Chickens Release Chemicals That Repel Malaria Mosquitoes

Chickens release volatiles that repel the host-seeking malaria mosquito Anopheles arabiensis, according to a study recently published in Malaria Journal. These volatiles, the authors wrote, “can provide protection to humans at risk of mosquito-vectored diseases in combination with established control [programs].”

In sub-Saharan Africa, intervention efforts with indoor residual spraying and insecticide-treated bed nets have markedly reduced malaria transmission by the mosquito Anopheles gambiae sensu stricto. However, malaria continues to be a significant public health problem in this region of the world, in part because these intervention efforts shifted malaria transmission to the outdoors. This shift led to A. arabiensis becoming the dominant malaria vector in sub-Saharan Africa.

A. arabiensis is an opportunistic feeder, feeding on indoor and outdoor vertebrate hosts like humans and livestock. Such feeding behavior, the study authors noted, underscores the need for novel mosquito control strategies.

Anopheles mosquitoes use factors such as host availability and volatile profiles when selecting hosts. The authors sought to identify the specific host selection process for A. arabiensis.

The study was conducted in three villages in western Ethiopia, where humans and livestock commonly share living quarters. Indoor- and outdoor-resting blood-fed mosquitoes were collected and identified; those identified as A. arabiensis were maintained in a research facility. Blood meals from freshly blood-fed mosquitoes underwent ELISA testing using antibodies against humans, cattle, goats, sheep, and chickens.

Volatile headspace collections of hair, wool, or feathers were obtained from one village’s livestock. Following chemical characterization of the volatiles, several electrophysiologic methods (gas chromatography, electroantennographic detection, electroantennography) were used to evaluate antennal responses to the volatiles. Antennae came from female non—blood fed A. arabiensis.

Field testing was conducted during Ethiopia’s rainy season. Mosquito traps with nearby vials of host and nonhost volatiles (determined prior to field testing) were suspended outside an untreated bed net, under which a human subject agreed to sleep. As a control, a caged chicken, determined to be a nonhost of A. arabiensis, was suspended like the vials.

A. arabiensis was the most abundant species of Anopheles mosquitoes caught during the study. Cattle were the most abundant host species, followed by humans and chickens. A. arabiensis selected humans most frequently for indoor blood meals and cattle most frequently for outdoor blood meals. It demonstrated no preference for indoor versus outdoor blood meals on goats and sheep, indicating random feeding on these species. Interestingly, A. arabiensis took only one blood meal from chickens, despite the chickens’ relatively high abundance.

Taken together, these results identified the hosts (humans, cattle, goats, sheep), and nonhosts (chickens) of A. arabiensis. They also provide insight into the host-selection process of this mosquito, namely that A. arabiensis prefers human blood when host seeking indoors but is opportunistic when seeking outdoor blood meals.

Several classes of compounds comprised the identified active volatiles, including hydrocarbons, ketones, and aldehydes. Of the active volatiles, 25 were detected in at least one of the nonhuman host species and 11 were detected in chickens. Four volatiles were chicken-specific.

Electroantennography demonstrated dose-dependent responses of female A. arabiensis antennae to all tested active volatiles.

During field testing, significantly fewer A. arabiensis were trapped with chicken-specific volatiles than with general volatiles. Similarly, the presence of the caged chicken significantly reduced the number of trapped A. arabiensis.

The ability of chicken-specific volatiles to repel A. arabiensis effectively suggests the usefulness of nonhost volatiles in conjunction with other mosquito control strategies. Given the problem of increased insecticide resistance, the authors stated that “it is incumbent on the international malaria community to embrace these novel control methods and products.”

Dr. Pendergrass received her doctorate in veterinary medicine from the Virginia-Maryland College of Veterinary Medicine. Following veterinary school, she completed a postdoctoral fellowship at Emory University’s Yerkes National Primate Research Center. Dr. Pendergrass is the founder and owner of JPen Communications, LLC.