FIG. 4.

Virus titers in the allantoic cavity and lethality for chicken embryos. Seven-day-old chicken embryos were infected with different doses of WT and delNS1 viruses and incubated at 37°C for 48 h. Dead and living embryos were identified by the movement activity of embryos and retained blood vessel network. Eggs were cooled at 4°C, and hemagglutinin (HA) titers of allantoic fluids were determined.