Effect of non-human hosts on the human biting rate of primary and secondary malaria vectors in Tanzania.

BACKGROUND: Malaria vectors vary in feeding preference depending on their innate behaviour, host availability and abundance. Host preference and human biting rate in malaria vectors are key factors in establishing zooprophylaxis and zoopotentiation. This study aimed at assessing the impact of non-human hosts in close proximity to humans on the human biting rate of primary and secondary malaria vectors, with varying host preferences. METHODS: The effect of the presence of non-human hosts in close proximity to the human host on the mean catches per person per night, as a proxy for mosquito biting rate, was measured using mosquito-electrocuting traps (METs), in Sagamaganga, Kilombero Valley, Tanzania. Two experiments were designed: (1) a human versus a calf, each enclosed in a MET, and (2) a human surrounded by three calves versus a human alone, with each human volunteer enclosed individually in a MET spaced 10 m apart. Each experiment was conducted on alternate days and lasted for 36 nights per experiment. During each experiment, the positions of hosts were exchanged daily (except the human in experiment 2). All anopheline mosquitoes caught were assayed for Plasmodium sporozoites using enzyme-linked immunosorbent assay. RESULTS: A total of 20,574 mosquitoes were captured and identified during the study, of which 3608 were anophelines (84.4% primary and 15.6% secondary malaria vectors) and 17,146 were culicines. In experiment 1, the primary malaria vector, Anopheles arabiensis, along with Culex spp. demonstrated a preference for cattle, while the primary vectors, Anopheles funestus, preferred humans. In experiment 2, both primary vectors, An. arabiensis and An. funestus, as well as the secondary vector Anopheles rivolurum, demonstrated behaviours amenable to zooprophylaxis, whereas Culex spp. increased their attraction to humans in the presence of nearby cattle. All anopheline mosquitoes tested negative for sporozoites. CONCLUSIONS: The findings of this study provide support for the zooprophylaxis model for malaria vectors present in the Kilombero Valley, and for the zoopotentiation model, as it pertains to the Culex spp. in the region. However, the factors regulating zooprophylaxis and zoopotentiation are complex, with different species-dependent mechanisms regulating these behaviours, that need to be considered when designing integrated vector management programmes.

Distance from human dwellings differentially affects the efficacy of a synthetic cattle urine odour lure to trap malaria vectors.

BACKGROUND: Cost-effective outdoor-based devices for surveillance and control of outdoor mosquito vector populations can substantially improve their efficacy when baited with synthetic human and animal odours. This study aimed at assessing the dose-dependent efficacy of a previously developed synthetic cattle urine odour to lure malaria vectors, and other mosquito species, to traps placed at different distances from human dwellings outdoors. METHODS: The efficacy of the cattle urine odour lure was assessed through a 5 × 5 Latin square design, using two sets of 5 Suna traps placed at either 1.5 m or 5 m from an adjacent human dwelling, in the rural village of Sagamaganga, Tanzania. Each trap was deployed with one of four doses of the synthetic cattle urine odour blend or a solvent control (heptane). Traps were rotated daily so that each dose and control visited each position twice over a period of 20 experimental nights. The relative attractiveness of each treatment dose and control was compared using a generalized linear mixed model for each species caught. RESULTS: A total of 1568 mosquitoes were caught, of which 783 were anophelines and 785 were culicines. Of the anophelines, 41.6 and 58.3% were primary and secondary vector species, respectively. Unfed and fed females of the primary vector, Anopheles arabiensis, were caught dose-dependently, close to human dwellings (1.5 m), whereas unfed, fed and gravid secondary vector Anopheles pharoensis females were caught dose-dependently, but at a farther distance from the dwellings (5 m). Females of Culex spp. were caught dose-dependently in similar numbers irrespective of the distance from human dwellings. CONCLUSIONS: This study further clarifies the factors to be considered for the implementation of outdoor trapping using the synthetic cattle urine lure to target exophilic and exophagic malaria vectors, for which efficient surveillance and control tools are currently lacking. The findings resulting from this study make significant progress in providing the needed information to overcome the regulatory obstacles to make this tool available for integrated vector management programs, including registration, as well as evaluation and regulation by the World Health Organization.

Seasonal variation in abundance and blood meal sources of primary and secondary malaria vectors within Kilombero Valley, Southern Tanzania.

BACKGROUND: Existing control tools have significantly reduced malaria over the past two decades. However, progress has been stalled due to increased resistance in primary vectors and the increasing role of secondary vectors. This study aimed to investigate the impact of seasonal change on primary and secondary vector abundance and host preference. Understanding the impact of seasonal dynamics of primary and secondary vectors on disease transmission will inform effective strategies for vector management and control. METHODS: Vector abundance was measured through longitudinal collection of mosquitoes, conducted monthly during the wet and dry seasons, in Sagamaganga, a village in the Kilombero Valley, Tanzania. Mosquitoes were collected indoors using CDC light traps and backpack aspirators, and outdoors using resting buckets baited with cattle urine. In addition, a direct measure of host preference was taken monthly using human- and cattle-baited mosquito electrocuting traps. A host census was conducted to provide an indirect measure of host preference together with monthly blood meal source analysis. All collected mosquitoes were assayed for Plasmodium sporozoites. RESULTS: A total of 2828 anophelines were collected, of which 78.5% and 21.4%, were primary and secondary vectors, respectively. The abundance of the primary vectors, Anopheles arabiensis and Anopheles funestus, and of the secondary vectors varied seasonally. Indirect measures of host preference indicated that all vectors varied blood meal choice seasonally, with the direct measure confirming this for An. arabiensis. All anopheline mosquitoes tested negative for sporozoites. CONCLUSIONS: At the study location, the abundance of both primary and secondary vectors changed seasonally. Indirect and direct measures of host preference demonstrated that An. arabiensis varied from being zoophilic to being more opportunistic during the wet and dry seasons. A similar trend was observed for the other vectors.