Limitation of using synthetic human odours to test mosquito repellents

Fredros O Okumu,Gerry F Killeen,Emmanuel Titus,Sarah J Moore,Edgar Mbeyela

doi:10.1186/1475-2875-8-150

Abstract

BackgroundGold-standard tests of mosquito repellents involve exposing human volunteers to host-seeking mosquitoes, to assess the protective efficacy of the repellents. These techniques are not exposure-free and cannot be performed prior to toxicological evaluation. It is postulated that synthetic lures could provide a useful assay that mimics in-vivo conditions for use in high-throughput screening for mosquito repellents.MethodsThis paper reports on a semi-field evaluation of repellents using a synthetic blend of human derived attractants for the malaria vector, Anopheles gambiae sensu stricto Different concentrations of known repellents, N, N diethyl-3-methylbenzamide (deet) and Para-methane-3, 8, diol (PMD) were added into traps baited with the synthetic blend, and resulting changes in mosquito catches were measured.ResultsAll test concentrations of deet (0.001% to 100%) reduced the attractiveness of the synthetic blend. However, PMD was repellent only at 0.25%. Above this concentration, it significantly increased the attractiveness of the blend. There was no relationship between the repellent concentrations and the change in mosquito catches when either deet (r2 = 0.033, P = 0.302) or PMD (r2 = 0.020, P = 0.578) was used.ConclusionIt is concluded that while some repellents may reduce the attractiveness of synthetic human odours, others may instead increase their attractiveness. Such inconsistencies indicate that even though the synthetic attractants may provide exposure-free and consistent test media for repellents, careful selection and multiple-repellent tests are necessary to ascertain their suitability for use in repellent screening. The synthetic odour blend tested here is not yet sufficiently refined to serve as replacement for humans in repellent testing, but may be developed further and evaluated in different formats for exposure free repellent testing purposes.

Highlights

Gold-standard tests of mosquito repellents involve exposing human volunteers to host-seeking mosquitoes, to assess the protective efficacy of the repellents
The addition of nylon strips impregnated with the two repellents, deet and PMD onto the synthetic lure affected the attractiveness of the lure, but in two different ways
All the tested concentrations of deet except 0.01% significantly reduced the attractiveness of the synthetic odour blend (P < 0.05); as significantly fewer mosquitoes were collected in traps where the synthetic blend was accompanied with deet (Figure 2)

Summary

Introduction

Gold-standard tests of mosquito repellents involve exposing human volunteers to host-seeking mosquitoes, to assess the protective efficacy of the repellents. These techniques are not exposure-free and cannot be performed prior to toxicological evaluation. Personal protection with insect repellents is a popular method for preventing contact with arthropod disease vectors While their modes of action may vary, repellents generally prevent host seeking vectors from landing on or biting the user. The vectors, which under normal circumstances would be attracted to the person, are either diverted away or disoriented in such a way that they fail to bite the host [1] These are the most important determinants of effectiveness of any repellent, though there are certainly other essential factors such as costs, availability and user acceptability [2]. Many haematophagous insects have preferences for different host emanations and some are more likely to bite certain parts of the body than others [6,7]

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