Dengue Alert

Letter published in The Straits Times, 22 Aug: Monitor drains more closely for mosquitoes

Letter published in the Straits Times, 22 Aug:

Monitor drains more closely for mosquitoes
Monday, 22 August 2005

Other than the blog of clogged drains started by Mr Jeffrey Ho (“Man starts blog to track mosquito sites”, ST, Aug 17), I hope the public will use the toll-free hotline to the National Environment Agency (NEA), 1800-CALL NEA (1800-2255632).

NEA investigations “show that 75 per cent of mosquitoes found breeding in drains belong to the Culex species, while just 25 per cent are dengue-carrying Aedes variety”, and that drains make up “just 6 per cent” of areas found breeding Aedes mosquitoes.

However, this last percentage relates only to breeding grounds known to NEA. It does not include unattended choked drains and the actual percentage is therefore higher.

Mr S. Satish Appoo of NEA writes of public drains that the “frequency of cleaning is adjusted to accommodate ground conditions” (“Aedes mosquitoes breed mostly in homes; ST, July 30). If roadsides need to be swept daily to keep them free of leaves, surely nearby drains should be cleared of leaves at least once a week, if not daily?

NEA contacted me after my letter was printed (“Mosquito breeding: public drains key culprit?”; ST, July 22), to check the location of the two drains. One was cleared on July 25 but was not touched again for at least two weeks after that.

The other drain I mentioned with stagnant water is about 100m long. If 25 per cent of the mosquitoes that breed in drains are the dengue-transmitting Aedes species, then of 100m of stagnant water in a drain, 25m (equal to more than 50 flowerpot plates) will breed Aedes mosquitoes.

How long does a flowerpot plate breed mosquitoes before it is detected by the home owner or NEA? Apparently, in a public drain, mosquitoes keep breeding until the water dries up or heavy rain flushes the drain clean.

Studies have shown that in this climate, mosquitoes need only seven days to mature from eggs. What if eggs or larvae are caught in moss or crevices in drains, and are not swept away by heavy rain? Then it will take them only a few more days to become adult mosquitoes (and not seven days after heavy rainfall).

By replying that “our studies have found that the mosquitoes that breed in the drains are mostly of the Culex species”, NEA surely does not mean it is all right for drains to breed mosquitoes? When “mostly” turns out to be 75 percent, one wonders how many Aedes mosquitoes make up the remaining 25 percent. One-quarter of all choked drains is a significant portion.

Even if some mosquitoes do not transmit dengue or malaria, their bites cause mush discomfort.

Gwee Jin Eng (Ms)

posted by Dengue Alert In Singapore @ 1:47 PM