SINGAPORE -The number of week-on-week dengue cases has fallen, but residents should still be alert to the mosquito-borne disease, said Second Minister for Environment and Water Resources Grace Fu at a media conference on Thursday.

There were 424 dengue cases last week, 18 fewer than in the previous week. This is also a marked fall from July, where dengue hit a high of 898 cases in a week.

However, the Den-2 virus is gaining a greater foothold in Singapore, rising from about 15 per cent of all cases in the first eight months of the year, to 25 per cent in August. The more prevalent Den-1 virus has dropped from 82 per cent to 75 in the same period.

Former dengue patients who were infected before with the Den-1 virus can be infected again by Den-2 so they are still vulnerable, cautioned Ms Fu.

Den-2 cases are not more deadly but "those who have dengue before should not let their guards down" she said.

The other two dengue viruses, Den-3 and Den-4, are not common in Singapore.

The biggest cluster now is in the Circuit Road area, where 231 cases have been reported. Eight out of 10 there are Den-2.

To reduce the spread, the Marine Parade Town Council and the National Environment Agency are stepping up dengue prevention measures in the area. Drains have been flushed and cleaning regimes intensified to avoid mosquito breeding. More than 4,000 homes were sprayed with insecticide, including some in Aljunied Crescent where dengue has been reported. More than 40 breeding sites in homes and HDB drains were destroyed.

Said Circuit Road resident Haji Abdul Razak, 34: "I use covers on bamboo pole holders and make sure there is no stagnant water around the house. Better be safe than sorry."