Eleven people in Guangzhou contracted dengue fever and two more tested positive for the disease after a work trip to Southeast Asia, the local health authority confirmed yesterday.

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 experienced high fevers, joint pains, nausea and rashes from April 11 to May 14. Some were diagnosed as having contracted malaria while 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 remaining 21 returned on May 30.

One member of the first party of workers to return 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 spokesman of the health bureau ruled out the possibility of the disease spreading, as the patient in the most recent case - identified on May 14 - had recovered, and no suspected cases were reported after three weeks of observation.

The disease control and prevention center has been closely monitoring the health of the company's staff and helped eradicate mosquitos 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 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.