Food and Environmental Hygiene Department consultant Ho Yuk-yin welcomed the lower figure, citing it as evidence of the success of the government's anti-mosquito campaign.

Some district councillors believed a major reason for the improvement was the unusually rainy weather in the past few months.

'Still water can be perfect breeding grounds for mosquitoes,' said Henry Chan Man-yu, chairman of Yau Tsim Mong District Council, 'But frequent and high rainfall can make it hard for mosquitoes to proliferate.'

More than 360mm of rain was recorded by the Observatory last month, about 37mm above average, following almost 894mm in June, double the monthly average and the fourth highest on record. Rainfall so far this year is more than 40 per cent above average.

Dr Ho said a reason for the decline was a better co-ordinated anti-mosquito campaign.

'Our new surveillance system, the Geographical Information Hub, has also smoothed the flow of information among districts.'

The ovitrap indices for April, May and June were all significantly lower than the average of the previous five years. Six areas where the indices were above 20 per cent in June saw the index fall to below 10 per cent.

Sheung Wan fell from 32.1 to 3.9 per cent, but Yau Ma Tei was at 27.1 per cent, from 1 per cent in June, and Shamshuipo at 21.2 per cent.

'Despite the improvements, we cannot afford any slackening in the fight against mosquito breeding as there are still chances for the disease vector to proliferate in the current rainy season,' Dr Ho said.

He urged people travelling to Southeast Asian countries to take protective measures against mosquito bites, as 11 imported cases of dengue fever had been reported this year.

The department will on Monday start the third phase of this year's anti-mosquito campaign.