Aug. 5, 2008 -- A drug originally made to fight HIV may help treat West Nile virus encephalitis, a dangerous brain infection.

West Nile virus is usually spread through mosquitoes; it can also be transmitted through blood transfusions, organ transplants, breastfeeding, and during pregnancy from mother to infant. Most people who get infected with West Nile virus don't get sick. Some have mild symptoms; serious symptoms such as encephalitis are rare.

The drug, called AMD3100, didn't pan out as an HIV drug. And it doesn't actually tackle West Nile virus itself. Instead, it paves the way for immune system cells to do the dirty work.

In lab tests, mice were injected with a virulent strain of West Nile virus. Some of the mice got a continuous infusion of AMD3100; for comparison, other mice got a placebo infusion.

All of the mice got sick, but half of the mice in the AMD3100 group got better within eight days, while all of the mice in the placebo group got sicker and died.

AMD3100 targeted the mice's blood-brain barrier, a fence that keeps infection-fighting inflammatory cells out of the brain, to help protect the brain from too much inflammation. By blocking a receptor in the blood-brain barrier, certain T-cells made by the immune system specifically to fight West Nile virus gained access to the brain.

AMD3100 might also prove useful against other causes of encephalitis, note the researchers, who included graduate student Erin McCandless and assistant professor of medicine Robyn Klein, MD, PhD, of Washington University School of Medicine in St. Louis.

They report their findings in this week's early online edition of Proceedings of the National Academy of Sciences.