Transcript Viruses, Prions and Viroids Infectious Agents of Animals

Viruses, Prions and Viroids
Infectious Agents of Animals
and Plants
Chapter 14
Structure and Classification of Animal
Viruses
 Classification of animal viruses

Taxonomic criteria based on

Genomic structure
 DNA or RNA
 Single stranded or double stranded

Virus particle structure
 Isometric
 Pleomorphic
 Helical

Presence or absence of envelope
Structure and Classification of
Animal Viruses
 Groupings based on route of transmission
 Disease causing viruses often grouped by route of
transmission
 Enteric viruses
 Generally transmitted via fecal-oral route
 Often cause gastroenteritis
 Some can cause systemic disease



Respiratory viruses
 Usually inhaled via infected respiratory droplets
 Generally remain localized in respiratory tract
Zoonotic viruses
 Transmitted from animal to human via animal vector
Sexually transmitted viruses
 Can causes lesions on genitalia or cause systemic
infections
Interactions of Animal Viruses
with Their Host
 Outcome of infection of eukaryotic cells depends on
factors independent of cell

Special importance are defense mechanisms of host
 Viruses may develop relationships with normal hosts
 No obvious disease or damage is caused to host
 State of balanced pathogenicity
 Relationships divided into two categories
 Acute
 persistent
Interactions of Animal Viruses
with Their Host
 Acute infections



Usually short in duration
Host may develop long lasting immunity
Result in productive infections


Produce large number of viruses during
replication
Disease symptoms result from tissue damage
and infection of new cells
Interactions of Animal Viruses
with Their Host
 Acute infections


Reproductive cycle of animal virus can
be compared to virulent bacteriophage
Essential steps include




Attachment
Entry into susceptible cell
Targeting site of reproduction
Uncoating of virion
 Removing protein coat exposing nucleic
acid






Replication nucleic acid and protein
Maturation
Cell lysis
Spreading within host
Shedding outside host
Transmission to next host
Interactions of Animal Viruses
with Their Host
 Persistent infections
 Viruses continually present in
host
 Releases from infected cell
via budding
 Can be divided into three
categories
 Latent infections
 Chronic infections
 Slow infections
 Categories distinguished by
ability to detect the virus
during period of persistence
Interactions of Animal Viruses
with Their Host
 Persistent infections
 Latent infections (presence of virus not always detectable)
 Infection is followed by symptomless period then
reactivation
 Infectious particles not detected until reactivation
 Symptoms of reactivation and initial disease may differ
 Example
 Herpes simplex viruses 1 and 2 (HSV1 and HSV2)
 Shingles (zoster)
Interactions of Animal Viruses
with Their Host
 Chronic infections



Infectious virus can be detected at all times
Disease may be present or absent during
extended times or may develop late
Best know example

Hepatitis B
 A.k.a serum hepatitis
Interactions of Animal Viruses
with Their Host
 Slow infections

Infectious agent gradually increases in amount
over long period of time


No significant symptoms apparent during this
time
Two groups of infectious agents cause slow
infections

Retroviruses which includes HIV & Prions
Viruses and Human Tumors
 Double stranded DNA viruses responsible for
most virus induced tumors in humans

Tumor viruses interact with host cells on one of
two ways



Virus can go through productive cycle and lyse cell
Virus can transform cell without killing it
Cancers caused by DNA viruses result from
integration of viral genome into host DNA

Transformed genes are expressed
 Uncontrolled growth results
Viral Genetic Alterations
 Genome exchange in segmented
viruses


Viruses can alter properties via
 Mutation
 Genetic reassortment
Genetic reassortment of viruses results
from two viruses infecting the same cell
 Each virus incorporates segments of
viral DNA
 One segment comes from one virion
 Rest of segment come from other virion

Reassortment responsible fro antigenic
shift and antigenic drift in influenza
virus
Methods of Studying Viruses
 First: Cultivate a host

Viruses multiply only inside host cell


Viruses are obligate intercellular parasites
Host cells are cultivated in the laboratory in
cell culture or tissue culture

Tissue culture prepared directly from an animal
host is termed primary culture
Methods of Studying Viruses
 Quantitation
 Plaque assay
 Determines number of viruses in
solution
 Know volume of solution added to
actively metabolizing cells
 Infection lyses cells and leads to
clear zone or plaque surrounded
by uninfected cells

Each plaque represents one
virion
 Plaques are only produced by
infected cells
Methods of Studying Viruses
 Quantitation
 Counting virions with
electron microscope
 Used with pure
preparations
 Concentration determined
by counting number of
virions in sample
 May distinguish infective
from non-infective agents
Methods of Studying Viruses
 Quantitation

Quantal assays


Provides and approximate concentration
Dilutions of virus preparation administered into
animal cells
 Chick embryos often used

Endpoint is dilution at which 50% of inoculated
host are infected or killed
 May be reported as either
 ID50 = infective dose
 LD50 = lethal dose
Methods of Studying Viruses
 Hemagglutination
 Some animal viruses clump or
agglutinate with red blood cells
 Termed hemagglutination
 The highest dilution showing
maximum agglutination is titer of
the virus
 i.e. Adding more virus does not
increase the agglutingation
Plant Viruses
 Number of plant diseases
are caused by viruses
 Can be of major
economic importance
 Infection may be recognized
via outward signs including
 Pigment loss
 Marks on leafs and fruit
 Tumors
 Stunted growth
 Plants generally do not
recover from viral infections
Plant Viruses
 Spread of plant viruses

Viruses infect plants through
wound in plant cell wall





Viruses do not attach to specific
cell receptors
Once started, infection spreads
from cell to cell through
plasmodesmata
Many viruses resistant to
inactivation
Viruses can be transmitted
through soil contaminated by prior
growth
Viruses spread through grafting
healthy plants to infected plants


Viruses can spread via
parasitic vine called dodder
Vine establishes
simultaneous connection
between two plants

Serves as conduit of
transfer
Other Infectious Agents
 Prions


Proteinaceous infectious agent
Linked to a number of fatal human diseases

All afflictions cause brain degeneration
 Brain tissue develops sponge like holes
 Disease termed transmissible spongiform encephalopathies

Symptoms may not appear for years after infection
Other Infectious Agents
 Prions
 Apparently arose following gene
encoding normal prion protein
 Mutation caused protein to have
different folding properties
 Mutated protein resistant to proteases
 Normal protein sensitive
 Resists UV light and nucleases
 Due to lack of nucleic acid
 Inactivated by chemicals that denature
proteins
Other Infectious Agents
 Viroids




Defines group of pathogens much smaller and
distinctly different from viruses
Consist solely of small single-stranded RNA
molecule
Varies in size
Have no protein coat

Allows them to be resistant to proteases
Other Infectious Agents
 Viroids
 Other viroid properties include




Replicates autonomously in susceptible cells
Single viroid capable of infecting a cell
Viroid RNA is circular and resistant to nuclease
digestion
All identified viroids infect plants

Diseases include
 Potato spindle tuber
 Chrysanthemum stunt
 Cadang-cadang