After only eight years since being identified in the United States, West Nile virus (WNV) has spread across the country, infecting over 3,400 humans in 2007 and causing the deaths of 98, reports the Wildlife Management Institute. The disease, however, has caused significant reductions in wild bird populations, affecting more than 300 different species, particularly crows and jays.

Earlier this year, geneticists at the University of California-Davis (UC-Davis) isolated a gene mutation that turned relatively mild forms of the virus into the highly virulent and deadly disease that it is to crows. Because crows are so common and so susceptible to the disease, they have been a sentinel species?alerting scientists to the spread of the virus across the country. Epidemiological studies have found that deaths of North American crows due to WNV are associated with higher rates of infection among mosquito populations and clusters of the disease in humans.

"The findings from this study highlight the potential for viruses like West Nile to rapidly adapt to changing environments when introduced to new geographic regions," said Aaron C. Brault, a virologist at the Center for Vectorborne Diseases in the Department of Pathology, Microbiology and Immunology of the UC-Davis School of Veterinary Medicine. "The study also suggests that the genetic mutations that create such adaptive changes may result in viral strains that have unexpected symptoms and patterns of transmission," Brault added.

WNV is a mosquito-borne infection that can cause encephalitis (inflammation of brain tissue) and/or meningitis (swelling of the tissue that encloses the brain and spinal cord). It is native to parts of Africa and the Middle East, and it was first detected in North America in the summer of 1999, in New York City. Within three months, WNV had spread to Connecticut and New Jersey, and animal cases have been identified in every state in the continental United States except Maine.