Download Chapter 18 Eucaryotic Viruses and Other Acellular Infectious Agents

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Chapter 18
Eucaryotic Viruses and
Other Acellular Infectious
Agents
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Viruses of Fungi and Protists
• Fungal Viruses:
• Most mycoviruses, viruses that infect fungi,
have been isolated from higher fungi such as
Penecillium and Aspergillus.
• They are infected with dsRNA viruses
– most cause latent infections. Some
mycoviruses include disease symptoms in host
such as the mushroom Agricus bisporous, but
cytopathic effects and toxic virus products
have not been observed
– Only few lower fungi infected by dsRNA or ssRNA
viruses
– Destruction and cell lysis
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Protists:
A protist is any organism that is not a plant,
animal or fungus
Protista = the very first
Algal viruses
the 4 genera recognized by ICTV have linear
dsDNA genomes
protozoan viruses
viruses of only 3 genera of protozoa have been
studied
giant dsDNA virus (tentative named a Mimivirus)
found in the amoeba Acanthamoeba polyphaga
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Insect Viruses
• Members of many virus families are known
to infect insects
– Eg. Baculoviridae, Reoviridae, Iridoviridae,
and Polydnaviridae
• Some use insects as agents for spreading to
populations of susceptible animals and
plants
• Yellow fever, West Nile disease, and several types of
encephalitis are the examples of human viral diseases.
• infection often accompanied by formation
of granular or polyhedral inclusion bodies
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Inclusion Bodies
Figure 18.18
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Insect larvae are infected when they feed on leaves
contaminated with inclusion bodies.
Insect viruses can persist in a latent state within the host for
generations while producing no disease symptoms.
Baculoviruses have potential as biological control agents for
insect pests
Received attention due to they attack only
invertebrates and have considerable host specificity
They are encased in protective inclusion bodies, these
viruses have a good shelf life and better viability when
dispersed in the environment
They are well suited for commercial productions
because they often reach extremely high concentrations in
larval tissue.
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Viroids and Virusoids
• viroids
– Smallest known infectious agents
composed only of RNA
• virusoids
– formerly called satellite RNAs
– infectious RNAs that encode one or
more gene products
– require a helper virus for replication
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VIROIDS: The Plant Invaders…
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• A VIROID is a…
• VIR(virus)OID(like) particle.
viroid, an infectious particle smaller
than any of the known viruses
cause plant diseases
They are an extremely small circular
single-standed RNAs, ~250-450
nucleotides long.
lacking the protein coat of a virus.
They do not code for any specific protein but
are able to replicate themselves in the nuclei
of infected cells.
Symptoms: Stunted or distorted growth and
sometimes death.:
Transmission: It can be transmitted by
pollen, seed, or farm implements.
Mechanically transmitted; often seed
transmitted
More than 40 viroid species and many
variants have been characterized
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Few Important Viroid Disease
Potato spindle tuber viroid
Chrsanthemum stunt viroid
Eggplant latent viroid
Tomato chlorotic dwarf viroid
Rice yellow mottle virus satellite
NOTE: Potato spindle tuber viroid was the
first to be identified.
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More about Viroids…
• unable to replicate itself because its
RNA does not encode gene products
– may be replicated by a host DNAdependent RNA polymerase
– RNA synthesis may use a rolling circlelike mechanism
• may cause disease by triggering
RNA silencing
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Virusoids
– Virusoids are similar to viroids in that they
are also covalently closed circular, ssRNA
molecules.
• Virusoids are circular single-stranded RNAs
dependent on plant viruses for replication and
encapsidation.
• The genome of virusoids consist of several
hundred nucleotides and only encodes
structural proteins.
• Virusoids are similar to viroids in size,
structure and means of replication (rollingcircle replication)
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Virusoids, while being studied in virology, are
not considered as viruses but as subviral
particles. Since they depend on helper viruses,
they are classified as satellites.
In the virological taxonomy they appear as
Satellites/Satellite nucleic acids/Subgroup 3:
Circular satellite RNAs.
The term virusoid is also sometimes used more
generally to refer to all satellites.
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Prions
Proteinaceous Infectious Particle
They contain no nucleic acid.
• examples of neurodegenerative
diseases in humans and animals
caused by prions
– Scrapie disease in sheep
– bovine spongiform encephalopathy
(BSE) or mad cow disease
– Creutzfeldt-Jakob disease (CJD) and
varient CJD (vCJD)
– kuru (Human Disease)
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What about Mad Cow Disease?
• prions cause bovine spongiform
encephalopathy (BSE or mad cow
disease)
• epidemic proportions in England in 1990s
• initially spread because cows were fed
meal made from all parts (including
brain tissue) of infected cattle
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Transmission
Spread of the disease is via horizontal
transmission, i.e., transmission from one
person to another, either directly or by
fomites or by ingestion of contaminated
meat.
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Variant Creutzfeldt-Jakob
(vCJD) v. CJD
• difference in diseases is origin
– eating meat from BSE infected cattle can
cause variant Creutzfeldt-Jakob (vCJD) in
humans
– CJD is caused by spontaneous mutation of
the gene that codes the prion protein
• all prion caused diseases
– have no effective treatment
– result in progressive degeneration of the
brain and eventual death
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The End
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