On February 13th, a study published in the open access journal PLOS ONE, by James Logan, Nina Stanczyk, and colleagues at London School of Hygiene & Tropical Medicine, UK, exposed a somewhat scary truth about our current methods for preventing bug bites and insect-carried diseases: mosquitos that are exposed to DEET, or N,N-Diethyl-m-toluamide, are developing a tolerance to the drug within three hours.
What We Thought…
For years prior to the study, the resistance of the aegypti mosquitos was believed to be due to a genetic trait that was passed down through generations. According to an article published in 2010 in the Nature Journal, researchers had used selective breeding to breed females with, what they believed to be, a genetic resistance to DEET. Furthermore, the researchers believed the gene to be dominant, meaning that only one parent has to have the gene for the offspring to inherit the DEET resistance (Stanczyk, 2010).
Now, after additional experiments, Logan and Stancyk conclude the resistance in these mosquitos is actually due to a habituation process similar to that in Humans.
But What Does This Mean?
When you walk into a room and are immediately presented with a foul smell, although absolutely intolerable at first, 20 or 30 seconds later, you no longer smell it. This is habituation.
Within our noses, we have receptors for different odors. When an intense odor enters our nose, the receptors that are sensitive to the odor saturate (respond maximally).This causes the receptor to, in a sense, readjust its baseline to ignore subsequent stimulation by the same odor.This preserves our ability to smell novel odors in the room that would be masked by the intense odor otherwise.
Logan and Stancyk have now concluded that the aegypti mosquitoes develop a resistance to DEET, in the same way as humans.
Logan and Stancyk repeatedly exposed 20 female aegypti mosquitos to DEET in four different experiments to look at their behavioral and physiological responses (Stancyk, 2012).
According to an excerpt from a press release from the Public Library of Science, Logan says “Our study shows that the effects of this exposure last up to three hours. We will be doing further research to determine how long the effect lasts.” This means that after one exposure, the mosquitoes showed insensitivity to subsequent DEET exposures–they attacked the hosts’ arm, although it had been re-sprayed with DEET. Additionally, supporting these behavioral findings, the researchers performed an electroantennography—a technique measuring the output from an antenna to the brain for a specific odor—and discovered that the olfactory receptor neurons actually did respond less to the DEET.
Since these neurons’ responses were altered by the pre-exposure to DEET, we now have proof that even one presentation of certain olfactory stimuli can modulate the olfactory system of certain mosquitoes, making the stimuli less effective, and creating potential problems for insect-carried illness prevention.
Implications for the Future…
Apart from the nuisance of having one, two or twelve bug bites, the newly discovered ineffectiveness of DEET leaves Humans in tropical areas at risk to illnesses such as West Nile Virus, Malaria, or Dengue Fever, carried by these mosquitos. Although the study specifically looked at the resistance of Aedes aegypti mosquitoes, other recent studies have also shown Drosophila melanogaster-the household fruitfly (Reeder, 2001)-and Rhodnius prolixus-a tritanopia bug (Sfara, 2011)- to also develop DEET resistances after primary exposure.
Although worrisome, Science News Daily emphasizes that Logan states “This doesn’t mean that we should stop using repellents — on the contrary, DEET is a very good repellent, and is still recommended for use in high risk areas. However, we are keeping a close eye on how mosquitoes can overcome the repellent and ways in which we can combat this.”