Antigenic shift
Encyclopedia
Antigenic shift is the process by which two or more different strains of a virus
, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigen
s of the two or more original strains. The term is often applied specifically to influenza
, as that is the best-known example, but the process is also known to occur with other viruses, such as visna virus
in sheep. Antigenic 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 mammal
s and bird
s, 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 for the emergence of new viral pathogen
s as it is a pathway that viruses may follow to enter a new niche
. It could occur with primate
viruses and may be a factor 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 protein
s, so they will be called, for example, H3N2 for type-3 hemagglutinin and type-2 neuraminidase. When two ifferent strains of influenza infect the same cell simultaneously, their protein capsid
s 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.
There are 16 different hemagglutinin
subtypes and 9 different neuraminidase
subtypes, all of which have been found among influenza A viruses in wild birds. Wild birds are the primary natural reservoir for all subtypes of influenza A viruses and are thought to be the source of influenza A viruses in all other animals. Most influenza viruses cause asymptomatic or mild infection in birds; however, the range of symptoms in birds varies greatly depending on the strain of virus. Infection with certain avian influenza A viruses (for example, some strains of H5 and H7 viruses) can cause widespread disease and death among some species of wild and especially domestic birds such as chickens and turkeys.
Pigs can be infected with both human and avian influenza viruses in addition to swine influenza viruses. Infected pigs get symptom
s similar to humans, such as cough, fever, and runny nose. Because pigs are susceptible to avian, human and swine influenza viruses, they potentially may be infected with influenza viruses from different species (e.g., ducks and humans) at the same time. If this happens, it is possible for the genes of these viruses to mix and create a new virus.
For example, if a pig was infected with a human influenza virus and an avian influenza virus at the same time, an antigenic shift could occur, producing a new virus that had most of the genes from the human virus, but a hemagglutinin or neuraminidase from the avian virus. The resulting new virus would likely be able to infect humans and spread from person to person, but it would have surface proteins (hemagglutinin and/or neuraminidase) not previously seen in influenza viruses that infect humans, and therefore to which most people have little or no immune protection. If this new virus causes illness in people and can be transmitted easily from person to person, an influenza pandemic can occur. The most recent 2009 H1N1 outbreak was a result of antigenic shift and reassortment between human, avian, and swine viruses.
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...
s of the two or more original strains. The term is often applied specifically to influenza
Influenza
Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...
, as that is the best-known example, but the process is also known to occur with other viruses, such as visna virus
Visna virus
Visna virus from the genus lentivirinae and subfamily Orthoretrovirinae, is a "prototype" retrovirus that causes encephalitis and chronic pneumonitis in sheep. It is known as visna when found in the brain, and maedi when infecting the lungs...
in sheep. Antigenic shift is a specific case of reassortment
Reassortment
Reassortment is the mixing of the genetic material of a species into new combinations in different individuals. Several different processes contribute to reassortment, including assortment of chromosomes, and chromosomal crossover. It is particularly used when two similar viruses that are infecting...
or viral shift
Viral shift
A viral shift is the sudden emergence of a seemingly novel strain of a virus. It is a prerequisite viral condition for pandemics. An existing virus undergoes a change and becomes a pathogen, such as the constantly mutating "A" strain of influenza in which the change is known as an antigenic shift...
that confers a phenotypic change.
In terms of virology
Virology
Virology is the study of viruses and virus-like agents: their structure, classification and evolution, their ways to infect and exploit cells for virus reproduction, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy...
, the marine ecosystem
Marine ecosystem
Marine ecosystems are among the largest of Earth's aquatic ecosystems. They include oceans, salt marsh and intertidal ecology, estuaries and lagoons, mangroves and coral reefs, the deep sea and the sea floor. They can be contrasted with freshwater ecosystems, which have a lower salt content. Marine...
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
Lysis
Lysis refers to the breaking down of a cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a "lysate"....
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
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
.
Antigenic shift is contrasted with antigenic drift
Antigenic drift
The immune system recognizes viruses when antigens on the surfaces of virus particles bind to immune receptors that are specific for these antigens. This is similar to a lock recognizing a key. After an infection, the body produces many more of these virus-specific receptors, which prevent...
, which is the natural mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
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
Influenzavirus A
Influenza A virus causes influenza in birds and some mammals and is the only species of Influenzavirus A. Influenzavirus A is a genus of the Orthomyxoviridae family of viruses. Strains of all subtypes of influenza A virus have been isolated from wild birds, although disease is uncommon...
, influenza B
Influenzavirus B
Influenzavirus B is a genus in the virus family Orthomyxoviridae. The only species in this genus is called "Influenza B virus".Influenza B viruses are only known to infect humans and seals, giving them influenza...
and influenza C
Influenzavirus C
Influenzavirus C is a genus in the virus family Orthomyxoviridae, which includes those viruses which cause influenza. The only species in this genus is called "Influenza C virus".Influenza C viruses are known to infect humans and pigs, giving them influenza...
. Antigenic shift, however, occurs only in influenzavirus A because it infects more than just humans. Affected species include other mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s and bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s, 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
Reassortment
Reassortment is the mixing of the genetic material of a species into new combinations in different individuals. Several different processes contribute to reassortment, including assortment of chromosomes, and chromosomal crossover. It is particularly used when two similar viruses that are infecting...
will change its phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
drastically.
Antigenic shift is important for the emergence of new viral pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s as it is a pathway that viruses may follow to enter a new niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
. It could occur with primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
viruses and may be a factor for the appearance of new viruses in the human species such as HIV
AIDS origin
AIDS is caused by the Human immunodeficiency virus , which originated in non-human primates in Sub-Saharan Africa and was transferred to humans during the late 19th or early 20th century....
. Due to the structure of its genome, HIV does not undergo reassortment, but it does recombine freely and via superinfection
Superinfection
In virology, superinfection is the process by which a cell, that has previously been infected by one virus, gets coinfected with a different strain of the virus, or another virus at a later point in time. Viral superinfections of serious conditions can lead to resistant strains of the virus, which...
HIV can produce recombinant HIV strains that differ significantly from their ancestors.
Flu strains are named after their types of hemagglutinin
Hemagglutinin
Influenza hemagglutinin or haemagglutinin is a type of hemagglutinin found on the surface of the influenza viruses. It is an antigenic glycoprotein. It is responsible for binding the virus to the cell that is being infected...
and neuraminidase
Neuraminidase
Neuraminidase enzymes are glycoside hydrolase enzymes that cleave the glycosidic linkages of neuraminic acids. Neuraminidase enzymes are a large family, found in a range of organisms. The most commonly known neuraminidase is the viral neuraminidase, a drug target for the prevention of the spread...
surface protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s, so they will be called, for example, H3N2 for type-3 hemagglutinin and type-2 neuraminidase. When two ifferent strains of influenza infect the same cell simultaneously, their protein capsid
Capsid
A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...
s and lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
envelopes are removed, exposing their RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
, 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
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
has difficulty recognizing the new influenza strain, it may be highly dangerous. Influenza viruses which have undergone antigenic shift have caused the Asian Flu
Asian flu
Asian Flu may refer to:* The Asian Financial Crisis of 1997, or* Asian Flu, the H2N2 virus...
pandemic of 1957, the Hong Kong Flu
Hong Kong flu
The Hong Kong flu was a category 2 flu pandemic whose outbreak in 1968 and 1969 killed an estimated one million people worldwide. It was caused by an H3N2 strain of the influenza A virus, descended from H2N2 through antigenic shift, a genetic process in which genes from multiple subtypes reassorted...
pandemic of 1968, and the Swine Flu
Swine flu
Swine influenza, also called pig influenza, swine flu, hog flu and pig flu, is an infection by any one of several types of swine influenza virus. Swine influenza virus or S-OIV is any strain of the influenza family of viruses that is endemic in pigs...
scare of 1976. Until recently, such combinations were believed to have caused the infamous Spanish Flu
Spanish flu
The 1918 flu pandemic was an influenza pandemic, and the first of the two pandemics involving H1N1 influenza virus . It was an unusually severe and deadly pandemic that spread across the world. Historical and epidemiological data are inadequate to identify the geographic origin...
outbreak of 1918 which killed 40~100 million people worldwide; however more recent research suggests the 1918 pandemic was caused by the antigenic drift
Antigenic drift
The immune system recognizes viruses when antigens on the surfaces of virus particles bind to immune receptors that are specific for these antigens. This is similar to a lock recognizing a key. After an infection, the body produces many more of these virus-specific receptors, which prevent...
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.
Role in transmission of influenza viruses from animals to people
Influenza A viruses are found in many different animals, including ducks, chickens, pigs, whales, horses, and seals. Influenza B viruses circulate widely principally among humans, though it has recently been found in seals.There are 16 different hemagglutinin
Hemagglutinin
Influenza hemagglutinin or haemagglutinin is a type of hemagglutinin found on the surface of the influenza viruses. It is an antigenic glycoprotein. It is responsible for binding the virus to the cell that is being infected...
subtypes and 9 different neuraminidase
Neuraminidase
Neuraminidase enzymes are glycoside hydrolase enzymes that cleave the glycosidic linkages of neuraminic acids. Neuraminidase enzymes are a large family, found in a range of organisms. The most commonly known neuraminidase is the viral neuraminidase, a drug target for the prevention of the spread...
subtypes, all of which have been found among influenza A viruses in wild birds. Wild birds are the primary natural reservoir for all subtypes of influenza A viruses and are thought to be the source of influenza A viruses in all other animals. Most influenza viruses cause asymptomatic or mild infection in birds; however, the range of symptoms in birds varies greatly depending on the strain of virus. Infection with certain avian influenza A viruses (for example, some strains of H5 and H7 viruses) can cause widespread disease and death among some species of wild and especially domestic birds such as chickens and turkeys.
Pigs can be infected with both human and avian influenza viruses in addition to swine influenza viruses. Infected pigs get symptom
Symptom
A symptom is a departure from normal function or feeling which is noticed by a patient, indicating the presence of disease or abnormality...
s similar to humans, such as cough, fever, and runny nose. Because pigs are susceptible to avian, human and swine influenza viruses, they potentially may be infected with influenza viruses from different species (e.g., ducks and humans) at the same time. If this happens, it is possible for the genes of these viruses to mix and create a new virus.
For example, if a pig was infected with a human influenza virus and an avian influenza virus at the same time, an antigenic shift could occur, producing a new virus that had most of the genes from the human virus, but a hemagglutinin or neuraminidase from the avian virus. The resulting new virus would likely be able to infect humans and spread from person to person, but it would have surface proteins (hemagglutinin and/or neuraminidase) not previously seen in influenza viruses that infect humans, and therefore to which most people have little or no immune protection. If this new virus causes illness in people and can be transmitted easily from person to person, an influenza pandemic can occur. The most recent 2009 H1N1 outbreak was a result of antigenic shift and reassortment between human, avian, and swine viruses.
See also
- 1918 flu pandemicSpanish fluThe 1918 flu pandemic was an influenza pandemic, and the first of the two pandemics involving H1N1 influenza virus . It was an unusually severe and deadly pandemic that spread across the world. Historical and epidemiological data are inadequate to identify the geographic origin...
- CoinfectionCoinfectionIn parasitology, coinfection is the term used to describe the simultaneous infection of a host by multiple pathogen species. In virology, coinfection can also refer to the simultaneous infection of a single cell by two or more virus particles...
- SuperinfectionSuperinfectionIn virology, superinfection is the process by which a cell, that has previously been infected by one virus, gets coinfected with a different strain of the virus, or another virus at a later point in time. Viral superinfections of serious conditions can lead to resistant strains of the virus, which...
- Viral ShiftViral shiftA viral shift is the sudden emergence of a seemingly novel strain of a virus. It is a prerequisite viral condition for pandemics. An existing virus undergoes a change and becomes a pathogen, such as the constantly mutating "A" strain of influenza in which the change is known as an antigenic shift...