Development and Calibration of an Exposure-Free Bednet Trap for the Sampling of Afrotropical Malaria Vectors

Abstract

Interventions aimed at reducing malaria vector populations through impregnated bednets, environmental management, residual indoor spraying etc., require monitoring of mosquitoes before, during and after the intervention. Various sampling methods have been developed, but collections by these do not reflect a true cross-section of the host-seeking population and are biased in various ways. The commonly accepted best estimate of the entomological inoculation rate (i.e. an indicator for malaria risk over a certain time period) relies upon human volunteers collecting biting mosquitoes from exposed feet and limbs. As some of these mosquitoes are inevitably infected with human malaria parasites, this represents an increased and undesirable risk to the volunteers. Therefore, effective and economical monitoring tools for African malaria vectors are urgently needed to assess the impact of malaria interventions on mosquito populations. The development of improved sampling systems based on an improved understanding of host-oriented behaviour is needed. The possibility of modifying bednets into traps for sampling malaria vectors would be a worthwhile endeavour. To pursue this possibility of modifying bednets into mosquito sampling devices, various objectives were designedformulated to: (i) Study the behaviour of mosquitoes around human-occupied bednets, (ii) to modify bednets into mosquito traps and (iii) To evaluate the efficacy and applicability of these traps aganist existing trapping techniques at different sites with different mosquito species and densities thereof. Results obtained from the study of mosquito behaviour demonstrated that temperature and relative humidity inside a human-baited bednet playa crucial role in the behaviour of mosquitoes around such nets. Mesh size of the material making the net was found to be a direct determinant of these parameters. From these studies of mosquito behaviour around human-baited bednets, an exposure-free bednet trap (the 'Mbita trap') for sampling of Afrotropical malaria vectors was developed. Its mosquito sampling efficacy was compared to the CDC miniature light-trap and human landing catches under semi-field conditions in a screen-walled greenhouse using laboratory reared Anopheles gambiae Giles sensu stricto (Diptera: Culicidae). When compared in a competitive manner (side by side), the Mbita trap caught 4.l times as many mosquitoes as the CDC light-trap, hung beside a human occupied bednet, and 43.2 % the number caught by human landing catches. The ratio of Mbita trap catches to those of the CDC light trap increased with decreasing mosquito density. Mosquito density did not affect the Mbita trap catch to human landing catch ratio. In a non-competitive comparison (each method independent of the other), the Mbita trap caught 89.7 % the number of mosquitoes caught by human landing catches and 1.2 times more mosquitoes than the CDC light trap. Differences in Mbita trap performance relative to the human landing catch under non-competitive versus competitive conditions were explained by the rate at which each method captured mosquitoes. The mosquito sampling efficiency of the new bednet trap (the Mbita trap) was compared with that of the CDC miniature light trap (hung adjacent to an occupied bednet) and the human landing catch in two different epidemiological settings (Lwanda and Ahero) in western Kenya. In Lwanda, the Mbita trap caught 48.7 % the number of Anopheles gambiae Giles sensu lato caught in the human landing collections and 27.4 % of the number caught by the light trap. The corresponding figures for An. funestus Giles were 74.6 % and 39.2 % respectively. Despite the clear differences in their sampling efficiencies, both the Mbita trap and light trap caught mosquitoes at rates directly proportional to human landing catches regardless of mosquito density. No significant differences in parity or sporozoite prevalence were observed between mosquitoes caught by the three methods for either An. gambiae s.l. or An. funestus. PCR identification of the sibling species of the An. gambiae complex indicated that the ratio of An. gambiae Giles sensu stricto to An. arabiensis Patton did not vary according to the sampling method used. In Ahero where An. arabiensis predominates, the Mbita trap caught only 17% of this species compared to the human landing catches but there was good proportionality between the two methods. The Mbita trap caught about 60% of the An. funestus caught in human landing catches. The CDC light trap, in contrast, caught about 60% of the An. arabiensis caught by the landing catches, with good proportionality and more An. funestus than in the landing catches. It is concluded that the Mbita trap is a promising tool for sampling malaria vector populations since its catch can be readily converted into equivalent human biting rates, can be applied more intensively, requires neither expensive equipment nor skilled personnel, and samples mosquitoes in an exposure-free manner. Such intensive sampling capability will allow cost-effective surveillance of malaria transmission at much finer spatial and temporal resolution than has been previously possible.