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Taiwan has long been a model of dengue fever prevention.

Over the past 30 years, doctors, academics, and epidemic prevention personnel have organized an “epidemic prevention brigade” to lead the effort, catching mosquitoes and spraying against the insects in local streets and homes. Overcoming political partisanship and public protests, this “brigade” has for the most part defeated this invasive dengue fever army year after year, preventing it from taking root in Taiwan.

Warm Winter Means Early Outbreak

This year, however, the outbreak of the disease came earlier than normal. The first case of the year was reported in February, and by the second week of July, a total of 47 domestic cases of the disease (people who caught the disease in Taiwan) had been reported.

Of those 39 occurred in Kaohsiung, 85 percent of which were concentrated in the city’s Sanmin District, and the other eight were seen in Tainan. There were also 212 imported cases of the disease, the most for the first six and a half months of the year in the past 20 years. (See table)

Dengue fever prevention has suddenly become a headache, especially with the central government and local authorities trading politically charged barbs over funding for prevention efforts. The Kaohsiung city government has accused the central government of being “unwilling to provide financial support,” only for the Centers for Disease Control (CDC) to issue a rebuttal and charge Kaohsiung with “not collecting the funds.”

This petty political squabbling has diverted attention from the growing dengue fever outbreak.

An academic familiar with dengue fever and how the disease spreads argues that this year’s outbreak, in fact, is not that serious, but he believes Kaohsiung authorities are particularly nervous because reported cases of the disease have emerged about one month earlier than normal. Spraying insecticides and testing for the disease do require large amounts of funding, but current requirements should still be within the means of local health authorities, he contends.

“The local government is worried about an outbreak, and there’s nothing wrong with that. But in the past it has only been when an epidemic has been at its peak or even spun out of control, or when case numbers have surged into the hundreds that central authorities have stepped in,” he says.

Hidden behind the political mudslinging lies the issue the public should truly be concerned about.

This year’s dengue fever outbreak occurred earlier than usual and mosquitoes have “evolved” not because of a breakdown in epidemic prevention or funding issues but because of a much more formidable enemy – climate change.

Lethal Insects

Mosquitoes may be less than 1.5 centimeters long, but they can be lethal.

The late Canadian-American historian William H. McNeill wrote in his book Plagues and Peoples (1976) that malaria and yellow fever, which are both spread by mosquitoes, have been Africa’s two major contagious diseases. Malaria can be spread over a wide area and leave behind heavy casualties, while yellow fever occurs suddenly and has a high mortality rate, McNeill wrote, noting that the latter would spread on a ship for months until the entire crew had withered away.

Taiwan does not have yellow fever but it is vulnerable to dengue fever, a disease that cannot be underestimated. There have been two outbreaks of more than 10,000 cases from 1980 to the present, and in both epidemics the chief culprit was the aedes aegypti mosquito, which loves still water, easily adapts to human household environments and can bite many people in one go.

Dengue fever infects roughly 400 million people a year and results in the deaths of 20,000 of them. To younger patients, it resembles a week-long cold, displaying such symptoms as a fever, muscle soreness, eye pain (typically behind the eyes) and headaches. Symptoms can appear three to eight days after a mosquito bite, but 70 percent of bite victims show no symptoms at all. In most cases, treatment involves supportive therapies, such as giving medicine to fight headaches or bring down fevers, and patients may be quarantined in hospitals.

Though the disease can be relatively harmless and symptomless, the medical community still considers dengue fever to be one of the world’s most serious contagious diseases. Senior CDC epidemiologist Hung Min-nan says dengue fever has a mortality rate of about 1 percent, but what makes it truly scary is its effect on high-risk groups, such as the elderly, pregnant women, diabetes or kidney dialysis patients, or people with heart disease. Dengue fever in those patients can easily trigger such complications as black stool, hematemesis (the vomiting of blood) or shock, or cause the patient to die, according to Hung.

Virus Spreads Faster the Hotter It Gets

Overseas studies have found that the higher the temperature, the faster the dengue virus spreads. In one study, it was discovered that a vector mosquito needed an incubation period of 25 days in temperatures of 26 degrees Celsius to spread the virus but an incubation period of only seven days in temperatures of 32 and 35 degrees to do the same. (See table)



Source: Watts et al., 1987

High temperatures and heavy rain present an even bigger challenge. Wen Tzai-hung, a professor in National Taiwan University’s Department of Geography, sees it as a vicious cycle. The heat spawns more mosquitoes, which are more interested in biting people, which shortens the incubation period, leading to faster transmission of the disease.

In Tainan for example, temperatures this year were already averaging around 27 degrees with highs of 31 degrees in early May. That early heat has been accompanied by more than 12 straight weeks of rain, conditions perfect for the rapid proliferation of mosquitoes.

The Center for Dengue Prevention and Control in Tainan and the National Mosquito-borne Diseases Control Research Center have teamed up to monitor vector mosquitoes by installing “ovitraps” – devices that entice mosquitoes to lay their eggs – around the city. Researchers estimate the growth in the mosquito population based on how many eggs have been laid in the traps.

What they have observed this year is that the number of eggs laid since the beginning of May has soared, coming not only earlier but also in greater numbers than in years past. The results so far have translated to a rough increase of more than 100,000 eggs from the same period a year earlier.

Mosquito Population on the Rise! Sharp Increase in Number of Eggs

Number of eggs laid by mosquitoes in the first 27 weeks of the year (until early July) in 2018 and 2019.



Note: The chart reflects the average number of eggs laid in the ovitraps. Ovitraps entice mosquitos to lay their eggs in the device and are used to collect data on the number of eggs being laid by mosquitoes in various neighborhoods and estimate mosquito populations. There were 3,456 ovitraps in 2018 and 3,096 in 2019.

Other non-climatic issues have been in play as well, such as local residents remaining in the habit of storing water and cities’ drainage systems creating buildups of stagnant water that are hard to get rid of. As the climate, the environment, and humans conspire to create a paradise for mosquitoes. Taiwan faces three particularly pressing risks.

Risk I: Hidden Vector Mosquitoes Moving Northward

Dengue fever has generally been concentrated in the metropolitan areas of Tainan and Kaohsiung because aedes aegypti mosquitoes normally congregate in urban areas south of the Tropic of Cancer. Northern Taiwan has traditionally been dominated by the aedes albopictus – known as the Asian tiger mosquito – which loves the outdoors, tends to only bite one person per cycle, is less effective at transmitting disease, and lives only half as long as the aedes aegypti mosquito.

In 2018, there were random cases of dengue fever in northern and central Taiwan, including in the Greater Taipei area and Taichung, and they were caused by the aedes albopictus. They were not cases of the aedes aegypti moving north, as many had feared.

“We have not seen a trend in that direction to date, but we do in fact fear that if temperatures continue to rise, the aedes aegypti will break through the “Maginot Line” established by the Tropic of Cancer and head north,” says Chen Chun-hong, a researcher with the National Research Health Institutes’ National Institute of Infectious Diseases and Vaccinology.

Tsai Kun-hsien, an associate professor in National Taiwan University’s Department of Public Health and a longtime student of mosquitoes, warned that Taiwan should not overlook the aedes albopictus. Though dengue fever is much less likely to occur in northern Taiwan than in the south, Tsai warned that if people did not change their habits, such as wearing short sleeves in hot weather, going out and planting vegetables, or storing water in ways that allow mosquitoes to breed, the risk of contracting dengue fever will rise significantly whenever temperatures rise.

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Risk II: A Virus Threatening to Become Homegrown

Dengue fever is not endemic to Taiwan. The first case reported in the country every year invariably is an imported case. But that could change in the future.

Chuang Ting-wu, an assistant professor in the Department of Molecular Parasitology and Tropical Diseases at Taipei Medical University, explains that the aedes aegypti mosquito is best suited to an environment ranging from 16 to 33 degrees. Too cold and the mosquito will not be active; too hot, and it will die more quickly.

Based on current conditions, Taiwanese winters still experience temperatures close to single digits that make it hard for the aedes aegypti mosquito to survive.

In National Taiwan University’s mosquito-borne disease laboratory, aedes albopictus mosquitoes are raised in a transparent box. (Photo by Justin Wu/CW)

Indeed, the vector mosquito has not been able to survive winters in the past, according to the CDC’s Hung Min-nan, but should global warming turn more severe, the mosquito could make it through the winter months. That means outbreaks could begin in February rather than being concentrated from June to November as is the case at present. An enemy present throughout the year would only exacerbate the pressure on prevention efforts.

If vector mosquitoes linger in the environment, enabling the virus to continually circulate between humans and mosquitoes, dengue fever will eventually emerge as a local illness. At that point, Taiwan will have to co-exist forever with the disease, and tracking its source and controlling outbreaks will be harder than ever. Even worse, the strategies current used to keep it at bay will likely become inadequate.

Risk III: ‘Globalization’ of Virus Complicating Prevention

Taiwan has a wealth of experience in trying to prevent dengue fever. Nurses from community health bureaus check their neighborhoods, searching for and cleaning mosquito breeding grounds and educating the public. Ovitraps are set to estimate mosquito populations, and the National Health Research Institutes sets up special units to test for mosquito numbers two weeks after areas have been sprayed to keep the insects under control.

“When we attend international conferences and report that this year’s dengue fever outbreak is serious, with more than 20 cases to date, everybody just laughs and asks us why we’re even bothering to report this. Other countries have 2.4 million infections,” NTU’s Tsai Kun-hsien says with a chuckle.

Future efforts to fight dengue fever, however, will depend on coordinated action across borders. No longer can the battle be limited to Taiwan; it will require a united front with Southeast Asian countries such as Vietnam, Indonesia, Thailand and the Philippines.

National Taiwan University associate professor Tsai Kun-hsien has studied mosquitoes for many years and has found a crustacean known as cyclops that eats mosquito larvae and can help fight dengue fever. (Photo by Justin Wu/CW)

Such alliances have become necessary because of the narrowing distances between countries. According to the Civil Aeronautics Administration, there were an average of nearly 1,400 flights a week (700 round-trips) between Taiwan and countries covered by the New Southbound Policy (Southeast Asia, South Asia, Australia and New Zealand) serving an average of just under 270,000 passengers a week in the first five months of 2019. Mosquitoes also travel between these destinations.

Since 2015, the CDC has promoted a “Dengue Fever Offshore Prevention Plan” that includes strengthening screenings of inbound passengers for fevers at international airports. Whenever an issue is discovered, the patient is treated and reported and prevention measures are expanded.

But the results have been mixed.

“The biggest problem is hidden infections. The more flights there are, the higher the risk, because most patients don’t get fevers and make it through the screening procedure, taking the virus into their communities,” the NHRI’s Chen Chun-hong says.

Using Technology to Fight Climate Change

The next step has been to enlist technology in the fight against mosquitoes. After a massive dengue fever outbreak in 2015, when there were nearly 44,000 confirmed cases in Taiwan, mostly in Tainan and Kaohsiung, and more than 200 deaths, the NHRI set up the National Mosquito-borne Diseases Control Research Center to find better ways to prevent the spread of mosquito-related diseases.

One finding involved natural bacteria known as “Wolbachia.” The center found that when Wolbachia are introduced into the aedes aegypti mosquito, it can shorten the mosquito’s life span and suppress the spread of the virus.

NTU’s Tsai said this can be done in two ways. One is to release large numbers of male mosquitoes carrying Wolbachia to mate with female mosquitoes not infected with the bacteria. That prevents them from reproducing, and the deaths of the male mosquitoes after mating also shorten life cycles, which gradually reduces vector mosquito numbers. Singapore and the United States have both adopted this strategy, and Taiwan tried it in Tainan for a month and was 70 to 90 percent effective in keeping mosquitoes in check.

The other strategy is to release both male and female infected mosquitoes and rely on the Wolbachia to sicken other mosquitoes and prevent them from replicating the virus, reducing the risk of dengue fever. The NHRI’s Chen recently returned from a seminar in France at which he learned that such a strategy was found in Australia to reduce dengue fever cases by 90 percent. This method is far cheaper than the first alternative, and Taiwan is considering delving further into it.

Regardless of which approach is used, public perception and acceptance are the biggest obstacles. Chen acknowledges that these strategies will take time, but if public fears of mosquito populations getting bigger rather than coming down can be eased, Wolbachia will offer the best solution.

As for preventive spraying, the NHRI currently is studying new, longer lasting pesticides. As new pesticides are developed overseas, the NHRI will also move to test them in Taiwan.

Among other methods already in use, Chen mentioned mosquito traps that can identify vector mosquitoes in 0.07 seconds and “sewer robots” that use cameras to detect pockets of stagnant water serving as mosquito breeding grounds.

Tsai also mentioned a biological control method introduced in 2012 using tiny freshwater crustaceans known as “cyclops.” Every time he engages in education campaigns in various communities, he brings a plush toy based on SpongeBob SquarePants character “Sheldon J. Plankton” resembling a cyclops that often draws exclamations of surprise from the public. In fact, the crustacean can eat up to 40 larvae in a day, reducing the proliferation of vector mosquitoes.

The SpongeBob SquarePants animated character Sheldon J. Plankton, with its two antennae and single eye, resembles cyclops crustaceans that scientists hope to use to fight mosquitoes. (Photo courtesy of Tsai Kun-hsien)

To more effectively combat the disease on a global basis, Chen suggested forming a “Southeast Asia Dengue Fever Information Platform” that would gather information on the most common strains of the virus at any particular time and information on the movement of people to understand the spread of the disease.

Once the information is complete, the system can quickly issue warnings and predict the next wave of the dengue fever virus. But that requires cooperation between countries to make information transparent.

“Every person involved in epidemic prevention should have a sense of mission. Controlling the spread of disease year in and year out is everybody’s expectation,” says the CDC’s Hung Min-nan.

People on the front lines of Taiwan’s dengue fever battle have confidence and a sense of responsibility. Now, armed with their new weapons, they hope they are adequately prepared to confront the challenges brought by climate change and keep mosquitoes, and mosquito-borne diseases, at bay.

Translated by Luke Sabatier

Edited by Sharon Tseng