The slaughter plant treatment increases the chance that a bird will inhale feces in the water, leading to a higher bacteria level in its meat, the lawsuit said.

The case against the US Agriculture Department was filed in federal district court in San Francisco. It seeks to broaden a 1958 law requiring the humane slaughter of cattle and pigs to include poultry.

The Humane Society and the East Bay Animal Advocates said the failure of USDA to include chickens, turkeys and other birds under the act has lead to inhumane treatment.

"These birds ... are being slaughtered by methods that are not humane," said Paul Shapiro, spokesman for the Humane Society of the United States. "It's only because the USDA fails to define poultry as livestock even though any dictionary definition demonstrates that farmed birds ought to be."

The National Chicken Council, a trade group for farmers and slaughter plants, called the lawsuit "little more than a publicity stunt" that is likely to be thrown out of court.

Steven Cohen, spokesman for the USDA's Food Safety and Inspection Service, said he could not comment on the lawsuit, but added that the slaughter process described by the Humane Society "is standard practice in the industry."

USDA veterinarians are assigned to poultry plants to ensure practices there do not violate the law, he said.

Shapiro estimated that 9 billion birds, or about 95 percent of domestic animals raised on farms, are unprotected during the slaughter process.

The Humane Society has advocated that chicken slaughter plants adopt the use of gas before birds are processed.

The lawsuit said recent reports of abuse in slaughter plants in West Virginia, Maryland and Alabama, where workers jumped on, kicked and slammed chickens against a wall, increased the need to protect poultry.