Could Alzheimer's disease be infectious?

While we know plenty about the risk factors for Alzheimer’s disease – smoking, not having enough friends, eating the wrong foods, being overweight – we’re still in the dark about what actually causes this and other brain diseases.

But now the discovery of a new disease similar to “mad cow” or and bovine spongiform encephalopathy and its human manifestation Creutzfeldt-Jakob disease (CJD), had scientist questioning once more whether diseases like dementia and Parkinson’s are able to be transmitted from one host to another.

It turns out scientists have always held this possibility in the backs of their collective minds.

Writing in The Conversation, Colin Masters, distinguished Florey Fellow at the Florey Instictute of Neuroscience and Mental Health, says, “Scientists have always kept – and still do – an open mind about whether Alzheimer’s disease and other neurodegenerative conditions are transmissible.”

The new disease is of the first human prion disease we’ve seen in 50 years. It was made by Stanley Prusiner, who won the Nobel prize in 1997 for his discovery CJD, could be transmitted by a “misfolded” protein.

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Prion diseases are a rare class of brain disorders that are transmissible between animals of any species, including humans. They were first identified in cannibals in Papua New Guinea who ate the brains and spinal cords of their deceased relatives.

The disease, called “Kuru”, affected mostly women and children who developed body tremors, balance problems and slurred speech. There’s no cure for kuru and sufferers always died. But it no longer strikes as cannibalism in the region has been eliminated.

This new prion disease, Shy-Drager syndrome (SDS) or multiple system atrophy (MSA), was first recognised in the early 1960s and has many features in common with Parkinson’s disease.

Professor Masters explains, “The most important of these is that a protein known as α-synuclein (α-syn) accumulates in the brain, in both Parkinson’s and SDS/MSA. This accumulation is very similar to what happens in CJD, where the prion protein accumulates, and also in Alzheimer’s disease and other dementias, where two types of proteins, known as amyloid beta (Aβ-amyloid) and tau, build up in the brain.

“The clumps and tangles of these various aggregated proteins cause neurons to degenerate and die. This is a cumulative process which takes between months and decades to manifest as overt disease.”

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Many of the neurodegenerative diseases of the ageing brain are associated with this build up and depositing of specific proteins.

“It has long been suspected that neurodegenerative diseases in general may all ultimately be caused by this process of proteins getting caught in the wrong process, and misfolding.

“Some but not all of these misfolded proteins gain the ability to be transmitted between people and animals. And we don’t yet know of ways to easily “dis-infect” or kill these proteins. All kinds of chemicals that kill bacteria and viruses do not harm prions.”

What they do know is that the Alzheimer’s disease-causing protein can cross-seed in genetically susceptible rodents. Mice carrying an unstable and genetically modified human protein can then be “infected” by giving them a dose of the human abnormal protein.

Professor Masters says there’s still no direct evidence that Alzheimer’s disease is transmissible between people, but as a precautionary measure, it’s likely we will see improved safety measures in neurosurgical clinics, such as the use of disposable stimulation electrodes for deep brain stimulation.

While the very idea that dementia and other brain diseases are transmittable is quite alarming, it’s worth remembering that anything that brings us closer to understanding these diseases brings us closer to a cure.