- Herpes zoster, also known as shingles, is a common disease among older adults caused by the varicella-zoster virus that also causes chickenpox.
- Some previous studies have linked shingles to an increased risk of dementia, but others have reported a lack of such an association.
- A new large population-based study shows that a past herpes zoster diagnosis was associated with a small decrease in the risk of dementia.
- Thus vaccinations against the varicella-zoster virus may not help reduce the risk of dementia.
It is estimated that around 25%-30% of the general population will develop shingles in their lifetime. Research suggests that viral infections could increase the risk of dementia, but evidence linking a history of shingles diagnosis to dementia is mixed.
A large population-based study published in the journal Neurology now suggests that the incidence of shingles does not increase the risk of dementia in individuals ages 40 years and older.
Dr. Charlotte Warren-Gash, an epidemiologist at the London School of Hygiene and Tropical Medicine, said, “Using data from national Danish registries, the study adds to growing evidence from other countries including the United Kingdom that shingles, despite leading to other complications including post-herpetic neuralgia, does not generally raise dementia risk.”
Dr. Warren-Gash said that in the study, they saw a small subgroup of individuals who had shingles that affected their central nervous system, e.g. leading to encephalitis, had a higher risk of dementia.
“However, encephalitis from other causes is also linked to raised dementia risk, so the association is not specific to shingles. While shingles vaccines effectively reduce the risks of shingles and its known complications, they are unlikely to be effective for dementia prevention,” she pointed out.
Shingles’ potential role in dementia
Herpes zoster is an infection caused by the varicella-zoster virus. Initial infection with the varicella-zoster virus causes chickenpox and the virus may not be completely eliminated by the immune system.
The varicella-zoster virus can then form a reservoir in sensory nerves and remain dormant for many years. Upon its reactivation, the varicella-zoster virus can cause shingles, which are characterized by a painful rash on one side of the body.
Researchers have hypothesized that viral infections, such as shingles, can increase the risk of dementia. A herpes zoster infection could potentially increase the risk of dementia by causing inflammation in the brain, directly infecting brain cells, or causing damage to blood vessels in the brain.
Such a positive association between herpes zoster and dementia risk would suggest that universal vaccination against the varicella-zoster virus in older individuals could reduce the risk of dementia. However, the evidence from epidemiological studies supporting an association between herpes zoster and the risk of dementia is mixed.
In the present study, the authors examined the association between herpes zoster incidence and the risk of dementia using Danish national registries.
No increase in dementia risk
In this study, the researchers used data from national registries on hospital diagnoses and drug prescriptions to identify individuals ages 40 years and older diagnosed with shingles and dementia between January 1997 and December 2017.
The study compared the risk of dementia in 247,305 individuals with herpes zoster with nearly 1.25 million age- and sex-matched individuals from the general population.
The researchers found that a previous herpes zoster diagnosis was associated with a small decrease in the risk of dementia in individuals compared with the general population.
The association between herpes zoster and dementia due to Alzheimer’s disease was also similar to the trend observed with all-cause dementia.
Dr. Sigrún Alba Jóhannesdóttir Schmidt, study author and an epidemiologist at Aarhus University Hospital, Denmark, explained:
“This was unexpected and we cannot explain the reason for the findings in our study. It is possible that it is simply due to missed diagnoses of zoster in people with undiagnosed dementia, because people with cognitive problems may not react to symptoms or seek healthcare for zoster. This would make it look like zoster leads to a lower risk of dementia even though this is not the case.”
“Another possibility is that antiviral drugs used to treat zoster actually protect against the development of dementia, but we could not look further into this with our data,” she added.
Dr. M. Arfan Ikram, an epidemiologist at Erasmus University Medical Center, Netherlands, told Medical News Today, “The authors did not find an association between herpes zoster infection and dementia (perhaps contrary to expectation).”
He said he was not surprised by this and that any effect the researchers found “would have been small.”
“[T]he role of infectious diseases in dementia risk has been reported, but in all likelihood will only be a modest contributor (and not in the same realm as for instance smoking or diabetes is),” he explained.
Infection of the central nervous system
A previous study reported that herpes zoster cases involving the infection of the ophthalmic nerves that innervate the eyes were associated with an increased risk of dementia. The ophthalmic nerves are one of the twelve pairs of cranial nerves that supply the head and the neck with nerves.
In the present study, the researchers found that herpes zoster infections involving these cranial nerves or the inflammation of the central nervous system were associated with a two-fold higher risk of dementia.
“This may be explained by severe inflammation in the brain and damage of brain tissue during the infection. However, because this complication to zoster is very rare, it may explain only a very small number of cases of dementia,” Dr. Schmidt said.
Given the lack of association between herpes zoster incidence and risk of dementia, Dr. Schmidt said, “Our study, therefore, does not change clinical practice. People with symptoms of zoster should still seek their doctor as soon as possible for evaluation of whether treatment should be initiated.”
“However, it would be interesting to see future studies looking into detail whether antiviral drugs for zoster play a role in the development of dementia,” he added.
The authors believe that universal vaccination of the elderly against the varicella-zoster virus is unlikely to lower the incidence of dementia.
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