US scientists say human blood tastes a 'little salty and a little sweet' to mozzies.
Experts identified mosquito neurons that sense our blood's 'delectable' flavour.
New drugs could potentially alter these neurons and mosquito's taste for blood.
Mosquitoes can spread fatal diseases such as malaria, dengue and yellow fever.
Human blood tastes a bit like salted caramel to hungry mosquitoes, according to a new study.
US researchers have described our blood as being 'a little salty and a little sweet' just like the hugely popular confectionery.
The team identified the individual neurons in a mozzie that sense our 'distinctive and delectable' flavour.
These neurons are triggered and send electrical impulses when the mosquito slurps blood, which transmits information about what we taste like.
The new study could help develop oral drugs that could mask our blood's flavour and stop us from being bitten, they claim.
Mosquitoes spread diseases like malaria, dengue, and yellow fever that kill at least half a million people each year via their bite.
While male mosquitoes only eat flower nectar, female mosquitoes eat both flower nectar and blood, as they need the protein in blood to develop eggs.
Female mosquitoes therefore need to distinguish between the sweet nectar they eat for most of their meals and the blood they gorge on before laying their eggs.
Females have a sense of taste that is specially tuned to detect a combination of at least four different substances in blood – including one that is 'incredibly exciting and rewarding' for the mosquito but tastes like absolutely nothing to humans.
This shows that female mosquitoes can 'taste things we can't', according to the researchers.
A neuron, also known as nerve cell, is an electrically excitable cell that takes up, processes and transmits information through electrical and chemical signals.
It is one of the basic elements of the nervous system.
In order that a human being can react to his environment, neurons transport stimuli.
The stimulation, for example the burning of the finger at a candle flame, is transported by the ascending neurons to the central nervous system and in return, the descending neurons stimulate the arm in order to remove the finger from the candle.
the diameter of a neuron is about the tenth size of the diameter of a human hair.
'There is nothing like this in the human experience,' said Dr Leslie B Vosshall at The Rockefeller University in New York.
'We think the taste of blood in mosquitoes is a totally unique experience, not accessible or familiar to humans – like the ability of honeybees to see ultraviolet and bats to hear ultrasonic sounds.'
In previous work, Dr Vosshall and her team found that mosquitoes can detect the repellent DEET with their legs and have identified an odorant receptor that mosquitoes use to distinguish between humans and non-humans.
But little is known about the insect's sense of taste, despite it being key to spreading illness to humans.
Mosquitoes have neurons in their brains, just as humans do.
'Somewhere in that female brain is the drive to sense humans, fly toward humans, land on humans, and bite and drink the blood of humans, said Dr Vosshall.
'Somewhere in that brain is where decision making, motivation and hunger reside.'
Dr Vosshall's lab previously built mosquitobrains.org, an online resource that provides the first map of the female mosquito brain, based on precise imaging technology.
With its 'mosquito brain browser', website visitors can select and view distinct regions of the brain belonging to the female aedes aegypti species of mosquito, which is also known as the yellow fever mosquito.
For this study, the team again focused on aedes aegypti, which is a huge threat to human health and transmits several different viruses, including the dengue virus.
Jové suspected that female aedes aegypti mosquitoes, unlike males, would be able to distinguish between the two substances – nectar and blood – by taste.
'If mosquitoes weren't able to detect the taste of blood, in theory they couldn't transmit disease,' said Veronica Jové at Rockefeller who led the work in Vosshall's laboratory.
In behavioural experiments, she found female mosquitoes have two feeding modes that use different mouthparts and detect different flavours.
A nectar-feeding mode detects sugars, and a blood-feeding mode uses a syringe-like 'stylet' to pierce the skin and taste blood.
Jové tricked the mosquitoes into the blood-feeding mode by offering them a mix of four compounds – glucose (a sugar), sodium chloride (salt), sodium bicarbonate (found in both blood and baking soda) and adenosine triphosphate (ATP), a compound that provides energy to cells.
There are four kinds of neurons that together let the mosquito taste blood:
1. Sodium chloride cell, a cell that likes the taste of salt
2. Sodium bicarbonate cell, a cell that likes the sour/fizzy taste of bicarbonate
3. ATP cell, a cell that likes the taste of ATP (which has no taste to humans)
4. The Integrator Cell – which only likes the taste of glucose (sweet) if both sodium chloride and sodium bicarbonate are present.
The ATP solution used in the lab 'doesn't have a taste at all', said Dr Vosshall, who tried it herself.
'ATP is this special mystery stuff that tastes like nothing to humans, but it's got to be incredibly exciting and rewarding for the mosquito,' she said.
Just as a human has taste buds that differentiate between salty, sweet, bitter, sour, and umami flavors, a mosquito's stylet has neurons specialised to respond to particular flavors.
Specifically, four different types of neurons need to be present to detect the four different substances in blood simultaneously – and help the mozzie distinguish it from nectar, which is merely sweet.
'Some of components of blood probably taste the same to humans and mosquitoes – the saltiness of sodium chloride, the sourness or fizziness of sodium bicarbonate, the sweetness of glucose,' said Dr Vosshall.
'But ATP is tasteless to humans and as mentioned, the whole experience is definitely different.'
To see these taste neurons in action, the researchers genetically modified mosquitoes with a fluorescent tag that glowed when a nerve cell was activated.
They then watched which cells in the stylet lit up in response to different meals.
Only a subset were activated by blood, including both real blood and the researchers' artificial mix.
'About half of the nerve cells in the syringe-like stylet are activated by blood – the other mysterious half were not activated by anything we offered them,' Dr Vosshall told MailOnline.
'We think the other half may taste bitter like insect repellents or other components of blood that we did not test in our study.'
The findings show just how specially adapted the female mosquito is to find blood in its quest to gather protein before reproducing.
Hopefully a better understanding of mosquito senses will lead to new ways to stop them from biting us and spreading disease.
For example, humans could take an oral drug before going to a mosquito-infested area.
The drug, which Vosshall likened to monthly flea and tick medication for dogs, could potentially interfere with a mosquito's taste for blood.
The study has been published in the journal Neuron.
The Aedes aegypti mosquito is the main vector that transmits the viruses that cause dengue.
The viruses are passed on to humans through the bites of an infective female Aedes mosquito, which mainly acquires the virus while feeding on the blood of an infected person.
Within the mosquito, the virus infects the mosquito mid-gut and subsequently spreads to the salivary glands over a period of 8-12 days.
After this incubation period, the virus can be transmitted to humans during subsequent probing or feeding.
The immature stages are found in water-filled habitats, mostly in artificial containers closely associated with human dwellings and often indoors.
Flight range studies suggest that most female A. aegypti may spend their lifetime in or around the houses where they emerge as adults and they usually fly an average of 1,300 feet (400 metres).
This means that people, rather than mosquitoes, rapidly move the virus within and between communities and places.
A. aegypti breed indoors and are capable of biting anyone throughout the day.
The indoor habitat is less susceptible to climatic variations and increases the mosquitoes’ longevity.
In September 2020, scientists reported that climate change could trigger the spread of potentially fatal mosquito-borne diseases around Africa and beyond.
Disease-carrying mosquito species, such as the aedes aegypti, are attracted to the excess heat caused by climate change.
They warn of 'public health disaster' if Africa fails to focus on strategies tailored to mosquito-borne diseases other than malaria.
They highlight that different species of the flying pest thrive at various temperature ranges and transmit different diseases.
Source: World Health Organisation