Brazil is using genetic engineering to help fight dengue fever, creating mosquitoes whose offspring die before they mature. Tests in two towns have been successful - but are there ecological implications?

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

But every year there are around half a millionserious cases,some of which prove fatal.The disease hasspread considerably in recent years. Even Europe is no longer safe. In 2010, more than 600 travellers returning to Europe from abroadwere diagnosed with dengue fever. "The number of unreported cases is estimated to be farhigher," says Jonas Schmidt-Chanasit from the Bernhard Nocht Institute for Tropical Medicine in Hamburg."We believe it could be ten times as many."2010 alsosaw 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 patternand is actually quite pretty, as insects go. Butit can carry andtransmit several viruses. It's one of the maincarriers of yellow fever, and for humans it can be disastrous. In the Spanish-American War of 1898, the number of US soldiers who diedof this kind of infectious diseaseis believed to have beenhigher than the number killed in battle. There is nowa vaccine against yellow fever, but none has yet been foundto preventdengue fever.

Fertile breeding ground for mosquitoes: a teaspoonful of standing water is enough

All attempts to fight the mosquitoes with the help of insecticide have failed. In Brazil, awareness campaigns warn peoplenot leavecar tires lying around where rain can collect inside them, and toflush toiletsregularly, even if they're not beingused.Thedangerous larvae generally breed in standing water,which people are advised to avoid - but the mosquitoes can also breed ina puddle, a hollow in a rock, or eventheheart ofa flower.A teaspoon of water isallthey needin order to deposittheir eggs.

Assistance fromgenetic engineering

British scientistswith the company Oxitec have now developed a genetically-modified male mosquito whose offspringare unable to survive into adulthood. The idea is that the genetically-modifiedmalesarereleased intoa natural environmentandallowed tomate with female mosquitoes. The fertilized eggs developinto larvae or pupae, andthen die.

Oxitec hasconducted successfulfield 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.

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Genetically modified mosquitoes combat disease