According to new research memory loss and Alzheimer's disease may be linked to the very viruses which cause the common cold.
In one of the first known laboratory studies that explores memory deficits associated with a viral infection of the central nervous system, researchers from the Mayo Clinic in Rochester, Minnesota, have found evidence that viral infections can lead to memory loss late in life.
The researchers suggest that the common viruses which infect around one billion people worldwide each year may be invading the central nervous system and damaging the brain.
They say that in later life this could lead to symptoms of memory loss and declining mental ability.
These particular viruses belong to a large family called picornaviruses which includes the rhinoviruses, which are linked to the common cold, enteroviruses, which are associated with respiratory and gastrointestinal ailments; encephalitis, an inflammation of the brain; myocarditis, inflammation of heart muscle; and meningitis.
Picornaviruses are the culprits in serious conditions such as encephalitis, meningitis, inflammation of the heart, hepatitis A and polio and foot-and-mouth disease.
A study of mice infected with a polio-like picornavirus showed evidence of harm to the brain as the mice had difficulty learning to navigate a maze designed to test various components of spatial memory.
The researchers say 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.
Dr. Charles Howe, a neuroscientist and co-author of the study, says that virus-induced memory loss might accumulate over a lifetime and eventually lead to clinical cognitive memory deficits.
It is quite common for people to suffer two or three rhinovirus or enterovirus infections each year and scientists already knew that in rare cases picornoviruses can infiltrate into the brains of children and cause long-lasting injury.
The best example of this is the polio virus which after entering the central nervous system, the virus damages the spinal cord and parts of the brain responsible for motor function, leading to paralysis.
The Mayo scientists believe picornoviruses might be having far 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.
Dr. Howe says the degree of brain damage in humans infected with a picornavirus infection is not known, but the evidence from the mouse study suggests this is an area of research that should be explored further.
According to Dr. Howe, picornaviruses are widespread and infection is common, and the potential for the viruses to damage the human brain presents a very real medical problem.
The study was funded by grants from the National Multiple Sclerosis Society; National Institutes of Health; a gift from Donald and Francis Herdrich; and the Mayo Graduate School.