The chief veterinarian of China's agricultural ministry say that samples from the dead animals have tested positive for porcine circovirus, a disease that is common in swine but harmless to humans.

So far, Shanghai officials had pulled more than 9,400 pigs from a water source, which provides 22% of the local drinking water, and more than 3,600 pig carcasses upstream in the neighbouring Zhejiang province.

Authorities remain puzzled about the exact source of the carcasses. Last week, 46 people are jailed after they are accused of selling pork from more than 1,000 diseased pigs which may not be related to the current infestation.

Only one unidentified hog producer is suspected of dumping carcasses into the river, according to reports.

Tests of water quality are also being stepped up even though recent reports indicate that the water is currently in a normal state.