The agriculture and environmental protection departments in Shanghai's Songjiang district have pulled more than 3,300 dead pigs out of the Huangpu River, which flows through the municipality, in the past week.
Porcine circovirus, a common hog disease that is not known to be infectious to humans, was found in a sample taken from the water, Shanghai's animal disease control department was quoted as saying by eastday.com  a major Shanghai news portal.
Tests by the Shanghai Animal Disease Prevention and Control Centre on five sets of internal organs taken from the dead pigs has ruled out five other diseases including foot-and-mouth, hog cholera and blue-ear.
The website quoted local water and environmental authorities as saying tests of water samples collected from six sites on the river showed that the poor water quality on the Songjiang district section of the Huangpu, a source of drinking water for Shanghai, was similar to the same period last year.
However, it said tap water from water suppliers in Songjiang and several neighbouring districts was in line with national standards.
According to the dead pigs' ear labels, most of them floated down the river from Jiaxing, Zhejiang. Some were also from Pinghu in Zhejiang or from neighbouring Jiangsu province, xinmin.cn quoted a staff member at the Songjiang District Agricultural Commission as saying.
China News Service quoted an official from Zhejiang's provincial agricultural department as saying that most of the dead pigs were young and they had died at least a month ago. He said that instead of dying from illness, most had died due to cold weather, even though average temperatures in Jiaxing in winter are above zero degrees Celsius.
The district government has asked for help from the municipal government for centralised treatmeant of the carcasses because there were too many for it to handle, the website said.
The Jiaxing Daily, the Communist Party's newspaper in Jiaxing, said the casual disposal of dead pigs was common in the Zhejiang city.
A report in the newspaper on Monday of last week said it had received many complaints from readers about dead pigs.
"The dead pigs appeared intact on the first day they were abandoned, but their legs disappeared on the second day," one reader was quoted as saying. "We don't know who got them and where they have gone. It would be horrible if they finally arrived on our dinner tables!"
In another report two days later, the newspaper said more than 18,000 pigs had died in one village, where hog raising accounted for 85 per cent of the villagers' income, in January and February.
"These days pigs are dying at a pace of over 300 a day," a villager was quoted as saying.
Deputy Agriculture Minister Chen Xiaohua told a news conference in Beijing that the incident reflected problems in the government's work.