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.

If, as the city authorities claim, there are carcasses in the river every year, the disposal of dead pigs in the upstream areas has long been a problem. But if farmers are resorting to throwing their dead pigs in the nearest stream or river, fishing them out is not enough.

A thorough investigation is the first thing that needs to be done in order to find out exactly why so many pigs have died in such a short period of time. Are there any problems in the way the pigs are kept or have the pigs been fed with anything that has affected their health? Whatever the reason, it must be addressed.

Then the system needs to ensure that farmers are willing to report dead pigs and able to get such pigs disposed of properly.

Whether local governments can effectively probe and solve the problems involved in raising pigs will be a test of not just their governing capability but also a measure of the concern they attach to the health of residents.