Figure 1.

Replication of influenza Italy/3 virus in vivo and in vitro. A, Three ferrets were inoculated with 10⁶ 50% egg infectious doses (EID₅₀) of Italy/3 virus, and nasal washes were collected from each ferret on the indicated days after inoculation (left bars) to assess viral replication. A naive ferret was placed in the same cage as each inoculated ferret at 24 hours after inoculation, and nasal washes were collected from each contact ferret on the indicated days after contact (right bars). The limit of virus detection was 10^1.5 EID₅₀/mL. Seroconversion to homologous virus was detected in the first and third ferret contact pairs shown on day 23 after contact. B, Primary human nasal epithelial, alveolar epithelial, or corneal epithelial cells were infected apically with Italy/3 virus at a multiplicity of infection of 0.01 for 1 hour, washed, and incubated at 37°C. Viral titers were determined at the indicated times. The limit of virus detection was 10 plaque-forming units (PFU)/ mL. The mean value (+SD) from triplicate independent cultures per virus is shown. C, Calu-3 cells were infected similarly to primary cells and incubated at 37°C or 33°C. Supernatants were removed at indicated times after inoculation, and titers of infectious virus were determined by standard plaque assay at the indicated times. The limit of virus detection was 10 PFU/mL. The mean value (+SD) from triplicate independent cultures per virus is shown.