The ruling, issued Friday, could force the closure of the last horse slaughtering plant in the United States.

"States have a legitimate interest in prolonging the lives of animals that their population happens to like," a three-judge panel of the U.S. Seventh Circuit Court of Appeals wrote. "They can ban bullfights and cockfights and the abuse and neglect of animals."

At Cavel International Inc.'s plant, located in the northern Illinois town of DeKalb, about 40,000 to 60,000 horses are slaughtered each year. Except for a portion sold to U.S. zoos, the meat is shipped to be eaten by diners overseas.

The plant has been forced to close twice since late May, when Gov. Rod Blagojevich signed into law a measure banning the slaughter of horses for human consumption, or the import, export or possession of horse meat designated for human consumption.

The plant was allowed to reopen during various challenges to the state law.

It was not immediately clear if Cavel would appeal the latest ruling, or if it would shut its doors Friday night. The company could ask the three-judge appellate panel to reconsider its ruling, or ask the full court to take on the case.

Messages left for Cavel and its attorney were not immediately returned.

The Illinois attorney general's office is handling the case on behalf of the state. The office was pleased the court upheld the constitutionality of the Illinois measure, and believes the state law can now be enforced by local authorities, according a statement released by a spokeswoman.

The Humane Society of the United States was "elated" by the ruling, according to Jonathan Lovvorn, the group's vice president of litigation.

"The court made it clear this is something that the states can ban," he said. "Hopefully this is the end after a very long road."

The Cavel plant has operated in DeKalb for about 20 years and employs about 60 workers. Cavel, a subsidiary of a Belgian company, buys horses for about $300 apiece.

Two other U.S. plants, both in Texas, closed earlier this year. A federal appeals court upheld a Texas law banning horse slaughter for the sale of meat for food, and the U.S. Supreme Court refused to take up the case.

Critics says the slaughterhouse process is inhumane. Some also argue the nation has no tradition of raising horses for meat, and shouldn't do so to satisfy foreign consumers.

In court papers, Cavel argued that Illinois' specific ban on human consumption of horse meat serves no purpose, because horses that are too old or no longer useful will be killed anyway.

"Even if no horses live longer as a result of the new law," the court wrote, "a state is permitted, within reason, to express disgust at what people do with the dead, whether dead human beings or dead human animals.

"There would be an uproar if restaurants in Chicago started serving cat and dog steaks, even though millions of stray cats and dogs are euthanized in animal shelters."

Cavel also argued the Illinois law violates the interstate and foreign commerce clauses of the U.S. Constitution because it bans importing or exporting horsemeat for human consumption.

The court said that provision of the law is not aimed at Cavel, but instead at any middlemen who would try to take horse meat out of or into Illinois.