A RELAXED ATTITUDE TOWARDS THE AEDES MOSQUITO COULD RESULT IN CONSEQUENCES IN THE MONTHS AHEAD:NEA WARNS
The number of dengue cases has remained below the warning level over the last few weeks. But the National Environment Agency (NEA) warns that a lack of vigilance now may lead to higher cases during the November to February period - even though these months are not the traditional dengue season.
The number of dengue cases crossed into epidemic level twice this year, the first time during Epidemiological Week 24 (10 to 16 June) when 401 cases were recorded, and the second during Epidemiological Weeks 26 to 28 (24 June to 14 July) when an average 397 cases were recorded weekly. Since then, the number of cases has been dropping. For E-week 35, the estimated number of dengue cases is 192 (as of 4pm, 3rd Sept).
This drop in number of cases over the past seven weeks would not have been possible without months of source reduction efforts and the cooperation of land agencies and the community. Other than sweeps for mosquito breeding in each constituency, a further 521,400 hours have been devoted to intensified source reduction efforts throughout the island over the past four months by the 500 Environmental Health officers (EHO) and 150 to 300 non-EHOs from other departments in NEA.
The NEA however warns against complacency as we are still seeing about 200 cases a week. It would be dangerous to assume that we can let our guard down just because temperature is cooler. Theoretically, cooler weather helps to slow down the mosquito’s reproduction ability. Studies have also shown that when mean temperature dips to 15 degree Celsius, the mosquito can take up to 41 days to develop1. But when mean temperatures are at 25 degree Celsius, the eggs take only 10 to 11 days to mature into adults.
Singapore's average temperature rarely goes below 25 degrees. Our cooler months thus still support dengue transmission - remaining conducive for mosquito breeding and virus replication in the mosquitoes. Allowing the mosquito population to build up during this period would lead to bigger numbers of cases in the next few months.
Research scientists at NEA also warn of the need to take proper vector control measures during a lull period. Said Dr Ng Lee Ching, Head of NEA’s Environmental Health Institute: “A lull period offers us the opportunity to bring future number of dengue cases down. It is important to suppress the mosquito population and subsequently dengue cases to as low a level as possible during this period. This will help to lower the baseline from which the number of cases will build upon during future warm months."
This phenomenon was experienced at the end of 2004 when weekly average dengue cases hovered around 280 a month in November and December leading to abnormally high numbers in January and February of around 276 cases. Previous patterns show average weekly cases to be below 100 during these months.
Dr Ng explains further: “Dengue viruses are transmitted from people infected with dengue to the mosquitoes which in turn pass it on to more people. A large number of dengue cases can lead to more mosquitoes being infected and to even higher rate of transmission. We must therefore remain vigilant in doing source reduction. This will help us to continue to keep cases low.”
A third reason to remain vigilant is that lesser numbers do not spell the end of localized outbreaks. Even now with cases below warning level, big clusters still develop. One example, the Bukit Batok cluster which has 73 cases. Cooler weather is also not an indication that serious clusters would not develop. Bukit Batok East Avenue 5/Street 52 was the scene of a big localized outbreak in December 2006, which registered 67 cases. The two clusters was a result of 109 common Aedes breeding habitats found in homes. Another 37 breeding habitats found outside homes also added to the problem.
The increase in homes found breeding compared to last year is a further cause of concern. The figure more than doubled from Jan to July this year compared to the same period last year from 1,616 to 3,289. This trend needs to be bucked.
The key to dengue prevention is source reduction, and it needs to be integrated into our daily living for the disease to be successfully managed.
~~ End of News Release ~~
1Contrast this with an eight days duration when temperature is at 30 degree Celsius and it is easy to understand why our tropical climate is conducive to mosquito breeding.