Story highlights Pig carcasses test positive for porcine circovirus

More than 2,800 dead pigs found floating in Shanghai river

Authorities say local tap water still safe to drink but residents worried

Reports suggests that more than 10,000 pigs died in a neighboring province earlier this year

At least 2,800 dead pigs have been fished from a Shanghai river since Friday, but authorities insist that tap water in the city is still safe to drink.

State news agency Xinhua said labels tagged to the pigs' ears indicated they came from the upper waters of the Huangpu River, which flows through the center of Shanghai and is a source of the city's drinking water.

It's not clear why the pigs had been dumped in the river, though local media reported earlier this month that a disease had killed thousands of pigs in a village south of Shanghai.

"We will continue to trace the source, investigate the cause, co-operate with neighboring areas and take measures to stop the dumping of pigs into rivers," the Shanghai Municipal Agricultural Commission said in a statement posted on their website on Monday.

As of Sunday, water quality on the Songjiang section of the river, where most of the pigs were found, remained normal and the incident has had "no significant effect on tap water supply," the commission added.

However, local residents and users of of the popular Twitter-like microblog service Sina Weibo have expressed concern that the dead pigs would make the city's tap water unsafe to drink.

"Huangpu river is the source of drinking water for more than 20 million Shanghai residents. And this horrific incident was only made public when residents started posting pictures on Weibo," business investor Xue Manzi said in a post on his account.

The agricultural commission said it had tested organ samples from the pig carcasses and the results suggested the animals had contracted a type of porcine circovirus.

According to Professor Fred Leung, who specialises in animal diseases at Hong Kong University, this is a fairly common disease in pigs and not usually fatal on its own.

Pictures showed sanitation workers with sticks retrieving the bloated bodies of small pigs caught up in reeds and debris at the side of the river.

A local newspaper in Jiaxing, a city in Zhejiang province south of Shanghai, reported on March 6 that tens of thousands of pigs had died of an animal disease in a major pig farming village in the past two months.

"According to our records, 10,078 pigs died in January, another 8,325 died in February. More than 300 pigs die everyday in our village, and we barely have any space left to dispose of the dead pigs," a local villager was quoted by the paper as saying.