To assess the pathogenicity of the viruses, the intravenous pathogenicity index (IVPI) for several isolates was determined according to standards established by the World Health Organization [28] . Briefly, 0.1 mL of a 1:10 dilution of infective egg chorioallantoic fluid in sterile PBS was intravenously injected into each of 10 six-week-old, specific pathogen-free (SPF) chickens. The birds were examined twice daily for 10 days, and individuals were scored on the basis of the observed morbidity (0 = normal, 1 = sick, 2 = severely sick, 3 = dead). Chickens were used in accordance with protocols approved by the Institutional Animal Care and Use Committee at St. Jude Children's Research Hospital (protocol 081). The study was approved by the Institutional Biosafety Committee at St. Jude Children's Research Hospital (#03A-137 and #02A-221).