Kolkata, Mar 12 : The influenza viruses are classified into types A, B and C on the basis of their nucleoproteins.
Only types A and B cause human disease of any concern. The subtypes of influenza A viruses are determined by envelope glycoproteins possessing either haemagglutinin (HA) or neuraminidase (NA) activity.
High mutation rates and frequent genetic reassortments of these viruses contribute to great variability of the HA and NA antigens. The majority of the currently identified 17 HA and 10 NA subtypes of influenza A viruses are maintained in wild, aquatic bird populations.
Humans are generally infected by viruses of the subtypes H1, H2 or H3, and N1 or N2. Minor point mutations causing small changes (“antigenic drift”) occur relatively often. Antigenic drift enables the virus to evade immune recognition, resulting in repeated influenza outbreaks during interpandemic years.
Major changes in the HA antigen (“antigenic shift”) are caused by reassortment of genetic material from different A subtypes. Antigenic shifts resulting in new pandemic strains are rare events, occurring through reassortment between animal and human subtypes, for example in co-infected pigs.
In 2009, global outbreaks caused by the A(H1N1) strain attained pandemic proportions, gradually evolving into a seasonal epidemiological pattern in 2010.
Respiratory transmission occurs mainly by droplets disseminated by unprotected coughs and sneezes. Airborne transmission of influenza viruses occurs particularly in crowded spaces. Hand contamination followed by direct mucosal inoculation of virus is another possible source of transmission.UNI