvectors and its susceptibility to insecticides, as well as long-lasting insecticidal net (LLIN) coverage, use and bio-efficacy.

Methods

A longitudinal entomological survey was conducted from July 2018 to April 2019. Adult mosquitoes were collected using the human landing catch (HLC) method and identified using morphological and molecular techniques.Anopheles gambiae(s.l.) larvae were sampled from several stagnant water pools throughout the village and reared to generate F1 adults. The presence ofP. falciparumcircumsporozoite antigen was detected in the heads and thoraces of mosquitoes collected as adults using an enzyme-linked immunosorbent assay. The insecticide susceptibility status of the localAn. gambiae(s.l.) F1 population to the pyrethroid insecticides deltamethrin 0.5% and permethrin 0.75% was determined using World Health Organization-tube bioassays, while the frequency of the knockdown resistance (kdr) mutation was determined by PCR. Coverage, use and physical integrity of LLINs were assessed in households, then cone assays were used to test for their bio-efficacy on both the reference insecticide-susceptible Kisumu strain and on field F1An. gambiae(s.l.)

Results

In total, 110Anophelesmosquitoes were collected, of which 59.1% were identified asAnopheles funestus(s.l.), 38.18% asAn. gambiae(s.l.) and 2.72% asAn. ziemanii.Anopheles funestuswas the most abundant species except in the long rainy season, whenAn. gambiae(s.l.) predominated (65.8%). In the dry seasons, vectors were principally endophagous (76% of those collected indoors) while they tended to be exophagous (66% of those collected outdoors) in rainy seasons. HighPlasmodiuminfection was observed inAn.gambiae(s.l.) andAn. funestus, with a circumsporozoitic rate of 14.29 and 10.77%, respectively.Anopheles gambiae(s.l.) was highly resistant to pyrethroid insecticides (mortality rates: 32% for permethrin and 5% for deltamethrin) and harbored thekdr-L1014F mutation at a high frequency (89.74%). Of the 80 households surveyed, only 47.69% had achieved universal coverage with LLNs. Around 70% of the LLINs sampled were in poor physical condition, with a proportionate hole index > 300. Of the ten LLNs tested, eight were effective against theAn. gambiaereference insecticide-susceptible Kisumu strain, showing mortality rate of > 80%, while none of these LLINs were efficient against localAn. gamabie(s.l.) populations (mortality rates < 11.5%).

Conclusion

A combination of elevatedP. falciparuminfection inAnophelesvector populations, insufficient coverage and loss of effectiveness of LLINs due to physical degradation, as well as high resistance to pyrethroid insecticides is responsible for the persistence of high malaria transmission in forested rural area of Mvoua, Cameroon.

Graphical Abstract