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REASSORTMENT OF INFLUENZA
VIRUS
DEFINITION
• Changing
of
genetic
material
especially genome of one virus with
other is known as reassortment
INTRODUCTION
• Mutation is an important source of RNA virus
diversity that is made possible by the errorprone nature of RNA synthesis.
• Viruses with segmented genomes, such as
influenza virus, have another mechanism for
generating diversity: reassortment.
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EXPLANATION
• Influenza virus infects a cell ,RNA segments enter the nucleus.
• The new RNA segments, are incorporated into new virus particles which
bud from the cell.
• If a cell is infected with two different influenza viruses, the RNAs of both
viruses are copied in the nucleus.
• When new virus particles are assembled at the plasma membrane, each of
the 8 RNA segments may originate from either infecting virus.
• The progeny that inherit RNAs from both parents are called reassortants.
• Reassortment can only occur between influenza viruses of the same type.
Why influenza A viruses never exchange RNA segments with influenza B or
C viruses is not understood.
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Antigenic drift
Antigenic drift is the term used to indicate minor
antigenic variations in HA and NA of the influenza virus
from the original parent virus.
• Antigenic drift (minor changes) occurs due to accumulation of point
mutations in the gene which results in changes in the amino acids in the
proteins. Sequence changes, occurring this way can alter the antigenic
site on the molecule. It is understood that a variant must sustain two or
more mutations before a significantly new (epidemiologically significant)
variant strain emerges.
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ANTIGENIC SHIFT
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Major changes in HA and NA which make the new virions significantly
different, are called Antigenic shift. The difference between the two
phenomena is a matter of degree.
•Changes which are extreme, and drastic (too drastic to be
explained by mutation alone) result in antigenic shift of the virus.
It has been seen that the segmented genomes of the influenza
viruses reassort readily in double infected cells. Genetic
reassortment between human and non-human influenza virus has
been suggested as a mechanism for antigenic shift. This bears
reference to the fact that Influenza B and C viruses have not
been seen to exhibit antigenic shift (because few related viruses
exist in animals
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DIFFERENCE BETWEEN ANTIGENIC
DRIFT AND ANTIGENIC SHIFT
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Antigenic Drift
Antigenic ‘drift’ occurs in HA and NA, and is associated with seasonal epidemics.
Each year’s flu vaccine contains three flu strains -- two A strains and one B strain -- that
can change from year to year.
After vaccination, your body produces infection-fighting antibodies against the three flu
strains in the vaccine.
If you are exposed to any of the three flu strains during the flu season, the antibodies will
latch onto the virus’s HA antigens, preventing the flu virus from attaching to healthy cells
and infecting them.
Influenza virus genes, made of RNA, are more prone to mutations than genes made of
DNA.
If the HA gene changes, so can the antigen that it encodes, causing it to change shape
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.”
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
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Antigenic Shift
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Antigenic ‘shift’ occurs in HA and NA and is associated with pandemics.
The genetic change that enables a flu strain to jump from one animal species to another,
including humans, is called antigenic shift. Antigenic shift can happen in three 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 2
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
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Antigenic Drift
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Antigenic Shift
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SWINE FLU
•. A human (or bird) influenza virus can infect a pig respiratory cell at the
same time as a swine influenza virus; some of the replicating RNA strands
from the human virus can get mistakenly enclosed inside the enveloped
swine influenza virus
•The total number of RNA types in one cell would be 16; four swine and
four human flu RNA segments could be incorporated into one particle,
making a viable eight RNA segmented flu virus from the 16 available
segment types
•.. Various combinations of RNA segments can result in a new subtype of
virus (known as antigenic shift) that may have the ability to preferentially
infect humans but still show characteristics unique to the swine influenza
virus
•). It is even possible to include RNA strands from birds, swine, and human
influenza viruses into one virus if a cell becomes infected with all three
types of influenza (for example, two bird flu, three swine flu, and three
human flu RNA segments to produce a viable eight-segment new type of
flu
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5/10/2017
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5/10/2017
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