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 Organisation, experts said.
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
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 disease is far larger than expected
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
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.
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
The researchers estimated that 70% of the world's
serious dengue cases were in Asia, with India alone accounting
for 34% of the total. The Americas - mainly Brazil and
Mexico - made up 14%, 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
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.