The push for a mad cow test
MAD cow-type diseases are insidious. The fatal brain disorders can incubate for decades before symptoms appear, yet there is no way to test a live animal, or human, for them.
Because a diagnosis can only be made after death, diseased animals with no obvious symptoms can enter the food chain, infecting other animals or people, and humans with the diseases can be misdiagnosed or infect others through blood or organ donations.
Now research teams across the country are making significant progress in devising a blood test that can detect the prions that appear to trigger these lethal ills. “A sensitive blood test is the holy grail, and recent discoveries are taking us steps closer toward achieving this goal,” says Niel Constantine, a prion expert at the University of Maryland School of Medicine in Baltimore.
Prions are a type of protein found in the nervous system of animals and humans. Sometimes they become abnormally twisted, and in this form, scientists believe, they cause the brain diseases with which they’re linked. When abnormal prions are eaten by animals or humans, they appear to convert normal prions into abnormal ones, spreading the original brain disease.
Such incurable neurological diseases are called scrapie in sheep, chronic wasting disease in deer and elk, bovine spongiform encephalopathy (BSE, or mad cow disease) in cows, and Creutzfeldt-Jakob disease in humans. Humans who eat beef with mad cow disease can develop a variant of Creutzfeldt-Jakob disease (vCJD).
About 180 people have been diagnosed globally with vCJD. Because of the up to 40-year incubation period of prion-type diseases, however, scientists have no idea of their actual magnitude; estimates of infected humans worldwide range from 300 to 300,000.
“While the number of people infected is seemingly small, these are people who have already developed symptoms,” says Dr. Jiri G. Safar, a neurologist and prion expert at UC San Francisco Medical Center. “We really don’t know how many carriers there are who could transmit the disease through blood transfusions or organ donations.”
This uncertainty lends urgency to the search for an effective blood test that could prevent the spread of these lethal diseases through the food and blood supply or through transplanted organs.
Devising a test has proved daunting, however. Because prions are neither bacteria nor viruses, they can’t be detected using standard tests. Compounding the problem is the fact that deadly prions are created by an alteration of normal cellular prion proteins found in all mammals. “All tissue, including the blood, contains large quantities of normal prion proteins, which are chemically identical to abnormal ones -- the only difference is they have slightly different shapes,” says Safar.
And because only infinitesimal numbers of renegade prions are in the blood -- they normally collect in the brain -- they are virtually impossible to detect until the infection is advanced. However, University of Texas scientists have invented an experimental blood test that circumvents this problem. Using a technology called protein misfolding cycle amplification, the abnormal prions in the blood are multiplied many times over, so the test can identify even a small number of the lethal particles.
In a 2005 study, the test accurately detected abnormal prions in 16 of 18 blood samples from infected laboratory animals. While not foolproof, this is the first test that was able to identify prions in the blood, says Claudio Soto, a neuroscientist who helped develop the test at the University of Texas Medical Branch at Galveston. The next step is to make the test even more sensitive, and determine whether it can amplify prions from human blood samples.
Other screening tools aren’t as far along in their development. At UC San Francisco, scientists have developed a diagnostic technique called the conformation-dependent immunoassay, a chemical test that can identify antibodies to the rogue prion proteins.
A 2005 study revealed that the immunoassay was significantly more sensitive than current tests. Using brain samples from 28 patients diagnosed on autopsy as having human forms of mad cow disease, the test identified prions in 100% of the samples, while the traditional assay found them in 22%, and routine tissue examinations detected only 17% of the prion-infected tissue. Researchers are now investigating whether the immunoassay can identify prions in blood and muscle.
University of Maryland School of Medicine researchers have developed the Immuno-PCR test, which uses a different method than the University of Texas test to multiply the number of lethal prions. A 2005 study of tissue samples revealed that the novel tool detected ultra-low levels of abnormal prions and was 10 times more sensitive than current tests. Future plans call for testing blood from animals known to be infected with the disease.
More research needs to be done, and a simple screening tool won’t be available for at least two years. “But different approaches are good in science because it will give us a way to choose the one that is most successful,” says Dr. Pierluigi Gambetti, a neuropathologist at Case Western Reserve University and director of the National Prion Disease Pathology Surveillance Center.
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A more efficient way of screening cattle
Currently a definitive diagnosis of BSE, or mad cow disease, is made only on autopsy, using samples of brain tissues from patients suspected of having the disorder. But a more efficient method of screening livestock may soon be available, preventing infected cattle from entering the human food chain.
Prion Developmental Labs’ Rapid BSE tester, which is composed of a chemically treated strip, can determine within five minutes whether abnormal prions are present in the brain tissue of slaughtered cattle, studies have shown. Current tests can take several days.
“We’re hoping to get clearance from the USDA within the year,” says Robert B. Petersen, a molecular biologist at Case Western Reserve University in Cleveland, who helped devise the test.
