The rapid growth of crowded cities has helped spread and increase the transmission of dengue around the world, health experts said on Tuesday, warning up to 3 billion people were already at risk.

They also disputed reports that climate change could become a factor in the spread of the disease from tropical areas because the mosquito that carries dengue has reached temperate regions due to rising temperatures.

Climate change has very little effect on the disease, Duane Gubler, director of Asia Pacific Institute of Tropical Medicine and Infectious Diseases at the University of Hawaii, told Reuters on the sidelines of a three-day dengue symposium in Manila.

Scientists and health experts are meeting to exchange practices and strategies to combat the disease that infects 50 million people every year, causing tens of thousands of deaths, mainly among children.

As early the 1980s, dengue fever had reached epidemic proportions in some countries in Asia long before climate change became an issue. Rapid urbanization, increase in air travel and lack of mosquito control are the main drivers of the disease.

Gubler said dengue spreads quickly in crowded cities with inadequate basic services, such as potable water, sanitation and waste-management and weak public health infrastructures.

In the 1950s, when the first dengue outbreak was reported in Manila, only 10 countries in Southeast and South Asia had dengue problems but the disease has now spread to about 100 states in the Pacific islands, Latin America and Africa due to rise in air travel.

About 57 percent of people across the globe are now living in cities, Gubler said, adding most urban areas in the region now have a population of over 5 million.

Gubler said only about 50 million people traveled every year in the 1950s, but the figure has risen to about 2 billion in the last six decades, helping spread the disease.

He said the disease could only be controlled if governments worked closely with the science and health sectors to improve public health services, make more people aware of the disease and eliminate mosquito breeding grounds.

Gubler said the world is rushing to develop an anti-dengue drug by 2012 and a vaccine in five to seven years, citing seven pharmaceutical and biotech companies that were at various stages of clinical tests to produce the drugs and vaccines.