Alzheimer's disease linked to the common cold

Viruses linked to the common cold could be causing memory loss and Alzheimer's disease in millions of people, new research suggests.

Experts suspect that common viruses which infect around one billion people worldwide each year may be invading the central nervous system and damaging the brain. In later life this could lead to symptoms of memory loss and declining mental ability.

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The viruses involved belong to a large family called picornaviruses. They include rhinoviruses, which are associated with the common cold, and enteroviruses, which cause stomach upsets. Picornaviruses are also responsible for serious conditions such as encephalitis, meningitis, inflammation of the heart, hepatitis A and polio.

Evidence of harm to the brain has emerged from a study of mice infected with a polio-like picornavirus.

The animals had difficulty learning to navigate a maze designed to test various components of spatial memory. Scientists found that the degree of impairment was directly related to the number of cells dying in the hippocampus brain region, which plays an important role in memory and learning.

Study leader Dr Charles Howe, from the Mayo Clinic in Rochester, Minnesota, said: "Our study suggests that virus-induced memory loss could accumulate over the lifetime of an individual and eventually lead to clinical cognitive memory deficits."

Many people suffer two or three rhinovirus or enterovirus infections each year. It was already known that in rare cases picornoviruses can get into the brains of children and cause long-lasting injury.

Polio is the best known example. After entering the central nervous system, the virus can damage the spinal cord and parts of the brain responsible for motor function, leading to paralysis.

However the Mayo scientists thinks picornoviruses might be having much more common brain effects that are being missed.

Clinical studies suggested that picornavirus infections might trigger damaging inflammation in the brain. Brain inflammation is associated with learning and memory loss, and is a key element of Alzheimer's disease.