Evans blue as a simple method to discriminate mosquitoes' feeding choice on small laboratory animals

Temperature, humidity, vision, and particularly odor, are external cues that play essential roles to mosquito blood feeding and oviposition. Entomological and behavioral studies employ well-established methods to evaluate mosquito attraction or repellency and to identify the source of the blood meal. Despite the efficacy of such methods, the costs involved in the production or acquisition of all parts, components and the chemical reagents involved are unaffordable for most researchers from poor countries. Thus, a simple and relatively low-cost method capable of evaluating mosquito preferences and the blood volume ingested is desirable.

By using Evans blue (EB) vital dye and few standard laboratory supplies, we developed and validated a system capable of evaluating mosquito's choice between two different host sources of blood. EB-injected and PBS-injected mice submitted to a number of situations were placed side by side on the top of a rounded recipient covered with tulle fabric and containing Aedes aegypti mosquitoes. Homogenates from engorged mosquitoes clearly revealed the blood source (EB- or PBS-injected host), either visually or spectrometrically. This method was able to estimate the number of engorded mosquitoes, the volume of blood ingested, the efficacy of a commercial repellent and the attractant effects of black color and human sweat.

Despite the obvious limitations due to its simplicity and to the dependence of a live source of blood, the present method can be used to assess a number of host variables (diet, aging, immunity, etc) and optimized for several aspects of mosquito blood feeding and vector-host interactions. Thus, it is proposed as an alternative to field studies, and it could be used for initial screenings of chemical compound candidates for repellents or attractants, since it replicates natural conditions of exposure to mosquitoes in a laboratory environment.