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What do cold sores have to do with Alzheimer's?

There's that nasty little bugger. A cold sore is caused by an outbreak of the herpes simplex virus 1.
There's that nasty little bugger. A cold sore is caused by an outbreak of the herpes simplex virus 1.
©iStockphoto.com/budgetstockphoto

It's morning, and the mirror is showing you something you'd rather not see -- a cold sore.

You were pretty sure you'd felt that familiar tingle on your upper lip a couple days ago, but hoped that it wasn't the prodrome -- the sensation that heralds the arrival of a cold sore. Here it is now, though, plain as day. In a few more days it will turn yellow and begin to ooze before it becomes crusty and eventually dries and fades like it was never there. You'll be able to return to your normal (and less embarrassing) life.

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Try not to feel too badly about it. Cold sores are very common and the virus that causes them is even more so. In the United States, 30 to 90 percent of adults have antibodies present in their bloodstream designed to ward off herpes simplex virus 1 (HSV1), the virus that causes cold sores [source: NLM]. This means that all of these people have been exposed to the virus and their immune systems have taken measures to prevent infection. Some people do have outbreaks (as your mirror told you) and many others go through life never knowing they're carriers.

Active herpes is transferred from person to person contact -- such as kissing a person with an infectious cold sore -- or from person to object, like sharing a razor with a person in the midst of an outbreak. The virus can also be exchanged from one carrier to another without either person ever suffering an outbreak. Once an outbreak has occurred, the virus infects nerve cells in the face, where it remains and can lead to further eruptions [source: Mayo Clinic].

In the nerve cells, the virus essentially hides from an attack of antibodies sent to destroy it. When a human's immune system is suppressed, like during menstruation or periods of stress, the virus replicates itself and travels to the skin cells to cause yet another cold sore.

The HSV1 specifically locates itself in the trigeminal ganglion, a bundle of nerves located by the jaw and temple. However, recent research has turned up something unusual; the virus can also be found in the brains of people suffering from Alzheimer's disease. This has some wondering if the relatively harmless HSV1 may have something to do with this mysterious and fatal brain condition.

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The plaques and tangles associated with Alzheimer's interfere with normal brain functions like memory recall and learning.
The plaques and tangles associated with Alzheimer's interfere with normal brain functions like memory recall and learning.
©iStockphoto.com/lisafx

Alzheimer's disease is a progressive and fatal brain disease that was first discovered in 1906 by German psychiatrist Alois Alzheimer, who reported on a woman who increasingly and inexplicably lost her memories and became unreasonably suspicious of her husband. While the disease was eventually named after Alzheimer, its underlying causes remain a mystery.

Medical science has been able to pin down what's behind the symptoms of Alzheimer's disease. From inspection of the brain tissue of people who have died from Alzheimer's, researchers have found two common characteristics that most patients have: plaques and tangles.

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Plaques are the remnants of an unprocessed protein called beta-amyloid that's normally removed from the brain as waste. Beta-amyloid is the result of the breakdown of the amyloid precursor protein, which researchers believe might be used to form the synapses that allow chemical communication between neurons in the brain. When these protein deposits build up, they form the plaque that characterizes Alzheimer's.

The tangles found in the brains of Alzheimer's patients are made up of another protein called tau. This protein fosters the creation of microtubules in nerve cells when they divide and form new ones. The microtubules provide structure for the new cell and when the cellular generation has taken place, the remnant tau, like the beta-amyloid, should generally be removed from inside the neurons as waste.

The development of these plaques and tangles are a normal part of aging. In the brains of Alzheimer's patients, however, this accumulation is much heavier than normal. They appear first in the areas dedicated to things like memory and learning. As the plaques and tangles accumulate around neurotransmitter receptor sites, researchers believe they interfere with proper communication within neural networks, essentially gumming up the brain's works. This leads to the symptoms associated with dementia, like forgetting where you live or failing to recognize your family members. It also destroys brain cells, leading to death.

So far, there's no cure for Alzheimer's. While medical science has concluded that the plaques and tangles found abundantly in the brains of Alzheimer's patients cause the degenerative disease, they remain puzzled why they accumulate more in some people's brains than others.

Some recent research linking Alzheimer's and HSV1 may explain one of the underlying causes of the disease.

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An artist's rendering of a nerve cell. Researchers from University of Manchester believe HSV1 travels to the nerve cells of the brain as we age and alter the brain's waste removal capabilities.
An artist's rendering of a nerve cell. Researchers from University of Manchester believe HSV1 travels to the nerve cells of the brain as we age and alter the brain's waste removal capabilities.
©iStockphoto.com/herbap

University of Manchester biophysicist Dr. Ruth Itzhaki has suspected a link between Alzheimer's and herpes simplex virus type 1 (HSV1) for several years. She co-authored a paper in 1999 showing that brain cells grown in culture develop beta-amyloid plaques when HSV1 is introduced [source: Coghlan]. She demonstrated the same results in laboratory experiments that used the brains of mice.

Most recently, Itzhaki led a research team that found that 90 percent of the plaques found in the brains of Alzheimer's patients are infected with the herpes virus that leads to cold sores. The HSV1 virus has already been shown to induce beta-amyloid accumulation as it destroys nerve cells. The presence of the virus in the plaque in Alzheimer's brain tissue suggests that the virus might be responsible for the excessive memory loss and forgetfulness associated with the disease. The virus may move from the facial area to attack neurons in the brain as our immune systems deteriorate due to age.

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As we further study human genetics, it becomes increasingly clear that genes play a major role in the prevalence of disease. It looks like Alzheimer's is no different.

In her research, Dr. Itzhaki found that all six of the Alzheimer's patients whose brain tissue she used as specimens in her research had a common variant of the APOE-4 gene. Itzhaki believes that mutation is responsible for an inability to dispose of the accumulating beta-amyloid deposits [source: Coghlan].

There are still questions over Itzhaki's findings. For one, how does one account for not only the plaques but also the tau protein tangles, which haven't been linked to the herpes virus? The research is hopeful, however. Itzhaki says that cheap, already available anti-viral medicines used to treat herpes can reduce the presence of beta-amyloid in human cells infected with HSV1.

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Sources

  • Coghlan, Andy. "Cold sore virus might cause Alzheimer's." New Scientist. December 9, 2008.http://www.newscientist.com/article/dn16234-cold-sore-virus-might-cause-alzheimers.html?full=true&print=true
  • University of Manchester. "Cold sore virus linked to Alzheimer's disease: new treatment, or even vaccine possible." December 7, 2008.http://www.sciencedaily.com/releases/2008/12/081207134109.htm
  • Alzheimer's Association. "Wht is Alzheimer's?" Accessed October 1, 2009.http://www.alz.org/alzheimers_disease_what_is_alzheimers.asp
  • The Mayo Clinic. "Cold sore." March 13, 2008.http://www.mayoclinic.com/health/cold-sore/DS00358
  • Drubin, DG and Kirschner, MW. "Tau protein function in living cells." The Journal of Cell Biology. 1986.http://jcb.rupress.org/cgi/content/abstract/103/6/2739
  • U.S. National Library of Medicine. "Herpes simplex." Accessed October 1, 2009. http://www.nlm.nih.gov/medlineplus/herpessimplex.html
  • Herpes Guide.ca. "What is a herpes simplex virus (HSV)?" August 1, 2007.http://www.herpesguide.ca/facts/herpes_simplex_virus.html

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