Influenza Virus, Structure, Replication and Pathogenesis

ORTHOMYXOVIRUSES ( INFLUENZA VIRUSES)

-         affinity of the viruses to mucins ( myxa: mucus)

-         4 genera: influenza A, B, C viruses and thogotoviruses

-         Mutability and high frequency of genetic reassortment.
 resultant antigenic changes in viral surface gp.

-         Influenza type A antigenically highly variable and responsible for most cases of epidemic influenza.

Structure / composition

-         virion spherical, pleomorphic (80-120 nm diameter), helical nucleocapsid (9m) SS RNA, (-) sense (13.6 kb genome size) of A and B viruses with and separate segments; infection C → 7 segments lacking neuraminidase gene.

-         Segment
1→ PB₂                   4→HA             7→M₁ and M₂

      2→PB₁                        5→NP             8→NS₁ and NS₂

      3→PA                   6→NA

Schematic representation of Influenza Virus Structure

→ 9 structural proteins; one non- structural

→ NP+ viral RNA → RNP (ribonucleoprotein)
→ PB1, PB2 and PA bound to RNP and responsible for transcription and replication.
→ M1 (matrix) protein forms a shell underneath the envelope.
→ HA and NA (surface gp) determine Ag- variation and host immunity.
→ M2 protein ( ion channel) allows PH changes in the endosome.