Saturday, March 14, 2009

Dengue in Singapore

There was a series of reports through Channelnewsasia about the dengue situation in Singapore arising from the recent Asia Pacific Dengue Workshop. Somewhat confusing I must say.

First on 10/3, they reported the alarming 26% year-on-year increase in dengue cases in the first 9 weeks of 2009. Then, on 14/9 they reported that the number of dengue cases in Singapore were down in the last few weeks. Duhhh..... such mindless reporting, just regurgitating information without processing it!

Fact is the apparent surge in 2009 was due to an unusually large spike of cases over the new year. Here are the actual numbers. Past the first week it has been actually same as 2008. Business as usual. Practically smack on the dot.Actually Singapore has done extremely well in managing the dengue problem. The Cairns Post in Australia carried commendations from medical entomologist Scott Ritchie. "The Far North must follow the lead of Singapore to drastically cut the chance of another large dengue fever outbreak, medical entomologist Scott Ritchie says. He
said a house index, measuring the percentage of houses breeding dengue fever mosquitoes was often used to determine the level of risk an area was at of a major outbreak. Cairns' house index was currently about 25 per cent, compared with 0.5 per cent in Singapore."

That's good stuff. Singapore has done extremely in managing the mozzie problem. We can certainly try to do better, but I seriously don't think we can ever be 100% free of the Aedes mosquito. That bug is an urban warrior. A smart one at that. He is comfortable in and completely adapted to his environment. So the reality is that we need to co-exist with the mosquito much as we loathe him and will likely have to live with a certain degree of baseline risk of dengue ... and chikungunya.

The other thing reporters often forget is to see that picture of dengue in greater totality. The usual dengue infections shouldn't worry us. It carries a certain morbidity no doubt, but it is self limiting and other than making you feel miserable, accounts for little. What is of concern is really the haemorrhagic form. This is the one that kills you....very quickly. How you differentiate one from another is what puzzles scientists. They talk of serotypes and superinfections, but in reality these are only guesses. Reasonable, educated guesses, but still just guesses.

The dengue infection rates fluctuate apparently in 5 year cycles. Don't ask me why, but they seem to do that. Here is a yearly plot of the dengue strike rates from 2007. As you can see we are way off the 2007 peak. Business as usual.

But what this graph doesn't tell you is that the haemorrhagic variety doesn't just follow the overall dengue strike rates. Here is my plot of the ratio of the haemorrhagic form to the non-haemorrhagic form over the last 8 years. As you can see, it also gyrates violently, and even that seems to be coming off a recent peak in 2005.The dengue problem is here to stay unfortunately. We shouldn't be complacent about it because 1-2 out of 100 cases will succumb to the infection and die. But we shouldn't need to be alarmed. Singapore is doing as best as can possibly be done globally. We would be at far greater risk when we travel to any city in tropical Asia.

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