Turns out that fighting mosquitoes with mosquitoes really works

The Temples of the Egyptians mosquito is not just a nuisance – it is a known carrier of dengue, yellow fever, chikungunya and zika viruses. The species is distinguished by the black and white stripes on its legs and is one of the most dangerous to humans.

In the Brazilian city of Indaiatuba, efforts are being made to eliminate these pests before they have a chance to spread disease. The weapon: more Temples of the Egyptians mosquitoes – but genetically engineered to kill their own kind. Made by British biotechnology company Oxitec, the mosquitoes seem to work.

The modified mosquitoes carry a synthetic self-limiting gene that prevents female offspring from surviving. This is important, because only the females bite and transmit disease. In a new studythe company’s scientists showed that their engineered insects affected the local population of Temples of the Egyptians up to 96 percent over 11 months in the neighborhoods where they were released.

“This is an area with a high degree of Temples of the Egyptians, and they have periodic dengue outbreaks,” said Nathan Rose, head of malaria programs at Oxitec. The Brazilian Ministry of Health even reported this this summer dengue fever continued to spread in all five regions of the country. Between January 1 and May 31, Brazil had more than 1.1 million cases — an increase of 198 percent compared to the same period in 2021. In those five months, the disease, which causes high fevers, rashes and muscle and joint pain, killed 504 people.

For the study, which was conducted in 2018 and 2019, the company chose four densely populated neighborhoods with many Temples of the Egyptians. In two, scientists released a “dose” of 100 male mosquitoes per resident per week. In the others, they increased that to 500.

The modified males mate with wild females, but the self-limiting gene prevents female offspring from surviving. This gene, which was developed in the laboratory but is based on elements found in E coli and the herpes simplex virus causes the cells of the female offspring to produce a lot of a protein called tTAV. This disrupts their cells’ ability to produce other essential proteins needed for development. As a result, the females die before reaching maturity and begin to bite. Male offspring survive and carry a copy of the self-limiting gene that they can then pass on.

To determine how effective these self-limiting male mosquitoes are, scientists need to poll the local mosquito population before and after the experiment. They lure, trap and count the number of adult mosquitoes in an area, or they set traps filled with water and then count the eggs that the females lay in them. Then they extrapolate to get a population estimate. (The Oxitec team used the egg method.)

This study found that during the mosquito peak season, which lasts from November to April in Brazil, treated mosquito populations were suppressed by an average of 88 percent, and in some cases up to 96 percent, compared to those in an untreated neighborhood that acted as a control.

Photo: Alexandre Carvalho/Oxitec

Interestingly enough, the dose of the mosquitoes didn’t seem to make a difference in how effective the method was. “There are a limited number of female mosquitoes in the environment, and the most important thing is that you maximize their chance of encountering one of these released ‘friendly’ male mosquitoes, as we call them,” says Rose. “We think that as long as you have more of these friendly male mosquitoes in the environment than the wild males, the chances are much higher that the female will find one of the Oxitec male mosquitoes.” In fact, Rose thinks it will be possible to even release fewer mosquitoes for a similar effect.

Like other countries, Brazil is widely spraying insecticides to control problematic mosquitoes. Temples of the Egyptians was once eradicated from much of South America following widespread use of the toxin DDT in the 1950s. But once the harmful health and environmental effects of the chemical came to light, the spraying was stopped and the mosquito quickly recovered. Nowadays, pyrethroids are often used for mosquito control, but the number of mosquitoes is increasing acquire resistance to them.

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