Fine-scale distribution of malaria mosquitoes biting or resting outside human dwellings in three low-altitude Tanzanian villages

Abstract

Background While malaria transmission in Africa still happens primarily inside houses, there is a substantial proportion of Anopheles mosquitoes that bite or rest outdoors. This situation may compromise the performance of indoor insecticidal interventions such as insecticide-treated nets (ITNs). This study investigated the distribution of malaria mosquitoes biting or resting outside dwellings in three low-altitude villages in south-eastern Tanzania. The likelihood of malaria infections outdoors was also assessed. Methods Nightly trapping was done outdoors for 12 months to collect resting mosquitoes (using resting bucket traps) and host-seeking mosquitoes (using odour-baited Suna® traps). The mosquitoes were sorted by species and physiological states. Pooled samples of Anopheles were tested to estimate proportions infected with Plasmodium falciparum parasites, estimate proportions carrying human blood as opposed to other vertebrate blood and identify sibling species in the Anopheles gambiae complex and An. funestus group. Environmental and anthropogenic factors were observed and recorded within 100 meters from each trapping positions. Generalised additive models were used to investigate relationships between these variables and vector densities, produce predictive maps of expected abundance and compare outcomes within and between villages. Results A high degree of fine-scale heterogeneity in Anopheles densities was observed between and within villages. Water bodies covered with vegetation were associated with 22% higher densities of An. arabiensis and 51% lower densities of An. funestus. Increasing densities of houses and people outdoors were both associated with reduced densities of An. arabiensis and An. funestus. Vector densities were highest around the end of the rainy season and beginning of the dry seasons. More than half (14) 58.3% of blood-fed An. arabiensis had bovine blood, (6) 25% had human blood. None of the Anopheles mosquitoes caught outdoors was found infected with malaria parasites. Conclusion Outdoor densities of both host-seeking and resting Anopheles mosquitoes had significant heterogeneities between and within villages, and were influenced by multiple environmental and anthropogenic factors. Despite the high Anopheles densities outside dwellings, the substantial proportion of non-human blood-meals and absence of malaria-infected mosquitoes after 12 months of nightly trapping suggests very low-levels of outdoor malaria transmission in these villages.

Impact of cattle on the abundance of indoor and outdoor resting malaria vectors in southern Malawi

“…arabiensis. The low catches with clay pots are contrary to findings elsewhere [35,61,62], but similar to studies by Mmbando et al [63], where resting buckets placed outdoors yielded relatively few An. arabiensis.…”

Trends in malaria epidemiological factors following the implementation of current control strategies in Dangassa, Mali

“…As expected in seasonal malaria transmission areas, this study’s results show higher EIRs at the end of the rainy season compared to the beginning [ 26 , 30 – 32 ]. There was a trend of decrease in both MHBR and EIR observed from 2015 to 2020 (with some fluctuation at the onset of the rainy season in 2018 and 2020), which can be related to the increase in malaria control intervention as described elsewhere [ 33 , 34 ].…”

“…Outdoor watching of television and/or other domestic activities may have allowed mosquitoes access to human blood sources despite LLIN universal coverage. In addition, outdoor resting behaviour developed by malaria vectors in this area may compromise the efficacy of indoor-based interventions, such LLINs [ 30 ].…”

Evaluation of the DN-Mini (miniaturized double net) trap for sampling host-seeking Anopheles mosquitoes in malaria-endemic villages of southern Tanzania

“…The study conducted by Mmbando et al 2020 [ 36 ] suggests that when employing the resting bucket (RBu), similar results are likely to be obtained, with the majority of outdoor mosquitoes being culicine and a proportion belonging to anopheline mosquitoes. It’s worth noting that the resting bucket is predominantly favored for outdoor use.…”

“…Hence, the HLC is a usual tool for understanding the transmission dynamic as well as host preference of mosquitoes, and such details cannot be observed from the HLC since mosquitoes collected might also feed on the HLC collector. Multiple blood feeding (taking more than one blood meal per gonotrophic cycle) was found to be the most important information when calculating human biting rate [ 36 ], such data that can easily be obtained from the DN-Mini trap. Though unlikely, the blood-fed mosquitoes may have been from the trap handlers or volunteers inside the traps.…”