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Introduction
Drift variants of low pathogenic avian influenza viruses (LPAI) have become a global challenge, inflicting heavy mortality in poultry (Li et al., 2005; Lu et al., 2011; Park et al., 2011), and are endemic in the Middle East and Asia (Swayne and Halvorson, 2008). Apart from reviewing the emergence of these drift variants, this paper focuses on their occurrence in India, typical lesions encountered, and the limitations of any vaccination programme to control variants.
Avian influenza virus
Avian influenza is a RNA virus. Its RNA is single-stranded, segmented, and of negative-sense. Its genetic material is composed of eight segments of single-stranded RNA. These eight segments, which are actually eight genes, encode 10 proteins of the virus (Swayne and Halvorson, 2008). The eight segments are surrounded by an envelope, which is covered by haemagglutinin (HA), and neuraminidase (NA) glycoproteins. Avian influenza viruses have 16 haemagglutinins and nine neuraminidases (Swayne and Halvorson, 2008). Each virus has one H and one N antigen subtype in any combination, creating 144 combinations. Most of the possible combinations have been isolated from avian species (Alexander, 2008).
The virus of avian influenza occurs in two forms: highly pathogenic avian influenza virus (HPAI) and the low pathogenic avian influenza virus (LPAI). The HPAI virus is highly virulent and is capable of causing mortality in birds up to 100% within a few days. The LPAI virus is mild and mortality is usually less than 5% (Swayne and Halvorson, 2008; Vegad, 2008). A characteristic feature of avian influenza viruses is that they are dynamic and continuously evolving. They can change in two ways: via antigenic drift and/or antigenic shift (Swayne and Halvorson, 2008), although, while antigenic drift changes occur all the time, antigenic shift is rare (Wikipedia, 2012a).
Antigenic drift
When a DNA virus replicates, its adenine must pair with thymine; and cytosine with guanine. If the correct pairing does not occur, an exact copy of the virus cannot be produced. The virus, by using its corrective, 'proofreading' mechanism (enzyme exonuclease), tries to rectify the mistakes, whereby the wrong base is cut and removed, and in its place a correct base is inserted. RNA viruses make a lot of wrong base pairings during replication due...