Shanghai's gruesome tide of dead pigs is a visible reminder of a far-reaching environmental crisis which could have dire effects on public health, experts and local people said.
The ministry of agriculture in Beijing has blamed the dumping of more than 13,000 pig carcasses in the Huangpu river, which supplies drinking water to Shanghai, on a lack of education among pig farmers.
The ministry has denied any pig epidemics have taken place in Jiaxing, where the dead pigs originated, blaming the die-off on common diseases and the changing seasons.
"Due to weak awareness and capacity in safe disposal of animals, when pigs die of diseases, some local farmers dump the carcasses into the river or on rural roads," China's chief veterinary officer Yu Kangzhen told reporters this week.
Wang Wen, a biochemist at Oxford University, said the official verdict that the pigs died from the cold, or seasonal diseases simply didn't stand up.
"For there to be such a large-scale die-off, there would have to be an epidemic, or it could be caused by air pollution," Wang said.
She said the authorities could be keeping quiet the emergence of a newly mutated pig pathogen, against which existing populations had no immunity.
"Infectious viruses like influenza are passed onto humans after being transmitted from birds to pigs," Wang said.
"In southern China, people and pigs live in close quarters, and there is a lot of opportunity for transmission," she added. "That's why most of the new influenza viruses originate in southern China."
Many Shanghai residents switched to drinking bottled water only after the pigs made the news, in spite of assurances by local officials that the drinking water was up to standard.
A Zhejiang-based journalist surnamed Gao said dead pigs were a common sight in lakes and rivers of the province, which neighbors Shanghai, although not usually on such a large scale.
He said it was likely the result of a recent clampdown on the sale of diseased pigs for meat.
"Before they would find a way to get rid of them, maybe by making them into sausages or something," Gao said. "Now, they are cracking down on all that, so that's not so easy any more."
"That's why they are throwing them in the river."
Gao said the widespread use of antibiotics by livestock farmers had led to widespread resistance among common pig pathogens, making diseases much more likely to be fatal.
While complaints and public protests about pollution and unsafe drinking water are increasingly common across China, the dead pigs have become a grizzly emblem of people's worries about the effects of pollution on their health.
On Monday, an official in Shanghai's Jiading district promised that the government would respond to public fears over the quality of drinking water after protesters clashed with police over a planned water treatment plant at the weekend.
"If local residents have different opinions about this project, then we will hear and make sure we understand them," a district official said on Monday following a protest by hundreds of local residents.
However, residents said they had marched to the district water resources bureau and government office buildings on Saturday, but were refused permission to enter.
Protesters said they feared that a planned waste water treatment plant would create further air and water pollution in Jiading, which is home to around 1.4 million people.
"We wanted the district chief to come out and talk to us but he didn't," said one protester surnamed Han. "We got angry after that, and we got out our banners in protest at the government, which doesn't care about people's welfare."
"Then the police came in and wouldn't let up put up the banners, and there were some clashes," she said.
Last month, data from Beijing showed that around 90 percent of ground-water in China is polluted, much of it severely, with activists blaming local governments for protecting polluting enterprises.
In a recent survey of water quality in 118 cities across China, 64 percent of cities had "severely polluted" ground-water, Xinhua news agency quoted experts from the ministry of water resources as saying.
Activists say lack of access to clean ground water has dire consequences for hundreds of millions of rural residents, who rely on such water both for personal use and for watering their crops.
Reported by Shi Shan for RFA's Mandarin service, and by Fung Yat-yiu for the Cantonese service. Translated and written in English by Luisetta Mudie.