Virus titers were assayed using an infectious cell culture assay as previously described [22] . Briefly, a specific volume of liver or spleen homogenate or serum was serially diluted and added to triplicate wells of Vero (African green monkey kidney; American Type Culture Collection, Manassas, VA) cell monolayers in 96well microplates. The viral cytopathic effect (CPE) was determined 7 to 8 days post-virus inoculation, and the 50% endpoints were calculated as described [23] . The assay detection ranges were 2.8 to 9.5 log 10 50% cell culture infectious doses (CCID 50 )/g of liver or spleen and 1.8 to 8.5 log 10 CCID 50 /ml of serum. In samples presenting with undetectable liver or spleen virus, a value of ,2.8 was assigned (,1.8 for serum). Conversely, in cases wherein virus exceeded the detection range, a value of.9.5 (.8.5 for serum) was assigned. For statistical analysis, values of 2.8 or 9.5 log 10 (1.8 or 8.5 for serum) were assigned as needed for samples with undetectable or saturated virus levels, respectively.