HANOI, May 22 (Xinhua) -- Nearly 700 of dengue fever patients, including one fatality, have been spotted in Vietnam's capital city so far this year, doubling the amount of the same period last year.

The disease cases used to surge from July to October, but now it occurs all year round and can break out irregularly, mainly due to climate change and abnormal weather patterns which facilitate reproduction of mosquitos and their larvae, the municipal Health Department said Monday.

Vietnam detected roughly 21,000 cases of dengue fever, including eight deaths, mainly in the southern region in the first four months of this year, according to the country's Health Ministry.