British scientists have developed a preliminary blood test that could one day be used to detect the proteins that cause the human form of mad cow disease, according to a new study.

The disease-carrying proteins, or prions, that cause variant Creutzfeldt-Jakob disease can remain dormant for decades and there are currently no reliable tests to detect them. After the mad cow scare that hit Britain in the 1980s, some experts fear there might be thousands of hidden cases in the country. The department of health estimates one in 4,000 Britons could be infected.

In a study published Thursday in the medical journal Lancet, scientists describe a new blood test that can identify tiny amounts of the prions that cause mad cow disease. John Collinge of the U.K. Medical Research Council Prion Unit and colleagues used their test on 190 blood samples, including 21 people with symptoms of the brain-wasting disease.

The new test correctly identified mad cow disease in 15 patients and was better at picking up small concentrations of the disease-causing proteins than the currently available test. The scientists thought their test might have missed the other six patients because the patients' levels of prion-causing disease were too low.

"Our findings demonstrate the ability to detect prion infection in blood and show that a donor screening test is technically feasible," Collinge and colleagues wrote. Mad cow disease can be spread by blood transfusions, surgery or dental procedures. Some experts have called for better ways to protect the population from the degenerative disease.

Variant Creutzfeldt-Jakob disease is usually seen in people who have eaten food products from infected cattle. Patients develop psychiatric symptoms including depression, schizophrenia or psychosis. Most are immobile or mute by the time the disease kills them.

In an accompanying commentary, Luisa Gregori of the U.S. Food and Drug Administration wrote that Collinge and colleagues' test "represents an important step" towards a diagnostic test for human mad cow disease.

But she warned that for a test to effectively screen large numbers of blood donors, it would need to be much more accurate and capable of detecting the disease in people carrying the disease who did not already have symptoms.