Central Nervous System Viral Infection Can Lead to Memory Deficits Late in Life

In one of the first known laboratory studies that explores memory deficits associated with a viral infection of the central nervous system, Mayo Clinic researchers have evidence that this infection can lead to memory loss late in life. The study, which was conducted in animal models, suggests that over the lifetime of an individual, a picornavirus-related infection could possibly have a permanent effect on memory late in life.

"Our study suggests that virus-induced memory loss could accumulate over the lifetime of an individual and eventually lead to clinical cognitive memory deficits," according to Charles L.

Howe, Ph.D., a Mayo Clinic neuroscientist and corresponding author of the study that appears on the cover of the November issue of Neurobiology of Disease. The study's first author is Eric J. Buenz, Ph.D., a recent graduate of the molecular neuroscience program at Mayo Graduate School.

Picornaviruses are the most common infectious viral agents in humans. They are a family of viruses that include rhinoviruses, which is a virus associated with the common cold; enteroviruses, a virus associated with respiratory and gastrointestinal ailments; encephalitis, inflammation of the brain; myocarditis, inflammation of heart muscle; and meningitis.

Other viruses in this family include those that cause foot-and-mouth disease, polio and hepatitis A. Researchers were intrigued by the possibility of a link between picornavirus infections and memory loss. Little has been published in this area, Dr. Howe says.

In the study, mice were infected with Theiler's murine (mouse) encephalomyelitis virus (comparable to the human poliovirus). Researchers looked for signs of spatial memory loss in the mice. Mice that contracted the virus had difficulty learning to navigate a maze designed to test various components of spatial memory. The degree of memory impairment, which ranged from no discernable damage to complete devastation, was directly correlated to the number of dead brain cells in the hippocampus region of the mouse's brain.

Picornaviruses infect more than one billion people worldwide each year. Generally individuals contract two or three enterovirus and/or rhinovirus infections each year. In some cases the viruses get into the brain, and in some children these viruses can cause long-lasting brain injuries. "We think picornavirus family members cross into the brain and cause a variety of brain injuries. For example, the polio virus can cause paralysis. It can injure the spinal cord and different parts of the brain responsible for motor function. In the murine virus we studied, it did the same thing and also injured parts of the brain responsible for memory," Dr. Howe says.

Enteroviruses and rhinoviruses are among the most common in the picornavirus family. Certain enteroviruses (gastrointestinal illnesses among others), such as enterovirus 71, are common in Asia, particularly southeast Asia where many children are infected. Once the virus infects the host due to unsanitary conditions, it can cross over into the brain and cause encephalitis, which can lead to conditions ranging from lethargy to a coma. In recent years, scores of children have died from infection.