Many farmers in Zhejiang Province illegally feed their pigs a toxic chemical that could be dangerous to humans if they eat the animals, it has emerged.

The revelation comes after more than 300 people were poisoned when they ate pork products containing the chemical, called clenbuterol.

Since September 13, Shanghai hospitals have reported a large number of patients poisoned by bad-quality pork, which resulted in the city's largest clenbuterol poisoning case. A total of 336 people were affected.

All the patients have now left hospital following treatment and there have been no new cases.

Local media reported the illegally practice of feeding pigs clenbuterol is a popular on many pig farms of East China's Zhejiang.

Around 10 per cent of pork products sold in Shanghai are from the neighbouring province.

Food authorities have traced the source of the contaminated pork to a batch of pigs provided by two pig farms located in the neighbouring cities of Jiashan and Jiaxing, and have banned pig trading from the two places.

But the exact number of poisoned pigs and why these products with healthy certificates are in the system remains a mystery.

Farmers say that adding clenbuterol (shouroujing in Chinese) to pig feed is still widely practised in farms, although the government banned the use of the toxic chemical in the 1990s after researchers found it can cause damage to people's nervous and cardiovascular systems.

Driven by high profits, farmers feed small amounts of the drug to pigs in the hope that it will greatly improve their muscle ratio and shorten the growing period.

"Many people are still using shouroujing. I've raised pigs for 10 years and almost all of them have been fed it," a farmer in Jiaxing was quoted as saying by the Oriental Morning Post.

Farmers and pork wholesale traders say that it is an open secret that farmers stop using the chemical several weeks before the pigs are slaughtered, which means it is hard to detect the chemical.

Official statistics issued this July show that around 1 per cent of pork products contained clenbuterol around the country.

Food authorities in Zhejiang say they do not test every pig because of the high cost. A test kit for 100 samples cost 3,000 yuan (US$375).

Food experts say the incident reflects many loopholes in the food inspection system, which involves too many government departments that do not co-operate.

"The quality and safety of food should be ensured since testing at market can't safeguard the quality," said associate professor Chai Chunyan, an expert in animal drug residues at Shanghai Jiaotong University.

Li Chunlin, a pork trader at the heart of the outbreak, has been detained by police.

He confessed to buying 274 pigs from two farms in Zhejiang, many of which were then transported to Shanghai.

However, Shanghai Food and Drug Administration officials said there are unlikely to be more contaminated pigs in the system.

Source: China Daily