David Brill
Efforts to control dengue in Singapore appear to have been a success this year, with early fears of a repeat outbreak proving to be premature.
Last year saw levels pass the epidemic threshold in early July, when 432 cases of dengue fever and dengue hemorrhagic fever were reported in a single week.
This year, however, the combined number of cases has so far remained comfortably below the warning level of 279 cases in a week.
That same week in July, the peak for 2007, saw just 160 cases reported in 2008, according to Ministry of Health (MOH) statistics.
Experts had raised concerns about the possibility of another epidemic when more cases were reported in the first 3 months of this year than last year.
In response the National Environment Agency (NEA) invested more resources into the Campaign Against Dengue, intensifying operations to identify and destroy mosquito breeding habitats.
In a joint statement with the MOH, the NEA said that this could be one of the reasons for the lower number of cases observed this year.
They also said that their annual ‘Intensive Source Reduction Exercise’ – an initiative targeting the town councils who manage the public housing estates – was launched earlier this year than last.
“We should nonetheless continue to monitor the dengue situation closely and sustain our efforts against the vector because the secular disease trend remains in the upward direction and epidemic transmission continues to be a problem,” the statement concluded.
Associate Professor Leo Yee Sin, head of the department of Infectious Diseases at Tan Tock Seng Hospital, said that the dengue patterns observed so far for 2008 are similar to typical non-epidemic years.
She also noted that last year’s outbreak was preceded by a distinct switch in the predominant circulating dengue strain from DEN-1 to DEN-2. The lack of epidemic this year could therefore be attributable to growing herd immunity against this strain, she said.
Leo added that the outbreak of the chikungunya virus in January this year prompted “massive search and destroy efforts” targeting the Aedes aegypti mosquito, which is also responsible for spreading dengue.
The worldwide incidence of dengue has “grown dramatically” in recent years according to the WHO, who estimate that two fifths of the global population is now at risk.
Singapore has experienced regular dengue epidemics since the first reported outbreak in 1901. These typically occur when there is a shift in the balance between the virus strains, all four of which are present in the country.
A recently-published analysis of the 2005 epidemic found that DEN-1 was responsible for 71.2 percent of cases and DEN-2 just 9.2 percent – a predominance that appears to have reversed in 2007. The development of an effective vaccine against all the strains could be the only definitive way to control dengue in Singapore in the long-term, the authors wrote. [Ann Acad Med Singapore 2008 Jul;37(7):538-45]
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