FLU H1N1

Influenza A Virus Subtype H1N1

Influenza A virus subtype H1N1, also known as A(H1N1), is a subtype of influenza virus A and the most common cause of influenza (flu) in humans. Some strains of H1N1 are endemic in humans, including the strain(s) responsible for the 1918 flu pandemic which killed 50–100 million people worldwide. Less virulent H1N1 strains still exist in the wild today, worldwide, causing a small fraction of all influenza-like illness and a large fraction of all seasonal influenza. H1N1 strains caused roughly half of all flu infections in 2006. Other strains of H1N1 are endemic in pigs and in birds.

Influenza

In March, April and May 2009, thousands of laboratory-confirmed infections and a number of deaths were caused by an outbreak of a new strain of H1N1

• 1 Nomenclature

• 2 Spanish Flu

• 3 Russian Flu

• 4 2009 Influenza A (H1N1) outbreak

Nomenclature

Influenza A virus strains are categorized according to two proteins found on the surface of the virus: hemagglutinin (H) and neuraminidase (N). All influenza A viruses contain hemagglutinin and neuraminidase, but the structure of these proteins differs from strain to strain due to rapid genetic mutation in the viral genome.

Influenza A virus strains are assigned an H number and an N number based on which forms of these two proteins the strain contains. There are 16 H and 9 N subtypes known in birds, but only H 1, 2 and 3, and N 1 and 2 are commonly found in humans.

Spanish flu

The Spanish flu, also known as La Gripe Española, or La Pesadilla, was an unusually severe and deadly strain of avian influenza, a viral infectious disease, that killed some 50 million to 100 million people worldwide over about a year in 1918 and 1919. It is thought to be one of the most deadly pandemics in human history. It was caused by the H1N1 type of influenza virus.

The Spanish flu caused an unusual number of deaths because it may have caused a cytokine storm in the body. (The recent epidemic of bird flu, also an Influenza A virus, had a similar effect.)^[8] The Spanish flu virus infected lung cells, leading to overstimulation of the immune system via release of cytokines into the lung tissue. This leads to extensive leukocyte migration towards the lungs, causing destruction of lung tissue and secretion of liquid into the organ. This makes it difficult for the patient to breathe. In contrast to other pandemics, which mostly kill the old and the very young, the 1918 pandemic killed unusual numbers of young adults, which may have been due to their healthy immune systems being able to mount a very strong and damaging response to the infection.

The term "Spanish" flu was coined because Spain was at the time the only European country where the press were printing reports of the outbreak, which had killed thousands in the armies fighting the First World War. Other countries suppressed the news in order to protect morale.

Russian flu

The more recent Russian flu was a 1977–1978 flu epidemic caused by strain Influenza A/USSR/90/77 (H1N1). It infected mostly children and young adults under 23 because a similar strain was prevalent in 1947–57, causing most adults to have substantial immunity. Some have called it a flu pandemic but because it only affected the young it is not considered a true pandemic. The virus was included in the 1978–1979 influenza vaccine.

2009 Influenza A (H1N1) outbreak

Minor outbreaks of swine influenza occurred in humans in 1976 and 1988, and in pigs in 1998 and 2007.

In the 2009 swine flu outbreak, the virus isolated from patients in the United States was found to be made up of genetic elements from four different flu viruses – North American Mexican influenza, North American avian influenza, human influenza, and swine influenza virus typically found in Asia and Europe – "an unusually mongrelised mix of genetic sequences." This new strain appears to be a result of reassortment of human influenzaswine influenza viruses, in all four different strains of subtype H1N1. However, as the virus has not yet been isolated in animals to date and also for historical naming reasons, the World Organisation for Animal Health (OIE) suggests it be called "North-American influenza". On April 30, 2009 the World Health Organization began referring to the outbreak as "Influenza A" instead of "swine flu", and later began referring to it as "Influenza A(H1N1)". Several complete genome sequences for U.S. flu cases were rapidly made available through the Global Initiative on Sharing Avian Influenza Data (GISAID). Preliminary genetic characterization found that the hemagglutinin (HA) gene was similar to that of swine flu viruses present in U.S. pigs since 1999, but the neuraminidase (NA) and matrix protein (M) genes resembled versions present in European swine flu isolates. The six genes from American swine flu are themselves mixtures of swine flu, bird flu, and human flu viruses. While viruses with this genetic makeup had not previously been found to be circulating in humans or pigs, there is no formal national surveillance system to determine what viruses are circulating in pigs in the U.S.

Illustration of influenza antigenic shift.

Antigenic shift

Antigenic shift is the process by which at least two different strains of a virus (or different viruses), especially influenza, combine to form a new subtype having a mixture of the surface antigens of the two original strains. The term antigenic shift is more often applied specifically, (but is not limited) to the influenza literature, as it is the best known example (e.g. visna virus in sheep). Antigenetic shift is a specific case of reassortment or viral shift that confers a phenotypic change.

In terms of virology, the marine ecosystem has been largely unstudied, but due to its extraordinary volume, high viral density (100 million viruses per mL in coastal waters, 3 million per mL in the deep sea) and high cell lysing rate (as high as 20% on average; marine viruses' antigenic shift and genetic recombination rates must be quite high. This is most striking when one considers that the coevolution of prokaryotes and viruses in the aquatic environment has been going on since before eukaryotes appeared on earth.

Antigenic shift is contrasted with antigenic drift, which is the natural mutation over time of known strains of influenza (or other things, in a more general sense) which may lead to a loss of immunity, or in vaccine mismatch. Antigenic drift occurs in all types of influenza including influenzavirus A, influenza B and influenza C. Antigenic shift, however, occurs only in influenzavirus A because it infects more than just humans. Affected species include other mammals and birds, giving influenza A the opportunity for a major reorganization of surface antigens. Influenza B and C principally infect humans, minimizing the chance that a reassortment will change its phenotype drastically.

Antigenic shift is important as it is a pathway that viruses may follow to enter a new niche, and so should not be overlooked in the emergence of new viral pathogens. It could occur with primate viruses and may be a factor to consider for the appearance of new viruses in the human species such as HIV. Due to the structure of its genome HIV does not undergo reassortment, but it does recombine freely and via superinfection HIV can produce recombinant HIV strains that differ significantly from their ancestors.

Flu strains are named after their types of hemagglutinin and neuraminidase surface proteins, so they will be called, for example, H3N2 for type-3 hemagglutinin and type-2 neuraminidase. When two different strains of influenza infect the same cell simultaneously, their protein capsids and lipid envelopes are removed, exposing their RNA, which is then transcribed to mRNA. The host cell then forms new viruses that combine their antigens; for example, H3N2 and H5N1 can form H5N2 this way. Because the human immune system has difficulty recognizing the new influenza strain, it may be highly dangerous. Influenza viruses which have undergone antigenic shift have caused the Asian Flu pandemic of 1957, the Hong Kong Flu pandemic of 1968, and the Swine Flu scare of 1976. Until recently, such combinations were believed to have caused the infamous Spanish Flu outbreak of 1918 which killed 40~100 million people worldwide^, however more recent research suggests the 1918 pandemic was caused by the antigenic drift of a fully avian virus to a form that could infect humans efficiently. One increasingly worrying situation is the possible antigenic shift between avian influenza and human influenza. This antigenic shift could cause the formation of a highly virulent virus.