NEW YORK (CNN/Money) - Shares of hamburger chains McDonald's and Wendy's rallied Monday after the fast-food chains said that the recent discovery of mad cow disease in the United States has not affected sales.

Shares of McDonald's (MCD: Research, Estimates), the world's largest restaurant chain, jumped 2.1 percent and ended near their highs for the session.

Wendy's (WEN: Research, Estimates) also bounced up after its announcement, with its stock closing up 1.5 percent.

"Of course we've been watching not just the volume of our sales but the specific McDonald's products being sold and we've noticed no reduction in the beef product being purchased," Walt Riker, a McDonald's spokesman, said in a statement over the weekend. "We continue to monitor the situation closely."

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J.P. Morgan followed the company's statement with an upgrade of McDonald's Monday to "overweight" from "underweight," saying the company should benefit from falling beef prices.

On Monday, Wendy's said its sales at U.S. stores open at least a year remained strong in December, following positive results in November. The company's official results are set to be released on Jan. 6.

"There has been no impact on Wendy's recent sales trends over the past three days (Friday through Sunday) since the announcement by the USDA that confirmed a single cow from a farm in Washington State tested presumptive positive for BSE," Jack Schuessler, chairman and CEO of Wendy's, said in a statement.

Wendy's officials said the company's beef supply is not affiliated with the meat plants where the diseased cow was detected

Privately held Burger King, the No. 2 U.S. hamburger chain, said Monday its beef sales have not been affected by the disclosure last week of the first case of mad cow disease in the United States.

"We haven't seen any impact to our beef sales as of yet," Laina Hanna, a spokeswoman for Burger King, told Reuters.

Officials from the Agriculture Department said Sunday that meat from the cow has been distributed to eight Western states and the territory of Guam.

About five tons of meat from the Washington state processing plant where the diseased cow was slaughtered have been recalled, with officials saying there's just about no risk to humans from the disease. (Click here for CNN.com's coverage)