The root cause of cognitive conditions such as Alzheimer’s disease is poorly understood, but new research suggests a common virus may be acting as a trigger.
The earliest stages of Alzheimer’s disease can be almost impossible to detect. Symptoms are often dismissed by patients and doctors alike as normal age-related forgetfulness.
Pinpointing exactly what causes these brain changes is also a challenge. But now, researchers from Tufts University and the University of Oxford have discovered one common virus that may trigger the onset of Alzheimer’s.
In a study published in the Journal of Alzheimer’s Disease, the research team showed varicella zoster virus (VZV), which commonly causes chickenpox and shingles, may activate herpes simplex (HSV), another common virus, to set in motion the early stages of Alzheimer’s disease.
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HSV-1, the most common variant of herpes simplex, usually lies dormant within neurons in the brain. However, when activated it leads to an accumulation of tau and beta-amyloid proteins – which are hallmarks of early stage Alzheimer’s.
The researchers created brain-like models inside six-millimetre-wide donut-shaped sponges made of silk protein and collagen.
They found that neurons grown in the brain tissue can be infected with VZV, but that alone did not lead to the formation of the signature Alzheimer’s proteins tau and beta-amyloid.
However, if the neurons already contained HSV-1, exposure to VZV led to a reactivation of HSV and a dramatic increase in tau and beta-amyloid proteins. Neuronal signals begin to slow down.
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“Our results suggest one pathway to Alzheimer’s disease caused by a VZV infection, which creates inflammatory triggers that awaken HSV in the brain,” says Dr Dana Cairns, co-author of the study.
“While we demonstrated a link between VZV and HSV-1 activation, it’s possible that other inflammatory events in the brain could also awaken HSV-1 and lead to Alzheimer’s disease.”
The World Health Organization (WHO) estimates more than 3.7 billion people worldwide are infected with HSV-1, although the majority will never experience symptoms and will likely remain unaware of their infection.
VZV infection is also very common, with the WHO estimating around 95 per cent of people are infected before reaching age 20. Many of these cases will appear as chicken pox in childhood.
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Later in life, VZV can again be reactivated and cause shingles – a painful condition characterised by blisters and nodules forming on the skin in a band-like pattern. The condition can last for weeks or even months, and around one in three adults will develop shingles in their lifetime.
“We have been working off a lot of established evidence that HSV has been linked to increased risk of Alzheimer’s disease in patients,” says Dr Ruth Itzhaki, another co-author of the study.
“We know there is a correlation between HSV-1 and Alzheimer’s disease, and some suggested involvement of VZV, but what we didn’t know is the sequence of events that the viruses create to set the disease in motion,” she says.
“We think we now have evidence of those events.”
Are you worried about developing Alzheimer’s? Do you think this discovery could help early detection? Let us know in the comments section below.
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