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Two common infections may trigger Alzheimer's, scientists warn – are you at risk?
SCIENTISTS have spent decades trying to understand what causes dementia.
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The evidence is mixed - though experts generally agree that there are several factors involved in the abnormal build-up of proteins called amyloid and tau, which are what cause symptoms.
A new review by Philadelphia College of Osteopathic Medicine, Pennsylvania, suggests two common infections may play a role.
Researchers found both chlamydia pneumoniae and SARS-CoV-2 increase the levels of substances in the brain called cytokines.
These trigger inflammation, which can "harm brain cells and may help speed up the buildup of harmful proteins linked to Alzheimer's".
Chlamydia pneumoniae is a "very common" type of bacteria that causes lung infections, including pneumonia.
It affects about 50 per cent of people by age 20, and 75 per cent by 65, and is passed on by sneezing and coughing, according to Superdrug.
It is not the same as the STI chlamydia, though both are highly infectious.
SARS-CoV-2 is the virus that causes Covid-19.
In 2022, seven in 10 people in England were thought to have been infected. That number is now likely much higher.
The report, published in Frontiers in Aging Neuroscience, found chlamydia pneumoniae and SARS-CoV-2 can invade the central nervous system through the blood-brain barrier and, "potentially more importantly", the olfactory route, which is responsible for our sense of smell.
"The olfactory system is lined with a specialised tissue called neuroepithelium that can serve as an entry point for pathogens to the brain," the authors said.
"After initial infection, these microbes can travel along the olfactory nerves, ultimately reaching the brain's olfactory bulbs, which are linked to areas of the brain for memory and cognition.
"This pathway is particularly relevant given that loss of smell is an early symptom in both Covid-19 and Alzheimer's disease."
We hope to create new avenues for prevention and treatment
Dr Brian J BalinProfessor of neuroscience
They looked at patients with certain genetic factors known to significantly increase a person's risk of developing Alzheimer's - the most common form of dementia.
This included APOE - a protein which transports fatty molecules like cholesterol to cells in our brain.
Everyone carries two copies of APOE, one inherited from each parent.
The three most common variants are APOE2, APOE3 and APOE4.
Having at least one APOE4 variant is said to double or triple the risk of developing Alzheimer's, and someone with two variants is eight to 12 times more likely to get it, according to Alzheimer's Research UK.
About one in 50 people carry two copies of APOE4.
Scientists also examined the cytokines IL-6 and CCL2, which are involved in inflammation and immune responses in the body.
Those carrying this gene variant appeared to be "more susceptible" to both chlamydia pneumoniae and SARS-CoV-2, "potentially amplifying" their risk for developing Alzheimer's.
Co-author Dr Brian J Balin, a professor of neuroscience and neuropathology and director of the Center for Chronic Disorders of Aging, said: "These findings bring us one step closer to understanding the complex interactions between infections and Alzheimer's disease.
"As we continue to learn more about the role infectious agents play in the development of this disease, we hope to create new avenues for prevention and treatment."
What causes Alzheimer's disease?
ALZHEIMER'S disease is thought to be caused by the abnormal build-up of proteins in and around brain cells.
One of these is amyloid, deposits of which form plaques around brain cells.
The other is tau, deposits of which form tangles within brain cells.
Unfortunately, it's not known exactly what causes this process to begin.
However, scientists suggest that the following factors are known to increase your risk of developing Alzheimer's:
Age - the single most significant factor. The likelihood of developing Alzheimer's disease doubles every five years after you reach 65.
Family history - the genes you inherit from your parents can contribute to your risk of developing Alzheimer's disease.
Down's syndrome - the genetic changes that cause Down's syndrome can also cause amyloid plaques to build up in the brain over time.
Head injuries - people who have had a severe head injury may be at higher risk of developing Alzheimer's disease, but research is still needed in this area.
Lifestyle factors and conditions - smoking, obesity, diabetes, high blood pressure are associated with a higher risk.
Other factors - research suggests that hearing loss, untreated depression, loneliness or social isolation and a sedentary lifestyle may also play a role.
About 980,000 people in the UK are living with dementia.
This number is expected to rise to more than 1.4million by 2040.
Alzheimer's is the most common form of dementia, accounting for between 60 and 80 per cent of all cases.
Symptoms often develop slowly over several years and they may not be obvious at first.
In the early stages, it can be difficult to tell the difference between memory problems caused by Alzheimer's, and mild forgetfulness that happens as we get older.
But memory loss is one of the most common early symptoms, and may include losing memories of recent events, asking the same questions repeatedly, or having difficulty following conversation and learning new information.
Patients may go on to regularly forget names and faces, repeat the same behaviours and routines, regularly misplace things, become confused about the date or time of day, feel disorientated in unfamiliar places, have problems finding the right words, or become low in mood, anxious or agitated.
As Alzheimer's progresses, it can impact other areas of life, including communication, sleep, movement, senses and day-to-day care.
If you are worried that you or a loved one are experiencing symptoms of Alzheimer's, speak to your GP.
While there is no cure, there are treatment options to help manage your symptoms for a time.
