The test was described today at the 226th national meeting of the American Chemical Society, the world's largest scientific society.
Critics argue that the standard immunoassay tests used to identify the infectious prion proteins that cause mad cow disease are inadequate for large scale screening of cattle. The tests can produce false readings and may take a week to yield results. A better test is needed, they say.
The new test, which has already undergone animal studies, seems to fit the bill. Called the conformation-dependent immunoassay (CDI), it can detect prion proteins with 100 percent accuracy at much smaller levels than conventional tests and only takes about five hours to produce results, according to the UCSF researchers.
Like conventional tests, the new test is designed for detecting prions in the brain tissue of cows only upon autopsy. Unlike other tests, however, the new test also shows promise for detecting the proteins in muscle tissue and even blood while the animal is still alive. If so, it could be used to identify precisely which animals are infected before they show symptoms and could help end the current practice of slaughtering whole herds, the scientists say.
"This represents a new generation of prion tests," says project leader Dr. Jiri G. Safar, M.D., an associate adjunct professor at UCSF. "It is the most promising test to date for accurately detecting prion proteins," says Safar, a member of the school's Institute of Neurodegenerative Diseases.
He says the test has been used in a field trial to check for signs of the disease in the brains of 11,000 slaughtered cows in Spain, the United Kingdom and Germany. Results were compared to those from standard immunoassays performed on the same animals. There were no discrepancies between the tests, he says.
"We had a perfect score. There were no false positives and no false negatives," says Safar. "We can't afford incorrect conclusions, and we didn't see that in our tests."
He says that the research group plans to use the test on an even larger scale among European cattle herds within the next year, checking them for signs of the disease upon autopsy. If further tests prove successful, he hopes it will eventually be used to evaluate dead cows in this country for mad cow disease, also known as bovine spongiform encephelopathy, or BSE.
Despite the fact that the CDI test is currently being done in dead cattle, Safar says the same test could eventually be used on live animals to determine the presence of prions. In lab tests, the researcher has used the CDI test to detect prions in the muscles of living mice.
The live test could eventually be used to screen patients for the human form of mad cow disease, known as variant Creutzfeldt-Jakob disease, which is thought to be acquired from eating infected beef. A tissue or blood test for live animals could be available in a year, says Safar. "We're not quite there yet," he adds. "We still need to validate the effectiveness of CDI in live farm animals."
CDI has other advantages. It is automated, allowing larger numbers of animals to be screened in a short period. The test can detect up to eight different strains of prions, including those that cause scrapie in sheep and chronic wasting disease in deer.
With the recent detection of mad cow disease in neighboring Canada and the temporary ban on beef imported from that country, critics have stepped up their call for better testing. To date, there has never been a case of mad cow disease detected in the U.S. Given the flaws of current testing, however, some experts believe it could be just a matter of time.
Safar's coauthor in this study is Dr. Stanley Prusiner, M.D., a professor of neurology and biochemistry at the university and director of its Institute for Neurodegenerative Diseases. Prusiner was the first to discover that abnormal prion proteins can cause disease, an accomplishment that won him the 1997 Nobel Prize in Physiology or Medicine.
CDI technology is now licensed to InPro Biotechnology, Inc., of San Francisco, a company founded by Prusiner.
Funding for this study was provided by grants from the National Institutes of Health, the United Kingdom's Department for Environment, Food and Rural Affairs and private sources.
The paper on this research, ANYL 12, will be presented at 2:30 p.m. on Sunday, Sept. 7, at the Javits Convention Center, Room 1A01/1A02, during the "Diagnostic Assays for Prion Diseases" symposium.
Jiri G. Safar, M.D., is an associate adjunct professor at the University of California-San Francisco and a member of the school's Institute of Neurodegenerative Disorders.
Stanley B. Prusiner, M.D., is a professor of neurology and biochemistry at UCSF and director of its Institute for Neurodegenerative Diseases. He is the recipient of the 1997 Nobel Prize in Physiology or Medicine.
-- Mark T. Sampson