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