As you recall, Alzheimer's Disease is a slowly progressive illness of neuronal degeneration. The cardinal symptoms are cognitive impairment and memory loss, but the condition eventually leads to death. I've dedicated many posts to Alzheimer's, which is a reflection of the amount of research out there. And for today's post, I'm going to begin with a nice review article from 2008, and over another post (or several) try to finish out with some more specific looks at the various papers.
Even if you don't find Alzheimer's that compelling, if infectious agents contribute to its pathology, then you have to open your mind to the idea that many neurodegenerative processes could be due to (or accelerated by) infection. Neurodegenerative diseases include many neurological illnesses, but also depression, bipolar disorder, schizophrenia, autism...
What is the argument against infection as an ongoing contributing factor? Well, where's the bug? Do a spinal tap - does the fluid grow any bacteria in culture? Are there white blood cells (sign of active infection). Is there an elevated level of protein (a sign of viral infection)? Over the last hundred years we've become pretty good at finding bugs. It's hard to imagine them hiding from us, even in the protected environment of the human skull. So if you bring up the idea of infections causing Alzheimer's disease to a physician friend and he or she scoffs at you - that's why. Where's the bug?
With that in mind, let's plunge forward into the review by Urosevic and Martins from the Journal of Alzheimer's Disease, "Infection and Alzheimer's Disease: The Apoe epsilon4 Connection and Lipid Metabolism."
The whole theory breaks down like so - there's a continuous, chronic infection supplying persistent live microorganisms, and their toxic products stimulate the host's (that's you) inflammation. The pathogen itself damages the neurons, and the brain's inflammatory response also damages the neurons. Remember that ApoE4 is the Alzheimer's vulnerable variant of apolipoprotein, the molecular key on the lipoprotein that invites cholesterol and triglycerides into the brain. As an added bonus, any infectious disease theory of Alzheimer's also has to explain why having ApoE4 makes you more likely to get Alzheimer's.
What infectious agents are we talking? Some viruses immediately come to mind, specifically herpes viruses. These little guys are exceedingly common and come in lots of different flavors, and are well known to hide out in nerve cells for the duration of the host's life (an easy example of "There's the bug.") HSVI, associated with cold sores, infects people early in life and hangs out in the trigeminal ganglia (the nerve root of the trigeminal nerve that innervates a good part of the face). Some people get cold sores, some people don't, but those who do are more likely to be ApoE4 carriers. People infected with HSVI are also more likely to develop Alzheimer's. ApoE4 mice were more likely to carry invasive HSVI and have brain colonization of the virus.
Other viruses implicated include human herpes virus 6 (cause of roseola, a common childhood illness of high fever followed by a characteristic rash), HIV, hepatitis C, and cytomegalovirus (a cause of mononucleosis-like illness and fatigue symptoms). It is well known that HIV causes a form of AIDS dementia (which happens to be more common in carriers of ApoE4), so it would make sense that other common viruses that infiltrate the neurons might lead to other types of dementia.
All the common inflammation players (TNFalpha, IL-6, nitric oxide synthase) are involved in fighting off viral infections. We know these players have a role in Alzheimer's pathology and in depression and bipolar disorder. Interestingly, as we get older, our immune response becomes less aggressive, and it is perhaps then that the infectious agents hold sway, leading to Alzheimer's pathology. Other inflammation-mediated brain disease occurs at different developmental stages - late adolescence and early adulthood for schizophrenia, and infancy for autism.
There are also suspected bacterial causes. The "spirochetes" are a type of sneaky bacteria that are known to infect nerves (as in syphillis and Lyme disease). Some spirochetes that cause gum disease are found in the mouths of Alzheimer's patients and healthy folks, but in the Alzheimer's patients, they are found in the brain more often than in healthy folks. Certain spirochetes have been found in the amyloid plaques in the brains of patients with Alzheimer's (once again - there's the bug).
Chlamydia pneumoniae is an intracellular bacterial pathogen also implicated in Alzheimer's dementia. Not surprisingly, it is better known as a common cause of pneumonia and other acute respiratory infections, but infected immune cells could presumably carry chlamydia pneumoniae from the upper respiratory tract to the brain if the blood brain carrier is compromised in some fashion. Chlamydia pneumoniae has been injected into mouse brains and causes amyloid deposits, which anyone will agree is suspicious behavior. Once again, ApoE4 comes up, as ApoE4 seems to allow for greater bacterial load and numbers of infected cells.
ApoE4 is not only a bad guy when it comes to Alzheimer's. Carriers are at greater risk of athersclerosis, stroke, and poor recovery from head injury. And ApoE's role as a molecular marker and director of where lipoproteins go is key. Lipoproteins are how fat and antioxidants are carried throughout the body. It is probably not a coincidence that major conditions leading to weight loss without any particular effort are infectious disease and Alzheimer's (along with cancer and melancholic depression). ApoE4 in particular is associated with poor clearance and recycling of lipoprotein particles. It's just not an efficient key. If everything is going along just fine, maybe we don't need an efficient key. Add stress or infection, though, and you have a greater need for meabolic efficiency. If you fall behind, garbage builds up.
The ability of a host (you, or me) to handle an infection depends on genetic, dietary, and other environmental factors such as age, stress, and immune status. Viruses and bacteria are sneaky - I daresay sneakier even than Homo sapiens, and the closeted nerve cells are a perfect place for an unnoticed infection to simmer for decades. Microogranisms can continuously release toxins, leading to chronic inflammation and damage. Treating a chronic infection, if it is found, could possibly be a useful way to fight a number of neurogenerative diseases, even Alzheimer's.
Thanks for posting this article. I have read a link ( I cannot find where ) between hip replacement & Alzheimer's caused by hospital infections. That certainly happened to my father, prior to his hip replacement he was fine, but he went downhill & died from the complications of Alzheimer's.
I wouldn't mind one bit if an infection were the underlying cause of Alzheimers. We usually have more success fighting germs than purely genetic defects.ReplyDelete
Further suggestive evidence for a role of infections in Alzheimers:ReplyDelete
The Alzheimer's Disease-Associated Amyloid β-Protein Is an Antimicrobial Peptide
Check out the comments on the article on PLoS site including the one about Vitamin D
As a neurosurgeon who has seen a ton of head injuries and AD I have believed for many yrs that AD was linked to inflammation and bad SAD. I have advocated for years the use of tumeric omega 3 and coconut oils to help combat the symptoms over things like Aricept. You have to fight the source and not treat the symptoms. Low carb ketotic diets tend to make these patients do much better. I truly believe this is why so many type two diabtetics get AD and why I expect the incidence and prevalance of this disease to skyrocket the next 20 yrs and be found in people much younger than before. If you asked me the number one reason for the spike in AD over the last 50 yrs I would link it to the industrial seed oil loaded with omega 6 and 9 oils. Lipid peroxidation causes 4-6 fold damage in the brain than glycation does and speeds the disease process up by messing with the lipoprotein biochemistry in the cerebral cortex. All my concussion patients, seizure patients and brain surgery cases are instructed to refrain from refined sugars flour all whole grains and industrial seed oils forever to speed their recovery.ReplyDelete
Research indicates Alzheimer's Disease is a prion disease almost identifical to Creutzfeldt Jakob Disease (and related to or caused by animal prions diseases such as mad cow BSE and Chronic Wasting Disease.ReplyDelete
P - sorry to hear about your Dad. I see a lot of depression worsening after orthopedic surgery - something about sawing through bone that releases a lot of inflammatory nasties into the circulation.ReplyDelete
Steve - I agree - and the compelling nature of the bugs in question is that they are intracellular or known to inhabit the CNS for long stretches, however, I'm guessing there are people with Alzheimer's who have no exposure to Chalmydia pneumoniae or HSVI or else this theory would have gained more traction. Often with these issues, "autoimmune" is enough of an explanatory paradigm, though perhaps not as satisfying as finding a nasty bug to kill.
Tom - thanks for the link. I have another post brewing that will go into those elements.
Dr K - if I have head trauma, I'm coming to see you!
hshields - interesting, though there are questions (theoretical questions, via Hypoerlipid) that prion disease is actually infectious via standard pathogens.