The economic impact of the deadly West Nile virus has been "substantially underestimated," according to a new report from the Centers for Disease Control and Prevention.

Researchers estimate that hospital visits, follow-up care, and lost productivity have cost the nation $778 million in the 14 years since the virus was first detected in the US. That works out to $56 million a year.

West Nile virus, which spreads via infected mosquitoes, was first detected in New York in 1999. Seasonal outbreaks are now expected every year. The virus can induce fever, meningitis, encephalitis, and acute flaccid paralysis in humans, which can cause death or permanent disability. There have been more than 37,000 cases in the US, resulting in 1,500 deaths. Vaccines are in development, but none are available on the market yet.

Studies like this are important in allocating resources to fight disease. This report suggests that West Nile may require more attention than previously thought, but it's still a relatively minor health care cost. The annual impact of the flu, for example, is estimated to be around $90 billion.

"The figures for economic burden reported in this paper will aid policy makers and pharma to assess the economics of vaccine and drug development," tropical disease specialist Alan D. T. Barrett wrote in an editorial accompanying the report in The American Journal of Tropical Medicine and Hygiene.