Study Probes West Nile Impact on Memory & Mood

New research solves the mystery of how a severe case of West Nile infection can cause memory loss and mood disorders.

Investigators at the University of Colorado Anschutz Medical Campus believe the discovery can lead to a variety of new treatment options for mosquito-borne illnesses.

Experts explain that 50 percent of patients who survive the most damaging kind of West Nile infection often go on to develop memory loss, learning difficulties, a lack of concentration, and irritability.

As published in the journal Nature, researchers discovered that the virus doesn’t kill off neurons but sparks inflammation that damages synapses, the connections that carry messages between nerve cells.

“What we found in mice, and later confirmed in humans, is that it’s not the death of cells that causes memory loss, it’s the loss of nerve cell connections,” said study co-author Kenneth Tyler, M.D., chairman of the department of neurology at the University of Colorado School of Medicine.

“The viral infection activates microglial cells and complement pathways which are helping to fight the infection but in turn end up destroying synapses.”

Bette K. DeMasters, M.D., professor and head of neuropathology at University of Colorado Anschutz, also co-authored the study.

The researchers found that mice infected with West Nile had a difficult time negotiating their way out of a maze that healthy mice figured out much faster. They later discovered that the infected mice suffered significant damage to their synapses.

The scientists then examined brain tissue from humans who had died from West Nile and found the same phenomenon.

Tyler said West Nile is the leading cause of acute viral encephalitis in the U.S. though still relatively rare. Only about one in 100 people infected with the disease develop the most severe form, he said.

West Nile appeared in the U.S. during the late 1990s and has remained a persistent threat. Last year, California had 730 cases, Texas 252 cases, and Colorado 101 cases.

In Colorado, that number included 57 neuro-invasive cases, the most serious kind, and two deaths.

“This discovery opens up the opportunity to test therapies and medications on mice as a precursor to humans,” Tyler said. “We already have some drugs that might be good candidates for treating this condition.”

The best way to avoid West Nile is to wear long sleeves, use mosquito repellent and steer clear of standing water. The mosquito that carries the virus is most active at dusk and dawn.

Source: University of Colorado Anschutz Medical Campus/EurekAlert