LONDON (REUTERS) - Around 390 million people are infected each year with dengue fever - the world's fastest-spreading tropical disease - more than triple the current estimate by the World Health Organization, experts said on Sunday.
The new finding, based on several years of analysis, underscores the growing burden of the mosquito-borne viral disease, which is also called "breakbone fever" because of the severe pain it can cause.
There is as yet no approved vaccine or specific drug to treat dengue, which is not normally fatal but lands many victims in hospital.
Researchers from the University of Oxford and the Wellcome Trust presented their results, along with a detailed map of dengue distribution, in the journal Nature.
Their new figure includes 96 million severe cases and approximately 300 million mild or asymptomatic episodes. That compares with the WHO's most recent estimate for overall infections of 50-100 million a year, The high number of relatively mild cases offers little cause for comfort, since it suggests the reservoir of disease is far larger than expected.
What is more, dengue is a disease that hits more than once and people who get it mildly first time are more likely to have a serious episode if bitten again by an infected mosquito.
"The asymptomatic patients, in terms of the future burden of disease, are a very important contributor," said Jeremy Farrar, director of Oxford University's tropical disease research unit in Vietnam.
Spread by the Aedes aegypti mosquito, dengue has grown rapidly along with urbanisation and globalisation because it thrives in tropical mega-cities and is easily spread in goods containing small puddles of water, such as used tyres.
Climate change is also making more parts of the planet habitable for the dengue-spreading mosquito.
As a result, half the world's population is now exposed to the disease, mostly in the developing world - but also in parts of southern Europe and the southern United States.
The researchers estimated that 70 percent of the world's serious dengue cases were in Asia, with India alone accounting for 34 percent of the total. The Americas - mainly Brazil and Mexico - made up 14 percent, while Africa's dengue burden was nearly as large.