SINGAPORE - The dengue outbreak continues to surge, with 1,678 people diagnosed with the viral infection last week, a 16 per cent increase over the previous week's figure of 1,449.
It was the fifth week running with more than 1,000 cases. Such high infection numbers have not been seen here before. The biggest number of people infected in a week before this year was 891 infections in 2014.
A total of 17,249 people here have been infected as at 3pm on Monday (July 13). This already exceeds the annual infection numbers for all previous years except for the epidemic of 2013-2014.
Experts warn that the number this year is likely to surpass even the 22,170 infections of 2013.
At least 16 people have died from dengue this year. The highest number of fatalities was in 2005 when 25 people died.
There are more than 370 active clusters, mostly in the east, with 133 considered high risk, while 17 have more than 100 people infected.
There are three clusters where more than 200 people living or working within 150m of each other have been infected within a two-week period.
The biggest cluster is in the Bukit Panjang-Woodlands area, with 242 infections, of which 42 occurred within the past fortnight. Block 608 in Senja Road, which is part of the cluster, has 33 people with dengue.
Professor Duane Gubler, a dengue expert at the Duke-NUS Medical School, said: "There are a number of factors that influence transmission dynamics and epidemics."
A surge could be caused by a change in the virus serotype. There are four dengue viral strains and getting infected by one doesn't protect a person from getting the other three.
In the early part of this year, the less frequently seen DenV-3 appeared to be gaining ascendancy and was believed to have caused the rise in infections as few people here are immune to it.
But it now seems to have given way to DenV-2, which is often the dominant strain here.
The National Environment Agency suggested some weeks ago that the unusual increase in cases could be due to the circuit breaker measures implemented to fight Covid-19.
The Aedes mosquito that spreads dengue is a day biter, so more people staying home could lead to increased numbers of infections.
The lockdown measures were lifted some time back but the surge in cases continues unabated.
Prof Gubler speculated that "if virus serotype and herd immunity, mosquito population densities and human behaviour have been ruled out, I would guess that it is the strain of virus".
"It has been demonstrated on several occasions that small mutations in the virus, combined with positive selection, can lead to the emergence of strains of virus of any serotype that have greater epidemic potential."
Dengue infections are expected to stay high for some months, with the hot and wet weather making it easier for the mosquitoes to breed.
Symptoms of dengue include a sudden onset of fever that lasts between two and seven days, severe headache and pain behind the eyes, nausea and vomiting, and joint and muscle ache.
Some people might get a skin rash that looks like red pinpricks, as well as bleeding from the nose and gums.
There is no specific treatment for dengue, but doctors can provide support and ease pain. Most people can be treated at outpatient clinics.
About one in five become so sick that they need hospital care.