Reports suggest the dead pigs may have been dumped from pig farms in Jiaxing, upstream of Shanghai.

BBC: China pulls nearly 6,000 dead pigs from Shanghai river

Farmers in the Jiaxing area of Zhejiang province, upstream from Shanghai, have denied dumping the pigs in rivers.

BBC: China pig deaths: Toll from Shanghai rivers nears 14,000

It is believed that the pigs may have come from Jiaxing in the neighbouring Zhejiang province, although the cause of their deaths is still not clear.

BBC: China pulls nearly 6,000 dead pigs from Shanghai river