Mosquitoes transmitting dengue fever and other sometimes severe viral diseases have developed high resistance to insecticides in parts of Asia, and new methods to stem their spread are urgently needed, according to a Japanese study recently published in
Science Advance
.
The spraying of insecticides in mosquito-infested areas is a common practice in tropical and subtropical regions.
Resistance was already a concern, but the extent of the problem was not precisely known until now.
Resistance due to genetic mutations
Japanese scientist Shinji Kasai and his team have studied mosquitoes from several Asian countries and Ghana, and found genetic mutations that make some of them immune to widely used insecticides like permethrin.
"
In Cambodia, more than 90% of Aedes aegypti mosquitoes - the main vector of dengue,
Zika
,
chikungunya
and
yellow fever viruses
- have a combination of mutations resulting in an extremely high level of resistance
", according to Mr. Kasaï.
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This director of the department of medical entomology at the Japanese National Institute of Infectious Diseases discovered that certain types of mosquitoes that were supposed to be 100% eliminated by insecticides were now only 7%.
And even a toxic dose ten times higher only killed 30%.
Additionally, resistance levels vary by region.
They “
differ completely between Cambodia and Vietnam, for example
,” said Mr. Kasai.
This study also revealed that in Ghana, parts of Indonesia and Taiwan, existing insecticides were still working for the time being.
Resistance to insecticides has also been observed in the tiger mosquito (
Aedes albopictus)
, but at lower levels.
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New eradication methods needed
This study published at the end of December by the journal
Science Advances
shows that “
commonly employed strategies may no longer be effective in controlling populations of harmful mosquitoes
”, explained Professor Cameron Webb, an expert from the University of Sydney.
According to the professor, new chemicals are needed, but authorities and scientists must also think about new methods of protection, such as vaccines.
Only a few dengue vaccines are currently available - that of the Japanese pharmaceutical group Takeda was approved last year by Indonesia and then the European Union, while the use of the French vaccine, developed by Sanofi, is very limited because it can make the disease worse in people who have never had this virus before.
Dengue fever cases have increased dramatically worldwide over the past 20 years, with between 100 and 400 million infections per year, according to the World Health Organization (WHO).
More than 80% of cases are mild or asymptomatic, but life-threatening complications exist.
Sterilize male mosquitoes
Shinji Kasai fears that the super-resistant mosquitoes identified could spread elsewhere in the world "
in the near future
".
To limit the spread, he recommends in particular to vary the insecticides more, but the problem is that their modes of action are often similar.
Alternatives are to step up efforts to eliminate mosquito breeding grounds or to sterilize male mosquitoes using Wolbachia bacteria, an innovative method that has already yielded encouraging results locally.
Exactly where and when insecticide resistance mutations in mosquitoes arose remains a mystery.
The next step for Dr. Kasai and his team is to expand their research to other parts of Asia and examine more recent samples from Cambodia and Vietnam to determine potential changes from the results of this study which focused on the period 2016-2019.
