U.S. federal health investigators suspect that they will find more farms that received tainted animal feed but stressed Tuesday that the threat to people is minimal.

The investigators are trying to get a handle on just how much pet food tainted with an industrial chemical called melamine made its way into products consumed by pets as well as by hogs and chickens.

'It was only a small portion of what the poultry was fed, and human consumers will only use poultry as a small portion of their diet.'—Dr. David Acheson, U.S. Food and Drug Administration

On Monday, they announced that byproducts from tainted pet food had been used in chicken feed on some farms in Indiana. A few days earlier, they said that hog farms in six states may have received tainted pet food for use as feed.

The pet food in question could be to blame for a wave of dog and cat deaths in March due to kidney failure. However, Dr. David Acheson, an assistant commissioner for the U.S. Food and Drug Administration, said the threat level to pets is greater than to livestock or humans.

"It was only a small portion of what the poultry was fed, and human consumers will only use poultry as a small portion of their diet," Acheson said.

By contrast, pets often eat the same product exclusively, he said.

No recall on poultry issued

Officials said that as many as three million young chickens out of nine billion slaughtered annually may have eaten feed that potentially included an ingredient containing the melamine. They have already been slaughtered for human consumption, but because there is no evidence that consumption is unsafe, no recall has been issued.

Acheson said that the investigation tracking contaminated pet food is complex and sweeping, which is why it could lead to the discovery of new states that are affected.

"There is a distinct possibility that it will broaden," Acheson said. "I'm not saying that it will, but we need to be prepared for that to happen."

FDA receives 17,000 calls about pet illness

Overall, the FDA has received about 17,000 calls alleging illness or death of a pet as a result of contaminated food. Of those, about 8,000 were entered into a database. Roughly half of those entered alleged an animal death.

The agency will investigate to determine whether the deaths are associated with the recalled products.