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Aina Meducci 2012


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Virology: Antigenic shift vs antigenic drift

Up until this moment, I'm still confuse with antigenic shift and drift. Ag shift caused by 2 virus combine together and form a new virus while Ag drift cause antigenic matter at the surface at the virus change due to mutation. Hmm..but that's not all!


Let's recap virus structure

*In this topic I will only discuss the mutation of virus associated with influenza

Simple structure of RNA virus (influenza)

Influenza viruses have eight RNA genes, and there are two types of spikelike protein on the surface of virus particles. Because of their very simple structure, influenza viruses cannot self-replicate. Instead, they invade human host cells and use the replication system of these cells skillfully.

The two types of spikelike protein on the virus surface are hemagglutinin (HA) and neuraminidase (NA). HA binds to glycoproteins on the surface of cells. Via this binding structure, viruses invade the cells (referred to as infection). On the other hand, NA plays a role in the separation/release of progeny viruses from the cells. The amino acids that constitute these two types of protein easily vary (referred to as mutation); 16 types of HA and 9 types of NA have been found thus far.

Thus, there are as many types of influenza virus as there are combinations of these proteins (H1N1 - H16N9). For example, the most fundamental virus infectious for humans is H1N1. The avian influenza virus, the pandemic of which has recently been of concern, is H5N1. The pathogenicity also differs among different types of influenza virus. For example, the Spanish flu that spread in 1918 and the Hong Kong flu detected in 1968 were both highly virulent influenza viruses that claimed many lives.

Antigenic shift (occurs in influenza and rotavirus-forming new virus)

Antigenic shift is a very large and sudden mutation (changing of many nucleotides/many genes), that happens all at once (occasionally). Antigenic shift happens when two different types of virus infect the same cell. The viruses’ RNA get mixed up, forming new viruses that have major changes in their genome.

The genetic change that enables a flu strain to jump from one animal species to another, including humans, is called antigenic shift.

antigenic shift

Antigenic shift:When two viruses infect the same cell at the same time, they could exchange RNA segments, making a new type of virus.

Antigenic shift can occur in 3 ways;

Antigenic Shift 1

  • A duck or other aquatic bird passes a bird strain of influenza A to an intermediate host such as a chicken or pig.
  • A person passes a human strain of influenza A to the same chicken or pig.When the viruses infect the same cell, the genes from the bird strain mix with genes from the human strain to yield a new strain.
  • The new strain can spread from the intermediate host to humans.

Antigenic Shift

  • Without undergoing genetic change, a bird strain of influenza A can jump directly from a duck or other aquatic bird to humans.

Antigenic Shift 3

  • Without undergoing genetic change, a bird strain of influenza A can jump directly from a duck or other aquatic bird to an intermediate animal host and then to humans.

**The new strain may further evolve to spread from person to person. If so, a flu pandemic could arise.In contrast to drift, pandemic viruses arise through a process known as antigenic shift. In this process, the surface existing viral H and N proteins are not modified, but are replaced by significantly different H and Ns. Since influenza A viruses that bear new (or novel) H or H/N combinations are perceived by immune systems as new, most people do not have pre-existing antibody protection to these novel viruses. This is one of the reasons that pandemic viruses can have such severe impact on the health of populations.

Antigenic shift in influenza H1N1

Timeline of antigenic shift

Antigenic drift (forming new strain)

Antigenic drift refers to the random, spontaneous mutations in RNA viruses that happen over time. These mutations are small, usually the alteration of one nucleotide at a time. Viruses will gradually accumulate more and more mutations in their RNA, eventually causing them to become new strains.

If the HA antigen changes shape, antibodies that normally would match up to it no longer can, allowing the newly mutated virus to infect the body's cells. This type of genetic mutation is called "antigenic drift."

antigenic drift

Antigenic drift: Virus RNA naturally mutates (changes) over time. The mutations in the RNA cause the virus to be altered slightly.
RNA mutations can cause changes such as altered infectious ability, and altered structure

DRIFT: a continual, gradual change in influenza A and influenza B virus ANTIGENS. It occurs as small changes results of point mutation in HA (hemagglutinin) and NA (neuromidase) that cause changes in surface protein. Produces new strains that antibodies to previous infection may not recognize. It is the rational for global surveillance to select strain for annual vaccines.

Influenza viruses can change through antigenic drift, which is a process in which mutations to the virus genome produce changes in the viral H or N. Drift is a continuous ongoing process that results in the emergence of new strain variants. The amount of change can be subtle or dramatic, but eventually one of the new variant strains becomes dominant, usually for a few years, until a new variant emerges and replaces it.

In essence, drift affects the influenza viruses that are already in worldwide circulation. This process allows influenza viruses to change and re-infect people repeatedly through their lifetime and is the reason the influenza virus strains

**that's explain why we get flu every year (antigenic drift).


Differences of ag shift and ag drift (summary)

Relationship between Ag shift and Ag drift

Difficulties in treating influenza

Current measures against influenza are prevention by vaccination and treatment with antiviral drugs such as Tamiflu®. Vaccines are prepared by predicting the type of influenza that is likely to spread each year. When a vaccinated person is infected with a virus, an immune reaction occurs immediately, thus preventing the virus from replicating in the body. However, vaccination has a disadvantage in that the vaccine is only effective against the corresponding type of influenza virus.

Tamiflu® is a drug that inhibits the release of viruses from infected cells (destroy NA). Unless a patient is administered Tamiflu® within 48 hours of infection, it has no effect on stopping the release of viruses. At any rate, there is no radical treatment for influenza, and the speed of recovery depends on the stamina of patients. Therefore, influenza is a threat for children and the elderly, who tend to have low stamina.

Summary of antigenic shift and antigenic drift

Antigenic shift

  • Forming new virus (virus A + virus B --> new virus)
  • Occurs once in a time
  • Large change in nucleotides of RNA
  • May affect lives
  • Difficult to treat (need new vaccine)
  • May jump from one species to another (animal-human)

Antigenic drift

  • Forming new strain of virus
  • Only one virus involve (Virus A* ----> Virus A#)
  • Occurs frequently
  • Small mutation of RNA (point mutation)
  • Changes in HA and NA proteins
  • Easy to treat (antibody and drugs available)
  • May infect animals of the same species

Sources: Aiming at development of new drugs against influenza virus; spring8.or.jp, Microblog; antigenic shift vs antigenic drift, mutation of avian flu virus; oracle think quest.org

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