VETS2006 Lecture Notes - Lecture 2: Virus Classification, Reassortment, Genetic Recombination
Viral genetics and diagnosis of viral diseases
Viral genetics
1. Mutation
- In viral infections, a few virions gain entry and replicate through any cycles to
produce millions of “progeny” virions.
- In each replication cycle, mutations occur. They may be point mutations
(substitution of a single nucleotide) or deletions or insertions (blocks of
nucleotides added or deleted)
- The “progeny” mutant virus any have advantage over others (e.g. Replicates
fasters or replicates in different cells or hosts or is able to evade the immune
response)
- These advantageous viruses will be selected over the others, leading to
evolution of the viral species.
2. Recombination
- When 2 different viruses simultaneously infect the same cell, genetic
recombination may occur.
- A) Exchange of nucleotide sequences: Between 2 related viruses or between
virus and host cell. (E.g. Fowl pox virus vaccines contain genes from another
avian virus)
- B) In viruses with segmented genomes, genetic reassortment can occur (an
exchange of segments) (E.g reassortments of segments from avian and
human influenza viruses can produce new strains of human influenza.)
Viral Quasispecies
- Mutations of the viral genome occur in every replication cycle, particularly in
RNA viruses where proof-reading mechanisms are not available, so errors
during replication of viral RNA are very common.
- Thus, after replication, the viral population becomes a cluster of viral genetic
variants known as ‘Quasispecies’
- Each Quasispecies variant has a slightly different genetic makeup to its
‘siblings’, but still belongs in the same species.
- There are some sections of the viral genome that remain unchanged
(conserved) amongst all variants because they code for essential proteins.
Variants with mutations in these sections would be unable to replicate. (A
‘lethal mutation.) Lethal to virus or animal?
- The Quasispecies unit is a genetic pool, undergoing continual change
competition and selection.
- The Quasispecies pool may be dominated by one major variant, which has
some kind of selective advantage (eg. Able to replicate a particular cell type,
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replicate more quickly than others, or able to avoid the hosts immune
response)
- The fittest variants are the most predominant, while variants capable of
responding to new environments are less numerous.
- If the host situation changes in some way, genetic variants that are able to
adapt to the new situation will emerge and become the predominant variant.
Quasispecies: An Example
Feline infectious peritonitis
- Cats can harbour a cluster of coronavirus Quasispecies replicating in their
intestinal epithelial cells, kept in check by the immune system.
- If a cat becomes immunosuppressed, a burst of virus replication occurs,
unchecked.
- This leads to emergence of viral variants that can grow in different cell types,
including the white blood cells > Virus invades the body.
- Virus replication throughout body > Inflammation and fluid in abdominal and
thoracic cavities.
Diagnosis of Viral Infections
- An animal suffering from a viral infection at show clinical signs suggestive of a
particular virus.
- However to make a definitive diagnosis, laboratory testing will likely be
required.
- The type of test depends on the nature of the viral disease and the availability
of different laboratory tests for that virus.
- General approaches to diagnosis of viral infections:
- 1. Detecting the virus in tissue or blood samples
- 2. Detecting antibodies in the virus
- 3. Detecting characteristic pathology caused by the virus.
1. Detecting the Virus
- Culture virus
- View virus (How?)
- Detect viral antigen (protein)
- Detect DNA/RNA
- Sample Type: Depends on the disease but is wherever virus is likely to be
(faeces, urine, respiratory secretion, reproductive tract, blood)
- Timing: When disease is occurring
But: Specific tests are not available for all viruses.
2. Detecting the Antibodies
- Test for antibodies using a serological test.
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Document Summary
In viral infections, a few virions gain entry and replicate through any cycles to produce millions of progeny virions. They may be point mutations (substitution of a single nucleotide) or deletions or insertions (blocks of nucleotides added or deleted) The progeny mutant virus any have advantage over others (e. g. replicates fasters or replicates in different cells or hosts or is able to evade the immune response) These advantageous viruses will be selected over the others, leading to evolution of the viral species. When 2 different viruses simultaneously infect the same cell, genetic recombination may occur. A) exchange of nucleotide sequences: between 2 related viruses or between virus and host cell. (e. g. Fowl pox virus vaccines contain genes from another avian virus) B) in viruses with segmented genomes, genetic reassortment can occur (an exchange of segments) (e. g reassortments of segments from avian and human influenza viruses can produce new strains of human influenza. )
