Factbox: Dengue fever a risk for 2.5 billion people

(Reuters) - British scientists have created genetically sterile mosquitoes who use sex to kill off others in their species and researchers say early field trials suggest the idea could help halt the spread of dengue fever.

Here are some facts about dengue fever:

* Dengue is transmitted by the bite of an Aedes aegypti mosquito infected with any one of the four dengue viruses. Symptoms appear in 3 to 14 days after the infective bite.

* Dengue fever is a severe, flu-like illness that affects babies, young children and adults.

* It is often known colloquially as bone-break fever for the severe muscle and joint pains it induces, the disease causes death only when it develops into a more severe form, dengue haemorrhagic fever.

* Early clinical diagnosis of dengue haemorrhagic fever and careful clinical management by experienced doctors and nurses often saves lives.

* Global incidence of dengue has grown dramatically in recent decades.

* Around 2.5 billion people -- two fifths of the world's population -- are now at risk from dengue. The World Health Organization (WHO) currently estimates there may be 50 million dengue infections worldwide every year.

* With no specific treatment for the four viruses, health officials focus on eradicating mosquitoes to prevent infections.

* More than 70 percent of the disease burden is in southeast Asia and the western Pacific area. Africa and the Eastern Mediterranean are much less affected.

* According to the WHO, the incidence and severity of disease are increasing rapidly in Latin America and the Caribbean.

* An increase in international air travel is facilitating the rapid global movement of dengue viruses. This increases the risk of dengue haemorrhagic fever epidemics by introducing new dengue viruses into susceptible populations.

* French drugmaker Sanofi-Aventis is one of various groups and firms seeking to develop a dengue vaccine and is currently testing it in late stage clinical trials.