People over 40 are eight times more likely than those younger to have been exposed to the potentially fatal Japanese encephalitis, a study has shown. Disclosing the results yesterday, the Centre for Health Protection said the overall exposure rate to the mosquito-borne disease was very low, confirming it is not endemic in Hong Kong. But people aged 40 and above have a 'significantly higher' Japanese encephalitis positive rate of 2.5 per cent, compared with younger people with 0.3 per cent. The centre said this was probably due to previous exposure, especially in the 1960s, when they lived on the mainland or in rural areas. The findings come from a 'seroprevalence study' in which blood samples taken from 2,047 people were tested from June 10 to July 12 to assess the risk of infection among different groups. The study was prompted by the confirmation of three local cases of the disease in June and last month, including an Indonesian domestic helper who died. Two men fell severely ill but are now in a stable condition in hospital. Those tested included 558 residents in areas neighbouring the location of the three cases, none of whom had been exposed. Only one of 48 Mai Po residents and none of the 51 pig farmers and abattoir workers tested positive. The Mai Po wetland and 300 pig farms are being monitored as high-risk areas for the Culex mosquito, which can transmit the disease to humans. Of 1,190 members of the general population - represented by people seen at outpatient clinics - 28 were positive. Meanwhile, the mosquito density index has eased. The index is an indirect measure of the risk of dengue fever, another possibly fatal mosquito-borne disease. The overall ovitrap index - a measure of mosquito eggs laid in special traps - was down to 21.1 per cent last month from 22 per cent in June. But 19 districts had indices higher than the target of 20 per cent, compared with 17 districts in June and 30 in May. Diamond Hill is still the highest-risk area with 42.3 per cent, followed by Shamshuipo with 38.3.