Study: Mosquitoes ignore insect repellents

Johns Hopkins Medicine scientists have uncovered the process that makes mosquitoes able to ignore common insect repellents.

The findings of the research were published in the journal 'Cell Reports'.

Unlike fruit flies, mosquitoes' odor sensing nerve cells shut down when those cells are forced to produce odor-related proteins or receptors, on the surface of the cell.

In contrast, when odor sensors in fruit flies are forced to express odor receptors, it prompts flight from some smelly situations.

The findings reveal the variation in insect olfactory systems and add to the growing body of research aimed at improving methods to repel mosquitoes from human skin.

Mosquito bites not only create irritating swelling and itching but, worldwide, they play a role in spreading rampant and often lethal diseases such as malaria and dengue fever, as well as Zika virus infections.

"When experiments don't go as predicted, there's often something new to be discovered," said Christopher Potter, PhD, associate professor of neuroscience at the Johns Hopkins University School of Medicine, describing the new study. It turns out, he said, that, "Mosquitoes are so much trickier than we thought."

Potter and former postdoctoral fellow Sarah Maguire, PhD, designed their research project suspecting they'd find that mosquitoes have the same reaction as fruit flies when their new odor sensors are forced to be expressed.

Other research showed that when odor receptors in fly olfactory neurons are abnormally expressed, a new signal, based on the expressed odor receptor, is delivered to the brain, and the bugs move away from an offending odor.

The researchers then tested this same scenario on female Anopheles mosquitoes, whose bite transmits parasites that cause malaria in humans. The idea was that if researchers could push mosquito odor neurons into a similar expression state, triggered by odorants already on the skin, the mosquitoes would avoid the scent and fly off.

In the mosquito experiments, the researchers used mosquitoes genetically modified to overexpress an odor receptor called AgOR2, which responds to animal odorants found on humans.

By measuring the neuron activity generated by mosquitoes' odor receptors, the scientists found that the mosquitoes with overexpressed AgOR2 receptors had very little response to common animal scents, benzaldehyde, and indole, as well as chemical odorants in general.

"AgOR2 overexpression threw a wrench in the whole system by inactivating olfactory receptors in these mosquitoes," said Potter.

Finally, in the current study, the researchers tested how mosquitoes modified to overexpress AgOR2 responded to odorants in common insect repellents, such as lemongrass. They found that the genetically modified mosquitoes were able to ignore insect repellents.

The researchers suspect that the odor receptor shutdown may be a kind of failsafe in mosquitoes, ensuring that only one type of odorant receptor is expressed at a single time.

Since Anopheles mosquito olfactory systems continue to develop into adulthood, about eight days after hatching, the researchers speculate that the insects' olfactory neurons might be susceptible to which olfactory receptors to express, based on their surrounding environment. This type of flexibility in a mosquito's olfactory neurons may allow the mosquito to adapt to its odor environment. The researchers are conducting experiments to test this theory.

Potter hopes that the current findings may advance the search for methods that can trick the mosquito olfactory system into no longer preferring the smell of humans.