The threat of dengue fever remains low, but authorities say the city should not let down its guard with the weather still favourable for mosquitoes to breed. The average monthly ovitrap index - which measures the prevalence of the dengue-carrying Aedes albopictus mosquito - was 4.4 per cent last month, up slightly from 3.5 per cent in August. The July index was 7 per cent. Only Tung Chung on Lantau Island, with an ovitrap index of 22.9 per cent, breached the dengue-alert level of 20 per cent. Wong Tai Sin Central recorded 11.7 per cent, Shamshuipo East 11.5 per cent and Wan Chai North 10.2 per cent. The index represents the proportion of traps found to contain Aedes mosquito eggs. The mosquitoes breed anywhere stagnant water collects. Food and Environmental Hygiene Department consultant Philip Ho Yuk-yin said that although the monthly average ovitrap index remained low, there should be no slackening in prevention efforts. 'The department is continuing its efforts to fight against mosquito breeding. Intensive on-the-spot inspections are conducted, followed by the elimination of mosquito breeding sources and the application of larvicides to potential breeding grounds, where appropriate,' Dr Ho said. He also reminded travellers to Southeast Asia - where there have been thousands of dengue cases this year - to take precautions, including using insect repellants. Among the countries worst-hit is Singapore, which has reported 10,951 cases, 13 of them fatal. There have been 21 cases of dengue fever in Hong Kong this year, all contracted overseas. The department is also monitoring the Culex tritaeniorhynchus mosquito, which spreads Japanese encephalitis. Samples were recently collected in Kowloon City, but no virus was detected in the specimens examined, Dr Ho said.