Influenza is one of the great epidemic diseases. From time to time it becomes pandemic and spreads through out the world. Influenza virus belongs to the family Orthomyxoviridae. Three immunological types are known. They are influenza type A, B and C viruses. Influenza type A is highly variable antigenically and is responsible
for most cases of epidemic influenza, B shows less variation and causes less frequent epidemics, C is antigenically stable and causes only mild infection.
Influenza virus is a RNA virus. There are eight separate single stranded negative sense fragments. Each segment is a gene and codes for a different protein viz. haemagglutinin and neuraminidase etc. Viruses are roughly spherical particles, medium size, 80-100 nm, with an envelope. The envelope contains radially projecting spikes of virus haemagglutinin and neuraminidase. Inside the envelope is situated nucleocapsid,
nucleic acid surrounded by protein capsomeres. The virus agglutinates the RBCs of many animal species. It grows on monkey kidney cell cultures and in amniotic cavity of developing chick embryo.
The virus attaches to the cell surface sialic acid via the receptor present on the tip of haemagglutinin. Viral particles are then internalized within endosomes. The endosome fuses with the cell membrane leading to uncoating. Viral nucleocapsids are released into the cell cytoplasm. Viral transcription occurs in the nucleus. Viral proteins aresynthesized in the cytoplasm. Viral RNA synthesis occurs with theproduction of positive strand complete copies of each segment. After viral assembly the mature virus is released by budding off the cell.
Virus enters by inhalation of respiratory secretions from an in-fected person. The incubation period is 1-4 days. The signs and symp-toms include fever, malaise, head ache, generalized aches, sometimes with nasal discharge and sneezing. There may be sore throat and hoarse-ness. The symptoms last for 4 days but tiredness and weakness persist for long. The virus multiplies in superficial epithelium of the upper and lower respiratory tract. Influenza causes damage to cilia and desqamation of the epithelium. Few patients may develop pneumonia. Secondary bacterial infections may also be seen.
Influenza virus undergoes antigenic variations from time to time. Epidemics are due to the emergence of new virus strains containing new haemagglutinin and neuraminidase. When major change occurs it is called antigenic shift. If minor change occurs it is called antigenic drift. At the time of pandemic, the speed with which new strain spreads makes it difficult to prepare sufficient quantity of vaccine. Current vac-cine contains inactivated virus grown in eggs and either purified or dis-rupted and purified. It is given subcutaneously and the protection is short lived.