Most people who are infected with dengue here are not diagnosed with the disease.
A study of blood samples collected from some 3,500 people living in six dengue hot spots has shown that only one in five victims is diagnosed with the disease.
The others either have only mild symptoms, which they brush off as a garden variety fever or cold, or show no symptoms at all, said the National Environment Agency (NEA), which conducted the study.
This is alarming, the agency added, as it means there are many more dengue carriers here than originally thought. Dengue is spread when an Aedes mosquito bites an infected person, and then transfers the virus to other people it bites.
To help combat the problem, doctors in dengue hot spots have been asked to conduct blood tests on patients who show flu-like symptoms.
People with symptoms such as fever and rash are also being encouraged to slap on mosquito repellent as a safety precaution.
This year, 6,318 people here have been diagnosed with dengue.
However, the study shows this figure is only 'the tip of the iceberg', said Dr Christina Liew, a medical entomologist from the Environmental Health Institute (EHI) - the NEA's research arm - who was involved in the research.
She has a message for anyone feeling unwell: See a doctor, and use mosquito repellent as a pre-emptive measure, so the disease is not spread further.
Dr Ng Lee Ching, head of the EHI, told The Straits Times that the ongoing study, which began in May, showed that half of those who tested positive for dengue showed no symptoms, and did not know they had been infected.
Of the rest, more than half had symptoms such as fever or aches, but were not diagnosed as having dengue.
Dr Ng said that getting more people tested for dengue will help increase the reporting rate for the disease and allow the NEA to identify problem areas faster, as well as assess the population's immunity.
The revelation that most of those who have dengue go unnoticed comes just as weekly figures for the disease here dipped below 200 for the first time since mid-May.
Last week, there were 192 dengue cases, the fourth week in a row that the figure has fallen below warning levels.
However, this level is 'not comfortable', and the fight is far from over, said Mr Tai Ji Choong, head of operations of the NEA's Environmental Health Department.
In fact, in some areas, such as Bukit Batok, the situation is still worrying.
The cluster around Bukit Batok Street 31, 32 and 34, for instance, could set a new nine-year record - there have been 73 cases there in the last 57 days, and it is still active. The current record was set in 2005, with 74 cases in Yishun Street 72.
And despite the cooler weather - mosquito breeding and activity dip along with the temperature - there are other factors at play, said the EHI's Dr Ng.
This year, for example, a new type of dengue - Den 2 - is dominant, so more people are susceptible as they have no immunity to the strain.
She added that cooler weather does not always translate into lower dengue figures.
In 2004, for example, figures remained at warning levels through the year-end period, although that is not the traditional dengue season because of the cooler, wetter weather. And from July to September the following year, even through temperatures fell, the number of cases went up.
High year-end figures also have implications for next year's dengue situation: When the warmer weather sets in, the situation will likely get worse.
'The more we suppress the dengue cases now during the lull period, the more we put ourselves at an advantage when the warmer months come,' said Dr Ng.
arti@sph.com.sg
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HIDDEN DANGER
'A lot of dengue victims are undiagnosed, and they can still transmit the disease because they are not taking the necessary precautions.'
DR CHRISTINA LIEW, a medical entomologist from the Environmental Health Institute, who was involved in the research
