Description
Influenza (flu) is a viral respiratory infection in mammals and birds. This virus is divided into three main types (A, B and C). Influenza A is found in a wide variety of mammalian and avian species and is associated with the major human pandemics. Influenza B is largely confined to humans and became unexpectedly prevalent in humans during 2000-2002. Influenza C infects humans, dogs and pigs and generally causes only mild upper respiratory tract infection. However, influenza A and B viruses cause a wide spectrum of severe disease including lower respiratory, tract infection, pneumonia and encephalitis. Influenza A is further divided into subtypes based on antigenic differences in the membrane proteins hemagglutinin (HA) and neuraminidase (NA). 16 HAs (H1-H16) and 9 NA (N1-N9) had been identified. While different combinations of the two antigens appear more frequently in some groups of birds than others, only few subtypes have established themselves in humans (HA: H1, H2, and H3; NA: N1 and N2).
The 2009 flu pandemic is caused by a new swine-origin influenza A (H1N1) virus which is a recombinated production by human H3N2, swine H1N1 and avian H5N1strains. The mixing of new genetic elements in swine can result in the emergence of viruses with pandemic potential in humans. As 2009 H1N1 influenza is a new virus and most people have no or little immunity this virus could cause more infections than are seen with seasonal flu. The virus spread worldwide by human-to-human transmission, causing the World Health Organization to raise its pandemic alert to the highest level 6. The HA, NA, and MP sequences of 2009 H1N1 flu (swine flu) have been placed on deposit at GISAID.
Hemagglutinin (HA), which binds to sialic acid (SA)-containing receptors on host cells, is the protein that produces neutralizing antibodies. Hemagglutinin plays a major role in the determination of host range restriction and virulence because human influenza HA preferentially binds to SA-α-2,6 while avian influenza HA preferentially binds to SA-α-2,3. The cleavage of HA into two disulfide-linked subunits, HA1 and HA2, is a prerequisite for initiating infection. Usually HA is restricted to be cleaved at respiratory tracts by limited proteases. Highly pathogenic avian influenza contains a stretch of basic residues adjacent to the HA cleavage site, enabling its HA to be cleaved by a wide range of proteases with ubiquitous tissue distributions. This process permits productive virus replication in organs outside of the respiratory and gastrointestinal tracts, including the brain, resulting in widespread disease and high mortality rates