Authorities in Shanghai plucked 611 dead pig carcasses Saturday from Huangpu river, which provides drinking water to the city's 23 million residents. In total, 8,965 dead pigs have been found in the river since March 8.
The swollen and rotting pigs are largely believed to be from the upstream city of Jiaxing in neighboring Zhejiang province, but Zhao Shumei, a deputy mayor, said it was inconclusive to say all the pigs were from her city.
Jiaxing where small hog farms are prevalent reported Friday night that it had recovered 3,601 dead pigs from its streams, according to state media.
The head veterinarian for China's Agriculture Ministry, Yu Kangzhen, who has traveled to the region to investigate the deaths, told state media Saturday that there has been no major swine epidemic, but said some samples tested positive for the common porcine circovirus and the epidemic diarrhea virus.
Yu also said cold weather and fluctuating temperatures have caused a spike in deaths among baby pigs.
Villagers have told state media that pig dumping is on the rise following police campaigns against the illicit trade of pork products harvested from diseased pigs that were illegally sold, instead of properly disposed of.
In Shanghai, authorities have repeatedly assured residents that tap water is safe, but locals remain worried about water contamination.
In 2012, about 130,000 Jiaxing hog farmers raised more than 7 million pigs, according to state media.