Epidemics And Pandemics Of Influenza Virus

 

Epidemics And Pandemics Of Influenza Virus

Influenza is enveloped, and is negative sense single stranded RNA whose genome is segmented into 8 segments. From the envelope there are two glycoprotein spikes; Haemagglutinin Spikes and Neuraminidase Spike.

Epidemics and Pandemic of Influenza is due to antigenic variation in both surface antigen (Haemagglutinin and Neuraminidase) Of Influenza. Surface antigens undergo independent antigenic variations which are:-                                                                                                                                                           a)Antigenic Drift                                                                                                                                           b)Antigenic Shift

a) Antigenic Drift

1. Accounts for the periodic epidemics of influenza.
2. It is gradual sequential change in antigenic structure occurring regularly at frequent intervals due to mutation and selection.(Minor change in antigenicity of the hemagglutinin and Neuraminidase proteins.)
3. Caused by point mutations in the H and N genes of Influenza A and B causing minor substitutions in the amino acid sequence of the proteins encoded.

b) Antigenic Shift

1. Accounts for the Pandemics of influenza
2. Discontinuous variation in the antigenic structure, such changes may involve either hemagglutinin or Neuraminidase or both.
3. Antigenic Shift results from genetic reassortment of the RNA segments that code for haemagglutinin during replication (i.e two viruses of  different subtypes ,one of human origin and one of animal origin infect all same cell, the eight genome segments of each virus replicate and when the progeny viruses are assembled one of human viral acquires it's haemagglutinin gene from second viruses.
4.  As a result of this reassortment, a new virus subtype emerges with a novel haemagglutinin to which population have no pre-existing antibody and cause pandemic.
5. Major Change and exclusive to Influenza A virus.

Antigenic Drift accounts for the periodic epidemics of influenza and Antigenic Shift accounts for the pandemic of Influence virus.