New York: Opening the door to potential new treatments for West Nile virus infection, researchers have discovered how the most severe forms of the mosquito-borne illness cause memory loss and mood disorders. Half 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. But exactly why this happens has been a mystery until now, the study said. Researchers discovered that the virus does not kill off neurons but sparks inflammation that prunes 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 from University of Colorado School of Medicine in the US. "The viral infection activates microglial cells and complement pathways which are helping to fight the infection but in turn end up destroying synapses," Tyler noted. The findings were published in the journal Nature. 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. When the scientists examined brain tissue from humans who had died from West Nile, they found the same phenomenon. There are currently no medications to treat or vaccines to prevent West Nile virus infection. The best way to avoid West Nile is to wear long sleeves, use mosquito repellant and steer clear of standing water. The mosquito that carries the virus is most active at dusk and dawn.