The US Agriculture Department received an "inconclusive" preliminary screening on a second animal indicating possible mad cow disease, but officials cautioned the test is so sensitive it does not mean another case has been found.

　　It's the second such discovery in five days as part of the government's rapid screening program. The only confirmed mad cow case in the country was discovered in Washington state last December, prompting more sophisticated screening programs.

　　Tissue samples from the cow in the latest case were being sent to the department's National Veterinary Services Laboratories in Ames, Iowa, so that more conclusive tests can be run. Results on samples from a case discovered last Friday have yet to be released.

　　Federal officials emphasized that the rapid screening tests - more than 7,000 have been conducted - are extremely sensitive and in themselves do not confirm a case of the brain-wasting mad cow disease, also known as bovine spongiform encephalopathy, or BSE.

　　"The inconclusive result does not mean we have found another case of BSE in this country," John Clifford, deputy administrator of the USDA's Animal and Plant Health Inspection Service, said in a statement late on Tuesday. "Inconclusive results are a normal component of screening tests."

　　Clifford said the carcass involved in Tuesday's test has been accounted for, and none of the meat is in the food supply.

　　As with the earlier case announced on Friday, officials did not disclose the location of the animal, the state it came from or the facility at which it was killed or tested. Follow-up tests could take four to seven days, Clifford said.

　　The more conclusive tests being conducted on the first animal are pending, said Clifford.

　　The rapid screening test is designed to give a quick early indication that there may be a problem with a particular cow. But USDA officials and industry representatives emphasized there is a high chance "that confirmatory results will be negative" once the tissues are more closely examined at the laboratory in Ames.

　　The lab in December diagnosed the nation's only confirmed case of made cow disease in a Holstein found in Washington state. The discovery caused turmoil in the beef industry and prompted some countries such as Japan to refuse to accept US beef.

　　"USDA remains confident in the safety of the US beef supply," said Clifford, adding that the animal tested on Tuesday did not enter the human food chain.

　　The government has issued rules that bar use of the most potentially at-risk cattle parts, such as brains and spinal cords, from entering the food chain. People who eat products containing the BSE protein can contract a rare but fatal disease similar to BSE, variant Creutzfeldt-Jakob disease.

　　Under an expanded BSE surveillance program begun after the disease was discovered in Washington state, the government has tested more than 7,000 animals. Clifford said as many as 268,000 animals may be screened as part of the program over the next 12 to 18 months.

　　But the tests are only meant to be a screening tool.

　　The screening effectively identifies tissue that could contain the BSE protein while not falsely identifying clean samples as contaminated, according to Norman Schwartz, president of Bio-Rad Laboratories, which designed a test used in the rapid screening program.

　　(The Associated Press)