Viruses, Prions and Viroids Infectious Agents of Animals and Plants Chapter 14

Structure and 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