GUANGZHOU, June 7 (Xinhua) -- Eleven people in Guangzhou have contracted dengue fever and two more have tested positive after a work trip to southeast Asia, the local health authority confirmed on Thursday.

    The 13 were among 23 employees of a Guangzhou-based survey company who left China in March to conduct preliminary surveys for a hydropower project in a southeast Asian country, said a spokesman at the municipal health bureau. He refused to name the country.

    Eleven of them suffered high fevers, joint pains, nausea and rashes from April 11 to May 14. Some were diagnosed as having "malaria" abroad and underwent treatment at local hospitals, he said.

    The company called back the group in two batches: two people arrived on April 14 and the other 21 on May 30.

    One of the first two arrivals came down with dengue fever on April 16 and the municipal disease control and prevention center immediately launched an investigation.

    The center found 12 of the 21 people arriving on May 30 carried the dengue-causing virus, and confirmed 10 had just recovered from an infection. The other two positive cases showed no symptoms of the disease.

    The health bureau spokesman ruled out the possibility of the disease spreading, as the most recent case on May 14 had recovered and no suspected cases were reported in the past three weeks of observation.

    The disease control and prevention center closely monitored the health of the company staff, and helped eradicate mosquitoes around its office building and employees' residences.

    Meanwhile, the city's health bureau has told all local health, quarantine and inspection, tourism and foreign trade authorities to be on heightened alert against the potentially fatal, mosquito-borne disease.

    Several southeast Asian countries have seen a surge in dengue fever cases in the recent weeks and Beijing reported this year's first infection -- an imported case from Malaysia -- early in May.     

    The Aedes mosquito, which carries the dengue-causing virus, breeds easily in pools of stagnant water in household gardens, rainwater gutters and other areas where water collects. Dengue causes internal bleeding and leads to death in severe cases.