An estimated 6,000 hogs from New York to California were likely given feed containing melamine, a chemical used in plastics and fertilizer, the U.S. Agriculture Department said Thursday.
A USDA official said that so far states have quarantined one farm each in South Carolina, New York, North Carolina and California.
Other states affected are Kansas, Oklahoma and Utah.
An estimated 105 million hogs are slaughtered in the United States each year, according to USDA figures.
"We're obviously tracking each farm and, frankly, each hog," the USDA official said.
USDA also said it believed a poultry feed mill in Missouri may have purchased contaminated feed.
States have the authority to quarantine farms and to test animals. The U.S. Food and Drug Administration has authority over feed, while USDA oversees products being sold.
The discovery of feed rations containing pet food tainted with melamine so far has not caused concern among importers of U.S. pork.
"We have not gotten any countries stopping our shipments yet," said Sally Klusaritz, a spokeswoman with USDA's Foreign Agricultural Service.
U.S. health officials said on Wednesday they are looking at whether humans may have consumed food containing melamine.
FDA officials said they would inspect imports of six grain products -- wheat gluten, corn gluten, corn meal, soy protein, rice bran and rice protein -- used in foods ranging from bread to baby formula for traces of the chemical that is believed to have killed and sickened cats and dogs.
Melamine has been found in wheat gluten and rice protein imported from China for use in some pet foods. More than 100 brands have been recalled. The following is a breakdown by state with information obtained from USDA: North Carolina - 1 farm with 300 pigs. 150 of the animals were sold to a plant in South Carolina where the product is on hold. Farmer also is holding the rest of the pigs. New York -1 farm, none of the animals have gone to slaughter. Kansas -195 hogs from one farm have gone to a processing plant; some product is on hold but some may have gone to commerce. California -26 hogs from one farm have gone to slaughter and are being held at a federally inspected facility. -50 hogs were custom slaughtered. They cannot be sold in retail stores, but purchasers are being contacted. Utah -2 farms where pigs are on voluntary hold, meat from one of the farms, totaling no more 100 hogs, may have gone into commerce through a federally inspected facility; the rest of product is on voluntary hold. South Carolina -1 farm, none of the pigs have gone to slaughter. Oklahoma -show hogs have eaten the feed but none have gone to slaughter. Poultry Missouri - a poultry feed mill in Missouri may have bought contaminated feed.