Resistance of mosquitoes to insecticides is a growing concern in Africa. Since only a few insecticides are used for public health and limited development of new molecules is expected in the next decade, maintaining the efficacy of control programmes mostly relies on resistance management strategies. Developing such strategies requires a deep understanding of factors influencing resistance together with characterizing the mechanisms involved. Among factors likely to influence insecticide resistance in mosquitoes, agriculture and urbanization have been implicated but rarely studied in detail. The present study aimed at comparing insecticide resistance levels and associated mechanisms across multiple Anopheles gambiae sensu lato populations from different environments.

To control malaria in Tanzania, two primary vector control interventions are being scaled up: long-lasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS). The main threat to effective malaria control is the selection of insecticide resistance. While resistance to pyrethroids, the primary insecticide used for LLINs and IRS, has been reported among mosquito vectors in only a few sites in Tanzania, neighbouring East African countries are recording increasing levels of resistance. To monitor the rapidly evolving situation, the resistance status of the malaria vector Anopheles gambiae s.l to different insecticides and the prevalence of the kdr resistance allele involved in pyrethroid resistance were investigated in north-western Tanzania, an area that has been subject to several rounds of pyrethroid IRS since 2006. Household collections of anopheline mosquitoes were exposed to diagnostic dosages of pyrethroid, DDT, and bendiocarb using WHO resistance test kits. The relative proportions of An. gambiae s.s and Anopheles arabiensis were also investigated among mosquitoes sampled using indoor CDC light traps. Anophelines were identified to species and the kdr mutation was detected using real time PCR TaqMan assays. From the light trap collections 80% of An. gambiae s.l were identified as An. gambiae s.s and 20% as An. arabiensis. There was cross-resistance between pyrethroids and DDT with mortality no higher than 40% reported in any of the resistance tests. The kdr-eastern variant was present in homozygous form in 97% of An. gambiae s.s but was absent in An. arabiensis. Anopheles gambiae s.s showed reduced susceptibility to the carbamate insecticide, bendiocarb, the proportion surviving WHO tests ranging from 0% to 30% depending on season and location. Anopheles gambiae s.s has developed phenotypic resistance to pyrethroids and DDT and kdr frequency has almost reached fixation. Unlike in coastal Tanzania, where the ratio of An. gambiae s.s to An. arabiensis has decreased in response to vector control, An. gambiae s.s persists at high frequency in north-western Tanzania, probably due to selection of pyrethroid resistance, and this trend is likely to arise in other areas as resistance spreads or is subject to local selection from IRS or LLINs.

Malaria and lymphatic filariasis (LF) are diseases of great public health important in East Africa. Malaria is transmitted by Anopheles while LF is transmitted by both Anopheles and Culex mosquitoes. There is limited evidence on the effects of physicochemical parameters on these mosquitoes in rural settings of Tanzania. This study aimed at assessing the effects of physicochemical parameters on Anopheles and Culex larvae abundance in different breeding sites in a rural setting of Muheza district. A cross-sectional study was conducted in 13 villages in Muheza district, between December 2015 and May 2016. Mosquito larvae were sampled using standard dipping techniques. Physicochemical parameters were measured by a Multi-parameter pH meter in different mosquito breeding sites. Mosquito larvae and pupa densities were compared between the ≤33th (lower) and ≥67th (upper) percentiles of physicochemical parameters. An. gambiae (s.l.) and An. funestus were identified by polymerase chain reaction (PCR) to reveal their sibling species. Abundance of Anopheles larvae was significantly higher (76.6%) than Culex (66.9%) χ 2 = 5.73, df = 1, P = 0.017). The presence of late instars of Anopheles was significantly higher (78.2%) than that of Culex (64.5%) (χ 2 = 0.984, df = 1, P = 0.017). A model adjusted for larval stage showed that the likelihood of finding Culex larvae was lower by 38.2% (95% CI: 16.9–54.1, df = 1, P = 0.001) compared to Anopheles. Upper percentiles of salinity (OR = 7.05; 95% CI: 1.19–41.88, P = 0.032) and conductivity (OR = 5.47; 95% CI: 1.01–29.67, P = 0.056) were significantly associated with the presence and with increased density of Anopheles larvae. PCR results showed that, within the gambiae complex, 53.3% (n = 136) were An. gambiae (s.s.) and 46.7% (n = 119) were An. arabiensis. In An. funestus group, 91.1% (n = 41) were An. funestus (s.s.), 4.4% (n = 2) were An. rivulorum, 2.2% (n = 1) were An. leesoni and 2.2% (n = 1) were An. parensis. High salinity and conductivity were significantly associated with increased density of Anopheles larvae. To our knowledge, this is the first study to demonstrate the occurrence of Anopheles larvae in polluted breeding sites in rural settings of Tanzania. The study has found both Anopheles and Culex mosquito larvae co-existed in breeding sites. The possible reasons for tolerance to a higher level of physicochemical parameters among Anopheles mosquitoes need to be ascertained.

Malaria vector control in Tanzania is based on use of long-lasting insecticide treated nets (LLINs) and indoor residual spraying (IRS), which both rely on the use of chemical insecticides. The effectiveness of these control tools is endangered by the development of insecticide resistance in the major malaria vectors. This study was carried out to monitor the susceptibility status of major malaria vectors to insecticides used for IRS and LLINs in mainland Tanzania. Mosquito larvae were collected in 20 sites of Tanzania mainland in 2015. Phenotypic resistance was determined using standard WHO susceptibility tests. Molecular assay were used to determine distribution of Anopheles gambiae sub-species. A microplate assay approach was used for identifying enzyme levels on single mosquitoes from each sites compared with a susceptible reference strain, An. gambiae sensu stricto (s.s.) Kisumu strain. Anopheles arabiensis was the dominant malaria specie in the country, accounting for 52% of the sibling species identified, while An. gambiae s.s. represented 48%. In Arumeru site, the dominant species was An. arabiensis, which was resistant to both pyrethroids (permethrin and deltamethrin), and pirimiphos-methyl, and had significant elevated levels of GSTs, non-specific esterases, and oxidase enzymes. An. arabiensis was also a dominant species in Kilombero and Kondoa sites, both were resistant to permethrin and deltamethrin with significant activity levels of oxidase enzymes. Resistance to bendiocarb was recorded in Ngara site where specie composition is evenly distributed between An. gambiae s.s. and An.arabiensis. Also bendiocarb resistance was recorded in Mbozi site, where An. gambiae s.s. is the dominant species. Overall, this study confirmed resistance to all four insecticide classes in An. gambiae sensu lato in selected locations in Tanzania. Results are discussed in relation to resistance mechanisms and the optimization of resistance management strategies.

Control of lymphatic filariasis (LF) in most countries of sub-Saharan Africa is based on annual mass drug administration (MDA) with a combination of ivermectin and albendazole, in order to interrupt transmission. We present findings from a detailed study on the effect of six rounds of MDA with this drug combination as implemented by the National Lymphatic Filariasis Elimination Programme (NLFEP) in a highly endemic rural area of north-eastern Tanzania.

Widespread and large scale use of ivermectin in humans and domestic animals can have unexpected effects on non-target organisms. As a search for a possible explanation for an observed longitudinal decline in density of anopheline vector mosquitoes, but not in Culex quinquefasciatus, in an area of north-eastern Tanzania which has been exposed to ivermectin mass drug administration, this study assessed and compared the effect of human ivermectin treatment on blood feeding Anopheles gambiae and Cx. quinquefasciatus. Consenting adult volunteers were randomized into two groups to receive either ivermectin or placebo. Twenty four hours after treatment, one volunteer from each group was concurrently exposed to 50 laboratory reared An. gambiae on one arm and 50 laboratory reared Cx. quinquefasciatus on the other arm for 15–30 minutes. Engorged mosquitoes were maintained on 10% glucose solution for 12 days and observed for survival and fecundity. The experiment was repeated 15 times. Two days after the blood meals, nearly half (average 47.7% for the 15 experiments) of the blood fed An. gambiae in the ivermectin group had died while almost all in the placebo group were alive (97.2%), and the difference in survival between these two groups continued to widen on the following days. There was no clear effect of ivermectin on Cx. quinquefasciatus, which had high survival in both ivermectin and placebo group on day 2 (95.7% and 98.4%, respectively) as well as on the following days. Ivermectin completely inhibited egg laying in An. gambiae, while egg laying and subsequent development of immature stages appeared normal in the other three groups. Blood meals taken on ivermectin treated volunteers significantly reduced survival and halted fecundity of An. gambiae but had only limited or no effect on Cx. quinquefasciatus. The result suggests that widespread use of ivermectin may have contributed to the observed decline in density of An. gambiae, without similar decrease in Cx. quinquefasciatus, in north-eastern Tanzania.