A new study has presented more evidence backing up a hypothesis suggesting viral infections in the olfactory system can accelerate the progression of Alzheimer's disease (AD). Instead of focusing on specific viruses, the research looked at the relationship between biomarkers of viral infections and neurodegeneration in the hippocampus.
For decades, researchers have reported an association between brain diseases and acute viral infections. The herpes simplex virus in particular has long been associated with the development of Alzheimer's disease. But exactly how viral infections could be influencing neurodegeneration has been unclear.
This new research focused specifically on the olfactory system. Loss of one's sense of smell has been found to be a potential early marker of Alzheimer's neurodegeneration. So, the question explored here is whether viral infections in the nose play a role in accelerating Alzheimer's decline.
To investigate this, the researchers analyzed post-mortem brain tissue from several individuals who died with Alzheimer's disease. Compared to tissue samples taken from age-matched cognitively healthy subjects, the Alzheimer's group displayed substantial markers of viral infection and inflammation in the olfactory tract – a key pathway that leads to the hippocampus.
"Overall, the transcriptional profile we have observed in this study may be representative of a routinely barraged olfactory system by pathogens and the resulting pathological ramifications such as amyloid deposition, microglia activation, and potentially myelination alterations," the researchers write in the new study. "Taken together with a parallel body of literature indicating that early AD is characterized by smell loss, amyloid deposition in the olfactory epithelium, and olfactory sensory neuron dysfunction, our study raises the possibility that viral infection of the olfactory bulb/olfactory tract accelerates Alzheimer's disease."
The study did not focus on what specific pathogens could be responsible for the persistent infections. However, the researchers do speculate herpes simplex virus type-1 (HSV-1) and varicella zoster virus (VZV) to be the most likely culprits. Both viruses have been previously implicated in the progression of Alzheimer's, and both are known to lead to latent life-long infections.
Lead author on the new study Andrew Bubak, hypothesizes this viral-induced loss of one's sense of smell ultimately speeds up hippocampal degeneration. So, rather than our sense of smell being damaged by Alzheimer's neurodegeneration, it's actually the process of losing our sense of smell that subsequently causes more dementia-related brain damage.
“We know that one of the early signs of Alzheimer’s disease is losing the sense of smell,” said Bubak. “The whole olfactory pathway goes to the hippocampus. If you decrease the signaling along that pathway then you get less signaling to the hippocampus. If you don’t use it, you lose it.”
The new study was published in Neurobiology of Aging.
