Odour-Based Strategies for Surveillance and Behavioural Disruption of Host-Seeking Malaria and Other Mosquitoes

Abstract

Malaria is one of the most important diseases that threaten the lives of a large proportion of the human population in the tropics. Currently, the application of novel technologies for malaria control is encouraged to provide reliable and robust tools for monitoring of anopheline vectors and to reduce the application of chemical insecticides. This thesis has investigated the potential of using odour-based strategies for surveillance and behavioural disruption of host-seeking malaria and other mosquitoes. All studies were done under semifield and field conditions in western Kenya. A study conducted in a semi-field enclosure demonstrated that nylon strips performed significantly better than low density polyethylene (LDPE) material in dispensing synthetic mosquito attractants up to 40 consecutive nights after treatment. The treated nylon strips attracted more Anopheles gambiae sensu stricto than LDPE sachets containing similar attractants up to one year post-treatment when re-used once a week. Additional volatile organic compounds and different bacterial populations were found on attractant-treated nylon strips after one year of intermittent exposures. Autoclaving of treated nylon strips prior to deployment did not affect their attractiveness for mosquito populations. Subsequent field results indicated that attractant-treated nylon strips and LDPE sachets deployed at weekly intervals remain attractive to indoor and outdoor host-seeking malaria vectors up to one year post-treatment. Indoor studies demonstrated that synthetic odour baits can be used to replace a human as a host stimulus in mosquito sampling devices. The interaction between visual and olfactory cues can present a more robust and reliable tool for sampling malaria and other mosquito vectors than either stimulus alone. Anopheles arabiensis was the most abundant member of the An. gambiae complex caught in Kigoche village, western Kenya. Unlike trapped An. gambiae s.l., a large proportion of trapped An. funestus had fed more on humans than bovines. One An. funestus tested positive for P. falciparum sporozoites. Addition of selected concentrations of 1-butylamine, 1-dodecanol or 2-pentadecanone to the Mbita blend + CO2 released by fermenting sugar increased the catches of indoor-biting An. gambiae s.l., An. funestus and Culex mosquitoes. The majority of female An. gambiae s.l. caught were either unfed or gravid while An. funestus were predominantly unfed suggesting that the odorant cues attracted different fractions of both species. Fermentation of molasses instead of sugar provided a sustainable and additional source of CO2 suitable for incorporation in odour baits for sampling unfed and blood-fed malaria mosquitoes. The use of locally available and regularly used textiles (i.e., cellulose + polyacrylate, cotton and polyester material) offered a similar or more efficient matrix for dispensing synthetic attractant odorants of malaria vectors compared to nylon. In general, the high numbers of mosquitoes caught indoors and outdoors imply that deployment of odour-baited technology may provide a complementary and sustainable tool for monitoring and possibly control of multiple malaria vectors having different abdominal conditions (i.e., unfed, blood-fed or gravid). This is likely to contribute to a reduction in mosquito bites, and transmission of malaria and other mosquito-borne diseases.