Disease experts yesterday warned that a severe form of dengue fever could emerge in Hong Kong if the outbreak on Ma Wan Island is not brought under control. Dengue haemorrhagic fever claims 12,000 lives worldwide each year, many of them children, in countries where dengue fever has become endemic. The potentially fatal complication triggers internal bleeding. The warning came as one more local case was confirmed yesterday, bringing the number to seven. The man, 36, who lives on Ma Wan, developed fever, muscle pain, joint pain and a rash in July and was admitted to Queen Mary Hospital from July 25 to August 2. He is the second Ma Wan resident to have contracted dengue fever locally. The five others are construction workers at the island's Park Island housing development. City University associate professor in microbiology Desmond O'Toole said dengue fever was a serious public health menace because it spreads rapidly, with a high risk of people being repeatedly infected. 'If within a year you get a second infection it becomes a really serious one [dengue haemorrhagic fever],' he said. Chinese University microbiology professor John Tam Siu-lun said antibodies produced after the first infection with dengue were unable to inactivate the virus when it reoccurred. 'Instead, the antibodies help the virus to enter the white blood cells, leading to more serious clinical symptoms,' he said. Legislator Dr Lo Wing-lok, an infectious disease specialist, said the first dengue fever outbreak showed up a gap in the disease surveillance system. Ma Wan, an outlying island off Lantau, is sparsely populated but there has been a lot of construction work where mosquito-breeding grounds have accumulated during the rainy season. 'But because Ma Wan is sparsely populated, the alert was not that sensitive. If in urban areas people see lots of mosquitoes, the alarm would have been raised,' Dr Lo said. The Hong Kong outbreak comes a year after the first outbreak of dengue in Macau, where nearly 1,200 people were afflicted. No one died. The experts said the mosquitoes could have travelled far with the sea breeze or been carried to the island in vehicles. Some victims could have been bitten by a mosquito when on holiday abroad. 'We hope the situation is still manageable. If the infected mosquitoes are still confined in Ma Wan, there is a chance we can eradicate dengue,' Dr Lo said. Dr Lo also suggested the Food and Environmental Hygiene Department conduct a year-round anti-mosquito campaign.