What do cold sores and Alzheimer’s disease have in common? A pair of studies suggests there could be a link between an increased risk of Alzheimer’s and the herpes virus that gives you cold sores that erupt in or near your mouth.
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In one study, researchers from Umeå University in Sweden found that being a carrier of herpes simplex virus 1 nearly doubled a person’s risk of developing Alzheimer’s disease. In a second study, the investigators followed 3,432 people for an average of 11.3 years, and found that a reactivated herpes simplex 1 infection doubled one’s risk of developing Alzheimer’s disease.
Herpes simplex virus 1 infections are very common. The majority of the population carries herpes simplex 1, the virus responsible for most cold sores. Once you are infected, you carry the virus for life. From time to time, the virus can become active, which causes the cold sores.
We talked with Alzheimer’s specialist Jagan Pillai, MD, about the studies to find out what they mean.
Both studies suggest a link between Alzheimer’s disease and herpes simplex virus. Is this something that researchers already knew?
Some earlier studies, which had smaller numbers of subjects, have reported a possible relationship between the herpes simplex 1 virus and Alzheimer’s disease. The link also has been reported in cellular lab studies.
What does this research mean for the general population?
What is notable about the research is that it was well-designed. Researchers studied a large group of people, there was a long period of follow up time — two decades — and the way in which they diagnosed Alzheimer’s used methods that we can reasonably trust.
A diagnosis of Alzheimer’s disease can be made with certainty only after death, though this is changing. Neuroimaging techniques, for example, now allow visualization of amyloid plaques, which is the hallmark of Alzheimer’s.
The studies show that for some as-yet unclear reason, immune changes related to herpes simplex 1 appear to be more common in older individuals (meaning older than age 60) with Alzheimer’s.
The research does not say, nor does it tell us, if the herpes simplex 1 virus caused Alzheimer’s. It could be that immune changes related to Alzheimer’s disease simply cause more reactivations of the virus.
So the bottom line is right now, there is no reason to be alarmed. Having the herpes simplex 1 virus does not mean that it’s certain you will go on to develop Alzheimer’s disease.
What significant issues do you have in assessing how either of the studies were conducted?
While the study’s conclusions are valid, there are some issues with the research. One is that Alzheimer’s was diagnosed clinically, meaning based on signs and symptoms. The Alzheimer’s diagnosis did not use tests, such as an amyloid PET scan or a cerebrospinal fluid test, to check for surrogate biomarkers of Alzheimer pathology. We know that Alzheimer’s, when diagnosed clinically only, is accurate up to about 70 percent of the time.
Because the number of cases in the studies that are driving the study’s significant statistical effect is small, the results could change with more careful parsing of the Alzheimer’s diagnoses.
These studies were done from data collected over a long period of time, so they did not have access to our current methods of Alzheimer’s diagnosis.
Are there any theories about why there is a link between herpes simplex 1 virus and Alzheimer’s disease?
It is unclear if herpes simplex 1 infection is special in any way. Many chronic neurological diseases are thought to be triggered by persistence of viral infections.
Herpes encephalitis, which is an uncommon, full-blown infection from the virus on the brain, tends to affect the hippocampus and temporal lobes. Scientists have observed that these brain areas also are typically involved early in Alzheimer’s disease.
More research is needed. Right now, we don’t know why infection with herpes simplex 1 is correlated to development of Alzheimer’s. It could be that Alzheimer’s disease is related to immune changes, making it more likely someone with Alzheimer’s could experience reactivation of the herpes simplex 1 virus.