Let’s Talk Public Health with Chew Ming Fai

To prevent dengue, vector control is key. We must prevent the breeding of Aedes mosquitoes, which transmit the virus. The main goal is a continuous reduction of the Aedes mosquito population. This is achieved mainly through good environmental management to prevent and remove potential breeding sites. So much of dengue control depends on the actions of individuals and communities to be continually vigilant in removing any and all stagnant water where the Aedes mosquitoes breed. This is no easy task in Singapore where it rains an average of 170 days a year.

NEA works with various stakeholders, including government agencies, town councils, grassroots organisations, volunteer groups, and the general public to continually raise awareness of vector control efforts and best practices. These partnerships have a powerful multiplier effect against mosquito breeding.

Project Wolbachia, the latest tool in our toolbox, involves releases of male Wolbachia-carrying Aedes aegypti (Wolbachia-Aedes) mosquitoes to suppress the dengue mosquito population in the community while complementing existing measures. When male Wolbachia-Aedes mosquitoes mate with urban female Aedes aegypti that do not carry Wolbachia, their resulting eggs do not hatch.

The results so far have been promising. In areas of Tampines and Yishun where releases have been ongoing for more than a year, we have seen up to 98 per cent suppression of the urban dengue mosquito population, and up to 88 percent reduction in dengue cases. In the current outbreak, these two towns have 70 per cent fewer dengue cases compared to similar areas without Wolbachia.

A person living in Singapore today is ten times less likely to acquire a first-time dengue infection compared to the 1960s. This is indicative of the success of our vector control programme. Paradoxically, however, this also means falling population immunity to dengue. As a result, it raises the risks of an explosive growth in the number of dengue cases and clusters in successive outbreaks.

Climate change and urbanisation are expected to create more conducive conditions for mosquito vectors, hence exacerbating the dengue situation in Singapore and globally. Increased temperatures and more intense rainfall can speed up mosquito breeding cycles and virus growth rates and potentially create more breeding habitats. The expansion of urban areas also favours Aedes aegypti mosquitoes, which have adapted to living and breeding in built environments.

Against this backdrop of low population immunity, favourable conditions for mosquito breeding, and the continued presence of dengue mosquitoes and viruses, Singapore will continue to experience regular dengue outbreaks. Our dengue control programme, including community mobilisation efforts and the incorporation of novel technologies, must continue to keep pace.

2020 was the first year of the COVID-19 pandemic and we also experienced the highest reported dengue cases on record. In 2020, there were 35,315 reported cases of dengue and 32 deaths.

There were a few reasons for the outbreak. Firstly, there was an increased transmission of the less common DENV-3 serotype. As this serotype was last dominant in Singapore about three decades ago, population immunity against the serotype is low, which likely contributed to the high case numbers.

The dengue situation was likely worsened by COVID-19 safe distancing measures. Singapore experienced a two-month “circuit breaker” from 7 April to 1 June 2020, where most of the population stayed at home. Around 95% of businesses were physically shut, schools were shut, and hospitality was shut. Aedes aegypti mosquitoes, which are generally day-biting, had easy access to blood meals as more people stayed home rather than go to work in air-conditioned offices.

Disruption to daily activities could also have affected cleaning and maintenance activities of all stakeholders, such as litter removal, housekeeping, landscaping works and so on.

While environmental management and source reduction remain the cornerstone of Singapore’s dengue control strategy, NEA continuously pursues new solutions and technologies to complement and enhance our integrated vector control programme. For example, we are testing improved vector surveillance tools such as smart traps that can automatically identify the gender and type of mosquitoes they capture. Data analytics and modelling capabilities will also help us to better forecast disease risk.

In the area of vector control, Wolbachia-Aedes suppression technology will complement traditional source reduction measures to further reduce transmission risk. Given the promising results of our ongoing trials, we will be expanding Wolbachia-Aedes releases to an additional eight locations in July 2022. Besides reducing the population of dengue mosquitoes in these areas, the expansion will allow us to understand the impact of a large-scale multi-site deployment of Wolbachia technology on dengue cases. With this expansion, Project Wolbachia will cover an estimated 300,000 homes, up from the current 160,000. This includes 31 per cent of all HDB blocks in Singapore, up from the current 19 per cent.

Promising dengue vaccine and therapeutic candidates are also being developed and evaluated by the healthcare industry. Together with vector control efforts, these will in time help equip countries with improved tools to combat dengue.

Viruses are unconstrained by geographical boundaries, and the growing dengue threat is a global one. Tackling it requires collective efforts, so international cooperation, coordination, and information sharing are critical.

Further, as a travel and trade hub, Singapore will continue to be exposed to viruses and vectors imported from overseas; it is therefore in our interest to work with international partners to build dengue control capacity regionally and globally.

For example, NEA’s Environmental Health Institute is a World Health Organization Collaborating Centre for Reference and Research of Arbovirus and their Associated Vectors. As part of this role, we share technical expertise, best practices, and lessons learned with vector-borne disease control teams in other countries, to enhance global outbreak preparedness.

We are also part of the UNITEDengue consortium, a regional network that facilitates cross-border sharing of case and virus surveillance data, as well as knowledge on dengue control and evaluation of novel methods.

I have taken on a keen interest in backpacking and hiking in recent years with visits to Tiger Leaping Gorge in Yunnan, Huashan (Mount Hua) in Shaanxi, Khong Lor cave in Central Laos and Chiang Dao in Thailand. With the gradual resumption of travel, I am looking forward to a trip to Southern Vietnam later this year.