September 8, 2003
Can a lifelong infection play a role in Alzheimer's disease? That's the provocative possibility raised by a new study out of Finland, in which elderly people with heart disease who were found to be infected with three common viruses -- the same viruses that cause cold sores, herpes, and a form of mononucleosis -- were more than twice as likely to suffer from mental decline and dementia as those who did not carry the viruses.
Some were found to be infected with herpes simplex type 1 virus (HSV-1), the most common cause of the unsightly and painful lip blisters called cold sores (also known as fever blisters). Others were infected with herpes simplex type 2 (HSV-2), the major cause of genital herpes. Still others had evidence of infection with a related herpes virus called cytomegalovirus that can cause a form of the "kissing disease" infectious mononucleosis.
The researchers, from the University of Helsinki, tested 383 seniors around age 80 for evidence of infection with one or more of these viruses. All of the men and women in the study had heart disease as well and were recruited from a larger trial known as the DEBATE, or Drugs and Evidence-Based Medicine in The Elderly, study. Participants receive yearly exams to test for orientation, memory and attention as well as regular physical and lab tests.
The researchers looked for antibodies to these viruses in the blood, an indication of past or current infection. Earlier studies had suggested a possible connection between these viruses and Alzheimer's, although the evidence was far from clear-cut, and some studies have found no link at all.
In the current study, more than half of the men and women had antibodies to two of the viruses, which put them at 1.8 times greater risk than those with evidence of only one or none of the viruses. A quarter were infected with all three, which increased their risk 2.3 times.
Most of the participants were probably infected as children or young adults, the researchers say. Once infected, people carry these herpes viruses permanently in their nerve cells in a latent form throughout their lifetimes, with no known long-term damage other than occasional flare-ups of skin or mouth sores.
Inflammation has been implicated in dementia, and viral infections could be a triggering factor, says lead author Timo E. Strandberg, M.D., Ph.D., of the department of medicine, geriatric clinic, at the University of Helsinki. Our findings should be tested in other studies, but if these viruses are involved, there are existing therapies such as vaccination and antiviral drugs that could be used to prevent or treat dementia.
Inflammation, Infection & Alzheimer's
Scientists increasingly suspect that inflammation may play a role in Alzheimer's disease and other maladies of old age, including heart disease and strokes. Harboring viruses in the nervous system, the researchers speculate, may inflame and destroy brain cells and could, in theory at least, predispose someone to Alzheimer's.
However, carrying these common viruses by no means assures that you will get Alzheimer's or another form of dementia. In the current study, about 5 percent of those infected with one or none of the viruses had signs of dementia on testing, compared to 16.2 percent of those infected with two of the viruses and 26.9 percent of those infected with all three.
Most people, up to 90 percent, become infected with one or more of these viruses by adulthood, but most do not appear to suffer long-term damage. And while up to 50 percent of seniors who live beyond age 85 will eventually develop Alzheimer's, not everyone who carries these viruses does. It is likely that genetic makeup and additional factors play a role in who develops mental decline.
The researchers also tested for infection with two common bacteria that cause pneumonia, Chlamydia pneumoniae and Mycoplasma pneumonia. Both have been linked to inflammation, hardening of the arteries and heart trouble. However, the scientists found no connection between infection with these bacteria and Alzheimer's.
More research is needed to help unravel the connection, if any, between infectious microbes and Alzheimer's disease. In the current study, for example, all of the participants had heart troubles or had earlier suffered a stroke, which could have contributed to mental decline. If the results are verified in a larger study, the researchers speculate, it may be possible to give virus-fighting drugs or vaccines to help ward off Alzheimer's in old age.
The study appeared in the September 1 issue of Stroke: Journal of the American Heart Association.
For more on the causes of Alzheimer's disease, visit www.ALZinfo.org.
By Toby Bilanow, Medical Writer, for www.ALZinfo.org, The Alzheimer's Information Site.
Reviewed by Samuel E. Gandy, M.D., Ph.D., Chairman of the Scientific Advisory Board, Fisher Center for Alzheimer's Research Foundation.