Alzheimer's Disease and risky sexual behavior: not quite a false alarm?

Nov 15 2007 Published by under CNS Diseases and Disorders

Abel over at Terra Sigillata got is writ in a tinger over bad grammar and was concerned that Alzheimer's Disease had been tied to risky sexual behavior. His conclusion was that he got a poorly worded email notice about two separate problems that were linked inappropriately by a semicolon. So everything's fine, all clear, right? He's free to engage in risky sexual behavior without fear of getting Alzheimer's Disease (AD) later in life!
That's not entirely true.
We've known that inflammation plays a huge role in the Alzheimer's disease process. Unfortunately, inflammation is caused by a number of sexually transmitted diseases, such as those of the herpes family. What's more, herpes viruses have a proclivity for neural tissue. Multiple herpes viruses have been found in the brains of AD patients; indeed (note the proper use of a semicolon, Abel), herpes simplex 1 (HSV-1) is a known risk factor for AD. Additionally, other herpes viruses have been studied and it appears that human herpes virus 6 (HHV-6) type B (but not type A) may be another with a strong link to incidence of AD. Cytomegalovirus (CMV) and HSV-2 seems to lack a relationship, and very few control or AD brains are infected with them.
Now now now, I know what you're thinking. You're saying "Aha Evil! HSV-2 is the simplex virus commonly associated with sexual activity!" While this is true, HSV-1 infection can and does result from sexual activity, just as HSV-2 is not only transferred via genitogenital contact. CMV is also transmitted sexually.
As I said, thankfully, HSV-2 rates seem to be quite low in AD brain and in elderly control brain. However, that could change in the future. As Tara at Aetiology mentions, the elderly are a population that is at an increasingly high risk for STD transfer, as the geriatric population is largely ignored by educational efforts, and they are less likely to practice safe sex or use condoms . An increase in the incidence of HSV-2 in the elderly could conceivably change that.
To make matters worse, those at risk for AD due to the presence of the apoE4 gene get a double whammy: HSV-1 is present at higher levels in the brains of those with the apoE4 allele, and HSV-1 shifts processing of amyloid precursor protein into Abeta (1-40) and (1-42) overdrive, which is unfortunate since Abeta protein fragments are what aggregate and form one of the pathological hallmarks of AD; the amyloid plaque. Fibrillar Abeta is thought to "gum up" synapses and contribute to neuronal dysfunction and cell death.
On a related note, CMV may also cause problems for the aging noggin-- it is transmitted sexually and also increase the risk of vascular dementia.
It is thought that merely having, say, HSV-1 isn't causative (obviously it isn't because 85% of people have HSV-1 from a young age and not that many people develop AD), but that HSV-1 "flareups" in the brain, over time, increase the inflammatory processes that contribute to AD, so that the effect is cumulative.
So Abel, you're sorta right and unfortunately sorta not. We don't have rock solid evidence that sex=Alzheimer's by a long shot, but there's definitely some cause for concern that STDs (or at least viruses that target the brain and can be transmitted sexually) may be contributing to the incidence of AD, and quite possibly to other neurodegenerative diseases.

7 responses so far

  • Thanks for the education - who knew?
    You, sir, are an Evil Genius!
    However, I won't be able to comment on your use of the semicolon until my bifocals arrive (or get back on Firefox and use control+).

  • Evil Monkey says:

    Oh man. Looks like you contracted ocular herpes. It is now only a matter of time until the virus eats directly through your twisted brain.

  • moonenite says:

    If you're really worried about HSV-1 as risk factor, shouldn't you're preventive measures concern kissing rather than risky sexual behavior, as that is the main route it comes to infect 85% of the population. Good luck trying to convince adolescents not to lock lips on fears of possibly boosting their chances for a disease that could affect them in their 70s.

  • Many people are infected even prior to adolescence, so just nixing the kissing won't guarantee protection. This is where a vaccine would come in handy.

  • Evil Monkey says:

    I don't believe I suggested any preventive measures in my post. I merely wanted to highlight the fact that there are consequences to contracting STDs that extend beyond not being able to get a date once your genitalia start falling off. 😀

  • Great post!
    I'm learning about Alzheimer disease right now in med school... it's nice to hear about these sorts of correlations.
    (And just to nitpick: ApoE epsilon4, not just ApoE4, is a risk factor for AD.)

  • Ben Harder says:

    Fascinating post. I suspect we only have the faintest glimmer of an understanding of how STDs affect the brain. From an evolutionary perspective, it just makes too much sense that an STD agent would 'want' to alter its host's behavior, including its sexual behavior. That might not be a factor in any CMV/HSV-1-AD link, but it could be for other pathogens. Look at syphilis, for example. And what about Toxoplasma gondii (which recently got my attention through the work of Robert Sapolsky)?

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