MANILA, July 23 (Reuters) - The World Health Organisation said on Monday the Western Pacific region could be at risk from a major outbreak of dengue unless governments improve efforts and cooperation to stamp out the virus.

About 40 percent of the world's population are at risk from dengue, and the mosquito-borne virus has spread rapidly across Southeast Asia this year due to warmer weather, heavy rains and crowded cities.

"Factors leading to the spread of dengue include population explosion, migration and rapid growth of urban areas, which place a heavy strain on public health services and access to drinkable water," said John Ehrenberg, a WHO regional adviser.

The virus, which is transmitted by the Aedes aegypti mosquito, causes severe fever, headaches, rashes and muscle and joint pain. Severe forms can cause haemorrhagic fever. There is no vaccine.

Dengue has killed hundreds in Southeast Asia and infected tens of thousands this year but Ehrenberg warned that tracking outbreaks was difficult because many incidences of the virus are not officially reported.

He called for countries in the region to increase dengue surveillance, properly manage patients, reduce mosquito breeding sites and prepare for outbreaks. Information-sharing was also key.

More than 50 million dengue infections, including about 400,000 cases of dengue haemorrhagic fever, are estimated to occur annually around the globe.

Dengue haemorrhagic fever is a leading cause of childhood death in many endemic countries, according to the WHO.

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