A World Mosquito Program project staff member hangs a mosquito-box from a tree in Colombo, Sri Lanka. | WORLD MOSQUITO PROGRAM
- BY ALEX JACKSON Jun 16, 2022
For Kasun Chameera, who lives in Sri Lanka’s densely populated capital Colombo, dengue fever is a disease which has afflicted many loved ones, including his brother.
“We fear death when we hear about dengue,” Chameera said. “It’s present almost everywhere in my district, and spreads faster in the city than in the villages.”
His brother “suffered a lot from it,” Chameera said. “For at least one to two months, he would be tired walking just 10 steps. We were very scared.”
Also known as breakbone fever because of the severe pain it can cause, the disease is a growing threat across Asia, where 70% of the world’s dengue cases occur. The virus is transmitted to humans through the bites of the female aedes aegypti and aedes albopictus mosquitoes, which thrive in both tropical and subtropical urban areas.
Worldwide, it is estimated to infect about 390 million people every year, with more than half of the global population now at risk.
Recent weeks have seen soaring cases in Singapore, Malaysia and Vietnam, despite the peak season usually lasting from June to October. Reported cases in Singapore topped the 10,000 mark in the first five months of this year, already exceeding the 5,258 cases reported in all of 2021.
In Japan, 461 cases were reported in 2019 — mainly found in travelers from Asian countries. But with the borders effectively closed during the pandemic, the number of cases dropped to 43 and eight in 2020 and 2021, respectively.
Dr. Ren Minghui, assistant director-general at the World Health Organization (WHO), says dengue outbreaks have been reported since the 1950s in several countries across Asia but that the threat has been growing during the past three decades.
“Unplanned urbanization, rapid movement of people and goods, and climatic and environmental changes have all led to an increase in the vector mosquito population and therefore a spread of the viruses,” he said.
No universal vaccine
One of the reasons behind the increased concern — the WHO considers dengue fever one of the top 10 threats to global health — is the fact that the virus has four distinct versions, or “serotypes,” and no universal vaccine.
This means people who recover from one serotype can still be infected by the other three, leading to more severe and lethal symptoms from a second dengue infection.
“The difficulty with developing the dengue vaccine is that you have to get a balanced immune response to all four serotypes,” says Dr. Sophie Yacoub, who leads the dengue research group at Oxford University’s Clinical Research Unit in Ho Chi Minh City, Vietnam. The only currently licensed vaccine, Dengvaxia, is approved for use in children and adults who have previously had dengue and live in endemic areas. The attempted age clarification for approval varies by nation but generally falls between 6 and 45 where licensed.
The risk of being infected by the different serotypes impacts the scale of epidemics, Yacoub said, noting that they vary depending on levels of immunity in the population and other factors including climate and mobility.
In both 2020 and 2021, Singapore reported cases of dengue serotype 3 (DENV-3) — the first time in many years. According to Yacoub, such high levels of dengue seem likely to happen again this year.
Common dengue symptoms include a high fever, headaches and aching joints, but the virus can also manifest itself more severely, which can lead to organ failure and shock due to low blood pressure. However, with more than an estimated 80% of people experiencing mild symptoms or being asymptomatic, dengue cases are underreported.
Endemic now in more than 100 countries, dengue is one of the few infectious diseases that has increased year-on-year over the past few decades, with the largest number of cases ever reported globally in 2019 — 5.2 million, compared with 505,430 in 2000.
And as cases grow, so does the economic cost.
Researchers have estimated the global annual cost of dengue to be $8.89 billion, and in Southeast Asia, it was reported to be between $610 million and $1.38 billion. The strain on local health systems is another “huge impact” of dengue, Yacoub said.
“During the rainy season, dengue is the No. 1 cause of admission to many hospitals including the intensive care units at the Hospital for Tropical Diseases (HTD) in Ho Chi Minh City. It impacts other patient admissions and their monitoring capabilities as well as putting a lot of strain on medical and nursing staff.”
Extreme climate and weather patterns such as droughts, floods, heat waves and rainfall are becoming more frequent and severe, providing favorable conditions for mosquitoes to breed in.
The latest report by the U.N. Intergovernmental Panel on Climate Change (IPCC) notes that warming temperatures and climate change will increase the range of Aedes mosquitoes and could help spread their viruses to higher latitudes and altitudes.
“It’s true that the next pandemic could be triggered by vector-borne diseases,” said Dr. Tedjo Sasmono, a senior research fellow at the Eijkman Institute for Molecular Biology in Jakarta. “Mainly because of the possible rapid expansion and geographic spread of the diseases’ vectors, driven by transportation and climate change.”
Researchers estimate that an additional 1 billion people could be at risk of dengue fever in the next century. Separate research also found that global temperature rises could lengthen the annual dengue transmission season by more than four months in the next 50 years.
“Growing urbanization provides an ideal environment for the rapid spread of diseases affecting a large number of the population, often quickly,” Minghui said. He added that frequent floods, poor water storage in homes and ineffective disposal of solid waste are other factors making the environment more hospitable to mosquitoes.
“The vectors (mosquitoes) have silently spread across continents and adapted very well to utilize urban areas to breed in and around human habitations.”
Through monitoring and identifying the hot spots of dengue, researchers discovered they could create predictive maps for future outbreaks for other diseases such as Zika and chikungunya. Data collected between 2008 and 2020 from cities in southern Mexico found there was a 62% overlap between hot spots of dengue and Zika, and 53% for cases of dengue and chikungunya.
In Southeast Asia, the Dengue Forecasting Model Satellite-based System (D-MOSS) gathers satellite data with local insights about dengue cases from partners on the ground, primarily in Malaysia, Vietnam and Sri Lanka. It aims to provide advanced intelligence to government officials in order to control outbreaks. Similarly, NASA scientists are working with local governments and public health officials in the U.S. to help map the locations of disease-transmitting mosquitoes and keep communities safe.
“The combination of vector control, vaccine introduction, and community involvement will be important to combat dengue,” Sasmono said. “Also, the use of new technologies for the control of Aedes mosquito vectors, such as Wolbachia or sterile insect techniques, hopefully can further decrease the disease burden and prevent possible future pandemics.”
Many of the methods used to combat dengue focus on the suppression of mosquito populations. They also need to be reapplied regularly to keep mosquito numbers in check, said Dr. Katie Anders, an epidemiologist and director of impact assessment at the nonprofit World Mosquito Program (WMP). These include both conventional approaches, including insecticide, as well as novel techniques such as the release of sterile male mosquitoes.
In Yogyakarta, Indonesia, a three-year randomized controlled trial saw female Aedes aegypti mosquitoes released into the community carrying Wolbachia — a common natural bacteria found in about 50% of insects — which reduces the ability of mosquitoes to transmit disease. The trial was carried out by WMP in partnership with Universitas Gadjah Mada.
The results, published in the New England Journal of Medicine, showed a 77% reduction in dengue incidences and an 86% reduction in hospitalizations in communities where the Wolbachia mosquitoes were introduced.
“Wolbachia changes the paradigm for the control of mosquito-borne disease, by reducing the ability of mosquitoes to transmit disease instead of killing them,” Anders said.
“This self-sustaining, safe and cost-effective method gives communities long-term resilience against the multiple diseases transmitted by Aedes aegypti mosquitoes.”
All of this gives hope to Chameera, whose brother has fortunately recovered.
“It brings me happiness to think of a dengue-free future,” he said.