Alzheimer's can be spread between humans through rare medical accidents, research has found, although experts stress that there is no evidence the disease can be spread from human to human through everyday activities or routine care. The new study, published in the journal Nature Medicine, provides the first evidence of Alzheimer's disease in living people that appear to have been medically acquired from deceased donors and due to transmission of a toxic protein that causes the condition. Researchers say a handful of people who received human growth hormone from the pituitary glands of the deceased have gone on to develop early-onset Alzheimer's likely because the hormones used were contaminated with proteins that seeded the disease in their brains.
"We're not suggesting for a moment you can catch Alzheimer's disease. This is not transmissible in the sense of a viral or bacterial infection," said Professor John Collinge, co-author of the study and director of the MRC Prion Unit, as per The Guardian.
"It's only when people have been accidentally inoculated, essentially, with human tissue or extracts of human tissue containing these seeds, which is thankfully a very rare and unusual circumstance," he added.
The team said that the findings may have important implications for understanding and treating Alzheimer's disease. According to researchers, the study also adds weight to the idea that the disease has similarities with prion disease, including the mechanism by which the proteins involved spread across the brain.
Notably, prion diseases are caused by infectious, misfolding proteins which propagate in the brain. These diseases typically occur spontaneously, however, more rarely they can arise from a genetic mutation, or be transmitted via infected brain or nervous tissue.
For the study, researchers reported how between 1959 and 1985, at least 1,848 patients in the United Kingdom received human growth hormone extracted from the pituitary glands of cadavers. This was used for various causes of short stature - when a child or a teen is well below the average height of their peers. However, this treatment was withdrawn in the 1980s after it emerged that some patients died as a result of hormone samples contaminated with Creutzfeldt-Jakob disease (CJD)-causing proteins. CJD is a rare fatal condition that affects the brain. Of the 80 such cases in the UK, some were also found to have a protein called amyloid-beta in their brains when they died - a hallmark of Alzheimer's disease.
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Researchers noted that it was unclear if these patients would have gone on to develop symptoms of Alzheimer's, but they cited other research that showed amyloid-beta was present in some of the hormone batches, and that these triggered Alzheimer 's-like disease if administered to mice. They also noted that once the cadaver-derived human growth hormone procedure was replaced with a synthetic growth hormone, it did not carry the risk of transmitting CJD.
"The patients we have described were given a specific and long-discontinued medical treatment which involved injecting patients with material now known to have been contaminated with disease-related proteins," Mr Collinge said.
"However, the recognition of transmission of amyloid-beta pathology in these rare situations should lead us to review measures to prevent accidental transmission via other medical or surgical procedures, in order to prevent such cases from occurring in future," he added.
Experts said that medical procedures should be reviewed to ensure rare cases of Alzheimer's transmission do not happen in future.
However, Andrew Doig, professor of biochemistry at the University of Manchester, said experts were already very careful about transmitting brain tissue between people. He also cautioned that only eight patients were involved in the study, some of whom lacked genetic data, while as yet there was no direct evidence for different strains of amyloid-beta.
