A mass swine epidemic is not what killed nearly 6,000 pigs that have floated downstream to Shanghai from Jiaxing, Zhejiang province, since last week, officials said on Tuesday.

"No abnormal animal epidemic was reported in Jiaxing," Jiang Hao, from the city's veterinary department, said at a news conference.

Local authorities are assisting Shanghai's veterinary departments in the investigation into what killed the pigs found floating on the Huangpu River, he said.