A pig virus has been found in a water sample of a river where 1,200 dead pigs had been fished out, Shanghai authorities said.

Workers retrieve dead pigs in Huangpu River in Shanghai. [Northnews.cn]

Laboratory tests found porcine circovirus (PCV) in one of the water samples taken from Huangpu River, a water source for city residents, sources with Shanghai municipal agricultural commission said Monday.

The virus causes porcine circovirus disease in pigs but does not spread to human beings, a commission statement said.

All other tests of the river water provided negative results, including tests for common pig-borne diseases such as foot and mouth, swine fever, hog cholera and epidemic diarrhea.

Authorities are investigating where the dead pigs came from. The commission said they are working with neighboring provinces to trace their source, and have warned riverside residents to refrain from dumping animals into the river.

Local media reports said the pigs mainly came from Shanghai's neighboring Jiangsu and Zhejiang provinces.

The city and Songjiang district governments retrieved the pigs from Friday night to Sunday.