A Closer Look
by Sean Green
A Fowl Smell Repels Mosquitoes fessor at the Swedish University of Agricultural Science heads up a study that was originally intended to determine how mosquitoes interact with other animals. Ignell sought to discover if certain animals attracted or repelled mosquitoes and conducted his studies in three Western Ethiopian villages in which humans and farm animals routinely share the same living space. The Mosquito studied, Anopheles arabiensis, feeds on both animals and human beings and is a significant malaria vector in Ethiopia. Collection and analysis of blood meals from over 1000 mosquitoes revealed that for the most part, Anopheles arabiensis strongly preferred human blood when seeking a host indoors and when outdoors, the mosquito feed randomly on other animals such as cattle, goat, and sheep and only one mosquito in the entire sample had a blood meal from a chicken. Furthermore the mosquitoes in the study not only appeared to avoid taking a blood meal from the chicken but consistently appeared to avoid the chicken entirely. With this new discovery the team of researchers collected fur, wool and feathers from common village animals to isolate and extracted the chemical components that characterize the animals scent. Once isolated, researchers could reproduce a pure essence of the scent of cattle, goat, sheep, and chicken. The essence that was produced was the focus of the second experiment in which thousands of villagers volunteered to serve as mosquito bait as researchers planted scent emitting devices in the sleeping quarters of the volunteer villagers, each villager was provided a device that emitted one of the four animal scents. Researchers found that in the rooms that included chicken scent devices there was a 90% to 95% reduction in mosquito counts when the mosquito traps were analyzed. Similarly, researchers conducted a test with over 6000 volunteers and live animals; sleeping quarters that featured a live caged chicken near the bed resulted in an 80% reduction of trapped mosquitoes.
Mosquitoes are problematic everywhere. It’s no secret they are the most dangerous animal in the world, responsible for more human deaths per year than human being themselves. In some countries such as Ethiopia, human death tolls from malaria reach tens of thousands each year. Modern research has expanded our understanding of the mosquito and thus armed our western civilization with tools to protect us from the onslaught of this tiny but potentially deadly insect. Contemporary solutions have their pitfalls, for example, toxins are effective, however, present biologic dangers to both pest and host; few natural predators feed on mosquitoes exclusively and therefore, become hard to sustain; bacterial and viral weapons can be species specific, but also prohibitively expensive. A recent study in Ethiopia has revealed interesting patterns in the feeding behavior of the malaria mosquito (Anopheles) thinning the veil of uncertainty and uncovering the potential for new methods of mosquito control. Scientists have discovered that the mosquito in the study seems to have an aversion to the scent of live chickens. A closer look at this study and the behavioral instincts of various mosquito species will be necessary to determine the extent to which new knowledge can be utilized in potential future solutions to combat mosquito borne illness. The new study published in the Malaria Journal last month reports new knowledge from old world Ethiopia, a country that suffers on average 70,000 deaths per year due to malaria. Rickard Ignell, a proWWW.INTHEFIELDMAGAZINE.COM
Every animal, including insects, evolves to avoid predators. Mice, for example, instinctively avoid the scent of natural predators such as cats and rats. Similarly, mosquitoes in the Culex genus have been observed avoiding bodies of water that include predators such as backswimmers (Notonecta), and mosquito fish (Gambusia). In this study, the mosquitoes responded to the scent of a chicken. Although the chicken is not a primary predator, its scent resulted in a distinct behavioral change nonetheless. While it is commonly known that mosquitoes use exhaled carbon dioxide, body odors and temperature to locate a blood meal, less is known about how the mosquito detects and avoids natural predators. A decade ago when the mosquito genome was first sequenced, scientists assumed the odorant receptors (ORs) on the mosquitoes antenna worked like the sensors in a mammals nose. It is now known that the mosquitoes ORs, are sensitive to specific predator-released kairomones (PRKs). The kairomones (pheremone like chemicals) that are released by a predator such as the backswimmers (Notonecta) are pheremone like compounds that trigger an instinctive response such as avoiding laying eggs in a body of water that has an abundance of predators, or seeking a blood meal from an animal that may feed on the mosquito such as the chicken. Researchers have discovered at least 75 different ORs, some of which are components of popular insect repellents containing DEET. The ORs do not mask the attractants in mammal breath and sweat, but rather, trigger an instinctual evolutionary behavior of avoidance. This study certainly does not provide a silver bullet for mosquito control, but it is a terrific beginning. Chemical identification of PRKs will facilitate the study of predator– prey interactions; taking a closer look can potentially enhance efforts to protect ourselves from the most dangerous animal in the world.
INTHEFIELD MAGAZINE
August 2016
79