The dengue fever epidemic appears to be on the decline, but the spread of another mosquito-borne disease is still going strong.

Close to 500 people have been infected with chikungunya this year - compared with a total of 60 cases in the three years between 2010 and last year.

In the week ended Aug 3, 21 people were infected - 16 in the Bukit Timah area, one in Jalan Papan in the Jurong area and one in Woodlands Industrial Park, said a spokesman for the Ministry of Health.

Although it is not known where the other three people were bitten, none of them had been overseas recently.

More than 85 per cent of this year's cases have occurred in the Bukit Timah and Sungei Kadut/Kranji areas.

Chikungunya was not found in Singapore until 2008, when the first local transmission occurred in Little India. There was a major outbreak that year, with 690 people infected.

The National Environment Agency was able to eradicate the virus and chikungunya is considered non-endemic in Singapore.

Immediate action is taken whenever an imported case is discovered, so the virus does not take root here. But it might not remain so, if the current epidemic continues unabated.

Like dengue, chikungunya is spread by the Aedes mosquito, and not from person to person. The two diseases have common symptoms such as fever, headache, fatigue, nausea, vomiting, muscle pain, rash and joint pain.

Fatality rates are higher with dengue whereas chikungunya is usually debilitating, with pain lasting for months in more serious cases.

The current chikungunya epidemic started in early April and peaked last month, with 45 new cases in one week.

Meanwhile, the number of weekly dengue infections has fallen from a high of 838 a week in June to 289 in the week ended Aug 3.

More than 14,000 people in Singapore have been diagnosed with dengue this year.