The number of dead pigs found in a river running through China's commercial hub Shanghai had reached more than 13,000, the government and state media said Monday, as mystery deepened over the hogs' precise origin. The Shanghai government said workers pulled 335 pigs out of the Huangpu river, which supplies 22 percent of the city's drinking water, on Monday, bringing the total to 9,795 since the infestation began earlier this month. Shanghai has blamed farmers in Jiaxing in neighbouring Zhejiang province for dumping pigs which died of disease into the river upstream, where the official Xinhua news agency said another 3,601 dead animals had been recovered so far. The Jiaxing government has said the area is not the sole source of the carcasses, adding it had found only one producer that could be held responsible. Shanghai had checked farms in its southwestern district of Songjiang, where the pigs were first detected, but found they were not to blame, the Shanghai Daily newspaper said on Monday. The scandal has spotlighted China's troubles with food safety, adding the country's most popular meat to a growing list of food items rocked by controversy. Samples of the dead pigs have tested positive for porcine circovirus, a common swine disease that does not affect humans. "Due to some farming households having a weak recognition of the law, bad habits, and lack of increased supervision and capability for treatment have led to the situation," the national agriculture ministry's chief veterinarian Yu Kangzhen said. Yu attributed a higher mortality rate among pigs to colder weather this spring, though he ruled out an epidemic, the ministry said in statement posted on its website over the weekend. The Shanghai government said in its statement that the quality of drinking water remained within national standards, despite widespread worries over water quality among the city's 23 million residents. The thousands of dead pigs have drawn attention to China's poorly regulated farm production. Animals that die from disease can end up in the country's food supply chain or improperly disposed of, despite laws against the practice. In Wenling, also in Zhejiang, authorities announced last week that 46 people had been jailed for up to six-and-a-half years for processing and selling pork from more than 1,000 diseased pigs. China faced one its biggest food-safety scandals in 2008 when the industrial chemical melamine was found to have been illegally added to dairy products, killing at least six babies and making 300,000 people ill. In another recent incident, the American fast-food giant KFC faced controversy after revealing that some Chinese suppliers provided chicken with high levels of antibiotics, in what appeared to be an industry-wide practice.