The Aedes aegypti the mosquito is not just a nuisance – it is a known vector of dengue fever, yellow fever, chikungunya and Zika virus. Distinguished by the black and white stripes on its legs, the species is one of the most dangerous to humans.
In the Brazilian city of Indaiatuba, an effort is underway to eradicate these pests before they have a chance to spread the disease. The weapon: more Aedes aegypti mosquitoes, but genetically modified to kill their own kind. Manufactured by British biotechnology company Oxitec, the mosquitoes seem to work.
Modified mosquitoes carry a synthetic self-limiting gene which prevents female offspring from surviving. This is important, as only females bite and transmit disease. In a new studycompany scientists have shown that their artificial insects are able to reduce the local population of Aedes aegypti up to 96% in 11 months in the neighborhoods where they were released.
“This is an area with high levels of Aedes aegypti, and they periodically have dengue fever outbreaks,” says Nathan Rose, manager of malaria programs at Oxitec. In fact, this summer the Brazilian Ministry of Health reported that dengue continues to spread in the five regions of the country. Between January 1 and May 31, Brazil recorded more than 1.1 million cases, an increase of 198% compared to the same period in 2021. During these five months, the disease, which causes a high fever, 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 high levels of Aedes aegypti. In two, the scientists released a “dose” of 100 male mosquitoes per inhabitant per week. In the others, they increased that up to 500.
The modified males mate with wild females, but the self-limiting gene prevents female offspring from surviving. This gene, which is designed in the laboratory but based on elements found in E.coli and the herpes simplex virus, causes female offspring cells to produce a lot of a protein called tTAV. This interferes with the ability of their cells to produce other essential proteins needed for development. As a result, females die before they mature and start biting. Male offspring survive, carrying a copy of the self-limiting gene which they can then pass on.
To determine how effective these self-limiting male mosquitoes are, scientists must assess the local mosquito population before and after the experiment. They either attract, 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 there. Then they extrapolate to get a population estimate. (The Oxitec team used the egg method.)
This study found that during the peak mosquito season, which lasts from November to April in Brazil, populations of treated mosquitoes were suppressed by an average of 88%, and in some cases up to 96%, compared to those of an untreated quarter that acted as a control.
Interestingly, the mosquito dose did not seem to make a difference in the effectiveness of the method. “There are a limited number of female mosquitoes in the environment, and the important thing is that you maximize their chances of encountering one of those released ‘friendly’ male mosquitoes, as we call them,” says Rose. “We believe that as long as you have more of these friendly male mosquitoes in the environment than wild males, there is a much higher chance that the female will find one of the male Oxitec mosquitoes.” In fact, Rose thinks it will be possible to go out even less of mosquitoes for a similar effect.
Like other countries, Brazil carries out large-scale insecticide spraying to control problematic mosquitoes. Aedes aegypti was eradicated in much of South America after widespread use of the toxin DDT in the 1950s. But once the chemical’s harmful health and environmental effects were discovered, spraying was stopped, and the mosquito quickly rebounded. Today, pyrethroids are commonly used to control mosquitoes, but mosquitoes are increasingly gain them resistance.