U.S. agricultural authorities Thursday said they are investigating what may be the second case of mad cow disease in this country, a development that hit shares of some food companies.

The first U.S. case of the brain-wasting disease, technically known as bovine spongiform encephalopathy, or BSE, was reported last December. The disease had ravaged herds in Europe, particularly the U.K, for parts of the 1990s, and affected those in Japan and Canada.

In a statement Thursday, the Department of Agriculture said: "Early this morning, we were notified that an inconclusive BSE test result was received on a rapid screening test used as part of our enhanced BSE surveillance program. The inconclusive result does not mean we have found another case of BSE in this country. Inconclusive results are a normal component of screening tests, which are designed to be extremely sensitive, so they will detect any sample that could possibly be positive."

The USDA statement on its Web site said tissue samples were en route for testing, with "confirmatory results" expected in the next four to seven days.

On Dec. 23, 2003, U.S. officials confirmed the first U.S. case of the deadly disease. That prompted an immediate selloff in beef producers and beef-oriented restaurant chains, as well as import bans on American beef by a host of trading partners. The case was discovered in Washington state, but the animal's origin was later traced to Canada.

McDonald's

(MCD) - Get Report

,

Wendy's International

(WEN) - Get Report

,

Tyson Foods

(TSN) - Get Report

and

Outback Steak House

(OSI)

were among the companies whose shares weakened on the news.