LOS ANGELES — The Humane Society of the United States sued the government Wednesday over what it says is a legal loophole that allows so-called "downer" cows to get into the food supply.

A U.S. Department of Agriculture rule change made in July allows some sick or crippled cows into the food supply, the society alleges in a lawsuit filed in U.S. District Court in Washington, D.C.

In 2004, the USDA tightened regulations to prohibit the slaughter of all "downer" cows — animals that cannot stand — after a case of mad cow disease was discovered in Washington state. The lawsuit alleges that under last year's change, cows that fell down after an initial veterinarian inspection but appeared otherwise healthy could be slaughtered.

Amanda Eamich, a spokeswoman for the USDA's Food Safety and Inspection Service, said she had not seen the lawsuit and could not immediately comment.

The USDA issued the largest beef recall in history Feb. 17 after the Humane Society released undercover video showing workers at Chino-based Westland/Hallmark Meat Co. shoving sick or crippled cows with forklifts to get them to stand.

The plant has since shut down and two workers face criminal charges in the case.