Transmitted by the bite of female mosquitoes, the disease is occurring more widely due to increased movement of people and goods - including carrier objects such as bamboo plants and used tyres - as well as floods linked to climate change, the United Nations agency said.

The viral disease, which affected only a handful of areas in the 1950s, is now present in more than 125 countries - significantly more than malaria, historically the most notorious mosquito-borne disease.

The most advanced vaccine against dengue is only 30 percent effective, trials last year showed.

"In 2012, dengue ranked as the fastest spreading vector-borne viral disease with an epidemic potential in the world, registering a 30-fold increase in disease incidence over the past 50 years," the WHO said in a statement.

Late last year, Europe's suffered its first sustained outbreak since the 1920s, with 2,000 people infected on the Portuguese Atlantic island of Madeira.

Worldwide, 2 million cases of dengue are reported each year by 100 countries, mainly in Asia, Africa and Latin America, causing 5,000 to 6,000 deaths, said Dr. Raman Velayudhan, a specialist at the WHO's control of neglected tropical diseases department.

But the true number is far higher as the disease has spread exponentially and is now present on all continents, he said.

"The WHO estimates that on average about 50 million cases occur every year. This is a very conservative estimate," Velayudhan told Reuters, adding that some independent studies put the figure at 100 million.

"Dengue is the most threatening and fastest spreading mosquito-borne disease. It is pandemic-prone, but it is a threat only. Definitely a bigger threat now than ever," he said

Malaria caused more deaths but was on the decline, affecting fewer than 100 countries.

SILENT EXPANSION

Speaking to a news briefing after the WHO released a report on 17 neglected tropical diseases affecting 1 billion people, Velayudhan said: "The mosquito has silently expanded its distribution.

"So today you have (the) aedes mosquito in over 150 countries. The threat of dengue exists all across the globe."

In Europe, the aedes mosquitoes that cause both dengue and chikungya disease have spread to 18 countries, often via the importation of ornamental bamboo or second-hand tyres, he said.

"But we are trying to address this in a more systematic way, by controlling entry of vectors at points of entry - seaports, airports, as well as the ground crossings," Velayudhan said, noting that it was hard to detect mosquitoes and their eggs.

Dengue causes flu-like symptoms that subside in a few days in some sufferers. But the severe form of the disease requires hospitalisation for complications, including severe bleeding, that may be lethal.

There is no specific treatment but early detection and access to proper medical care lowers fatality rates below 1 percent, according to the Geneva-based WHO.

"You have to bear in mind that it has no treatment and vaccines are still in the research stage," Velayudhan said.

The most advanced, being developed by French drugmaker Sanofi SA (SASY.PA), proved only 30 percent effective in a large clinical trial in Thailand, far less than hoped, according to results published in September.

But researchers said it did show for the first time that a safe vaccine was possible.

The WHO also said aims to eliminate globally two neglected tropical diseases, dracunculiasis, known as guinea worm disease, in 2015, and yaws, or treponematoses, in 2020. (Reporting by Stephanie Nebehay; Editing by John Stonestreet)

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