Influenza, more commonly known as the flu, is a virus that infects the respiratory system of mammals. Viruses are obligate parasites, meaning that they cannot exist without a host cell. This is because, although viruses have a genome that encodes proteins and enzyme for their survival and function, they cannot replicate on their own, they need the cellular machinery available in a host cell. Viruses encode proteins that facilitate their attachment and entry into host cells, which affects their virulence and lethality. Thus, viral strains are named depending on the proteins encoded by the viral genome as this differentiates between their abilities to infect, sicken, and kill.
Influenza Types
The flu virus occurs in three types: A, B, and C. The type is designated by the host the virus infects and the type of mutations its genome undergoes. Type C influenza virus does not cause pandemics or outbreaks as it is an extremely mild infection in humans. Type B influenza viruses are less mild than type C virus and have, on occasion, caused outbreaks, but the strains only mutate via slow genetic drift, with the new strain replacing the old. Type B influenza usually only infects humans.
Type A viruses are the more virulent strains and currently circulate in the human population. Type A strains also infect birds and swine. These strains undergo both antigenic drift and shift, being a more versatile type of influenza virus from a genetic and host standpoint. These strains are capable of sharing genes with strains that infect other species, making the circulating type A virus variable from year to year and by geography. The type A virus is further subtyped into strains.
Type A Influenza Strains
Two proteins encoded by the viral genome are hemagglutinin (H) and neuraminidase (N). These surface proteins aid in the infection of respiratory cells by the flu virus. The infection of a particular host requires a particular H and N combination because of the expression of surface proteins by respiratory cells that can differ between species, so some type A subtypes usually infect a particular animal. However, numerous strains circulate in the human population and can cross the species barrier, allowing for antigenic shift. An example is the current H1N1 swine flu strain and potentially the current H5N1 avian flu strain.
For more information on type A influenza strains visit the Centers for Disease Control and Prevention website.
Swine, Avian, and Human Flu
As already mentioned, some strains infect a particular host. This is how some influenza viruses become known as swine flu, avian flu, or human flu – more commonly known as seasonal flu. Further designations are given to type A strains based on when they emerge and in which species, because there are other proteins besides H and N. H1N1 and H3N2 are the common seasonal flu strains, but the seasonal H1N1 strain is different from the 2009 H1N1 strain. The seasonal strain contains genes known to be human influenza genes, i.e. infective mainly to humans. However, the 2009 strain also contains swine and avian influenza genes – making it genetically different from the strain to which humans were previously exposed. However, these differences are not specifically noted when naming flu viruses beyond designation of the year, species of emergence, and geographic location of any emergence.
Many of the names given to influenza overlap, but they all designate some particular characteristic of its infectivity on a given level – host, surface protein expression, and/or genetic mutation capability.
Determining Influenza Type
Polymerase chain reaction (PCR) is a standard laboratory technique used to determine whether particular genes and mutations are present in a genome. The specific type of influenza strain in a host is diagnosed based on the PCR results for select influenza genes, including H and N. DNA sequencing is also used to determine new mutations and was used to establish the genetic structure of the influenza genome. According to the International Society for Infectious Diseases ProMED-mail discussions on the 2009 H1N1 pandemic, some faster rapid real-time(rRT) PCR methods are being used but are only 90% accurate and ideally only used in the case of epidemics when faster containment and treatment is needed. However, reliable PCR methods are more widely used, though very few diagnostics are 100% accurate and research on influenza and its strains is ongoing.
Alicia Mae Prater - Alicia received her doctorate in Experimental Pathology in 2007. She has been a freelance writer and scientific editor since 2008.
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