Emergency Measures Set on Dengue Fever

SINGAPORE Even as Asia struggles to head off a bird flu pandemic, a much more common disease is sweeping the region, borne on the wings of mosquitoes and commercial aircraft: dengue fever.

At least 127,000 people have been infected by dengue so far this year from eastern India to Indonesia and the Philippines, with at least 990 deaths.

Warning of a dengue epidemic, Malaysia on Wednesday joined Singapore in declaring emergency measures to combat a surge in cases that have killed at least 70 people in Malaysia this year. And while the two countries are experiencing record infection rates, the country worst affected has been Indonesia, with more than 48,000 cases and over 600 deaths.

"Dengue is now one of the most important tropical diseases," said Duane Gubler, Director of the Asia-Pacific Institute for Tropical Medicine and Infectious Diseases at the University of Hawaii's John A. Burns School of Medicine. "While it doesn't kill that many people, it has tremendous economic and social impact."

A member of the same family of viruses that causes West Nile, Japanese encephalitis and yellow fever, dengue fever is a potentially fatal disease for which there is no vaccine or treatment despite estimates that it infects at least 50 million people a year. Those with acute cases suffer a painful fever and debilitating lethargy, with about 1 percent developing hemorrhagic fever or shock, sometimes with gastrointestinal bleeding and, in rare cases, brain hemorrhages.

Unlike other tropical diseases like malaria that are found only in rural areas or in urban slums, dengue is carried by mosquitoes that can thrive in modern, clean and prosperous cities in warm, humid climates like Singapore.

Dengue is therefore an unfortunate byproduct of Asia's economic development and emerging prosperity.

The two species of mosquitoes that carry dengue, Aedes aegypti and Aedes albopictus, can breed in minuscule pools of fresh water in homes and gardens, in addition to plastic containers and lids that often litter Asia's urban landscape.

If a female mosquito bites a person already infected with dengue, it can pass the virus on to the next person it bites. If that person then flies to another country with Aedes mosquitoes, the person can unknowingly spread the virus even further.

Dubbed "breakbone fever" when it was first diagnosed more than three centuries ago, because it causes extreme pain in the joints, dengue began its global spread around Asia during World War II, when it traveled with warring armies from country to country. After the war, Aedes mosquitoes and dengue flourished along with Asia's rapid population growth and urbanization and then was carried aboard ships and planes to Africa and the Mediterranean.

When the use of the insecticide DDT in Latin America was stopped in the 1970s after the apparent eradication of yellow fever, which Aedes mosquitoes also carries, dengue was able to stage a comeback in the New World.

Complicating prevention efforts is the fact that 90 percent of those infected with dengue develop only mild symptoms or no symptoms at all, thus inadvertently serving as a reservoir for the virus. Even those who do fall ill are infectious days before symptoms appear, making any potential dengue quarantine useless, experts say.

Researchers are divided over why outbreaks are rising in number and virulence. While the popularity of international air travel is one catalyst, some believe that global warming is expanding the Aedes mosquitoes' habitat. Officials in Singapore have noted what they say is a correlation between dengue outbreaks and rising temperatures, particularly during the El Niño weather phenomenon. But other experts, including Gubler at the University of Hawaii, say mosquito breeding depends more on rainfall than temperature.

Very little is known about the virus itself.

"There was very little drug discovery effort, because it affects a lot of people, but mostly in the tropics," said Paul Herrling, head of corporate research at Novartis, the drug manufacturer.

Little is known about why the virus can depress the amount of blood-clotting platelets, which can result in hemorrhaging, or why it causes a temporary form of hepatitis.

In 2004, Novartis set up an institute of tropical diseases here with funding from the Singapore government to conduct research into potential treatments and cures of various diseases.

Using state-of-the-art computer methods, researchers are trying to sequence the dengue genome in order to create drugs that would prevent it from replicating in the body.

But progress is slow. The research is impeded by the fact that dengue produces a hemorrhagic reaction in no other species but man, meaning scientists cannot use other animals to test potential treatments.

Several vaccines against dengue have been developed, but testing is likely to take as long as seven years and is complicated by one of dengue's other paradoxes: with four strains of the virus, those infected with one strain appear to become more susceptible to a hemorrhagic reaction if infected with any of the others. Any potential vaccine, therefore, has to be simultaneously effective against all four strains.

Until researchers succeed, the only hope of reducing dengue outbreaks, experts say, is to control the mosquito population. Yet even in this area, the best efforts have met with mixed success.

Singapore requires building owners to routinely spray insecticide, and officials conduct periodic inspections of residences to ensure they are not harboring mosquitoes. Officials also monitor new cases to pinpoint new dengue hot spots. In response to the spike in cases this year, the health authorities have dispatched additional inspection teams to search out and destroy breeding sites.

Yet, despite having one of the world's most aggressive campaigns combating dengue, Singapore is experiencing a record outbreak this year, with almost 9,500 reported cases and 11 deaths so far.

"Singapore probably underscores the need for regional controls," Gubler said.

Visiting workers and tourists can keep Singapore continually re-stocked with fresh supplies of the virus, he said.