Chickens that ate bird feed made with a small amount of contaminated pet food are safe for human consumption and can be released for slaughter and sale, federal health officials said yesterday.

That decision emerged from a government risk analysis completed over the weekend involving 20 million chickens that officials said Friday had inadvertently been fed the tainted feed in several states.

"There is very low risk to human health" from consuming the chickens, according to a synopsis of the findings released by the Agriculture Department and the Food and Drug Administration.

Even if a person were to eat the chickens for breakfast, lunch and dinner, scientists concluded, the amount of melamine consumed in one day would be 1/2,500 of the minimum dose thought capable of posing a health risk.

"In other words, it was well below any level of public health concern," the Agriculture Department said in a statement.

As a result, all 20 million chickens held since Friday are eligible for standard USDA inspection and sale. An additional 100,000 breeder chickens, which have been held in Indiana for more than a week, remain on hold pending further analysis because they are thought to have eaten higher doses of melamine. Thousands of California hogs also remain on hold pending further studies.

Melamine is an industrial chemical not intended for human consumption. In March, Chinese wheat gluten and rice protein imported for use in pet food was found to have been spiked with the chemical, apparently to make the products appear to have more protein than they did.

Thousands of pets are suspected of having been sickened or killed by the toxin. Human health risks became a concern when it became clear about a week ago that some farm animals had also been fed the tainted food.

The new risk analysis -- conducted on an emergency basis by the USDA and the FDA with scientists from the Environmental Protection Agency, the Centers for Disease Control and Prevention, U.S. Customs and Border Protection, and the Department of Homeland Security -- took into account such factors as the small volume of tainted pet food in the chicken feed (less than 5 percent); the fact that melamine is largely excreted by chickens and does not accumulate significantly in their tissues; and the presumption, based on studies in rats, that melamine by itself is minimally toxic in people. Questions remain as to whether that toxicity is being amplified by another contaminant, cyanuric acid, that has been found in pet food from China.

Officials said the amount of melamine in the feed consumed by the 20 million chickens was so low that the chemical was undetectable in the birds, even using a highly sensitive experimental test that can detect as few as 10 parts of melamine per billion parts of tissue.

A larger proportion of pet food was added to the feed eaten by the Indiana breeding birds, so those chickens are being kept off the market until measurements of melamine in their tissues are complete, officials said. But government scientists said they were encouraged by the fact that none of the birds had grown ill -- and by the recent finding that the kidneys of pigs that ate the contaminated chow appeared normal under a microscope. Kidneys are often the first organs to show damage from melamine poisoning.

The USDA and the FDA are creating a science advisory board to review their risk assessment and to contribute to future such analyses.

Robert Buchanan, a senior science adviser with the FDA's Center for Food Safety and Applied Nutrition, who was involved in the risk analysis, said he was gratified to find that "the safety margins are very large" -- that is, the conclusion that there is minimal risk to human health would hold true even if the scientists' assumptions were found to be off by tenfold or more.

He said the analysis will be posted online.

"Our intention is to make the risk assessment totally available to the public," Buchanan said.

Washington Post