Aedes mozzies can't be wiped out, say experts

Evolution has made the insect highly adaptable to the urban environment; it is also able to breed very fast

National Environment Agency officers pouring oil into drains. Most of the Aedes aegypti's breeding habitats, such as gutters and crevices, are small and difficult to locate, making it hard to eradicate the mosquitoes.
National Environment Agency officers pouring oil into drains. Most of the Aedes aegypti's breeding habitats, such as gutters and crevices, are small and difficult to locate, making it hard to eradicate the mosquitoes. ST PHOTO: LIM SIN THAI

With new Zika infection cases in Singapore emerging every day, the public and experts are debating whether Aedes aegypti - the mosquito responsible for the spread of dengue, Zika, chikungunya and yellow fever - can be wiped out for good.

The answer is no, say scientists, and there are a number of reasons that have allowed the insect to thrive and evolve to become the world's deadliest animal.

For one thing, evolution has made the insect highly adaptable to the urban environment, enabling it to breed in any small accumulation of stagnant water, explained Assistant Professor Roman Carrasco from the National University of Singapore's biological sciences department. Most of its breeding habitats, such as gutters and crevices, are small and difficult to locate.

  • History of failed campaigns

  • Decades of research and millions of dollars spent on eradication efforts have failed to eradicate nature's deadliest animal.

    In 1947, the Pan American Sanitary Bureau, now the Pan American Health Organisation, embarked on an ambitious campaign of gargantuan proportions - to eradicate the Aedes aegypti mosquito across the Americas and put a stop to yellow fever.

    Extensive eradication efforts saw pools of stagnant water removed and insecticide used liberally across the Western hemisphere. After 15 years, the bureau seemed to have a textbook success story: it eradicated the Aedes mosquito in 18 Latin American countries and drastically reduce the spread of dengue and yellow fever. The celebrations, however, were short-lived.

    The mosquitoes returned in full force, and in Cuba in 1981 they brought back with them the first epidemic in the region of the more severe form of dengue - dengue haemorrhagic fever.

    Rachel Oh

"It is able to breed very fast, with females laying 100 to 200 eggs per batch and five batches in a lifetime," explained Prof Carrasco.

As it cannot do without human blood, it makes sense for the mosquito to live around people.

The Aedes aegypti has evolved to feed exclusively on humans, said Professor Ooi Eng Eong, deputy director of the Emerging Infectious Diseases Programme at the Duke- NUS Medical School.

According to Dr Hwang Wei Song of the Lee Kong Chian Natural History Museum, the mosquito's natural ability to adapt to changes in the environment is what makes it such a difficult target.

Total eradication of mosquitoes in Singapore is not a feasible course of action, he pointed out. "We simply don't have the tools now to eradicate all mosquitoes without seriously damaging the natural and human environment," he said.

Prof Ooi also said eradicating Aedes aegypti is not the "best path to go down" under present circumstances as it is not sustainable and will be very costly.

"When you say you would need to spend millions to combat dengue and Zika, no one would bat an eyelid when there are lots of cases.

"But after eradication, you still will need to spend millions to sustain the mosquito control programme to prevent reintroduction and re-emergence of dengue and other Aedes-borne diseases... We have only finite resources," he said.

Minister for the Environment and Water Resources Masagos Zulkifli pointed out in Parliament on Tuesday that while fogging is helpful in killing adult mosquitoes with the dengue or Zika virus, it "would not be wise to conduct fogging indiscriminately" outside clusters.

"I know everyone likes fogging because it's very optical - everyone can see it and everyone feels better. But it does not solve the problem," he said.

The news is not all bleak though.

Trials are already ongoing to evaluate the potential of genetically modified mosquitoes and Aedes aegypti mosquitoes infected with Wolbachia bacteria to control the Aedes aegypti population.

Wolbachia is a bacterium which can be found in many insects, and when Wolbachia-carrying male mosquitoes mate with wild females, the females produce eggs which do not hatch.

The World Health Organisation (WHO) said these new methods could play a "significant role" in long-term dengue prevention.

After a six-month study of the technique here, it could be rolled out to high-risk areas by 2020. Further down the road, it may be possible to selectively remove the Aedes aegypti mosquito from the urban ecosystem at a low cost and without damaging the environment.

Prof Carrasco highlighted the Sterile Insect Technique as a species-specific method of biological insect control, whereby large amounts of sterile insects are released into the environment.

"It is used a lot against agricultural pests with remarkable large area eradication success histories," he said. "It is certainly more affordable than current control methods."

The technique, however, has yet to be fine-tuned to combat the Aedes aegypti mosquito population.

"When this happens, we will have our most powerful mosquito control technique ever ready," said Prof Carrasco.

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A version of this article appeared in the print edition of The Straits Times on September 15, 2016, with the headline Aedes mozzies can't be wiped out, say experts. Subscribe