Authorities have detected a sometimes-fatal disease in at least one of the pig carcasses fished out of the Huangpu River in Shanghai's Songjiang District since last week.
The disease is associated with the porcine circovirus, which is widespread in pigs but doesn't affect other livestock or humans, officials said yesterday.
The dead pigs were mainly from Jiaxing in neighboring Zhejiang Province, the Shanghai government said yesterday.
By Sunday evening, more than 2,800 pigs had been removed from the river in Songjiang and Jinshan districts, authorities said.
Shao Qiliang, secretary general of the Shanghai Agricultural Commission, said the pigs had been dumped in the Huangpu River from family farms Jiaxing and drifted downstream.
The total number of carcasses is expected to increase. Workers were continuing to collect dead pigs from the river and the government was closely monitoring water quality, although no pollution has been found so far.
Shao said the commission had informed agricultural authorities in Zhejiang Province, who have started intercepting and retrieving dead pigs in their area.
Pigs that have died of disease should be either incinerated or buried, but some unscrupulous farmers and animal control officials have sold problematic carcasses to slaughterhouses, with the pork ending up in markets.
As a food safety problem, this has drawn attention from the Ministry of Public Security, which has made it a priority to crack down on gangs that purchase dead or sick pigs and process them for profit.
Zhejiang police said officers have been investigating the trade in pork from sick pigs and their efforts were stepped up ahead of the lunar New Year celebrations last month.
In one operation last year in a village in Jiaxing, police arrested 12 suspects and confiscated nearly 12 tons of tainted pork meat.
Shao said: "We retrieve dead pigs from Jiaxing almost every year. There were around 200 a year in the past, but the number is particularly high this year."
An unnamed villager told the Jiaxing Daily newspaper: "Ever since the police stepped up efforts to crack down on the illicit market of sick pigs since last year, no one has come here to buy dead pigs, and the problem of pig dumping is worse than ever this year."
Shanghai collects dead pigs from farmers for biological treatment and farmers can receive some compensation for their losses.
There is no such mechanism in Zhejiang and pig farmers there simply discard dead pigs in rivers, according to the newspaper.