SINGAPORE'S dengue fever epidemic has entered its 12th week and is looking serious.
There are three strains of the virus that are almost equally active, and this is a rare occurrence.
As a result, weekly infection cases are at a six-year high, and more than double the figures seen in the first three months of the last three years.
Experts fear that if these trends continue, this year's outbreak could be worse than in 2005, when 14,000 people fell ill and 25 died.
So far this year, more than 3,100 people have been infected by the mosquito-borne disease, with a quarter landing in hospital. There have been no deaths so far.
Last week, 308 people were diagnosed. Usually, there are fewer than 100 infections a week this early in the year.
Latest figures from the Ministry of Health show that the long dormant Den-3 strain of the virus has resurfaced, and was responsible for 33 per cent of infections last month. The Den-2 strain - the most common strain since 2007 - made up 39 per cent, and the Den-1 strain accounted for 26 per cent.
Dengue sufferers develop an immunity to the particular strain they contract that usually arrests further spread of the disease.
As the Den-3 strain has stayed low key for more than a decade, people in Singapore do not have immunity to it, said internal medicine specialist Doshi Mukund of Parkway East Hospital.
To make matters worse, the presence of three strong strains means a dengue sufferer who recovers can more easily be re-infected with a different strain.
"Those who have been exposed to previous two viruses and contract the new virus will be at risk of having a more severe disease," said Dr Mukund.
Clinical director of the Communicable Disease Centre Leo Yee Sin noted that the two newly-active strains - Den-1 and Den-3 - are infecting more than half the patients.
"This requires close monitoring," she said.
Historically, dengue epidemics come in five-year to seven-year cycles, with each peak significantly higher than the previous one.
Experts expect this year's numbers to rise further, as dengue cases usually peak during the hotter months of May to July.
"There is typically a lag period from the wet months to the peak of the dengue cases," noted Dr Indumathi Venkatachalam, an infectious diseases expert at the National University Hospital.
If this is the lead-up to the usual mid-year highs, then the number of infections this year could exceed Singapore's worst outbreak in 2005. That was the only time weekly infections topped 300 a week this early.
A Health Ministry spokesman said yesterday that although the number of infections is up, the number of people with haemorrhagic fever is low and nobody has died.
The biggest hot spot is Tampines, where 167 people are down with it.
Responding to queries, the National Environment Agency (NEA) said it is stepping up ground-level checks nationwide to spot and eradicate potential mosquito breeding grounds.
At the two biggest clusters - both in Tampines - NEA's search and destroy operations "are being extended to another 20 blocks outside each cluster zone to create a wider buffer to prevent further spread of the virus".
As a preventive measure, it will also send more than 60 officers to check homes and outdoor areas of about 100 blocks of flats between these clusters.
