August 31, 2009
Defeating Malaria: A Look At An Innovative Natural Approach
By Michael D. Shaw
Carbon dioxide (CO2) is on the minds of many people these days, as it is the prime forensic evidence of our supposedly profligate lifestyle. But did you know that CO2 is vitally important in the lives of many insect species—some beneficial and some immensely destructive?
The hawkmoth (gardeners are well familiar with its larval form as the tomato hornworm) is attracted to particular flowers of the poisonous and hallucinogenic Datura wrightii plant by sensing the amount of CO2 emitted. Newly opened flowers emit more CO2 and offer more nectar. On the other hand, honeybees are turned off by CO2, and will fan the hive with their wings to drive it off. Mosquitoes are attracted by CO2, which has been called the GPS leading this insect to a host.
Fruit flies are CO2 averse, and will emit the compound when they are under stress, as a danger signal to their fellow flies. This is curious in that most fruit also emits CO2, and as we all know fruit flies are attracted to fruit, not to mention other food sources that emit the compound.
This problem intrigued University of California Riverside researcher Anandasankar Ray and one of his graduate students Stephanie Turner.
In a letter to Nature entitled “Modification of CO2 avoidance behaviour in Drosophila by inhibitory odorants,” published online August 26th, the group describes how it has identified a new class of odorants present in food that directly inhibit CO2-sensitive neurons in the fruit fly’s antennae. Importantly, they have demonstrated that related odorants are effective inhibitors of the CO2 response in Culex mosquitoes that transmit West Nile fever and filariasis.
“CO2 emitted in human breath is the main attractant for the Culex mosquito to find people, aiding the transmission of these deadly diseases,” Ray said. “In our experiments we identified hexanol, and a related odor, butanal, as strong inhibitors of CO2-sensitive neurons in Culex mosquitoes. These compounds can now be used to guide research in developing novel repellents and masking agents that are economical and environmentally safe methods to block mosquitoes’ ability to detect CO2 in our breath, thereby dramatically reducing mosquito-human contact.”
Ray’s interest in insect olfaction as a way of short-circuiting mosquitoes stems from his own childhood bout with malaria, in his native India. It was further piqued when his wife contracted dengue fever on a trip to India a few years ago.
“Mosquito-borne diseases that seem exotic and dangerous here in the U.S., like dengue fever and malignant malaria, are extremely common there. In India, if you go into a classroom and ask how many people had had a mosquito-borne illness, 50 percent would raise their hands,” he said. As it is, each year there are approximately 300 million clinical cases involving the five major disease transmitted by mosquitoes. And each year more than 1.5 million fatalities can be attributed to them. Regular readers of this column are aware of my feelings on the banning of DDT.
An encouraging finding of Ray’s work is that inhibition of the CO2 response seems to persist.
“To our surprise, we found that exposure to a long-term CO2 response inhibitor can exert a profound and specific effect on the behavior of the insect, even after the inhibitor is no longer in the environment,” Ray said. “This means this odorant could potentially be used to keep mosquitoes at bay for longer periods of time, benefiting people in areas where mosquito-transmitted diseases are prevalent.”
Hexanol is a relatively benign compound that is said to have the odor of freshly-mowed grass, and is used in the fragrance industry. Butanal (also called butaraldehyde) is less benign, but would be used at very low concentrations. In any event, mosquitoes are far more dangerous.
Ray’s work recently garnered him a $100,000 grant from the Bill and Melinda Gates Foundation to combat mosquito-borne illnesses.
“Investments in global health research are already paying big dividends. An incredible number of new vaccines, drugs and other tools are becoming available to improve health in developing countries,” said Dr. Tachi Yamada, president of the Gates Foundation’s Global Health Program.
Perhaps were are on our way to controlling the world’s most prolific serial killer: the mosquito.