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New ways of curbing mosquito population

Dr Derrick Aarons

Sunday, February 17, 2019

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Insect-borne illnesses are increasing across the globe, and the insects that carry them are also spreading and getting harder to kill. As a result, scientists have been looking for new, high-tech ways to prevent these insects from spreading those diseases. New technologies are needed because traditional ways of controlling mosquitoes have not been reducing the spread of disease lately.

In California, USA, researchers are trying to change the genes of mosquitoes to prevent them from carrying the parasites that cause malaria.

In Brazil and south-east Asia, scientists are working to infect mosquitoes with bacteria that may keep them from spreading dengue fever. Dengue is sometimes deadly virus which in recent times has been reaching beyond its typical boundaries within the tropics.

Crowding out the dengue virus

The World Mosquito Programme, a non-profit organisation, has found that the bacteria Wolbachia pipientis works by crowding out the dengue virus in the Aedes aegypti mosquito species that carry the disease. In addition, when Wolbachia-infected males mate with female mosquitoes, their eggs do not develop. Further, when males and females both carry Wolbachia, the bacteria are passed on through their eggs. Within three to four months, Wolbachia-carrying mosquitoes thus begin to crowd out the dengue-carrying mosquitoes.

This programme is reportedly being tested in Australia, Vietnam, and Indonesia, including a randomised, controlled clinical trial in the Indonesian city of Yogyakarta. In neighbourhoods where Wolbachia was introduced, reportedly more than 90 per cent of mosquitoes now carry the bacteria, and fewer cases of dengue are being reported.

The findings of this research were presented at the Centers for Disease Control and Prevention's International Conference on Emerging Infectious Diseases in Atlanta, USA, in August last year. Similar tests are reportedly being launched in Brazil and Colombia in South America.

Other initiatives

Dengue fever affects more than 50 million people each year, killing about 10,000 of them. Further, malaria killed about 445,000 of the world's population in 2016, according to the World Health Organization. Efforts to control the anopheles mosquito species that carries the malaria parasitic illness through pesticides and chemical-treated mosquito nets have been making good progress in recent years; however, there have also been increases in malaria cases in some countries, particularly in Africa, as mosquitoes have been becoming more resistant.

Consequently, there is now an anti-malaria initiative that is trying to splice certain genes into the genes of anopheles mosquitoes in an attempt to head off the malaria disease. These genes seek to prevent the mosquito from transmitting the disease to humans.

The strategy is to engineer the mosquitoes in such a way that they are incapable of becoming infected with malaria parasites and then introducing these mosquitoes into the natural population. The goal is that the modified mosquitoes carrying that gene would spread it to their offspring and thus replace the natural, parasite-prone insects.

CRISPR

This idea has been around since the 1990s, but with the recent development of the CRISPR-Cas9 gene editing technique, the matter has now been brought closer to reality. Mosquitoes that shut down the parasite have been produced in the laboratory, but it is likely to be some three years before any specimens are ready to be released into the environment, with the latter occurring under tightly regulated conditions.

The programme is currently considering an initial testing on a small island off the coastline of Africa, where the disease is common, then moving on to larger islands, and eventually on to the Africa mainland. Understandably, any testing would have to be developed in consultation with officials within the host country, the World Health Organization, and other UN agencies.

Dengue fever and malaria

In addition to dengue, the Aedes aegypti mosquito can also spread Zika, West Nile, and chikungunya viruses, and its range is spreading as climate change is producing a warmer climate and making it easier for the insect to survive in places that they were not able to do so before.

Mosquitoes are also getting more resistant to the pesticides that have been used to kill them over past decades. In fact, insecticide resistance in anopheles mosquitoes has been increasing in several countries which have the biggest problems with malaria.

Scientists over the world, therefore, ought to work together on this venture (that is, collaborative research), since the genes they are currently trying to implant into mosquitoes should not be carried from one species to another or affect any other trait, and tests of how the modified mosquitoes interact with other species should be done within cages before any insects are released into the wild.

Dr Derrick Aarons MD, PhD, is a consultant bioethicist and family physician; a specialist in ethical issues in health care, research, and the life sciences; the health registrar and head of the health secretariat for the Turks & Caicos Islands; and a member of UNESCO's International Bioethics Committee (IBC).


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