Influenza pandemic
An influenza pandemic is an epidemic of an influenza virus that spreads across a large region (either multiple continents or worldwide) and infects a large proportion of the population. There have been six major influenza epidemics in the last 140 years, with the 1918 flu pandemic being the most severe; this is estimated to have been responsible for the deaths of 50–100 million people. The 2009 swine flu pandemic resulted in under 300,000 deaths and is considered relatively mild. These pandemics occur irregularly.
Influenza pandemics occur when a new strain of the influenza virus is transmitted to humans from another animal species. Species that are thought to be important in the emergence of new human strains are pigs, chickens and ducks. These novel strains are unaffected by any immunity people may have to older strains of human influenza and can therefore spread extremely rapidly and infect very large numbers of people. Influenza A viruses can occasionally be transmitted from wild birds to other species, causing outbreaks in domestic poultry, and may give rise to human influenza pandemics.[1][2] The propagation of influenza viruses throughout the world is thought to be in part by bird migrations, though commercial shipments of live bird products might also be implicated, as well as human travel patterns.
The World Health Organization (WHO) has produced a six-stage classification that describes the process by which a novel influenza virus moves from the first few infections in humans through to a pandemic. This starts with the virus mostly infecting animals, with a few cases where animals infect people, then moves through the stage where the virus begins to spread directly between people, and ends with a pandemic when infections from the new virus have spread worldwide.[3]
One strain of virus that may produce a pandemic in the future is a highly pathogenic variation of the H5N1 subtype of influenza A virus. On 11 June 2009, a new strain of H1N1 influenza was declared to be a pandemic (Stage 6) by the WHO after evidence of spreading in the southern hemisphere.[4] The 13 November 2009 worldwide update by the WHO stated that "[a]s of 8 November 2009, worldwide more than 206 countries and overseas territories or communities have reported [503,536] laboratory confirmed cases of pandemic influenza H1N1 2009, including over 6,250 deaths."[5]
Variants of Influenza A virus are identified and named according to the isolate that they are like and thus are presumed to share lineage (example Fujian flu virus like); according to their typical host (example Human flu virus); according to their subtype (example H3N2); and according to their deadliness (e.g., Low Pathogenic as discussed below). So, a flu from a virus similar to the isolate A/Fujian/411/2002(H3N2) is called Fujian flu, human flu, and H3N2 flu.
Variants are sometimes named according to the species (host) the strain is endemic in or adapted to. Some variants named using this convention are:[19]
The Influenza A virus subtypes are labeled according to an H number (for hemagglutinin) and an N number (for neuraminidase). Each subtype virus has mutated into a variety of strains with differing pathogenic profiles; some pathogenic to one species but not others, some pathogenic to multiple species. Frequently, a newer strain will completely replace an older strain.[20]
Because of the impact of avian influenza on economically important chicken farms, a classification system was devised in 1981 which divided avian virus strains as either highly pathogenic (and therefore potentially requiring vigorous control measures) or low pathogenic. The test for this is based solely on the effect on chickens - a virus strain is highly pathogenic avian influenza (HPAI) if 75% or more of chickens die after being deliberately infected with it. The alternative classification is low pathogenic avian influenza (LPAI).[21] This classification system has since been modified to take into account the structure of the virus' haemagglutinin protein.[22] Other species of birds, especially water birds, can become infected with HPAI virus without experiencing severe symptoms and can spread the infection over large distances; the exact symptoms depend on the species of bird and the strain of virus.[21] Classification of an avian virus strain as HPAI or LPAI does not predict how serious the disease might be if it infects humans or other mammals.[21][23]