Influenza pandemic
An influenza pandemic is an epidemic of an influenza virus that spreads across a large region and infects a large proportion of the population. There have been five major influenza pandemics 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. 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 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.
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 by the WHO after evidence of spreading in the southern hemisphere. The 13 November 2009 worldwide update by the WHO stated that "s of 8 November 2009, worldwide more than 206 countries and overseas territories or communities have reported laboratory confirmed cases of pandemic influenza H1N1 2009, including over 6,250 deaths."
Influenza
Influenza, commonly known as the flu, is an infectious disease of birds and mammals. It was thought to be caused by comets, earthquakes, volcanoes, cosmic dust, the rising and setting of the sun, vapors arising from the air and ground, or a blast from the stars. Now we know that it is caused by an RNA virus of the family Orthomyxoviridae. In humans, common symptoms of influenza infection are fever, sore throat, muscle pains, severe headache, coughing, and weakness and fatigue. In more serious cases, influenza causes pneumonia, which can be fatal, particularly in young children and the elderly. While sometimes confused with the common cold, influenza is a much more severe disease and is caused by a different type of virus. Although nausea and vomiting can be produced, especially in children, these symptoms are more characteristic of the unrelated gastroenteritis, which is sometimes called "stomach flu" or "24-hour flu."Typically, influenza is transmitted from infected mammals through the air by coughs or sneezes, creating aerosols containing the virus, and from infected birds through their droppings. Influenza can also be transmitted by saliva, nasal secretions, feces, and blood. Healthy individuals can become infected if they breathe in a virus-laden aerosol directly, or if they touch their eyes, nose or mouth after touching any of the aforementioned bodily fluids. Flu viruses can remain infectious for about one week at human body temperature, over 30 days at, and indefinitely at very low temperatures. Most influenza strains can be inactivated easily by disinfectants and detergents.
Flu spreads around the world in seasonal epidemics. Ten pandemics were recorded before the Spanish flu of 1918. Three influenza pandemics occurred during the 20th century and killed tens of millions of people, with each of these pandemics being caused by the appearance of a new strain of the virus in humans. Often, these new strains result from the spread of an existing flu virus to humans from other animal species, so close proximity between humans and animals can promote epidemics. Additionally, the wartime practice of packing together the severely ill in field hospitals can promote the spread of more virulent strains of influenza when such people would otherwise be isolated from others during peacetime.
Vaccinations against influenza are most commonly given to high-risk humans in industrialized countries and to farmed poultry. The most common human vaccine is the trivalent influenza vaccine that contains purified and inactivated material from three viral strains. Typically this vaccine includes material from two influenza A virus subtypes and one influenza B virus strain. A vaccine formulated for one year may be ineffective in the following year, since the influenza virus changes rapidly over time and different strains become dominant. Antiviral drugs can be used to treat influenza, with neuraminidase inhibitors being particularly effective.
Variants and subtypes of ''Influenza A virus''
Variants of Influenza A virus are identified and named according to the isolate that they are like and thus are presumed to share lineage ; according to their typical host ; according to their subtype ; and according to their deadliness. So, a flu from a virus similar to the isolate A/Fujian/411/2002 is called Fujian flu, human flu, and H3N2 flu.Variants are sometimes named according to the species the strain is endemic in or adapted to. Some variants named using this convention are:
The Influenza A virus subtypes are labeled according to an H number and an N number. 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.
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 or low pathogenic. The test for this is based solely on the effect on chickens - a virus strain is highly pathogenic avian influenza if 75% or more of chickens die after being deliberately infected with it. The alternative classification is low pathogenic avian influenza. This classification system has since been modified to take into account the structure of the virus' haemagglutinin protein. 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. 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.
Nature of a flu pandemic
Some pandemics are relatively minor such as the one in 1957 called Asian flu. Others have a higher Pandemic Severity Index whose severity warrants more comprehensive social isolation measures.The 1918 pandemic killed tens of millions and sickened hundreds of millions; the loss of this many people in the population caused upheaval and psychological damage to many people. There were not enough doctors, hospital rooms, or medical supplies for the living as they contracted the disease. Dead bodies were often left unburied as few people were available to deal with them. There can be great social disruption as well as a sense of fear. Efforts to deal with pandemics can leave a great deal to be desired because of human selfishness, lack of trust, illegal behavior, and ignorance. For example, in the 1918 pandemic: "This horrific disconnect between reassurances and reality destroyed the credibility of those in authority. People felt they had no one to turn to, no one to rely on, no one to trust."
A letter from a physician at one U.S. Army camp in the 1918 pandemic said:
It is only a matter of a few hours then until death comes . It is horrible. One can stand it to see one, two or twenty men die, but to see these poor devils dropping like flies . We have been averaging about 100 deaths per day . Pneumonia means in about all cases death . We have lost an outrageous number of Nurses and Drs. It takes special trains to carry away the dead. For several days there were no coffins and the bodies piled up something fierce .
Wave nature
Flu pandemics typically come in waves. The 1889–1890 and 1918–1920 flu pandemics each came in three or four waves of increasing lethality. Within a wave, mortality was greater at the beginning of the wave.Variable mortality
Mortality varies widely in a pandemic. In the 1918 pandemic:In U.S. Army camps where reasonably reliable statistics were kept, case mortality often exceeded 5 percent, and in some circumstances exceeded 10 percent. In the British Army in India, case mortality for white troops was 9.6 percent, for Indian troops 21.9 percent. In isolated human populations, the virus killed at even higher rates. In the Fiji islands, it killed 14 percent of the entire population in 16 days. In Labrador and Alaska, it killed at least one-third of the entire native population.