McDonald's Corp. (MCD), the world's largest fast-food company, Tuesday said its supply chain is not linked to a case of mad cow disease (search) found at a U.S. farm in Washington state.

"The meat packer in question has no connection whatsoever to McDonald's supply chain," said Lisa Howard, a McDonald's spokeswoman, in a prepared statement. "We are confident the USDA and the Department of Agriculture (search) are investigating this situation very seriously and are taking all the appropriate steps necessary to safeguard the beef supply."

Shares of McDonald's fell as low as $24.20 in post-market Instinet trading from a close of $25.28 on the New York Stock Exchange (search) when a USDA official said the first U.S. case of mad cow disease, a brain-wasting disorder, was found in just one cow. The diseased cow was tested on Dec. 9. The stock later recovered on Instinet to trade at $24.34.