Transfluthrin eave-positioned targeted insecticide (EPTI) reduces human landing rate (HLR) of pyrethroid resistant and susceptible malaria vectors in a semi-field simulated peridomestic space

Background Volatile pyrethroids (VPs) are proven to reduce human–vector contact for mosquito vectors. With increasing resistance to pyrethroids in mosquitoes, the efficacy of VPs, such as transfluthrin, may be compromised. Therefore, experiments were conducted to determine if the efficacy of transfluthrin eave-positioned targeted insecticide (EPTI) depends on the resistance status of malaria vectors. Methods Ribbons treated with 5.25 g transfluthrin or untreated controls were used around the eaves of an experimental hut as EPTI inside a semi-field system. Mosquito strains with different levels of pyrethroid resistance were released simultaneously, recaptured by means of human landing catches (HLCs) and monitored for 24-h mortality. Technical-grade (TG) transfluthrin was used, followed by emulsifiable concentrate (EC) transfluthrin and additional mosquito strains. Generalized linear mixed models with binomial distribution were used to determine the impact of transfluthrin and mosquito strain on mosquito landing rates and 24-h mortality. Results EPTI treated with 5.25 g of either TG or EC transfluthrin significantly reduced HLR of all susceptible and resistant Anopheles mosquitoes (Odds Ratio (OR) ranging from 0.14 (95% Confidence Interval (CI) [0.11–0.17], P < 0.001) to 0.57, (CI [0.42–0.78] P < 0.001). Both TG and EC EPTI had less impact on landing for the resistant Anopheles arabiensis (Mbita strain) compared to the susceptible Anopheles gambiae (Ifakara strain) (OR 1.50 [95% CI 1.18–1.91] P < 0.001) and (OR 1.67 [95% CI 1.29–2.17] P < 0.001), respectively. The EC EPTI also had less impact on the resistant An. arabiensis (Kingani strain) (OR 2.29 [95% CI 1.78–2.94] P < 0.001) compared to the control however the TG EPTI was equally effective against the resistant Kingani strain and susceptible Ifakara strain (OR 1.03 [95% CI 0.82–1.32] P = 0.75). Finally the EC EPTI was equally effective against the susceptible An. gambiae (Kisumu strain) and the resistant An. gambiae (Kisumu-kdr strain) (OR 0.98 [95% CI 0.74–1.30] P = 0.90). Conclusions Transfluthrin-treated EPTI could be useful in areas with pyrethroid-resistant mosquitoes, but it remains unclear whether stronger resistance to pyrethroids will undermine the efficacy of transfluthrin. At this dosage, transfluthrin EPTI cannot be used to kill exposed mosquitoes.

Biochemical Mechanism of Insecticide Resistance in Malaria Vector, Anopheles gambiae s.l in Nigeria

Background: Malaria is a parasitic vector-borne disease endemic in the tropical and subtropical countries of the world. The aim of this study was to investigate the current activities of the detoxification enzymes in resistant and susceptible Anopheles gambiae s.l. in northern Nigeria. Methods: Anopheles larvae were collected from northeast and northwestern Nigeria between Aug and Nov 2018. Biochemical analyses was carried out on the mosquitoes exposed to various insecticides (deltamethrin, DDT, bendiocarb, malathion) to measure and compare the enzymatic activities of the major detoxification enzymes (P450, GSTs, Esterase). Results: High levels of resistance was observed; DDT 37%-53% (95%, CI: 29-61), bendiocarb 44%-55% (CI: 39- 60) and deltamethrin 74%-82% (CI: 70-86). However, these mosquitoes were found to be susceptible to malathion 99%-100% (CI: 98-100). The P450 and GSTs enzymes were found to be elevated in the resistant mosquitoes exposed to deltamethrin (1.0240±0.1902); (1.3088±1.2478), DDT (1.7703±1.4528); (1.7462±0.9418) and bendiocarb (1.1814±0.0918); (1.4479±1.0083) compared to the Kisumu strain (0.764±0.4226); (0.6508±0.6542), (0.3875±0.3482); (0.4072±0.4916) and (0.6672±0.3949); (0.7126±0.7259) at P<0.05. Similarly, the resistant mosquitoes expressed increased activity to esterase (0.7606±1.1477), (0.3269±1.1957) and (2.8203±0.6488) compared to their susceptible counterpart (0.6841±0.7597), (0.7032±0.5380) and (0.6398±0.4159) at P<0.05. The enzyme ratio was found to be: P450 (1.341, 4.568 and 1.77); GSTs (2.011, 4.288 and 2.031); Esterases (1.111, 0.469 and 4.408). One way Anova and single sample t-test were also conducted to determine the effect of the enzymes on the resistant and susceptible strains. Conclusion: High level of insecticide resistance was observed with significant elevation of detoxification enzymes activities in the resistant mosquitoes.

Insights into factors sustaining persistence of high malaria transmission in forested areas of sub-Saharan Africa: the case of Mvoua, South Cameroon

Background In Mvoua, a village situated in a forested area of Cameroon, recent studies have reported high prevalence of Plasmodium falciparum infection among the population. In order to understand factors that can sustain such a high malaria transmission, we investigated the biology of Anopheles vectors 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 of P. falciparum circumsporozoite 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 local An. 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 F1 An. gambiae (s.l.) Results In total, 110 Anopheles mosquitoes were collected, of which 59.1% were identified as Anopheles funestus (s.l.), 38.18% as An. gambiae (s.l.) and 2.72% as An. ziemanii. Anopheles funestus was the most abundant species except in the long rainy season, when An. 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. High Plasmodium infection was observed in An. gambiae (s.l.) and An. 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 the kdr-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 the An. gambiae reference insecticide-susceptible Kisumu strain, showing mortality rate of > 80%, while none of these LLINs were efficient against local An. gamabie (s.l.) populations (mortality rates < 11.5%). Conclusion A combination of elevated P. falciparum infection in Anopheles vector 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.

Short Persistence and Vector Susceptibility to Ficam 80WP (bendiocarb active ingredient) During Pilot Application of Indoor Residual Spraying in Burkina Faso, West Africa

Indoor residual spraying (IRS) was applied in addition to the use of long-lasting insecticidal nets in the South West in Burkina Faso, where Anopheles gambiae s.l. the major malaria vector was resistant to pyrethroids. This study was designed to evaluate the efficacy and residual life of bendiocarb (active ingredient) used for spraying on different wall surfaces (mud and cement). Cone bioassays were done monthly with the susceptible An. gambiae ‘Kisumu’ strain and the local wild populations to determine the duration for which insecticide was effective in killing mosquitoes. Cone bioassay data showed low efficacy and short persistence of bendiocarb applied on mud and cement walls, lasting 2 mo with the susceptible insectary strain and less than 1 mo with An. gambiae wild populations. In addition, WHO tube assays confirmed resistance of An. gambiae wild populations to 0.1% bendiocarb with mortality rates less than 80% in both study sites (sprayed and unsprayed sites). The pilot study of IRS with bendiocarb showed that the residual efficacy of bendiocarb was very short, and resistance to bendiocarb was confirmed in wild populations of An. gambiae s.l. Therefore, Ficam 80 WP was not suitable for IRS in this area due to the short residual duration related mainly to vectors resistance to bendiocarb. While waiting for innovative malaria control tool, alternative insecticide (organophosphate or neonicotinoid classes) or combinations of insecticides have to be used for insecticide resistance management in Burkina Faso.

Anopheles gambiae attraction to floral and human-skin-based odours and their combination is modulated by previous bloodmeal experience

Background: Mosquitoes use odours to find energy resources, blood hosts and oviposition sites. These odour sources are normally spatio-temporally segregated in a mosquito’s life history. This study was carried out to determine the attractiveness of Anopheles gambiae to flower- and human-mimicking synthetic volatiles.Methods: This study was carried out in Muheza District in northeast of Tanzania. Anopheles gambiae s.s. Kisumu strain was used in all experiments. The chemicals/odour blends used were Vectrax, skin lure and the combination thereof. The attractiveness of Vectrax and Skin Lure were compared to each other in an insectary and semi-field settings. Semi-field experiments were then conducted using mosquito-spheres. In the two settings, parous or non-parous female mosquitoes, were simultaneously offered four treatments: Skin Lure, Vectrax, a combination of the two, and a control (no attractant). Results: In the insectary nulliparous and parous mosquitoes preferred skin odours and combinations thereof over floral odours. However, in semi-field settings significantly more nulliparous mosquitoes more caught with floral odours, whereas no differences were observed for parous females. Combining floral and human volatiles did not augment attractiveness.Conclusions: Nulliparous and parous An. gambiae s.s. are attracted to combinations of odours derived from floral and human volatiles. However, combinations of floral and skin odours did not augment attraction in semi-field settings, in spite of the fact that these blends activate distinct sets of sensory neurons. Instead, mosquito preference appeared to be modulated by blood meal experience from floral to a more generic attraction to odour blends. Results are discussed both from an odour coding, as well as from an application perspective.

Water Quality andAnopheles gambiaeLarval Tolerance to Pyrethroids in the Cities of Douala and Yaoundé (Cameroon)

The poor management of the urban environment in sub-Saharan Africa is affectingAnopheles gambiaesusceptibility to insecticides. A study was undertaken to assess the influence of breeding sites physicochemical parameters on malaria vectors population tolerance to insecticides. A total of 18, 262 larvae collected from 104 breeding sites were exposed to diagnostic concentrations of permethrin and deltamethrin. Larvae originating from cultivated sites were more tolerant than larvae from polluted or nonpolluted sites. No significant difference was observed between polluted and nonpolluted sites. Field larvae were 142 to 325 times and 6.08 to 9.57 times more tolerant to deltamethrin and permethrin, respectively, than larvae of theA. gambiaeKisumu strain used as control. A low but significant correlation was detected between physicochemical parameters and larval insecticide tolerance. Cultivated sites were negatively and significantly correlated to larval tolerance to both deltamethrin (r=−0.421;P<0.0001) and permethrin (r=−0.392;P<0.0001). Dissolved oxygen (r=+0.466;P<0.0001) and ammonia (r=−0.205;P=0.04) appeared significantly correlated to larval tolerance to deltamethrin. The data suggest a direct correlation between some characteristics from the breeding sites and larval tolerance to pyrethroids.