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Viruses, Prions and ViroidsInfectious 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
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
Structure and Classification of Animal Viruses
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
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
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)
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
Interactions of Animal Viruses with Their Host
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
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
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
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
Other Infectious Agents