Dengue fever

Dengue is a tropical fever with similar symptoms to the flu: fever and shivering, headache and joint pain, and a rash. Most infections are comparatively mild and last no longer than a week.

But every year there are around half a million serious cases, some of which prove fatal. The disease has spread considerably in recent years. Even Europe is no longer safe. In 2010, more than 600 travellers returning to Germany from abroad were diagnosed with dengue fever. "The number of unreported cases is estimated to be far higher," says Jonas Schmidt-Chanasit from the Bernhard Nocht Institute for Tropical Medicine in Hamburg. "We believe it could be ten times as many." 2010 also saw the very first cases of infection in France and Croatia.

The root of the problem

Aedes aegypti, the mosquito's scientific name, has a black and white pattern and is actually quite pretty, as insects go. But it can carry and transmit several viruses. It's one of the main carriers of yellow fever, and for humans it can be disastrous. In the Spanish-American War of 1898, the number of US soldiers who died of this kind of infectious disease is believed to have been higher than the number killed in battle. There is now a vaccine against yellow fever, but none has yet been found to prevent dengue fever.

All attempts to fight the mosquitoes with the help of insecticide have failed. In Brazil, awareness campaigns warn people not leave car tires lying around where rain can collect inside them, and to flush toilets regularly, even if they're not being used. The dangerous larvae generally breed in standing water, which people are advised to avoid - but the mosquitoes can also breed in a puddle, a hollow in a rock, or even the heart of a flower. A teaspoon of water is all they need in order to deposit their eggs.

Assistance from genetic engineering

British scientists with the company Oxitec have now developed a genetically-modified male mosquito whose offspring are unable to survive into adulthood. The idea is that the genetically-modified males are released into a natural environment and allowed to mate with female mosquitoes. The fertilized eggs develop into larvae or pupae, and then die.

Oxitec has conducted successful field trials on the Cayman Islands and in Malaysia. In 2011, the biotech company Moscamed in the Brazilian city of Juazeiro joined the project. Here, in the hinterland of Brazil's Bahia state, dengue is more common than almost anywhere else in the world.

"Brazil is the first country to produce and use mosquitoes on a large scale", says biologist Margareth Capurro from São Paulo University. She coordinates the Project for Genetically-Modified Aedes. More than 550,000 genetically-modified mosquitoes are being bred here each week and released in two test districts in the town. The project has been extremely successful. Six months on, the scientists say the mosquito population has been reduced by 90 percent.

In future, the plan is to produce around four million mosquitoes "on the assembly line" every week, and to help eradicate the disease in other areas of Brazil, too. The next town will be Jacobina, which has 80,000 inhabitants. "We only release the males. They can't bite, and they transmit the mutant gene to their offspring," Capurro explains.

Biologists and virologists are skeptical

The management of Moscamed stresses that the tests cause no harm to the ecosystem. But virologist Schmidt-Chanasit is skeptical. "This mosquito, the Egyptian mosquito, has existed in Brazil for decades now. It's been completely integrated," he says. That means it's become a part of the food chain. And Schmidt-Chanasit also warns that the consequences of genetic modification in general are still very hard to predict.

The virologist is also very doubtful as to whether the project will be successful in the long term. "The virus is variable and can adapt to other mosquitoes," he says. After all, aedes aegypti may be the most common carrier of the virus today, but it's far from the only one. The biologist Capurro, however, disagrees. She believes that the chances of the virus adapting, or of a different mosquito becoming the main carrier, are zero.

And although Schmidt-Chanasit is wary of using genetically modified species in nature, he can't wait to see the results. "If the project is successful, it could serve as a model for other countries, and in the fight against similar diseases like malaria," he says.