Even after decades of research, the cause of the most common form of dementia, Alzheimer's disease, is unclear. Evidence that pointed to plaques and tangles of disordered proteins called amyloid beta and tau has been called into question since therapeutics that aim at the protein clumps have not been particularly effective at relieving disease. In recent years, scientists have also found evidence that viral infections may be to blame for some serious long-term diseases; multiple sclerosis, for example, seems to only develop in people who have been infected with Epstein-Barr virus (EBV). The pandemic coronavirus SARS-CoV-2 also appears to have neurological impacts in some people.
Researchers have now used the power of biobanks that contain health data from hundreds of thousands of people to look for links between viral infections like influenza and EBV and neurodegenerative diseases including Alzheimer’s disease, amyotrophic lateral sclerosis (ALS), generalized dementia, and Parkinson’s disease. Data from FinnGen on over 300,000 people was used to look for associations that were then assessed in the nearly half a million people in the UK Biobank for confirmation. An additional cohort of almost 100,000 people who had not had any neurodegenerative disorder was used as a control group. COVID-19 hospitalizations were not included in this study. The findings have been reported in Neuron.
The research showed that there is a connection between viral infections and neurodegeneration, which is particularly significant for viruses that are able to cross the blood brain barrier and infect the central nervous system, perhaps unsurprisingly. The study authors suggested that these so-called neurotropic viruses may be causing neurodegeneration because of neuroinflammation.
A first pass of the data suggested there were 45 significant associations between viral infections and neurodegenerative disease. After querying the UK Biobank, the researchers refined it to 22 associations.
The top candidate was generalized dementia, which was linked to infections with six different viruses: all influenza, flu and pneumonia, viral pneumonia, viral encephalitis, viral warts, and other viral diseases seem to raise the risk of generalized dementia. The research also showed that anyone who had viral encephalitis was 20 times more likely or more to receive an Alzheimer's diagnosis compared to people who have not had viral encephalitis.
Severe flu was connected to a greater likelihood of different types of neurodegeneration. Influenza and pneumonia exposures were also linked to an increased risk of all neurodegenerative disorders except multiple sclerosis.
Senior study author Michael Nalls, Ph.D., the NIH Center for Alzheimer's Related Dementias (CARD) Advanced Analytics Expert Group leader, also noted that the viral infections that are being considered in this study were quite severe and led to hospitalizations; they were not mild cases of the common cold.
"Nevertheless, the fact that commonly used vaccines reduce the risk or severity of many of the viral illnesses observed in this study raises the possibility that the risks of neurodegenerative disorders might also be mitigated," said Nalls.
"Our results support the idea that viral infections and related inflammation in the nervous system may be common and possibly avoidable risk factors for these types of disorders," added study co-author Andrew B. Singleton, Ph.D., the director of CARD.
Sources: National Institutes of Health, Neuron