Shanghai may suffer high numbers of mosquitoes this summer because of the warm winter and higher density of mosquitoes detected in previous months, local health officials warned yesterday.

The density of mosquitoes in November was 18.6 percent higher than the same month in 2010, said the Shanghai Center for Disease Control and Prevention.

Moreover, the frequent rains in autumn and relatively high temperatures this winter have provided ideal conditions for mosquitoes, which don't hibernate and still breed during the colder season.

"Mosquitoes are a big annoyance for local residents," said Xu Renquan from Shanghai Center for Disease Control and Prevention's vector control department. "Instead of purely itches and poor sleep, they are a threat to people's health and safety. We have found more eggs of one type of mosquito which spread dengue fever in our recent checks."

Data provided by Putuo District showed that 72 percent of 1,153 indoor areas checked in mid December, such as garages and Metro stations, were found to contain overwintering mosquitoes.

A citywide campaign was carried out yesterday, targeting warm and humid places.