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 9 proteins. Segments size range from 0.9 to 2.3 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 cleaved from cellular mRNAS by Cap snatching. mRNA are polyadenylated by the viral polymerase stuttering on a poly U track. NS mRNA can be spliced, giving rise to mRNA coding NEP protein. Polyprotein p42 is cleaved in M1 and CM2 proteins.
- Virus attaches to sialic acid receptor through HEF protein and is endocytosed in the host cell.
- Endosome acidification induces fusion of virus membrane with the vesicle membrane; encapsidated RNA segments migrate to the nucleus.
- Transcription of genomic segments by the viral polymerase produces mRNAs.
- genomic (-)RNA is replicated through antigenomic (+)RNA intermediate.
- High level of M1 protein induces genomes segments export from nucleus by NEP protein.
- Virus assembly and budding occurs at the plasma membrane.