An index that measures growth in the numbers of a type of mosquito that could transmit dengue fever and the Zika virus has exceeded an “alert” level in eight districts and reached a new high for the city overall. A spokesman for the Food and Environmental Hygiene Department, which monitors the growth of the Aedes albopictus species, has warned Hongkongers to “remain vigilant” to dangers posed by the pests as dengue fever remains prevalent in neighbouring countries and rainy summer weather brings favourable conditions for breeding. According to the latest figures released by the department, the Ovitrap Index, which records the number of eggs laid by the mosquitoes, rose to a reading of 13.5 per cent citywide last month from 9.3 per cent in May. Among 52 locations surveyed for the index, eight exceeded the “alert” level of 20 per cent. Tseung Kwan O North was highest at 35 per cent, followed by Tsing Yi North. When compared with index readings from June last year, Tsuen Wan Town, which recorded 26.8 per cent last month, saw the biggest rise. An index reading between 20 and 40 per cent means infestation of the mosquito has “exceeded one-fifth of the area” in question. The spokesman said an interdepartmental anti-mosquito mechanism had been activated in the eight areas with an “alert” reading. In addition to the usual anti-mosquito programmes carried out weekly, measures including applying larvicide to stagnant water and stepping up inspections would also be carried out to eliminate breeding grounds. Groups that have subscribed to an Ovitrap rapid alert system will post notices in common areas of buildings to remind occupants and staff to take preventive measures. Meanwhile, intensive preventive and control exercises will be carried out across the city between August 1 and October 31 by various government departments, during which time a number of people are expected to travel to Brazil for the Olympic and Paralympic Games. The spokesman said such actions would reduce the risk of any spread of dengue fever or the Zika virus, which could be possible if a person were infected elsewhere and bitten by a local mosquito. Another 10-week anti-mosquito campaign by the department targeting areas of specific concern such as public markets and hawker bazaars will begin on August 15. As of last Thursday, 58 dengue fever cases, all imported to the city, had been confirmed for the year.