West Nile virus risk may involve heredity

WASHINGTON -- Only about one in five people infected with the West Nile virus develop a severe, life-threatening illness. A study in mice suggests a gene variation may be the reason some become very sick from the mosquito-borne virus, while others recover easily.

Experts said the research is an important step toward finding a drug to treat West Nile, a virus that has caused 11 deaths in the United States this year. The Centers for Disease Control and Prevention reported Saturday there have been at least 251 human cases of the disease this year.

West Nile is carried by mosquitos whose bite can spread the virus to birds, horses and humans.

For most people, West Nile causes only flu-like symptoms. But for some patients, particularly the young and the elderly, West Nile can be a killer, causing a swelling of the brain that can be lethal.

Just why some people are more susceptible to serious illness from the virus has intrigued researchers and prompted scientists at the Pasteur Institute in France to search for a gene variation that might explain the difference.

The French first tested a series of laboratory mouse strains to find a genetic type that was most likely to die after being exposed to the West Nile virus.

They found that animals in the mouse strain called BALB/c all died within 13 days when injected with the West Nile virus. When these animals were mated with other mouse strains, some of the offspring died from the virus, while others were little affected.

By analyzing the genes of both the BALB/c mice and the mixed strains with a high rate of West Nile deaths, the researchers isolated a specific gene variation that increased the susceptibility to the virus, the researchers report. They called the variation the West Nile gene.

Dr. Jean-Louis Guenet of Pasteur, a co-author of the study appearing this week in the Proceedings of the National Academy of Sciences, said the West Nile gene allows the virus to cause disease by blocking production of a group of proteins that normally prevent viruses from reproducing inside a cell.