Table 1.

Reassortants of higher pathogenicity than both parental viruses (pathogenicity group I)

“r” in virus names denotes reassortant. The numbers in the virus names indicate gene segments derived from the BJ09 virus as follows: 1, PB2; 2, PB1; 3, PA; 4, HA; 5, NP; 6, NA; 7, M; and 8, NS. The virus segments derived from the HB08 virus were not assigned numbers. Segments derived from BJ09 virus are in yellow, and those derived from HB08 virus are in blue. The pathogenicity of the reassortant viruses was determined as described in Materials and Methods. Maximum mean weight loss was determined from three mice (percentage weight loss relative to day 0 p.i.) following infection with 10⁵ pfu of virus. MST, mean survival time of mice infected with 10⁵ pfu.