SINGAPORE - Low immunity among the population against the dengue virus is likely to be contributing to the surge in dengue infections this year, especially as mosquito numbers remain persistently high, said local health and environment experts on Saturday (June 22).

This comes amid a spike in dengue cases around the region, which indicates that the Aedes mosquito - the carrier of the disease - may be thriving with a warmer weather pattern this year, said experts.

At a media sharing session in commemoration of ASEAN Dengue Day 2019 a week ago, a panel of experts also raised concerns about the disease among patients over 65, who tend to suffer more, yet typically do not display the usual dengue symptoms.

These patients are likely to have contributed to the small uptick in the average hospitalisation rate for the disease in recent years, they said.

Singapore is in the midst of a dengue outbreak, with five reported deaths from the disease so far this year. As of last Monday (Jun 17), there were 5,261 infections reported, a 60 per cent increase over the total for the whole of last year, and close to double 2017's overall numbers.

Associate Professor Ng Lee Ching, director of National Environment Agency's (NEA) Environmental Health Institute, said Singapore has low herd immunity against the disease as good mosquito control over the years means many people have never been infected.

A study by NEA on the prevalence of the disease in healthy adults here showed that only about 16 per cent of Singapore's youth aged between 16 and 20 years old in 2009 had had dengue before, she added. The study, which is the latest one available on the topic, was published in 2015.

A recovered patient is immune to the particular strain of dengue he or she was infected with.

"When we say we have low herd immunity and that we are very sensitive to outbreaks, that means that we just need a few mosquitoes to have an outbreak," said Prof Ng.

Professor Leo Yee Sin, executive director of the National Centre for Infectious Diseases, noted that a particular group of concern was the elderly.

A study published last month that reviewed dengue hospitalisations showed that a higher proportion of elderly dengue patients were hospitalised from 2003 to 2017, than in younger patients. In 2017, 47.2 per cent of dengue patients aged 65 and above were admitted to hospitals.

This has probably contributed to the slight increase in the average hospitalisation rate for dengue, from 2014's low of 25.6 per cent, to 2017's 35.9 per cent, said Prof Leo.

The elderly tend to face more complications with the disease because of existing chronic conditions, and they are more susceptible to secondary infections, she said.

"We are now in this peculiar situation where we see more older people with dengue, and we anticipate that they would have more severe disease," said Prof Leo.

Another factor which compounds the problem is the difficulty of diagnosing dengue in the elderly, as they tend not to display the typical symptoms such as fever, body aches and rash, said experts.

Dr Wong Wei Mon, a senior physician with the College of Family Physicians Singapore, said the elderly may display only fever or nausea when they visit the doctor, or may even have symptoms not listed as common dengue symptoms.

"Doctors, if they are not alert enough, may not trigger the definitive test to confirm the presence of dengue," said Dr Wong.

"The elderly tend to fall into the risk of getting complications of severe dengue, so we may miss early diagnosis, and by the time we pick it up it is actually quite dangerous."

To combat this, the authorities have been advocating for doctors to keep a keen eye out for dengue among the elderly, said the experts.

The NEA too has been ramping up efforts to prevent mosquito breeding, such as introducing Aedes moquitoes infected with the Wolbachia bacteria into the wild to help reduce the mosquito population, said Dr Christina Liew, NEA's medical entomologist.

"As the mosquito evolves and becomes smarter, we have to make sure that we do continue efforts to think of ways to outsmart it," said Dr Liew.

Prof Ng added: "It is the community and everybody's effort, it is not just effort on one part. Mosquito breeding sites could be everywhere.

"We need everybody's efforts to reduce this population."