Number of mosquito-transmitted dengue fever infections triples

More than three times as many people are infected with the tropical disease dengue fever annually than world health officials expected, researchers said Sunday.

The figures, based on years of research, show how quickly the mosquito-born viral disease is spreading.

The World Health Organization estimated that there were 50-100 cases of dengue fever, also known as "break-bone fever," every year. 

But the new figures from the University of Oxford in England put the number of severe cases at 96 million annually and the number of mild or asymptomatic cases at 300 million. 

The high number of asymptomatic cases shows just how far the disease has spread. People who have been infected with the virus once, even if they don't show any symptoms, are far more likely to contract the infection again more severely.

"The asymptomatic patients, in terms of the future burden of disease, are a very important contributor," Jeremy Farrar, director of Oxford University's tropical disease research unit in Vietnam and one of the study authors told Reuters.

The WHO, which was not involved in the study, said the new figures aren't surprising.

"We fully agree the spectrum of dengue is very wide and there was every chance we were missing cases," Raman Velayudhan, the agency’s global dengue coordinator told the Associated Press.

Like malaria, dengue fever is passed from person to person by infected mosquitos. There are no vaccinations to prevent contracting dengue and it is difficult to treat.

The virus causes a high fever, rash and severe joint and muscle pain.

According to the World Health Organization, dengue fever kills about 12,000 people, mostly children, each year.

The study was published online Sunday in the journal Nature.