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There’s an old joke that Alzheimer’s is something you catch from your teenaged children. However, there is mounting evidence that bacterial and viral infections contribute to age-related cognitive decline.

New findings from the Northern Manhattan Study (NOMAS) suggest that loss of cognitive abilities with age may be related to repeated infections.

Dr. Mira Katan, a physician and lead researcher for the study, says that infections may contribute to a kind of “common ground” between the two most common causes of intellectual decline in the elderly, vascular disease and dementia.

What We Already Know About Age-Related Cognitive Decline

Medical science emphasizes two types of intellectual impairment in the elderly. Vascular disease can be thought of as a kind of “plumbing problem.” In older people who have had brain injuries or mini-strokes, tiny areas of damaged tissue can accumulate in the brain. The process of inflammation removes damaged brain tissue, but removes surrounding, healthy brain tissue with it. Eventually so much of the brain can be destroyed that mental functioning is impaired. This form of age-related cognitive impairment is diagnosed along a spectrum from relatively mild to relative severe symptoms: mild vascular cognitive impairment, vascular dementia due to a single infarct (clot causing death of brain tissue), multi-infarct dementia, vascular dementia due to lacunar lesions (essentially “holes” in the functional tissue of the brain), vascular dementia due to hemorrhagic lesions (bleeding in the brain), Binswanger disease (causing destruction of the larger blood vessels in the brain), and subcortical dementia.

There can also be mixed dementia, attributable to a combination of vascular problems and the more common source of intellectual impairment in the elderly, Alzheimer’s disease. In Alzheimer’s, tangles of proteins (usually) “strangle” neurons so that the brain slowly loses connections and memory and movement are lost.

The NOMAS study suggests that there is a third category of age-related intellectual decline “in between” vascular dementia and Alzheimer’s. Very specific infections have been tied not just to age-related problems in cognition but also to stroke.

Zeroing In On Brain-Specific Infections

Between 1993 and 2001, Dr. Katan and her colleagues recruited 1625 residents of the northern reaches of the island of Manhattan in New York City. All of the participants in the study were free of stroke, 58 percent were Hispanic, and 65 percent were women. Their average was 69 years when they joined the study.

The NOMAS team assessed intellectual function when participants were brought into the study with the Mini-Mental State Examination (MMSE), and once a year with a quiz that could be administered by telephone. At the beginning of the study, the researchers also took blood samples to measure cumulative exposure to common infections including Chlamydia, herpes (both oral and genital), cytomegalovirus (CMV), and Helicobacter pylori, a common bacterial infection of the stomach and duodenum. The researchers also did genetic testing to determine the APOE genotype, which indicates some of the links between diet and vascular health and also future risk of Alzheimer’s disease.

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