Planet Earth's most contagious diseases include measles, tuberculosis, everyone's winter nemesis, the flu, and now Covid-19. At the same time, many diseases that plague humans arise due to mishaps in our bodies, our genetic makeup, or the lifestyle choices that we make.
So what about Alzheimer's? Is there a chance it could be transmitted? Or is it the result of something else?
Alzheimer's disease (AD) is a neurodegenerative condition that leads to the deterioration of cells in the brain. The hallmark symptom is memory loss, but it can lead to other issues like speech problems and difficulty moving too. Unfortunately, there is currently no cure, and 5.8 million Americans over the age of 65 are currently living with the disease.
So, what causes it? At the moment, we’re not entirely sure. Experts think it’s likely due to a combination of genetic, lifestyle, and environmental factors – one specific cause has never been identified.
It has long been debated that Alzheimer's could have some sort of transmissible element, and while this is unlikely to be the case, there is some evidence to back up the argument.
From the dead to the living
Those with the disease have build-ups of misfolded amyloid beta proteins and tau proteins in their brains, but we still don’t know whether these protein aggregations are a cause or just a side effect of the disease. But could they be transmissible?
It may be possible for brain proteins to be passed from person to person, although not in a conventional way. Between 1958 and 1985, growth hormone treatments for children were created using hormones taken from cadavers. Over time, it became apparent that many patients who had received these growth hormone treatments were becoming ill with a disease known as Creutzfeldt-Jakob disease (CJD). This is a rare and fatal condition that causes brain degeneration.
CJD is thought to be caused by a type of brain protein called a prion, and leads to a build-up of abnormal proteins in the brain. It seems that certain growth hormone treatments were contaminated with prions from the cadavers, causing the unlucky patients to effectively 'catch' the disease.
So what about the proteins linked to Alzheimer's?
A 2018 study led by University College London analyzed the brains of eight people who had died from CJD after receiving the growth hormone treatments. Interestingly, the team found the presence of beta-amyloid proteins and tau proteins – the two proteins linked to AD – in the brains of all eight people, likely transferred from the cadavers. The proteins were not found in the brains of those who had received growth hormone extracted from cadavers via an alternative method.
Therefore, there does appear to be some evidence that Alzheimer’s-related proteins might be somewhat transmissible, but it’s important to remember that those studied literally received injections derived from dead people. Even if the proteins can be transmitted in this way – and more evidence is needed to confirm they can – you won’t catch the proteins from someone coughing or via skin-to-skin contact like you would a contagious disease.
"The transmission probably requires direct contact with the brain or the presence of circulating pathogenic forms of the proteins in the bloodstream," Seth Love, professor of Neuropathology at the University of Bristol and co-leader of Bristol's Dementia Research Group, told IFLScience. "I am not aware of any evidence that the disease is transmissible in the usual sense of passing from one person to another in the absence of injection or implantation of brain tissue.
"I’d add that the pathogenic protein aggregates in Alzheimer’s disease are not nearly as resistant to degradation as those in prion diseases such as Creutzfeldt-Jakob disease."
Meanwhile, just as proteins might not actually cause Alzheimer's, new evidence suggests that prions might not actually be the cause of CJD.
"The idea that AD is caused by infectious proteins is not scientifically valid since the idea of replicating proteins in CJD on which that premise is based does not make scientific sense," Frank O. Bastian, clinical professor of Neurosurgery and Pathology at Louisiana State University, told IFLScience. Bastian's own research suggests that the root cause of CJD is actually spiroplasma, a wall-less bacterium.
"Several bacteria are suspect in AD so time will tell,” he added.
Could bacteria play a role?
Weirdly enough, a number of studies have found a link between gum disease and Alzheimer’s. That’s not to say that the infection causes Alzheimer’s, but it may be a contributing factor in certain people. A study from earlier this year found the bacterium Porphyromonas gingivalis, the main cause of chronic gum disease, in the brains of deceased people who suffered from Alzheimer’s.
The bacterium’s DNA was also found in their spinal fluid and toxic enzymes that it produces were found in their brains. The same brains were also infiltrated by tau proteins and ubiquitin, both of which are linked to Alzheimer’s. Looking at mice, the team found that P. gingivalis infections spread to their brains, resulting in some of the hallmark signs of Alzheimer’s.
Although more research needs to be done to establish the link further, if gum disease was shown to be an important factor in Alzheimer’s development, could it be transmissible? While gum disease generally isn’t passed from person to person, it can sometimes be via kissing.
Still, there’s no need to panic about catching Alzheimer’s from your partner just yet.
“We know diseases like Alzheimer’s are complex and have several different causes, but strong genetic evidence indicates that factors other than bacterial infections are central to the development of Alzheimer’s, so these new findings need to be taken in the context of this existing research,” David Reynolds, chief Scientific officer at Alzheimer’s Research UK, said of the research at the time.
Sticky proteins and surgical instruments
It has previously been suggested that amyloid proteins, which are particularly sticky, might be transmitted to people's brains during surgery. A 2018 study found that a number of people who developed a disease called cerebral amyloid angiopathy (CAA) at a surprisingly young age had all undergone brain surgery in their youth. CAA is the result of a build-up of amyloid protein in the brain's blood vessels, which causes them to leak or even burst. The condition normally appears in old age, but those affected developed it when they were younger, some were only in their 30s. As these patients had no other risk factors for developing CAA, the researchers believe that the amyloid proteins spread to their brains during their surgeries by sticking to surgical instruments previously used on Alzheimer's patients.
However, none of those affected actually developed Alzheimer's disease, just CAA, although it seems the proteins involved were transferred to their brains. Previous research suggests that amyloid proteins are resistant to sterilization techniques such as boiling and formaldehyde exposure, so lead researcher Sebastian Brandner told New Scientist that improving these methods or introducing single-use instruments might be a good idea.
Can AD spread via the blood?
A study published in 2017 found that when two mice share a blood supply, Alzheimer’s plaques can spread from an affected mouse to a healthy mouse, eventually leading to damage to the brain tissue. Mice don’t naturally suffer from Alzheimer’s, so the researchers genetically engineered them to possess a gene that produces the human version of the beta-amyloid protein.
The team found that when attached to a mouse with AD, the once healthy mice developed brain plaques and experienced changes in brain areas related to memory and learning.
However, while the findings are certainly intriguing, the study involved literally sewing mice together so that they shared a blood supply. Unless you plan on sewing yourself to someone with Alzheimer’s, there’s no need to worry.
Still, as Reynolds pointed out at the time, "If these findings do prove to be relevant to people, it highlights the continued importance of developing treatments that target amyloid in the body as well as the brain."
So, what's the consensus?
Overall, while there is evidence to suggest that infectious proteins or bacteria might be involved in the development of Alzheimer's disease, based on current evidence it seems unlikely the condition is transmissible. Even if the condition can spread through direct contact with the brain, it certainly doesn't spread in the ways that most infectious diseases tend to – sneezing, skin contact, sex, etc.
“My personal belief is that it’s highly unlikely Alzheimer’s disease can be transmitted from person to person under normal conditions," Joel Watts, assistant professor of Biochemistry at the University of Toronto, told IFLScience. "Experiments in genetically modified mice and the analysis of individuals that received growth hormone injections prepared from cadavers clearly show that amyloid-? pathology can be transmitted under special circumstances.
"However, to the best of my knowledge, there is no evidence that the full spectrum of clinical and pathological features that defines Alzheimer’s disease can arise due to transmission between individuals."
Professor Love added: “There is, I think, convincing evidence that pathogenic forms of the proteins that accumulate within the brain in Alzheimer’s disease are transmissible in a limited sense, in that when introduced by injection or implantation of brain tissue they can be taken up by normal cells in the brain and can induce the formation and further spread of those pathogenic forms within the brain."
So, unless you're planning on being sewed to someone with Alzheimer's disease or injecting the proteins directly into your brain, you're not going to catch Alzheimer's anytime soon. For now, it seems the disease is the result of a combination of genetic and environmental factors. Future research will hopefully tell us more about the elusive cause of the disease, and confirm whether it can be transmitted, and if so, how.
"This is an important question that certainly deserves to be thoroughly investigated,” noted Watts.