Many diseases spread to people by mosquitoes, including the Zika virus, West Nile virus, dengue, and malaria, are a great threat to humans, resulting in about 700,000 deaths yearly. As a countermeasure, many mosquito repellents have been used by humans. One of which has been used worldwide is a chrysanthemum flower head extract named pyrethrum. The exact molecular basis of pyrethrum repellency remains unknown, but a recent study performed by a Duke University neurotoxicologist, Ke Dong, and her colleagues, presents a deeper understanding of pyrethrum repellency against mosquitoes.
“We’re very excited because we are finally beginning to understand how a popular natural insect repellent, used worldwide, keeps mosquitoes from biting people”, said Dong.
Dong and her team attached electrodes to the hairs on mosquitoes’ antennae and measured how the insects’ individual odourant receptors in nerve cells respond to repellents. Many disease-carrying mosquito species contain around 100 or more such receptors. However, it was discovered that the pyrethrum activates only one receptor, called Odourant receptor 31, or Or31. Genetically engineered mosquito species lacking the Or31 receptor had no response to the pyrethrum substance.
Dong said that, unlike many other odourant receptors, Or31 appears in all known disease-carrying mosquito species. She further explained other natural repellents, which involve the activation of several receptors, are not well understood. Therefore, the Or31 receptor could act as a clear, universal target for developing more effective repellents.
Through chemical analysis, the research team identified two main components of pyrethrum, (E)-β-farnesene (EBF) and pyrethrin that work synergistically to activate the Or31 neurons of Aedes aegypti mosquitoes. Experiments demonstrated that pyrethrin induces the activation of voltage-gated sodium ion channels, enhancing the repellency by intensifying nerve signalling.
Dong is planning to investigate the neural circuits behind the repellency induced by pyrethrum and other similar naturally occurring substances. They will also focus on testing other potential repellent molecules, including the main component in citronella oil, which also activates Or31. Christopher Potter, a neuroscientist at Johns Hopkins University, commented that the findings could ultimately result in “super mosquito repellents”. Untangling the specific neurons that determine mosquitoes’ responses to certain odours could lead to ways to manipulate their behaviour. “Perhaps one day, we could identify how to turn this dial-up even further or how to trick mosquitoes into becoming repelled by other odours — such as those that normally attract them to humans”.
The revelation of this dual-target mechanism presents the potential in developing a new generation of repellents against major mosquito vectors of infectious diseases.
Edited By: Park Jihye and Park, Changmo
References Dong, K., Wang, Q., Xu, P., et al. (2021, May 5). A dual-target molecular mechanism of pyrethrum repellency against mosquitoes. National Library of Medicine. Retrieved October 11, 2021, from https://pubmed.ncbi.nlm.nih.gov/33953207/ Nuwer, R. (2021, August 1). Natural Mosquito Repellent’s Powers Finally Decoded. Scientific American. Retrieved October 11, 2021, from https://www.scientificamerican.com/article/natural-mosquito-repellents-powers-finally-decoded/#