Regular readers know an interest of mine are infections that cause more than just the typical acute spectrum of disease. For example, I've written on the role microbes might play in obesity, or the role of viruses in chronic disease such as cancer and, of course, AIDS. Still, typically, infections are thought of as acute and self-limited; that is, they infect the individual, cause illness, and are resolved in a matter of days or weeks, even though we know that this doesn't always happen. And increasingly, we're finding that infections are associated with all kinds of long-term diseases or conditions, either causally or as a co-factor. A recent article highlights one area of investigation: how viral infections can influence memory problems.
(More after the jump)
A family of viruses that cause a range of ills from the common cold to polio may be able to infect the brain and cause steady damage, a team at the Mayo Clinic in Minnesota reported on Monday.
"Our study suggests that virus-induced memory loss could accumulate over the lifetime of an individual and eventually lead to clinical cognitive memory deficits," said Charles Howe, who reported the findings in the journal Neurobiology of Disease. [Article can be found here; --TS]
This may seem somewhat far-fetched at first, but we know that viruses certainly can cause severe damage to the brain and nervous system. Therefore, this lends plausibility to the idea that they can cause more minor damage as well. As they note, polio (a picornavirus) is a prime example of this, with the potential to affect the brain and the spinal cord and result in paralysis. A related virus in mice, called Theiler's murine encephalomyelitis virus, has also been used to induce damage to the nervous system (for example, it's used as a mouse model for multiple sclerosis), and researchers infected mice with this and then performed cognitive tests to see if it had more subtle effects as well:
Infected mice later had difficulty learning to navigate a maze. Some were barely affected, while others were completely unable to manage, and when the mice were killed and their brains examined, a correlating amount of damage was seen in the hippocampus region, related to learning and memory.
Now, it's always a bit difficult to extrapolate directly from an animal model to humans, but the study is certainly intriguing and can open doors for epidemiological studies of viruses and cognition in humans.
One virus particularly likely to cause brain damage is enterovirus 71, which is common in Asia, the researchers said. It can cross over into the brain and cause encephalitis, a brain inflammation that can lead to coma and death.
"Our findings suggest that picornavirus infections throughout the lifetime of an individual may chip away at the cognitive reserve, increasing the likelihood of detectable cognitive impairment as the individual ages," the researchers wrote in their report.
"We hypothesize that mild memory and cognitive impairments of unknown etiology may, in fact, be due to accumulative loss of hippocampus function caused by repeated infection with common and widespread neurovirulent picornaviruses."
This is interesting stuff, and brings in a confluence of factors to explain the neurodegeneration that frequently comes with aging. How much of it is our own body's "program," and how much of it is due to external forces such as microbes? This is already a growing field, and likely only will increase in the coming years.
Tara, the first link is screwed up - there is another url and a breakline in front of the correct one.
Thanks Kristjan--should be fixed now.
Viruses hurting memory? How redi.....uhm......what was I talking about?
ba dum dum crash!
Yea, the unification of chronic and infectious disease epidemiology continiues.
Found this post from Leslie's roundup on blogher, so Howdy! Would the reports several years ago that a percentage of Alzheimer's patients brains showed signs of a chlamydia infection (not the std form.. one we usually get as children?) be part of this same discussion?
Memory loss is a tough thing to tie down as to causation. Alzheimer's has become a catch-all for any sort of memory problem but the most recent literature points to an error rate of as much as 48% among family physicians attempting to diagnose a patient's memory loss. Some research indicates that hearing loss may play a role in diagnoses of Alzheimer's as well as cognitive slowing as a natural consequence of aging. The "clock test" is usually cited as one of the more reliable tests.
Tara, I know you're already aware of the general notion of microbial pathogens as putative causes of mental illness (e.g. your post at http://aetiology.blogspot.com/2005/09/study-reinforces-link-between.html), but the burgeoning literature in this field really points up the plausibility of incredibly subtle microbial effects on higher level cognition. See for example:
-maternal in utero infection and schizophrenia
Arch Gen Psychiatry. 2004;61:774-780
Am J Psychiatry 2005; 162:767-773
-neonatal exposure to Borna disease virus and ?autism
Behavioural Brain Research 176 (2007) 141-148
Keep up the great blogging!
Thanks Ben. I definitely keep an eye out for papers like that, so I'll add those to the file.
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