Evaluation of potential vector control methods that target outdoor feeding anopheles mosquitoes

Abstract

Insecticide-Treated Nets (ITNs/Long Lasting Insecticide-Treated Nets (LLINs) and Indoor Residual Spraying (IRS) have contributed to halving global malaria incidence and mortality rates. Achieving zero malaria transmission using these methods which target indoor-feeding mosquitoes is hampered by residual malaria transmission attributable to outdoor-feeding mosquitoes. This study evaluated potential malaria vector-control methods that target outdoor-feeding Anopheles mosquitoes to contribute to efforts to eliminate residual malaria transmission. Specifically, it set out: (i) To assess community knowledge and awareness on malaria and its vectors, (ii) To determine spatial distribution of Anopheles adults and larvae, (iii) To determine effects of combined use of chemo-attractants and non excito-repellant insecticide (Chlorfenapyr) applied to contaminating devices in attracting and killing outdoor Anopheles mosquitoes and (iv) To evaluate method of attracting gravid Anopheles gambiae s.s to oviposit in artificially-created ovicidal breeding sites. To meet specific objective (i), a cross-sectional questionnaire study which explored knowledge and awareness on malaria was carried out in Dodoma and Morogoro on 400 respondents. The questionnaire study revealed that a vast majority (78.8%) were not aware that early outdoor biting of mosquito was a risk factor for malaria transmission. The main Anopheles breeding sites were found to be rice paddies (25.2%), ditches (23.3%) and septic tanks/pits (18.8%). Adult Anopheles mosquitoes were collected mainly from ceiling (42.3%) and stored/piled junks (16.3%) in the hot-wet season and from under beds (32.1%), undisturbed curtains (25.3%) and store rooms (23.7%) in cold-dry seasons. On testing a novel Umbrella-topped Mosquito Contaminating Device (UtMCD) developed in the present studies as a tool for attracting and killing outdoor Anopheles mosquitoes (specific objective iii) it was revealed that, adjusting for season and study location, the number of Anopheles mosquitoes caught were significantly associated with type of UtMCD set-up used. From among the UtMCD set-ups A-D and device E tested (A: Device alone, B: Device with attractants alone, C: Device with insecticide alone, D: Device with insecticide and attractants and E: Device (Okomu), UtMCD set-up D significantly caught more Anopheles mosquitoes than UtMCD set-up A (AMR=2.96,p<0.0001). Mortality analysis showed that UtMCD set-ups D, C and device E had higher percentage mortality: D (87.7%); C (89.4%) and E (84.9%) than UtMCD set-up A (19.2%) and UtMCD set-up B (17.4%). In the studies on artificially-created breeding sites it was found that the odds of not hatching for eggs deposited in hay infusion +vinegar sites was significantly greater than that of eggs deposited in water alone (AOR=80.6, p<0.001). In conclusion, the studies revealed there is a need for increased awareness on malaria transmission by outdoor early-feeding Anopheles and furthermore, the novel UtMCD (with chemo-attractants and insecticide) as well as artificial breeding sites with attractants and an ovicidal agent (vinegar) have potential for use in integrated vector and larval source management to eliminate residual malaria transmission.