Dengue fever cases are climbing in much of Southeast Asia this year. As a result, health authorities in the region are stepping up the fight against the mosquito-borne disease as the monsoon season sets in.

Dengue fever sickens thousands in Asia each year, especially during the wet season now under way in the tropics. In Vietnam, dozens of people have died from the mosquito-borne illness this year. Sixteen-thousand more people have been infected - mostly in communities along the Mekong river.

The disease also killed more than 600 people in Indonesia this year and infected close to 60-thousand more. In Hong Kong, health authorities warn of a higher risk of an outbreak because of an increase in the number of mosquitoes that carry dengue.

Peter Cordingley, spokesman for the World Health Organization in Asia, says the current outbreak in Southeast Asia so far is not alarming. "You normally find it in the wet season. It comes around every two or three years," he says. "There's nothing that we have noted that suggests at this stage that we have an exceptional situation in our hands."

The dengue viruses are transmitted to humans through the bites of infected female Aedes mosquitoes. Dengue fever is prevalent in tropical and sub-tropical regions, although in recent years it has spread to drier areas, such as the Middle East.

In recent years, it has become one of the major causes of hospitalizations and deaths among children in Asia. The United Nations health agency says the rapid rise in urban populations is bringing more people into contact with Aedes mosquitoes, which breed in stagnant water standing in containers such as flowerpots and empty cans.

Patients with dengue fever suffer muscle and joint aches, headaches and rashes. In severe cases, it can lead to bleeding and organ failure. With proper treatment, dengue fever seldom causes death, but left untreated, its severe form kills 20 percent of those infected.

Scientists are still searching for a vaccine against the disease. Mr. Cordingley says vigilance is key in curbing its spread. "This mosquito breeds in stagnant water, so quite clearly the advice is to ensure that they don't have stagnant water standing around," he says. "Nobody should be waiting for a dengue outbreak to start looking at the problem of stagnant water. It should be an on-going, year-round application of good old-fashion sanitary common sense."

In response to the outbreak, Vietnam has launched a clean-up drive to prevent mosquito breeding. Late last month Sri Lankan authorities also began a campaign to clean mosquito breeding grounds in the capital Colombo, which saw more than six hundred cases this year. Singapore imposes heavy fines on households where stagnant water is found standing.