It is rainy season in Southeast Asia, and the region is facing its annual battle with dengue fever and other mosquito-borne diseases. The World Health Organization says such diseases are a leading cause of hospitalization in several Asian nations.
Dengue is found predominantly in urban areas, where mosquitoes breed in man-made containers that collect rainwater. Dengue, malaria and other parasitic and so-called vector-borne diseases are considered major health problems.
In Hong Kong, as in other Southeast Asian cities, summer heat and monsoon rains create a sharp spike in the risk of dengue fever.
In its milder forms, the disease causes acute flu-like symptoms, including high temperatures, severe headaches, rash and muscle pain. The more severe form, known as dengue haemorrhagic fever, can be fatal, particularly in children. Both forms of the disease can be successfully treated.
Dengue is caused by any of four related viruses, meaning a person can theoretically be infected four separate times in a lifetime. Repeated infections may increase the chance of contracting the fatal form of the disease.
The World Health Organization says about 2.5 billion people in the Americas, the Asia-Pacific region, Africa, and the Eastern Mediterranean are at risk of infection.
Hong Kong health officials say last year was the first year a dengue outbreak was traced to local sources.
Twenty locally-transmitted cases were detected in all, 17 of which were traced to a single construction site.
Health officials say it is still too early to conclude that the disease has become endemic to the territory.
Hong Kong authorities say none of the cases of dengue fever reported in Hong Kong so far this year can be traced to a local source. Instead, they have been imported from the Philippines, Thailand, Indonesia, Singapore, Malaysia, Cambodia and Sri Lanka.
Dr. Ho Yuk-yin, a consultant for Hong Kong's Food and Environmental Hygiene Department, says, up to now, most dengue fever has reached Hong Kong with people who travel here. "For the last 10 years, or so, all the other cases are imported from our neighboring countries, particularly from Southeast Asian countries," he said.
Dengue is transmitted between humans by female mosquitoes, which draw in the virus that causes the disease while feeding on the blood of one person, and then passing the virus on when they bite another. The females can also recycle the disease by passing it on to new generations in their eggs.
The mosquito that most often carries dengue is called aedes albopictus, which is common in tropical areas.
Dr. Hitoshi Oshitani, an Asia regional advisor for the WHO, says dengue spreads swiftly in places where lots of humans meet lots of aedes mosquitoes, meaning cities. "Many dengue cases are occurring in the urban or suburban areas," said Dr. Oshitani. "The rural environment is not optimal for the dengue spread."
Sparsely populated rural areas have plenty of mosquitoes, but the low density of humans lessens the risk of transmission. In urban areas, with their high concentrations of humans, doctors say, the spread of dengue can be exponential.
Southeast Asia, with its juxtaposition of high population densities and tropical climates, is a perfect environment for the spread of dengue. Dense urban clusters are interspersed with areas of lush tropical vegetation. Dr. Ho says breeding grounds for the mosquitoes can be found everywhere in the modern urban environment. "They can breed in empty lunch boxes, empty cans, the small containers containing less than 100 mils [milliliters] water," he explained. "They can survive and breed."
Hong Kong's government warns of this frequently in a campaign of public service announcements. "Change the water inside vases and containers of watered plants, and change them at least once a week," urges one of the ads.
The government monitors the mosquito population with a simple device called an "ovitrap," a suspended cylinder filled with a moist solution that is attractive to female mosquitoes. By counting the eggs that are laid in it, inspectors compile what they call an "ovitrap index" to track monthly changes in the mosquito population.
The authorities say keeping the mosquito population low is the best-known preventive measure against dengue. They scrutinize districts where index levels are high, especially construction sites, where hollow bamboo scaffolding collects summer rains.
Dr. Ho says keeping mosquitoes away from humans is good public health policy in general. "Dengue endemic areas, many of them, are also endemic for other mosquito-borne diseases, like malaria and Japanese encephalitis," he noted.
In the longer run, the World Health Organization is sounding an optimistic note about the fight against dengue. The organization says, if progress continues at its current rate, a vaccine could be available within several years.
This report is part of VOA's series on World Health