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2 months ago
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They are responsible for more than 700,000 deaths around the world every year.
But scientists now warn that mosquitoes could become even more dangerous as they discover mutant bugs with a resistance to insecticide.
For the first time, malaria-carrying mosquitoes in Tanzania have genetically mutated to survive the long-banned pesticide DDT.
Researchers from the University of Glasgow and the Ifakara Health Institute in Tanzania now warn that this development could jeopardise the fight against malaria.
Lead author Joel Odero, a PhD student at the University of Glasgow, says: 'The emergence of new resistance mechanisms could threaten decades of progress made in reducing malaria transmission and mortality.'
Mosquitoes from the species anopheles funestus are the main carriers of malaria in Eastern and Southern Africa.
Scientists have warned that mutant anopheles funestus mosquitoes (pictured) could jeopardise efforts to combat malaria as they develop a resistance to insecticides (file photo)
Malaria kills over 600,000 people each year, mainly in Africa. In Tanzania, the disease is controlled by the use of mosquito nets but chemical treatments to kill mosquitoes are still used widely. Pictured: A doctor tests a child for malaria at a hospital in Arusha, Tanzania
While the use of mosquito nets is believed to avoid 633 million cases of the disease each year, chemical treatments still play a critical role in controlling this insects population and preventing the spread of malaria.
However, when researchers sampled mosquitoes from 10 regions across Tanzania they discovered that some populations had developed a worrying insecticide resistance.
Due to a genetic mutation called 'L976F' some mosquitoes gained 'knock-down resistance' to the pesticide DDT.
In particular, mosquitoes collected from the Morogoro region in the East of the country only died 68 per cent of the time after exposure to DDT - compared to almost 100 per cent for other mosquitoes.
By genetically sequencing mosquitoes from the region between 2017 and 2023, the researchers found that 90 per cent of Morogoro mosquitoes had the genes for resistance at one time.
Worryingly, this is the first time that malaria-carrying anopheles funestus mosquitoes have developed knock-down resistance to any chemical treatment.
Scientists discovered that mosquitoes in the Morogo region (pictured) had developed a resistance to the now-banned insecticide DDT
Not all mosquitoes are malaria spreading and some also spread other diseases. In Eastern and Southern Africa, the disease is mainly spread by the anopheles funestus mosquito (pictured right)
Mr Odero says: 'Our discovery raises concerns for the effectiveness of current malaria control methods, which rely heavily on insecticides.
Mr Odero says: 'Our discovery raises concerns for the effectiveness of current malaria control methods, which rely heavily on insecticides.
'Understanding the development of insecticide resistance is key to combating malaria, a disease that kills hundreds of thousands of people annually, mostly in Africa.'
Just like antibiotic resistance in bacteria, insecticide resistance emerges quickly when a new chemical treatment is used to target a species of insect.
The new pesticide creates a strong 'evolutionary pressure' meaning that only insects with the mutation for resistance can survive.
This pressure causes the mutation to spread quickly through the population and the treatment to become ineffective.
Mosquitoes from the Morogo region (pink) only died 68 per cent of the time when exposed to DDT, compared with nearly 100 per cent for mosquitoes in other regions
What makes this discovery so unusual is that the use of DDT has been banned in Tanzania since 2008.
Before research revealed the chemical's severe health impacts, DDT had been used around the world to reduce mosquito and other insect populations.
An astonishing 1.34 billion tonnes of the chemical was sprayed across the United States in the years between 1946 and 1962.
However, it later emerged that exposure to DDT was linked to breast and other cancers, male infertility, miscarriages and low birth weight, developmental delay and nervous system damage.
These discoveries led to many countries banning the pesticide's use from the 1970s onwards.
Since DDT is no longer being used, this should mean that there is no evolutionary pressure to drive up the rates of insecticide resistance among Tanzanian mosquitoes.
In response to a rise in cases of West Nile virus in the US, four cities began spraying pesticides to reduce the mosquito population. If mosquitoes begin to develop a resistance to insecticides, this kind of intervention may become less effective
However, up until 2012, Tanzania still had a stockpile of 1,500 tonnes of obsolete pesticides including hundreds of tonnes of DDT.
The researchers realised that a stockpile holding 30 tonnes of the dangerous pesticide was located just 50km (31 miles) from where the mutant mosquitoes were found.
In their paper, published in Molecular Ecology, they argue that historic exposure to DDT in the environment was enough to trigger the spread of the mutation.
The researchers found that there were large stockpiles of DDT near each of the points where knock-down resistance was discovered (red dots). This map shows Tanzania's historical DDT stockpiles as green circles
Co-author Dr Francesco Baldini, of the University of Glasgow, says: 'Our discovery sheds light on the far-reaching and unintended consequences of historical insecticide use, highlighting how past environmental contamination can shape the evolution of vector populations and impact current public health interventions.'
Additionally, while rates of the L976F gene reached a very high peak, the researchers found that the gene had almost vanished by 2023.
The researchers attribute this decline to the Tanzanian government's successful campaign to clean up the last remaining DDT stockpiles.
Co-author Professor Fredros Okumu, of the University of Glasgow and Ifakara Health Institute, is now calling for 'urgent' research into whether this type of resistance could emerge for other pesticides.
What is insecticide resistance?
When a new insecticide is first used it is very effective at killing the target insect.
However, rare genetic mutations sometimes create natural resistance to these chemicals.
When a population is treated with insecticides, only those insects with the mutations survive and go on to reproduce.
This leads to the mutation becoming common and the insect becoming resistant to the insecticide.
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.
