Across the world, scientists are modifying mosquitoes to fight the spread of infectious diseases. When mosquitoes are bred to carry a bacterium called Wolbachia, their ability to reproduce and spread disease-causing viruses is inhibited. After successful trials in 14 countries, the release of Wolbachia-infected mosquitoes is projected to scale up in the coming years.

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Above: Mosquitoes lay eggs in a World Mosquito Program Factory in Medellín, Colombia. Image courtesy of AP News.

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Above: A female Aedes aegypti mosquito. Image courtesy of Wikipedia.

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How Do Mosquitoes Transmit Diseases?

The Aedes aegypti mosquito is the top transmitter of dengue fever, yellow fever, chikungunya, and Zika viruses. However, mosquitos do not naturally carry diseases. Instead, they contract them by biting infected people. After becoming infected, the mosquito spreads the virus to the next person it bites, leading to dengue outbreaks such as those seen in Brazil in 2019. Interestingly, male mosquitoes don’t bite because they rely on flower nectar, rather than blood, for nutrition. Only female mosquitoes feed on blood, so only they can spread diseases by biting!

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Above: 112 countries and territories have reported cases of dengue as of 2018. Image courtesy of The New York Times.

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The Dengue Crisis

Mosquito-borne diseases are most common in tropical and subtropical regions, but global warming has enabled the spread of these illnesses to previously unaffected areas. Consequences of climate change such as rising temperatures, precipitation, and humidity are broadening the regions and seasons in which mosquitoes thrive. As a result, mosquito-borne diseases are also spreading. With a 30-fold increase in just the last 50 years, dengue is now the world’s fastest-spreading tropical disease. Just over a year ago, Brazil experienced the most severe dengue outbreak in history with 1.3 million cases and almost 1,000 deaths. According to the World Health Organization, dengue is now surging towards pandemic status.

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Above: Dengue cases are on the rise in the Americas. Image courtesy of Nature.

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Top: Wolbachia bacterium. Image courtesy of StoryMD. Bottom: Mosquito larvae. Image courtesy of AP News.

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How Are Wolbachia Mosquitoes Bred?

Wolbachia is a common type of bacteria that is found in up to 60% of insects. However, it is not naturally present in A. aegypti. Scientists create Wolbachia-infected mosquitoes by injecting Wolbachia bacteria into eggs. When the eggs hatch, the mosquitoes contain the bacteria and will pass it on to their offspring. Researchers sort mosquito larvae into groups of males and females and raise them into adults. 

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Above: Mosquito eggs are injected with Wolbachia bacteria. Image courtesy of ScienceLine.

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Above: World Mosquito Program volunteers distribute packages containing adult mosquitoes and mosquito eggs. Image courtesy of the World Mosquito Program.

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Wolbachia Mosquito Field Trials

The World Mosquito Program (WMP)—a non-profit effort led by Monash University in Australia—has spearheaded global efforts to test the safety and efficacy of releasing Wolbachia mosquitoes into the environment. The organization conducted its first tests in 2011 with the release of mosquitoes into several small towns in northern Australia. Today, the project has expanded to 14 countries around the globe including Singapore, Colombia, and Brazil. Before distribution, adult mosquitoes are packaged into boxes for release into communities highly impacted by mosquito-borne diseases. WMP volunteers deliver these packages to neighborhoods either by foot, motor vehicle, or drone. In addition to adult mosquitoes, WMP gives locals dehydrated Wolbachia mosquito eggs. By adding these eggs to a cup of water along with the provided nutrition packet, residents can help grow disease-fighting mosquitoes in their own backyards. After three years of testing, researchers found that dengue infection rates were reduced by 77% and hospitalizations were down by 86% in areas with Wolbachia, compared to nearby control areas. 

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Above: Punnett square displaying mating outcomes for Wolbachia-infected and wild-type mosquitoes. Image courtesy of the University of Melbourne.

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How Do Modified Mosquitoes Fight Dengue?

Wolbachia helps prevent the spread of dengue in two key ways: by limiting the population of A. aegypti and by reducing viral reproduction within mosquitoes. The four mating combinations between unmodified wild-type (WT) and Wolbachia-infected mosquitoes yield three potential genotypic outcomes: WT offspring, Wolbachia-infected offspring, or no viable offspring. The release of Wolbachia males into the wild leads to decreased production of viable offspring and ultimately a decline in A. aegypti populations. When Wolbachia males and females find each other and mate, their offspring have a reduced probability of spreading viruses to humans. Although the exact mechanism by which Wolbachia prevents viruses from reproducing is not fully understood, evidence suggests that Wolbachia competes with the virus for a limited supply of subcellular fatty acids—a resource necessary for viral reproduction. Outcompeted by Wolbachia, the virus is less likely to spread to a new (human) host.

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Above: Cases of dengue (per 100,000) in Medellín, Colombia from 2010 to 2022. Image courtesy of The New York Times.

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Safety, Sustainability, and Efficacy

Wolbachia mosquitoes have shown immense promise as a safe, effective, and affordable method for curbing the spread of mosquito-borne diseases. With a population of three million people, the field trial in Medellín, Colombia is the WMP’s biggest project yet. Medellín was projected to have a peak year of dengue cases in 2021. Instead, the city experienced the lowest number of cases it had seen in the last 20 years, supporting the efficacy of Wolbachia mosquitoes. Unlike current methods such as spraying insecticides, the release of Wolbachia mosquitoes does not produce any observable harm to the environment or human health, making it a safer option. Additionally, after the initial release, Wolbachia mosquitoes will slowly replace wild populations and thus require no additional maintenance to sustain their presence in the environment. Although arboviruses may eventually evolve to resist the effects of Wolbachia—at which point scientists will have to identify a new method—Australian entomologist Dr. Scott O’Neill predicts this mutation won’t occur for thousands of years. In terms of costliness, the WMP insists that this process will “pay for itself” by reducing current costs of healthcare and insecticide maintenance. 

Above: Proportion of mosquitoes with Wolbachia in a six-month trial. Image courtesy of the University of Melbourne.

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Above: World Mosquito Program factory located in Brazil. Image courtesy of the World Mosquito Program.

Next Steps

Similar projects have begun in California and Texas, leading to a decline in A. aegypti mosquito populations in these areas. In 2024, the WMP plans to set up its largest operation to date: a Wolbachia mosquito production factory in Brazil that will produce up to five billion mosquitoes per year. If successful, the World Health Organization may issue an official statement on this technology, which would encourage other countries to adopt it. In the coming years, we can expect an increase in the release of Wolbachia mosquitoes all across the world and, hopefully, significant strides in the fight against mosquito-borne diseases.