Tue May 8, 2007 5:29PM EDT

By Christopher Doering

WASHINGTON (Reuters) - U.S. fish farms may have been given meal laced with melamine, expanding an investigation that already has found the industrial chemical in food for pets, pigs and chickens, officials said on Tuesday.

China shipped tainted ingredients to Canada, which produced fish meal that was later sold to several fish farms in the United States. The U.S. Food and Drug Administration said it was contacting those farms to determine the number of fish affected, what farmers are doing with the fish given the meal and whether any has entered the food supply.

"As with the situation with the poultry and hogs the (small) levels that we're seeing in the fish meal are very comparable and... we do not believe there is any significant human health risk associated with consuming these fish," said FDA Assistant Commissioner for Food Protection David Acheson.

Investigators so far believe the melamine will not harm hogs, chickens and fish. A final review of the impact on these species is expected in about a week.

About 8,500 complaints of related pet deaths had been reported to the FDA as of May 3, but the agency said that only 16 deaths of cats and dogs have been confirmed.

The U.S. Agriculture Department said earlier this week there was no need to quarantine livestock on farms where melamine could not be detected in animal feed.

An estimated 20 million chickens being held on farms over the weekend for testing that may have received contaminated feed were allowed on Monday to be sent to slaughter. Some 6,000 hogs in six states that had a ban prohibiting them from being slaughtered have had that restriction lifted.

USDA is still collecting information on 100,000 chickens from eight breeder farms in Indiana and 50,000 hogs at three facilities in Illinois. Those animals are currently on hold and can not be processed, officials said.

"MISLABELED"

Melamine, a chemical used in plastics and fertilizers, has been found in vegetable protein shipments from China that were used to make pet foods. More than 100 brands of pet foods have been recalled, and the pet food was later purchased by poultry, fish and hog feeders for use in their feeds.

Separately, FDA also said it learned that the contaminated shipments of wheat gluten and rice protein concentrate that the agency has been investigating for the past month were mislabeled and actually contained wheat flour. The agency said it is considering possible enforcement action.

The U.S. Customs and Border Protection agency is now taking samples for testing from imports of wheat gluten, corn gluten and rice protein concentrate.

"There is no evidence to suggest that the bulk of these products have any risk or contamination beyond what the FDA has already identified from the Chinese companies," Vera Adams, director of commercial enforcement for the border agency said.

FDA officials acknowledged the discovery that the imported material was mislabeled may force them to expand their own testing. "I don't exclude the possibility that we would get into sampling wheat flour but that is not the top priority right now," Acheson said.

He also emphasized the FDA must target its testing of imported of ingredients for animal and human food. "We're trying to expand our assessment based on where we see the risks," Acheson said. "This is shifting constantly."

The FDA investigators now in China are scheduled to return to Washington early next week.

Officials also said they are looking at how to improve the overall system of regulating feed imported for pets, livestock, fish and other animals.

Existing animal feed regulations are designed to detect specific contaminants, such as cattle parts that can be contaminated with bovine spongiform encephalopathy, said Steven Sundlof, director of the FDA's Center for Veterinary Medicine.

"We have not had really an overall comprehensive program ... that would catch problems like what we've seen with the melamine," Sundlof said.

(Additional reporting by Julie Vorman)