SINGAPORE – Dengue infections have been going up for six consecutive weeks, with 396 cases reported in the week of Jan 7 to 13 – the highest weekly figure in more than a year. Six weeks ago, there were just 143 cases.

While this is a far cry from the 1,552 cases a week at the peak of the infection in 2022, the steady increase in cases has the National Environment Agency (NEA) urging people living in dengue cluster areas to take immediate action to suppress the Aedes mosquito population and help break disease transmission.

It also asked them to cooperate with NEA officers inspecting their property for mosquito larvae.

There were almost 10,000 dengue cases reported in 2023, down from the more than 32,000 cases in 2022. The largest number of this Aedes mosquito-borne viral infection seen here was in 2020, when there were 35,315 cases and 32 deaths.

Dengue caused three deaths here in the first nine months of 2023. In 2022, 19 people died.

However, experts say the recent storms could lead to a drop in dengue cases.

“The general trend is for dengue cases to dip after heavy rainfall and this dip can go on for as much as six weeks,” said Professor Hsu Li Yang, an infectious diseases expert and vice-dean for global health at the National University of Singapore Saw Swee Hock School of Public Health.

While wet weather resulting in standing water increases mosquito breeding, heavy rain causes a “flushing effect” where the larvae are washed away, killing most of them.

The experts are also not overly worried about the two changes in the dominant dengue serotype in 2023, although they warn that a second infection could prove more serious.

There are four dengue serotypes. DEN-3, the dominant strain behind the bulk of infections in 2021 and 2022, was overtaken by DEN-1 in June 2023, and later DEN-2 in September.

People do not get reinfected by the virus from the same serotype, but can be infected by the other three strains.

Professor Paul Tambyah, president of the United States-based International Society for Infectious Diseases, said the switch in dengue serotype is unlikely to result in a higher number of infections, although it may be too early to be sure.

“There was concern (back in 2021) that DEN-3 had not been circulating for some time and thus immunity levels would be low,” he said.

“DEN-2, on the other hand, has been circulating almost continuously for decades. And thus, this year’s peak may be lower than previously, like last year’s, when DEN-1 was dominant during the mid-year season,” said Prof Tambyah, who is also a senior consultant in infectious diseases at the National University Hospital.

However, both he and Prof Hsu said people who are infected a second time – meaning the infection is caused by a different serotype – are likely to become more seriously ill.

Said Prof Tambyah: “The phenomenon of antibody-enhanced infection is now well accepted, and thus a second infection in many cases – not all – can be more serious than a first.”

Antibody-dependent enhancement refers to the body producing suboptimal antibodies – in the case of dengue, the antibodies target the earlier serotype but are not the best for a subsequent infection by a different viral serotype – that may actually increase the ability of a virus to enter cells and cause a worsening of disease.

Prof Tambyah added: “There is also the question as to which combination of viruses – meaning, DEN-1 followed by DEN-2, or the other way around – is more serious. The bottom line is that we should avoid getting bitten and getting any infection at all.”

Symptoms of dengue include a sudden onset of fever, headache, muscular and joint aches, nausea and vomiting.

Anyone with such symptoms should see a doctor to prevent the infection from taking on a more severe form, such as dengue haemorrhagic fever or dengue shock syndrome.

There are currently 80 active dengue clusters, of which 17 have 10 or more cases. The biggest cluster, in the Boon Lay-Corporation Road-Jurong West area, has recorded 200 cases, of which 48 were within the past fortnight.

The NEA said it had observed a “fast rate of dengue transmission” in this cluster, as well as two in Pasir Ris and one in Jalan Chegar in Upper Thomson.

It urged people to use insect repellent to protect themselves from getting bitten by the Aedes mosquito.

Anyone diagnosed with or suspected to have dengue is also advised to avoid further mosquito bites, to prevent passing on the dengue virus to mosquitoes and other people in their neighbourhood, NEA added.