Like other insects, mosquitoes use their antennae (a pair of sense organs located on their head) for searching. The olfactory sensory (sense of smell) in their antennae is mainly responsible for locating humans. They also use their vision to spot a host and thermal sensory information to detect human body heat. Thus, mosquitoes combine the information through olfactory, visual and thermal systems/senses as clues to map out the path to getting blood from human targets. However, mosquitoes can also pick up other clues that signal that a human is nearby.

From recent research, it has been discovered that a certain olfactory receptor (sense of smell) in their antennae serves as a detector of humans, responding to smell chemicals in human sweat. Scientists have identified a unique olfactory receptor that is used to detect these odours in mosquitoes. The scent of human sweat is like a mouthwatering aroma to mosquitoes. Scientists, recently, have discovered how human sweat is used as delectable odour by mosquitoes. It is the odour from lactic acid and other acidic volatiles found in human sweat, and mosquitoes are attracted to them.

Scientists, even in the 1960s, knew that the sweat and lactic acid is used by mosquitoes for detecting humans, but no one knew how it actually functions. For more than 40 years, people had been looking for the answer to how it worked. Fortunately, this receptor has now been identified and is known to scientists as Ionotropic Receptor (IR8a).

A mosquito can sense exhaled carbon dioxide from a distance of more than 30 feet. By detecting carbon dioxide, it begins to trace human odour. The mosquito follows this odour and, when it gets very close, starts to detect body heat. Once mosquitoes land on the human body, they can actually taste the skin with their legs and then they look for a place to bite.

Scientists, in recent research, by creating the first-ever mutant mosquitos through Crispr/Cas9 technology (gene editing tools), were able to remove a gene and investigate how the absence of that olfactory gene (Ir8a) changed the mosquito’s behaviour. They genetically altered mosquitoes to block the activity of this specific olfactory receptor (Ir8a). The result showed that the blood feeding female mosquitoes were no longer attracted to lactic acid.

Accordingly, a variety of lab tests have been done to see if disrupting this receptor would make mosquitoes less responsive to humans. Scientists asked people to put their hands into a device called an “olfactometer” that let mosquitoes smell them from a distance. Captive mosquitoes could fly through the device to get close, but not close enough to bite. The tests showed that mosquitoes that were genetically altered to disable this olfactory receptor in them were significantly less likely to fly toward the human skin. The researchers also had people in the study wear nylon sleeves for about 12 hours to collect sweat. Then they put these sleeves into the olfactometer. Again, mutant mosquitoes were much less attracted to the scent than the normal mosquitoes.

Mosquitoes are known to transmit dangerous and sometimes deadly diseases to humans, including dengue, chikungunya, yellow fever and Zika. Scientists are finding ways to overstimulate parts of the mosquitoes’ human-detection system that might help to create a powerful repellent.

Blocking the IR8a pathway could be an important strategy in this repellent design. Removing the function of this receptor removes approximately 50 percent of the host-seeking activity of mosquitoes. The ultimate goal of the research is to create a life-saving perfume to protect humans from mosquito bites. So, we can hope that in the near future, there may be a kind of a perfume or something similar that we can spray to prevent mosquitoes from detecting our sweat and in the process, manage to keep them at bay.

Dr Md Asaduzzaman Miah is a visiting scientist, Anastasia Mosquito Control District, Florida, USA, and Associate Professor (Deputation), Entomology, PSTU, Bangladesh.Email: [email protected]

Source: The Daily Star.