Enveloped. Usually rounded but can be filamentous. The virions are 80-120 nm in diameter.


segmented ssRNA(-) linear genome, encapsidated by nucleoprotein (NP) Contains 7 segments coding for 7 proteins. Segments size range from 0.76 to 2.32 kb. Genome total size is about 10 Kb


Viral RNA polymerase (PB1, PB2 and PA) transcribes one mRNA from each genome segment. Transcription is primed by cap snatching. mRNA are polyadenylated by the viral polymerase stuttering on a poly U track.



  1. Virus attaches to sialic acid receptor though HEF protein and is endocytosed.
  2. endosome acidification induces fusion of virus membrane with the vesicle membrane; encapsidated RNA segments migrate to the nucleus.
  3. Transcription of genomic segments by the viral polymerase produces mRNAs that are capped and polyadenylated by the viral polymerase.
  4. Replication of genomic segments.
  5. Virus assembly and budding occurs at the plasma membrane.