A Chinese shopper pays cash for eggs at a market in Chengdu, southwest China, on Tuesday. (Color China Photo/Associated Press)

Animal feed producers in China commonly add the industrial chemical melamine to their products to make them appear higher in protein, state media reported Thursday, an indication that the scope of the country's latest food safety scandal could extend beyond milk and eggs.

The practice of mixing melamine into animal feed is an "open secret" in the industry, the Nanfang Daily newspaper reported in an article that was republished on the websites of the official Xinhua News Agency and the Communist Party mouthpiece People's Daily.

Publicizing such a problem is rare in the Chinese media and appears to be a tacit admission by China's central government that melamine contamination is widespread.

The news comes after four brands of Chinese eggs were found to be contaminated with melamine, which agriculture officials have speculated came from adulterated feed given to hens. The discovery of the tainted eggs followed on the heels of a similar crisis involving compromised dairy products that sent tens of thousands of children to the hospital and was linked to the deaths of four infants.

That scandal was triggered by dairy suppliers who added melamine, a chemical used to make plastics and fertilizer, to watered-down milk in order to dupe quality control tests and make the product appear rich in protein.

Health experts say ingesting a small amount of melamine poses no danger, but in larger doses, it can cause kidney stones and lead to kidney failure.

It is forbidden to deliberately add melamine to food and animal feed, but its apparent prevalence highlights the inability of authorities to keep the food production process clean of toxins despite official vows to raise safety standards.

The Ministry of Agriculture and the General Administration of Quality Supervision, Inspection and Quarantine did not immediately respond to faxed requests for comment. Phones rang unanswered at the Ministry of Health.

Chemical plants used to pay companies to treat and dispose of excess melamine, but about five years ago began selling it to manufacturers who repackaged it as "protein powder," the Nanfang Daily report said, citing an unnamed chemical industry expert. Melamine is high in nitrogen, and most protein tests test for nitrogen levels.

The inexpensive powder was first used to give the impression of higher protein levels in aquatic feed, then later in feed for livestock and poultry, the report said.

"The effect far more exceeds the milk powder scandal," the newspaper said.

Melamine found in 4 brands of eggs

In the past week, melamine has been discovered in at least four brands of Chinese eggs, and officials in China's largest city, Shanghai, said they had begun checks on all eggs sold in local markets.

No one has been sickened and it was not immediately clear how many eggs have been recalled.

China's leading egg processor, Dalian Hanwei Enterprise Group, was among the companies found to have tainted eggs, which were first identified by Hong Kong food safety regulators.

The reputation of Chinese products has in the past year come under fire after high levels of chemicals and additives were found in goods ranging from toothpaste to milk powder. In the milk scandal, Chinese authorities and a leading dairy producer delayed reporting the problem for months.

The Ministry of Health said Wednesday that 2,390 children remained hospitalized after drinking tainted milk, including one in serious condition, and 48,514 had been treated at hospitals and released.