By Ben Hirschler
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 urbanization and globalization because it thrives in tropical mega-cities and is easily spread in goods containing small puddles of water, such as used tires.
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.
Last year Europe experienced its first sustained transmission of dengue fever since the 1920s with around 2,000 people infected in the Portuguese archipelago of Madeira.
Farrar said more such outbreaks were likely in future, since the mosquito was already present in southern Europe and there were increasing numbers of people travelling to and from dengue-infected areas.
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.
The prevalence of the disease in Africa is worrying, since dengue has not generally been seen as a major problem on the continent. The research team said the impact of disease in Africa was being masked by symptomatically similar illnesses, such as malaria.
Hopes for an effective dengue vaccine suffered a setback last year when an experimental shot from Sanofi proved far less effective than hoped in a mid-stage clinical trial in Thailand.
Further large trials of the Sanofi vaccine - the most advanced in development - are still continuing and scientists have not given up hope that it may yet have a role of play.
A number of other experimental vaccines are also in development, although at a much earlier stage.
(Reporting by Ben Hirschler)