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In fact, being a carrier of certain antibodies to the virus can double the risk of Alzheimer's disease, the researchers found.
"The identification of a treatable cause [herpes simplex] of the most common dementia disorder is a breakthrough," said lead researcher Dr. Hugo Lovheim, an associate professor in the department of community medicine and rehabilitation at Umea University in Sweden.
But others aren't so sure that there's a clear cause and effect relationship between herpes simplex and Alzheimer's disease.
Dr. Sam Gandy, director of the Center for Cognitive Health at Mount Sinai Hospital in New York City, doubts that herpes and Alzheimer's disease are connected.
"From time to time data such as these appear in the literature, but they do not address causality or mechanism. The new data are likewise not definitive, and they do not say anything new about the association," he said.
"I do not disbelieve the data. I simply do not know whether the association has anything to do with the cause of Alzheimer's disease," Gandy added.
Herpes simplex is a common infection, affecting up to 90 percent of the population, according to background information in the article. It's the virus that causes cold sores, according to the American Academy of Dermatology (AAD). Once you have herpes simplex, you have it for life, though the virus isn't always active, according to AAD.
The theory linking the herpes virus and Alzheimer's disease is that the virus weakens the immune system, allowing the virus to spread to the brain, which may start the process toward dementia, the researchers said.
In one study of nearly 3,500 people followed for an average of 11 years, Lovheim's team found that having certain antibodies to a herpes infection doubled the risk of developing Alzheimer's disease.
In the second study, blood samples were taken from 360 Alzheimer's patients an average of 9.6 years before being diagnosed with the disease. The researchers compared these with samples taken from people without Alzheimer's disease. When comparing all of the samples, the researchers found no association between Alzheimer's and herpes infection. However, when they looked only at people who'd had their blood taken at least 6.6 years prior, there was a significant association between the herpes virus and the risk of developing Alzheimer's disease.
"I think herpes virus causes a significant proportion of all cases of AD -- about 40 to 50 percent -- according to our data," Lovheim said.
Lovheim said that these studies indicate that the role the herpes virus plays in the development of Alzheimer's disease is more than a chance association, but instead indicate a causal relationship.
"I think a causal relationship is likely, but like all epidemiological studies, there might always be confounders one has not thought about or not measured," he said. "In a few years we hope we will be able to start clinical studies to investigate whether antiviral drugs might slow the progression of Alzheimer's disease," he said.
However, Greg Cole, the associate director of the Geriatric Research and Clinical Center at the UCLA Alzheimer Disease Research Center in Los Angeles, isn't convinced.
"More than 90 percent of the population has antibodies to herpes, and they are not all destined to develop Alzheimer's disease," he said.
However, an immune response or infection connection between herpes and Alzheimer's disease is possible, Cole noted.
"Recent genetic studies have implicated variants of several genes controlling immunity with increased Alzheimer's disease risk. These new results warrant a closer look in larger populations," Cole said.
The studies were published in the journal Alzheimer's & Dementia.
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SOURCES: Hugo Lovheim, M.D., Ph.D., associate professor, department of community medicine and rehabilitation, Umea University, Sweden; Sam Gandy, M.D., Ph.D., director, Center for Cognitive Health, Mount Sinai Hospital, New York City; Greg Cole, Ph.D., associate director, Geriatric Research, Education and Clinical Center, UCLA Alzheimer Disease Research Center, Los Angeles, Calif.; July 17, 2014 and Oct. 7, 2014, Alzheimer's & Dementia online