Pandemic influenza

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 most recent, 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 in part to be 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.

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. The 13 November 2009 worldwide update by the WHO stated that " of 8 November 2009, worldwide more than 206 countries and overseas territories or communities have reported 03,536laboratory 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 (the influenza viruses). 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 (or surfaces contaminated with those fluids). Flu viruses can remain infectious for about one week at human body temperature, over 30 days at 0 °C (32 °F), and indefinitely at very low temperatures (such as lakes in northeast Siberia). 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. In addition, epidemiological factors, such as the WWI practice of packing soldiers with severe influenza illness into field hospitals while soldiers with mild illness stayed outside on the battlefield, are an important determinant of whether or not a new strain of influenza virus will spur a pandemic. (During the 1918 Spanish flu pandemic, this practice served to promote the evolution of more virulent viral strains over those that produced mild illness.) When it first killed humans in Asia in the 1990s, a deadly avian strain of H5N1 posed a great risk for a new influenza pandemic; however, this virus did not mutate to spread easily between people.

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 (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:
* Bird flu
* Dog flu
* Horse flu
* Human flu
* Swine flu

Avian variants have also sometimes been named according to their deadliness in poultry, especially chickens:
* Low Pathogenic Avian Influenza (LPAI)
* Highly Pathogenic Avian Influenza (HPAI), also called: deadly flu or death flu

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. Most known strains are extinct strains. For example, the annual flu subtype H3N2 no longer contains the strain that caused the Hong Kong flu.

Influenza A viruses are negative sense, single-stranded, segmented RNA viruses. "There are 16 different HA antigens (H1 to H16) and nine different NA antigens (N1 to N9) for influenza A. Until recently, 15 HA types had been recognized, but recently two new types were isolated: a new type (H16) was isolated from black-headed gulls caught in Sweden and the Netherlands in 1999 and reported in the literature in 2005." "The other, H17, was isolated from fruit bats caught in Guatemala and reported in the literature in 2013."

Nature of a flu pandemic

Some pandemics are relatively minor such as the one in 1957 called Asian flu (1–4 million dead, depending on source). 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. But 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.

Influenza pandemics

A 1921 book lists nine influenza pandemics prior to the 1889–1890 flu, the first in 1510. A more modern source lists six.

Asiatic flu (1889–1890)

The 1889–1890 pandemic, often referred to as the ''Asiatic flu'' or ''Russian flu'', killed about 1 million people out of a world population of about 1.5 billion.

It was the last great pandemic of the 19th century, and is among the deadliest pandemics in history. The most reported effects of the pandemic took place from October 1889 to December 1890, with recurrences in 1891 to 1895.

Spanish flu (1918–1920)

The 1918 flu pandemic, commonly referred to as the Spanish flu, was a category 5 influenza pandemic caused by an unusually severe and deadly Influenza A virus strain of subtype H1N1.

The Spanish flu pandemic lasted from 1918 to 1920. Various estimates say it killed between 17 million and 100 million people This pandemic has been described as "the greatest medical holocaust in history" and may have killed as many people as the Black Death, although the Black Death is estimated to have killed over a fifth of the world's population at the time, a significantly higher proportion. This huge death toll was caused by an extremely high infection rate of up to 50% and the extreme severity of the symptoms, suspected to be caused by cytokine storms. Indeed, symptoms in 1918 were so unusual that initially influenza was misdiagnosed as dengue, cholera, or typhoid. One observer wrote, "One of the most striking of the complications was hemorrhage from mucous membranes, especially from the nose, stomach, and intestine. Bleeding from the ears and petechial hemorrhages in the skin also occurred." The majority of deaths were from bacterial pneumonia, a secondary infection caused by influenza, but the virus also killed people directly, causing massive hemorrhages and edema in the lung.

The Spanish flu pandemic was truly global, spreading even to the Arctic and remote Pacific islands. The unusually severe disease killed between 10 and 20% of those infected, as opposed to the more usual flu epidemic mortality rate of 0.1%. Another unusual feature of this pandemic was that it mostly killed young adults, with 99% of pandemic influenza deaths occurring in people under 65, and more than half in young adults 20 to 40 years old. This is unusual since influenza is normally most deadly to the very young (under age 2) and the very old (over age 70). The total mortality of the 1918–1920 pandemic is estimated to be between 17 and 100 million people, constituting approximately 1–6% of the world's population. As many as 25 million may have been killed in the first 25 weeks; in contrast, HIV/AIDS has killed 25 million in its first 25 ''years''.

Asian flu (1957–1958)

The Asian flu was a category 2 flu pandemic outbreak caused by a strain of H2N2 that originated in China in early 1957, lasting until 1958. The virus originated from a mutation in wild ducks combining with a pre-existing human strain.Greene, Jeffrey. Moline, Karen. 006(2006) The Bird Flu Pandemic. . The virus was first identified in Guizhou in late February; by mid-March it had spread across the entire mainland. It was not until the virus had reached Hong Kong in April, however, that the world was alerted to the unusual situation, when the international press began to report on the outbreak. The World Health Organization was officially informed when the virus arrived in Singapore, which operated the only influenza surveillance laboratory in Southeast Asia, in early May. From that point on, as the virus continued to sweep the region, the WHO remained attuned to the developing outbreak and helped coordinate the global response for the duration of the pandemic.

This was the first pandemic to occur during what is considered the "era of modern virology". One significant development since the 1918 pandemic was the identification of the causative agent behind the flu. Later, it was recognized that the influenza virus changes over time, typically only slightly (a process called "antigenic drift"), sometimes significantly enough to result in a new subtype (" antigenic shift"). Within weeks of the report out of Hong Kong, laboratories in the United States, the United Kingdom, and Australia had analyzed the virus and concluded that it was a novel strain of influenza A. Chinese researchers had already come to a similar conclusion in March, but as China was not a member of the WHO nor a part of its network of National Influenza Centers

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