West Nile virus in the United States is thought to have a 4% mortality rate among those infected who present with serious symptoms affecting the central nervous system. West Nile virus was introduced into the United States about 18 years ago, now allowing for health officials to measure some of the potential long term effects of infection and serious symptoms.
Study findings from a recent paper at the American Society of Tropical Medicine and Hygeine allude to a potentially higher fatality rate among those infected individuals that presented with more serious symtoms. Greater levels of mortality were most notably recorded within the first six years after infection, often related to kidney disease or failure or death from another infectious disease.
New Study Ties West Nile Virus to Risk of Shorter Life Span: Death Rate Rises from ‘Delayed’ Fatalities Long After Recovery. http://www.astmh.org/CMSPages/GetFile.aspx?guid=4154182f-7606-493b-8acb-823709f1dfeb