Police are investigating reports that 10 per cent of Hong Kong's ovitraps, which measure mosquito prevalence, have been tampered with. The investigation follows allegations that Food and Environmental Hygiene Department staff ordered sub-contractors to clean out the traps so they could report zero levels of dengue fever-carrying mosquitoes on their patch. Director of Food and Environmental Hygiene Eddy Chan Yuk-tak said yesterday that 200 of the 2,000 ovitraps placed this month had been tampered with. 'We have now referred the matter to the police for further investigation to see if there is any criminal element involved,' Mr Chan said. He said measures had been taken to prevent further tampering and contractors had been told to inform department headquarters if they received instructions to interfere with the equipment. Ovitraps are used to assess the mosquito population of an area by counting how many eggs are laid in stagnant water in the trap. Police said last night the Regional Crime Unit of New Territories South was investigating whether ovitraps in Kwai Tsing district had been tampered with amid reports that 'criminal intimidation may have been involved'. The ovitrap index for Kwai Chung fell from 17.8 in June to zero in August, while Tsing Yi's dropped from 36.5 in May to zero in June and July. Mr Chan said traps that had been interfered with would be replaced and the department would consider redesigning the devices to prevent future tampering. He said the fact that there had been no cases of local dengue fever in Hong Kong since September 2003 and that the number of complaints about the insects had dropped from 9,500 in the first eight months of 2003 to about 7,300 in the first eight months of this year 'indicated a progressively decreasing trend'. Mr Chan would not speculate as to the motive behind the tampering but said there were no monetary rewards for staff who reported zero presence of mosquito ova in their area. Legislator for the medical sector Kwok Ka-ki said the ovitrap tampering had dealt a huge blow to the public's confidence in the government and the department. He called for an independent monitoring body to examine the work of the department and the data it released. 'How can we believe what they say now? The next time they release information about the indices, how can we know it's true,' Dr Kwok asked. He said the tampered with ovitraps and the resultant skewed indices results had 'a great impact on public health' as it gave a false impression of the city's cleanliness. The government allocated HK$200 million for anti-mosquito work for 2006-07.