Swine flu (S-OIV) pandemic.

Influenza is a disease of global dimension occurring in annual epidemics, and infrequently as pandemic with high attack rates. The annual epidemics result in the host population updating its pool of humoral immunity by exposure to the most recent viruses carrying altered antigenic specificity caused by 'anti-gen shift' in the haemagglutinin (H) ligand. Pandemics are caused by 'antigen drift'. They occur at intervals and give rise to high morbidity and mortality. Influenza viruses are RNA viruses with a segmental genome comprising eight parts. 'Antigen drift' and 'shift' arise from these characteristics (Fig. 1). The RNA genome is transcribed by the polymerase that the virus carries. The transcription is associated with many point mutations persistently causing changes in virus proteins including surface proteins HA and NA (neuraminidase), and thereby causing the 'antigen drift' [1]. Antigen drift enables the virus to escape onslaught by the host's immune defences. Segmentation of the genome facilitates reassortment when two or more viral variants happen to infect the same cell. The progeny virus gets a mixed genome with a different viral structure. The emergence of H3N2 virus, as indeed of several other variants, has been traced to such a genomic reassortment [2]. Two envelope proteins stick out as spikes which the virus uses for entry into host cells (H, agglutinin) and exit by the progeny of the virus (N, neuramini-dase). Influenza A viruses are subtyped into 16 H (H1 to H16) and 9 N (N1 to N9) giving a theoretical possibility of 144 serological subtypes. Up to now 105 influenza A subtypes have been discovered, of which all are endemic in water birds (ducks, geese and gulls). Some subtypes have adapted to other birds (chickens) and mammals (pigs, horses and humans) [3]. Influenza pandemics occur when an influenza virus strain is transmitted to humans from another animal species. Species that are considered important from this respect are pigs, chicken and ducks. For example , in 1997 during an outbreak in chickens in Hong Kong the avian influenza virus H5N1 crossed the species barrier and infected 18 persons of whom six died. Since then antigenic variants of the virus have continued to appear in domestic and wild birds. During the 20th century, four pandemics of influenza have been recorded (Table 1). The current swine flu pandemic is the first one of the 21st century. Viruses of the H5N1 subtype are highly pathogenic for humans [4]. Currently both …