The decomposing pig carcasses recovered from Shanghai's main river have raised concerns not only about the safety of the drinking water source for the country's largest metropolis, but also about the way pigs are reared.

Despite the city authorities' claim that the river water is safe and there has been no swine epidemic reported upstream, there are still questions that need to be answered, such as where did the carcasses come from and what has happened to cause the death of about 10,000 pigs in such a short space of time?

China is the world's largest consumer and producer of pork; more than 4 billion pigs were raised nationwide last year. In the area around the city of Jiaxing, which is the suspected source of the carcasses in Shanghai's Huangpu River, about 130,000 households raised more than 7 million pigs last year. There are so many pigs in this area their excrement has become a source of pollution, and each year the number of dead pigs that are unfit for consumption and need to be disposed of can be as high as 100,000.