doi:10.1038/nindia.2013.132 Published online 3 October 2013
Anandashankar Ray had a personal score to settle with mosquitoes. He suffered a bout of malaria as a youngster in the eastern Indian state of Odisha (then Orissa). His wife Anupama Dahanukar was down with dengue in a hospital for a week.
The entomologist had been trying to figure out the mechanism behind how bugs detect the insect repellent DEET (N,N-diethyl-meta-toluamide). Along with his team mates at University of California, Riverside, he seems to have finally cracked the code .
Essentially, Ray and his colleagues (two others of Indian-origin — Pinky Kain, a PhD from the National Centre for Biological Sciences in Bangalore and Ray's wife Anupama Dahanukar, also a professor at UC, Riverside) have found out just how bugs detect DEET, considered the gold-standard bug repellent since the 1940s when the U.S. Army developed it. Besides understanding this, the team also reported several other safe and cheap chemical compounds that could help fight the war against bugs such as mosquitoes.
Ray, who grew up in Ranchi, travelled to the Shimlipal Tiger Reserve in Orissa with friends during his Presidency College days in Kolkata. "Several of us went down with malaria after returning back. My wife was affected by Dengue in 2007 when we were on vacation in India, before starting my lab." The finding, therefore, is also a personal victory against the bugs for him.
Ray says scientists have not been able to improve upon DEET since they had no clue about how insects detect the chemical in the first place. Scientifically speaking, "the receptors causing olfactory repellency" were unknown. Also it is exorbitant to test whether new chemicals are safe for human use. The lab mates got together to experiment the chemical on genetically modified fruit flies Drosophila melanogaster. They tweaked the bugs genetically so that special nerve cells in their antennae would glow a fluorescent green when they reacted with DEET.
The team zeroed in on some "DEET-sensitive neurons" in a pit-like structure called the sacculus inside the antenna of the frutflies. These were identified as the key part of the nerve cell that detect DEET and thus signal the bug to go away from DEET. They repeated their experiments with close to 200 natural compounds as candidate repellents. These new compounds were also found to be strong repellents for dengue-causing female Aedes Aegypti mosquitoes. The newly identified compounds contain chemicals that are inexpensive and "smell mildly like grapes", with three (derived from plum, orange and jasmine) considered safe in human foods by the U. S. Food and Drug Administration.
"Our findings pave the way to discover new generations of repellents that will help fight deadly insect-borne diseases worldwide," the team report in their paper.
Neurobiologist Pinky Kain, a co-author of the paper who grew up in New Delhi, observed these bugs closely under the microscope. The fruitfly has been a fascinating model animal for since her PhD days under Veronica Rodrigues at the National Centre for Biological Sciences (NCBS-TIFR), Bangalore, whom she sorely "misses at this moment of scientific achievement".
"I am really blessed because I don't get bitten by mosquitoes. But I have seen my own people and others suffering because of these little monsters," Kain, who was the first to see the bugs' antennae go green during the ecxperiment, says.
"We are taking a step further to find better and cheaper solutions. We can now test this new class of affordable and safe repellents to limit the contact between insects and humans," she told Nature India.
DEET has has been reported earlier to have little effect on disease control in endemic regions due to the high costs and inconvenience of continuous application on skin at high concentrations. The chemical also dissolves some plastics, synthetic fabrics and painted surfaces. Earlier instances of DEET resistance have also been reported in flies and mosquitoes.
The new compounds, which the team members coated on nettings and tested on their arms, will pave a way for new formulations to fight against insects and provide an important line of defence, Kain hopes.