Science

Bloodthirsty mosquitoes can smell out their prey in more than one way, scientists find

Do you find you still get covered in bites during the summer even when you have covered yourself in mosquito repellent?

Scientists may have figured out why, as it the bloodthirsty pests have evolved the ability to sniff out human body odour in more ways than one.

Most animals smell with the ‘olfactory receptors’ in their nose or antennae, which each detect a single, unique scent particle.

The receptors are connected to ‘olfactory neurons’ that transmit information about that particular odour to the brain.

However, researchers from Rockefeller University in New York, USA, have discovered that the neurons in mosquito antennae are connected to multiple types of receptor. 

This means that their neurons are activated by more than one chemical produced by humans, so if one type of receptor is knocked out, they can still track us down.

Professor Leslie Vosshall, a senior author of the study, said: ‘You need to work harder to break mosquitoes because getting rid of a single receptor has no effect. 

‘Any future attempts to control mosquitoes by repellents or anything else has to take into account how unbreakable their attraction is to us.’

Female mosquitoes are able to track down humans by the carbon dioxide we exhale, and the chemicals in our body odour, including 1-octen-3-ol and amines (stock image)

Most animals smell with the ‘olfactory receptors’ in their nose (or ‘maxillary palp’) and antennae, that each detect a single, unique scent particle. The receptors are connected to ‘olfactory neurons’ that transmit information about that particular odour to the brain. Glomeruli are the connections between the olfactory neurons and the nerves in the brain

Most animals smell with the ‘olfactory receptors’ in their nose or antennae, that each detect a single, unique scent particle, and this was what was expected for mosquitoes

However, researchers from Rockefeller University in New York, USA, have discovered that the neurons in mosquito antennae are connected to multiple types of receptor. This means that their neurons are activated by more than one chemical produced by humans, so if one type of receptor is knocked out, they can still track us down

HOW DO MOSQUITOES SMELL HUMANS? 

Mosquitoes have smell receptors in their antennae that are connected to neurons which transmit scent information to their brain.

Each receptor detects a specific smell particle, which triggers the neuron it is linked to.

In most animals, each neuron has receptors that detect the same scent particle. 

However, the New York-based researchers have discovered that mosquito neurons have receptors that detect different particles.

This means losing one or more receptors does not affect the ability of mosquitoes to pick up on human smells. 

Lead author Professor Meg Younger, from Boston University, said: ‘This project really started unexpectedly when we were looking at how human odour was encoded in the mosquito brain.’

Mosquitoes are able to track down humans by the carbon dioxide we exhale, and the chemicals in our body odour, including 1-octen-3-ol and amines.

Professor Younger’s team initially used the gene-editing technology CRISPR on female mosquitoes, Aedes aegypti, to deactivate groups of human-odour receptors on their antennae.

They expected this would entirely prevent their olfactory neurons from firing up in response to the human scent.

However, when they measured neuronal activity as the mosquitoes were exposed to human odour, they found the insects could still detect the smell.

The researchers then used RNA sequencing to discover what was happening at a cellular level, and found that the neurons that are stimulated by 1-octen-3-ol are also stimulated by amines.

Therefore, the cocktail of chemicals in human odour was still managing to activate olfactory neurons through receptors that had not been deactivated.

‘This may be a general strategy for insects that depend heavily on their sense of smell,’ says Vosshall. 

It could also explain why insect repellents that work by blocking a specific scent receptor are not effective, as their neurons can still be triggered by their other receptor types.

The researchers used RNA sequencing to discover what was happening at a cellular level when mosquitoes detect human odour, and found that the neurons that are stimulated by 1-octen-3-ol are also stimulated by amines. Pictured: Mosquito antenna with fluorescently labeled olfactory neurons

The findings, published today in Cell , suggest that gene editing out their human-scent detectors is not the most effective way of preventing the spread of mosquito-borne disease, like malaria and yellow fever. Pictured: A revised model of scent detection in Ae. aegypti based on this study

The findings, published today in Cell , suggest that gene editing out their human-scent detectors is not the most effective way of preventing the spread of mosquito-borne disease, like malaria and yellow fever. Pictured: A revised model of scent detection in Ae. aegypti based on this study

This goes against all existing rules of how animals smell, suggesting mosquitoes evolved this ability as a fail-safe of sniffing out valuable human blood. 

The findings, published today in Cell, suggest that gene-editing out their human-scent detectors is not the most effective way of preventing the spread of mosquito-borne disease, like malaria and yellow fever.

Instead, the authors suggest we should focus on creating more potent traps and repellents that work with how the mosquitoes process human odour.

Future research will look more deeply at why the pests have developed multiple receptors on single olfactory neurons.

WHY DO MOSQUITOES BITE SOME PEOPLE AND NOT OTHERS?

Around 20 per cent of people are more prone to mosquito bites.

And while scientists are yet to find a cure, they do have some ideas as to why the insects attack some of us more than others.  

Blood type

Certain blood types are more attractive to taste buds of mosquitoes. 

Research has shown that people who have Type O blood – the most common blood type – tend to get bitten twice as much as those with Type A. People with Type B blood get bitten somewhere in the middle. 

Exercise and metabolism

Working up a sweat during exercise can also make a person more susceptible to a mosquitoes bite.

Strenuous exercise causes higher body temperatures and a buildup of lactic acid, which emit yummy signals to the insects.     

Beer    

A cold glass of beer makes you sweat and your body release ethanol, which may be why mosquitoes like to land on beer drinkers. 

Skin bacteria 

Levels of bacteria on the human skin can entice mosquitoes to bite, particularly where bacteria clusters like on the ankles and feet. 

Having different types of bacteria on the skin, however, tends to turn the insects off. 

Body odour 

Mosquitoes use even the faintest of human body odours when searching for potential victims.

It’s been known for some time that female mosquitoes use specific sensors around their mouths to detect carbon dioxide being exhaled from humans and animals.

But a few years ago, researchers from the University of California Riverside discovered the blood-sucking insects also use these same sensors to detect body odours – especially the smell of feet. 


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