They are responsible for more than 700,000 deaths around the world every year.
But scientists are now warning that mosquitoes could become even more dangerous if they discover mutated insects that are resistant to insecticides.
For the first time, malaria mosquitoes in Tanzania have been 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 jeopardize the fight against malaria.
Lead author Joel Odero, a PhD candidate at the University of Glasgow, said: ‘The emergence of new resistance mechanisms could threaten decades of progress made in reducing malaria transmission and mortality.’
Scientists have warned that mutated Anopheles funestus mosquitoes (pictured) could jeopardize efforts to combat malaria as they develop resistance to insecticides (file photo)
Malaria kills more than 600,000 people every year, mainly in Africa. In Tanzania, the disease is controlled by the use of mosquito nets, but chemical treatments to kill mosquitoes are still widely used. In the photo: a doctor tests a child for malaria in a hospital in Arusha, Tanzania
Mosquitoes of the species Anopheles funestus are the most important vectors of malaria in East and Southern Africa.
Although the use of mosquito nets is believed to prevent 633 million cases of the disease every year, chemical treatments still play a crucial role in controlling this insect population and preventing the spread of malaria.
However, when researchers sampled mosquitoes from ten regions in Tanzania, they found that some populations had developed worrying resistance to insecticides.
A genetic mutation called ‘L976F’ gave some mosquitoes ‘knock-down resistance’ to the pesticide DDT.
In particular, mosquitoes collected in the Morogoro region in the east of the country died only 68 percent of the time after exposure to DDT – compared to almost 100 percent for other mosquitoes.
By genetically sequencing mosquitoes from the region between 2017 and 2023, the researchers found that 90 percent of Morogoro mosquitoes once harbored the genes for resistance.
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 resistance to the now banned insecticide DDT
Not all mosquitoes spread malaria and some also spread other diseases. In East and Southern Africa the disease is mainly spread by the Anopheles funestus mosquito (photo right)
Mr Odero said: ‘Our discovery raises concerns about 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 every year, especially in Africa.’
Like antibiotic resistance in bacteria, insecticide resistance develops quickly when a new chemical treatment is used to target an insect species.
The new pesticide creates strong ‘evolutionary pressure’, meaning only insects with the resistance mutation can survive.
Mosquitoes from the Morogo region (pink) died only 68 percent of the time when exposed to DDT, compared to almost 100 percent for mosquitoes in other regions
This pressure causes the mutation to spread quickly through the population and makes treatment ineffective.
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 serious health effects, DDT was used around the world to reduce mosquito and other insect populations.
Between 1946 and 1962, an astonishing 1.34 billion tons of the chemical were sprayed across the United States.
However, it was later found that DDT exposure was linked to breast and other cancers, male infertility, miscarriages and low birth weight, developmental delay and damage to the nervous system.
In response to a rise in cases of West Nile virus in the US, four cities have started spraying pesticides to reduce mosquito populations. If mosquitoes begin to develop resistance to insecticides, this type of intervention may become less effective
These discoveries led many countries to ban the use of the pesticide starting in the 1970s.
Since DDT is no longer used, this should mean that there is no evolutionary pressure to drive insecticide resistance among Tanzanian mosquitoes.
However, until 2012, Tanzania still had a stockpile of 1,500 tons of outdated pesticides, including hundreds of tons of DDT.
The researchers realized that a 30-ton stockpile of the dangerous pesticide was just 50 kilometers from where the mutated mosquitoes were found.
In their paper, published in Molecular Ecology, they argue that historical exposure to DDT in the environment was sufficient to trigger the spread of the mutation.
The researchers found that there were large supplies of DDT near each of the points where knock-down resistance was discovered (red dots). This map shows Tanzania’s historical DDT stocks as green circles
Co-author Dr Francesco Baldini, from the University of Glasgow, said: ‘Our discovery sheds light on the far-reaching and unintended consequences of the historic use of insecticides, highlighting how past environmental pollution can influence the evolution of vector populations and the current can affect public health. interventions.’
Furthermore, although the numbers of the L976F gene peaked very high, the researchers found that the gene was almost gone by 2023.
The researchers attribute this decline to the Tanzanian government’s successful campaign to clear the last remaining DDT stockpiles.
Co-author Professor Fredros Okumu, from the University of Glasgow and the Ifakara Health Institute, is now calling for ‘urgent’ research into whether this kind of resistance to other pesticides could emerge.