By Dr Monica Chan and Professor Leo Yee Sin
Singapore is in the midst of a dengue outbreak, with weekly cases quadrupling in the last three months and hitting a three-year high of 467 cases in the week ending June 15. The 5,184 dengue cases as of June 15 already far exceeds the 3,285 cases reported in 2018 and the 2,772 cases in 2017.
What could be behind this spike and how can we better manage dengue from a medical point of view?In 2012, the World Health Organisation (WHO) ranked dengue as the "most important mosquito-borne viral disease in the world" with an estimated 40 per cent of the world population at risk. The frequency of dengue infection has increased 30-fold in the past 50 years and now circulates in more than 100 countries in the Americas, Caribbean, Africa and Asia, where three-quarters of global dengue disease cases occur.
Singapore launched a comprehensive nationwide programme in 1968 targeting the Aedes mosquito, the main mosquito vector of dengue. This control programme combines environmental management, source reduction, public health education and law enforcement. Since then, the number of homes found to be breeding Aedes mosquito fell sharply and remains low.
Dengue, however, remains endemic in Singapore. Since the 1990s, there have been periodic spikes in cases occurring in five- to six-year cycles. This year's spike could be a case in point, coming after the epidemics in 2013 and 2014, which saw over 22,000 and 18,000 cases respectively.
Several theories have been proposed to explain this cyclical pattern. Dengue in Singapore typically rises in warmer months, from April, peaking in July or August, before declining in September or October. Abnormal climate changes, such as the El Niño weather phenomenon, may affect the mosquito breeding environment, accelerate the life-cycle of Aedes mosquito, shorten the incubation period of the dengue virus and drive up cases.
The dengue virus has four different serotypes, and a switch in the predominant virus serotype has been a historical precursor of dengue epidemics in Singapore. For example, two large epidemics, each stretching over two years in 2004–2005 and 2013–2014, were associated with a switch in the main dengue virus serotype from serotype 2 to serotype 1.
Population factors also contribute to this endemic. The decrease in dengue transmission due to the successful implementation of the Aedes control programme in the 1970s and 1980s reduced incidence of dengue infection in the population. As herd immunity to dengue is consequently low, a large proportion of the Singapore resident population today remains susceptible to infection by any of the four dengue serotypes.
The average age of patients with dengue infection has shifted from approximately 14 years in 1973 to above 30 years in the last decade. Primary infections in young children are generally mild or silent, while infections in adults are clinically overt and symptomatic which contribute to increasing detectable cases.
On the other hand, dengue in the elderly tends to present atypically, with fewer of the common symptoms of dengue such as headache, fever, rashes and muscle ache, making clinical diagnosis challenging.
To exacerbate this situation further, older adults frequently have pre-existing chronic ill health, placing them at risk of more severe secondary dengue infections.
The presence of concomitant chronic co-morbidities and multiple medications complicates the management of dengue. The elderly are consequently at higher risk of developing severe dengue and have higher risk of mortality, as demonstrated unfortunately by the four fatal cases this year. The elderly are also more likely to have a prolonged hospital stay with greater risk of hospital-acquired infections.
How, then, can we help in the fight against dengue? While there are innovative ways of mosquito control such as the Wolbachia mosquito that may prevent large scale dengue epidemics, it is crucial that the community continues basic mosquito control measures and embraces the "five-step Mozzie Wipeout" throughout the year to protect us from dengue. We must ensure homes, work, school and common premises are free of stagnant water which provides for potential mosquito breeding sites.
Vaccination is also a prevention mechanism. But dengvaxia, the only commercially available dengue vaccine for people aged 12-45 years old provides incomplete protection and those vaccinated can still be infected with the disease. Dengvaxia is recommended only for those who have contracted dengue before. Large clinical trials have found that those who have never had dengue before are at higher risk of getting severe dengue infections if they receive the vaccination. It is for this reason that the WHO recommends vaccination programmes only for highly endemic areas where at least 80 per cent of the population has had dengue before. In the case of Singapore where the population has not reached this threshold, its use is recommended only to those whom have evidence of past infections.
The medical community too needs to be mobilised. In commemoration of Asean Dengue Day which falls on June 15 every year, the National Centre for Infectious Diseases looks forward to engaging the medical community at our annual seminar on Saturday (June 22). Our emphasis continues to be placed on the provision of optimal care at appropriate levels with focus on raising the index of suspicion, early testing, early diagnosis, close monitoring, and appropriate referral to tertiary care.
Recognising that the elderly population may not manifest typical signs and symptoms of dengue, it is even more critical for doctors to have a keen sense to suspect, test and review. Since 2006, dengue on-site diagnostic tests with quick turnaround time have been made available to primary care clinics to facilitate the early diagnosis of dengue. Surveys of primary care practices in 2011 and 2014 have demonstrated the increased confidence and better management of dengue among primary care practitioners.
The fluctuation of blood pressure and blood sugar levels are also common during acute dengue and add to the difficulties in managing those with pre-existing high blood pressure, insufficient heart function and diabetes. Close monitoring of those at risk of severe dengue and its complications, such as patients on blood thinners with risk of bleeding, need to be built into existing care processes.
Dengue shows no signs of abating; more research is needed on countermeasures. Although achieving zero dengue death is a stretch goal, it is a necessary one that requires a concerted effort from all of us.
Dr Monica Chan is a Senior Consultant at the National Centre for Infectious Diseases and Professor Leo Yee Sin is the Executive Director of the National Centre for Infectious Diseases.