They reiterated that monitoring results showed the quality of both the Huangpu water and tap water produced by water plants in the affected area was normal.

Shanghai's animal control authority found porcine circovirus, a common disease among hogs that was not known to infect humans, in 13 of 20 samples of internal organs taken from dead pigs retrieved from the Huangpu, the Ministry of Agriculture said on its website.

In Zhejiang , the animal control authority found the same virus in 16 of 36 samples drawn from dumped pig carcasses in Jiaxing. It also found evidence of diarrhoea in seven of them.

The area witnessed an abnormally high number of dead hogs, especially young ones, mainly because of the two diseases plus changeable weather this winter, the website quoted the ministry's chief veterinarian, Yu Kangzhen , who has led a team to Jiaxing, as saying.

Jiaxing was a big hog producer but most pig farms were very small and unprofessional, he said, so the number of dead hogs was unusually high.

The city's vice-mayor, Zhao Shumei , said on Friday night that some local farmers were in the habit of dumping dead pigs in inappropriate places.

"Most of the dead pigs retrieved died more than two weeks ago," Zhao said. "The deaths didn't occur within a short period."

The carcasses, which originally sank to the river beds, gradually floated up as the weather became warmer, she said.

A total of 130,000 households raised more than seven million hogs in Jiaxing last year - with 89 per cent of them running small farms with capacity for fewer than 50 pigs.

The city's deputy environmental chief, Yu Hongwei , said water samples collected from eight spots in rivers bordering Shanghai were of normal quality, and "some indexes were better than the same period last year".