Shanghai authorities say tap water is safe to drink as efforts stepped up to remove bloated carcasses from Huangpu river

More than 2,800 pig carcasses have been discovered floating in a river that runs through Shanghai and feeds into its tap water supply, according to China's state media.

The number of dead pigs found in the Huangpu river rose from a few dozen on Thursday to more than 1,200 on Sunday, and again to over 2,813 on Monday afternoon as the city's cleanup effort intensified.

While the cause of the incident is still under investigation, water quality tests along the river have identified traces of porcine circovirus, a virus that can affect pigs but not humans. No signs of other diseases such as E coli, foot and mouth disease, or hog cholera have been found, and authorities say the city's tap water is still safe to drink.

Pollution has emerged as a source of widespread anger in China. Photograph: Aly Song/Reuters

China's toxic smog, rubbish-strewn rivers and contaminated soil have emerged as a source of widespread anger over the past few weeks, as profit-minded officials jostle with aggrieved internet users over how to balance the country's economic development with its environmental concerns.

Experts say the groundwater in half of all Chinese cities is contaminated, most of it severely, and that soil pollution could be widespread in 15 of the country's 33 provinces.

Villagers found the first pig carcasses near a water treatment plant on a creek upstream from Shanghai on Tuesday, but clean-up efforts did not begin until the weekend, according to the news portal Xinmin Online. Shanghai initially dispatched six barges to remove the corpses, and added another six when the problem's scope became clear.

Dead pigs collected by sanitation workers from Shanghai's main waterway. Photograph: Peter Parks/AFP/Getty Images

Pictures online show the bloated carcasses floating by the shore of the murky river and workers wearing blue uniforms fishing them out with long-handled farming tools. "The overpowering stench of the pigs from strong sun exposure and heat in Shanghai these days has made most reporters on the scene sick," reported the popular China news blog Ministry of Tofu.

Xu Rong, director of Shanghai's Songjiang district environmental department, told China's state broadcaster CCTV that she saw dead pigs floating along an 11km stretch of the river's Pingshen waterway, which extends to a cement plant in nearby Zhejiang province.

Judging by identification tags on the the pigs' ears, she said, they most likely floated into Shanghai from farming communities upstream.

"You can see dead pigs here every year, but there are more now than in the past few years," a local man told the station.

The Jiaxing Daily newspaper in northern Zhejiang province quoted a villager as saying that over the past two months almost 20,000 pigs in his village have died of unknown causes. While Shanghai compensates its farmers for properly disposing of dead swine, the newspaper said, Zhejiang and Jiangsu provinces lack a comparable incentive system, so farmers there often dump their pig carcasses directly into local rivers.

"The local authorities are conducting co-ordinated efforts to stop the dumping of dead pigs from the source," said China's official newswire Xinhua.