Does Herpes Cause Alzheimer's Disease?

In June, 2018 we posted about a possible connection between the herpes virus and Alzheimer's disease.  (In February 2019 we posted about a bacterium that might also be implicated.)  As we noted on January 15 of this year, 2019 saw new interest in the possible role of the herpes virus in the etiology of Alzheimer's disease. 

So what are we to make of the role of pathogens in general and herpes in particular in what sparks Alzheimer's disease and what then drives its development?

The answer is that it depends on who you ask.  This week's ALZForum carried an interesting article that attempted to assess where we're at with this question: "Herpes Viruses and Alzheimer's -- the Debate Continues."  As we noted in the June 2018 article, the herpes virus can be found in the brains of people who died of Alzheimer's dementia, suggesting that it was part of the etiology -- although it noted that our brains begin accumulating viruses of many types from when we were children.  But the new article points out that there are different ways of assessing the presence of viruses in an autopsy, and the different ways yield different conclusions.

A problem with the story that's been evolving since the important research in 2018 is that it's not clear that there are more herpes viruses in the brain of a person who died with Alzheimer's dementia than in that of a normal person.  Some studies say there is, but other credible studies (using a different methodology) find that there isn't.  On the other hand, there seems to be a consensus that viruses may well play a role in the development of Alzheimer's disease.

It's not so much about finding the viruses are still active, but rather finding the damage they caused. 

Gene regulatory networks are systems that control how genes are expressed in proteins.  (As a consequence, they control, for example, whether you are a human or a reptile.)  Damaging them won't cause you to turn into a lizard, but damage in a gene regulatory network may cause a gene to express in a manner that promotes Alzheimer's disease.  And damaged networks may focus their influence on the genes we have learned to associate with risk for Alzheimer's disease.

The evidence for a connection between herpes and Alzheimer's disease is strong, based on data used in the study we discussed in June 2018.  Remember that Alzheimer's disease begins years, maybe decades before the first symptoms appear.  So, while there may not be evidence of an unusual presence of herpes at death, it may have been much stronger long ago.  The virus could have done its damage to the gene networks then, and the consequences were realized much later.