Evolution of Influenza
Encyclopedia
The virus causing influenza
is one of the best known pathogens found in various species
. In particular, the virus is found in birds as well as mammal
s including horses, pigs, and humans. The phylogeny, or the evolutionary history of a particular species, is an important component when analyzing the evolution of influenza. Phylogenetic trees are graphical models of the relationships between various species. They can be used to trace the virus back to particular species and show how organisms that look so different may be so closely related.
and genetic drift
.
were mixed. Infecting tissue cultures can demonstrate how pathogenic qualities can evolve for a particular species even though the reassorted virus may be nonpathogenic for another species. A prime example of evolution under natural conditions is the reassortment of two avian influenza strains that were discovered in dead seals back in 1979.
or antigenic drift. Mutation
and selection
for the most advantageous variation of the virus takes place during this form of evolution. Antigenic mutants can evolve quickly due to the high mutation rate in viruses. Influenza antigenic drift happens when two influenza viruses infect on cell. When new ones come out they have a segment from the others genome that could let some but not enough antibodies to bind. Also the receptor could not bind to antibodies. This evolution occurs under the pressure of antibodies or immune system responses.
Analysis can also feature relationships between species. The 1918 Spanish influenza virus demonstrates this. Even though the hemagglutinin
(HA) gene was closer in relation to avian strains than mammalian ones, it was, in fact, mammalian. The gene may have been adapting in humans even prior to 1918. Breaking down the phylogenetic history of the influenza virus shows that there is a common ancestor that reaches back before the 1918 outbreak that links the current human virus to the swine virus. The ancestor was derived from an avian host.
In current years, there has been a huge increase in the amount of resistance to certain drugs, including the antiviral compound adamantine
. In fact, its resistance has recently climbed from 2 percent to nearly 90 percent. These records of built up resistance infer that drugs, such as adamantine, will not be useful against the influenza virus in the future.
Various lineages may continue their presence and reassort indicating the importance of a complete-genome approach to determine new influenza strains and future epidemics. In terms of vaccine strain selection, antigenic clades evolve by reassortment, not by antigenic drift. This was shown in the 2003-2004 influenza outbreak.
Phylogenetic trees can help determine what codons in the HA gene of the influenza A virus have changed in past outbreaks. The more mutations there are in a virus strain, the more likely that strain is to be a generator of a new lineage in future influenza seasons.
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...
is one of the best known pathogens found in various species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
. In particular, the virus is found in birds as well as 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 including horses, pigs, and humans. The phylogeny, or the evolutionary history of a particular species, is an important component when analyzing the evolution of influenza. Phylogenetic trees are graphical models of the relationships between various species. They can be used to trace the virus back to particular species and show how organisms that look so different may be so closely related.
Mechanisms of evolution
Two common mechanisms by which viruses evolve are reassortmentReassortment
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...
and genetic drift
Genetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...
.
Reassortment
Reassortment allows new viruses to evolve under both natural conditions and in artificial cultures. In fact, the 1957 evolution of the H2N2 virus is thought to be a result of reassortment. In this case, human H1N1 strains and avian influenza A genesGênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
were mixed. Infecting tissue cultures can demonstrate how pathogenic qualities can evolve for a particular species even though the reassorted virus may be nonpathogenic for another species. A prime example of evolution under natural conditions is the reassortment of two avian influenza strains that were discovered in dead seals back in 1979.
Drift
New viruses can also emerge by drift. Drift can refer to genetic driftGenetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...
or antigenic drift. 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...
and selection
Selection
In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...
for the most advantageous variation of the virus takes place during this form of evolution. Antigenic mutants can evolve quickly due to the high mutation rate in viruses. Influenza antigenic drift happens when two influenza viruses infect on cell. When new ones come out they have a segment from the others genome that could let some but not enough antibodies to bind. Also the receptor could not bind to antibodies. This evolution occurs under the pressure of antibodies or immune system responses.
Species and barriers
The transmission, or how the influenza virus is passed from one species to another, varies. There are barriers that prevent the flow of the virus between some species ranging from high to low transmission. For example, there is no direct pathway between humans and birds. Pigs however, serve as an open pathway. There is a limited barrier for them to spread the virus. Therefore, pigs act as a donator of the virus relatively easily.Geographic differences
Phylogenetic maps are a graphical representation of the geographic relationships among species. They indicate that the human influenza virus is minimally impacted by geographic differences. However, both swine and avian influenza does appear to be geographically dependent. All three groups (avian, swine, and human) show chronological differences. The human influenza virus is retained in humans only, meaning it does not spread to other species. Some lineages and sublineages of the virus emerge and may be more prevalent in certain locations. For instance, many human influenza outbreaks begin in Southeast Asia.Phylogenetic analysis
Phylogenetic analysis can help determine past viruses and their patterns as well as determining a common ancestor of the virus. Past studies reveal that an avian virus spread to pigs and then to humans approximately 100 years ago. This resulted in human lineages further evolving and becoming more prominent and stable.Analysis can also feature relationships between species. The 1918 Spanish influenza virus demonstrates this. Even though the 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...
(HA) gene was closer in relation to avian strains than mammalian ones, it was, in fact, mammalian. The gene may have been adapting in humans even prior to 1918. Breaking down the phylogenetic history of the influenza virus shows that there is a common ancestor that reaches back before the 1918 outbreak that links the current human virus to the swine virus. The ancestor was derived from an avian host.
Future impact
Looking at the past phylogenetic relationships of the influenza virus can help lead to information regarding treatment resistance, selecting vaccine strains, and future influenza strains.In current years, there has been a huge increase in the amount of resistance to certain drugs, including the antiviral compound adamantine
Adamantine
Adamantine is a mineral, often referred to as adamantine spar. It is a silky brown form of corundum. It has a Mohs rating of 9.Adamantine is also used as an adjective to refer to non-metallic, brilliant light reflecting and transmitting properties, known as adamantine luster...
. In fact, its resistance has recently climbed from 2 percent to nearly 90 percent. These records of built up resistance infer that drugs, such as adamantine, will not be useful against the influenza virus in the future.
Various lineages may continue their presence and reassort indicating the importance of a complete-genome approach to determine new influenza strains and future epidemics. In terms of vaccine strain selection, antigenic clades evolve by reassortment, not by antigenic drift. This was shown in the 2003-2004 influenza outbreak.
Phylogenetic trees can help determine what codons in the HA gene of the influenza A virus have changed in past outbreaks. The more mutations there are in a virus strain, the more likely that strain is to be a generator of a new lineage in future influenza seasons.