DEET may chemically cloak humans from malaria-carrying mosquitoes, rather than repel them, researchers report. Since its invention during the Second World War for soldiers stationed in countries with high malaria transmission rates, researchers have worked to pinpoint precisely how DEET actually affects mosquitoes. Past studies have analysed the chemical structure of the repellent, studied the response in easier insects to work with, such as fruit flies, and experimented with genetically engineered mosquito scent receptors grown inside frog eggs. However, the Anopheles mosquitos neurological response to DEET and other repellents remained largely unknown because directly studying the scent-responsive neurons in the mosquito itself proved technically challenging and labour-intensive work.
Hidden Skin Odours
For the new study, published in Current Biology, researchers applied a genetic engineering technique to the malaria-transmitting Anopheles mosquito, which allowed them to peer at the inner workings of the insects nose. Repellents are an amazing group of odours that can prevent mosquito bites, but its been unclear as to how they actually work. Using our new, engineered strains of Anopheles mosquitoes, we can finally ask the question, How do the smell neurons of a mosquito respond to repellent odours?’ says Christopher Potter, associate professor of neuroscience at Johns Hopkins University School of Medicine. Our results from Anopheles mosquitoes took us by surprise. We found that Anopheles mosquitoes smell neurons did not directly respond to DEET or other synthetic repellents, but instead these repellents prevented human-skin odours from being able to be detected by the mosquito. In other words, these repellents were masking, or hiding, our skin odours from Anopheles. We found that DEET interacts with and masks the chemicals on our skin rather than directly repelling mosquitoes. This will help us develop new repellents that work the same way, says first author Ali Afify, a postdoctoral fellow.
DEET Scent Trap
When researchers then puffed a scent that the mosquitoes could detectsuch as the chemicals that make up the scent of human skinonto the insects antennae, fluorescent molecules engineered to be expressed in the antenna would light the neurons up and a camera would record them, showing that the mosquitos nose detected the signal. Using this odour-detecting setup, the researchers found that different scents, including chemical bug repellents such as DEET, natural repellents such as lemongrass, and chemicals found in human scent had different effects on the neurons. When the researchers puffed the scent of DEET alone onto the mosquitoes antennae, the fluorescent molecules in the mosquitoes neurons did not light up, a sign that the mosquitoes could not directly smell the chemical. When exposed to the chemicals known to make up human scent, the neurons lit up like a Christmas tree, Potter says. And notably, when researchers mixed human scent with DEET, simulating the effect of applying the repellent to the skin, it tempered the neuronal response to the mixture, resulting in a much lower responseabout 20% the power of the response to human scent alone. Looking to gain insight into why this happened, the researchers measured the number of scent molecules in the air reaching the antenna to find out how much smell was present for the insects to respond. They found that when combined with DEET, the number of human scent molecules in the air decreased to 15% of their previous amounts. We therefore think that DEET traps human scents and prevents them from reaching the mosquitoes, says Afify. The researchers say they suspect that this effect is enough to mask the human scent and keep it from ever reaching the mosquitos odour detectors.
Varied Senses of Smell
The investigators caution that their study did not address the possibility that DEET and similar chemicals likely also act as contact repellents, possibly deterring Anopheles through taste or touch. The group also did not look at DEETs effect on other species of mosquitoissues they say they plan to tackle in future experiments. The sense of smell in insects is quite remarkable in its variety, and it is certainly possible that other types of mosquitoes such as Aedes mosquitoes, which can transmit Zika or dengue, might actually be able to detect DEET. A key question to address would be if this detection is linked to repulsion, or if its perceived as just another odour by the mosquito, Potter says. The researchers say they also plan to study the specific chemical receptors in the brain responsible for detecting natural odours like lemongrass. Anopheles mosquitoes are the most prevalent carrier of the malaria-causing parasite Plasmodium, which spreads from person to person through infected bites. Malaria killed an estimated 435,000 people in 2017, according to the World Health Organization (WHO). Additional researchers are from Durham University, Virginia Polytechnic Institute and State University, and Johns Hopkins. The Department of Defence, the National Institute of Allergy and Infectious Diseases, and Johns Hopkins funded the work. The authors declare no competing interests.
Futurity, 21 October 2019