Philip Ho Yuk-yin, consultant for the Food and Environmental Hygiene Department, said the findings were cause for alarm because the five areas represented a third of the 16 districts assessed so far. The department has to assess a total of 38 districts.

In April last year, only four out of 38 districts had an index of 10 per cent and higher.

Dr Ho warned that the index would continue to rise next month and in June when the rainy season started. He said Hong Kong faced a high risk of dengue fever infections this year as previous patterns showed that the ovitrap index peaked every two years. The index averaged 3.8 per cent last year compared with 10.8 per cent in 2004.

Pest control officer-in-charge Yuen Ming-chi said that the five districts shared similar problems such as stagnant water near housing estates, at hillsides and construction sites.

Mr Yuen said the department had stepped up measures to eliminate mosquitoes and improve hygiene in those areas.

Dr Ho also warned of another mosquito borne disease, Chikungunya fever, after a 66-year-old man was infected with the disease after visiting Mauritius last month.

The man developed fever, chills and muscle pain after returning to Hong Kong. He was sent to Princess Margaret Hospital from Prince of Wales Hospital for observation and had been discharged, according to the department.

'The risk of Chikungunya fever outbreak in Hong Kong remains low as the disease is found mostly in African and Indian Ocean Islands,' he said.

'But it would be important for Hong Kong people who travel to those areas to take precautions such as using mosquito spray to avoid mosquito bites. In Hong Kong, it is important for people to control the breeding of mosquitoes to avoid the spread of the diseases.'