Chapter 14 Viruses, Prions, and Viroids:
Infectious Agents of Animals and Plants
Animal Virus Classification
l Reflects ______relationships
l Inherently difficult, changing
l Based mainly on
n ______structure
n Virus ______structure
n Presence or absence of an ______
Families of Viruses
l Members of families share common ______
n Relationship between families more complex
l Families further subdivided into genera
l 14 RNA-virus families infect vertebrates
l 7 DNA-containing families infect vertebrates
l Family name ends in “-viridae”
Animal Virus Classification and Taxonomy
l Family “-______”
n Named for appearance – Coronaviridae
n Named for place found – Bunyaviridae
l Genus“-______”
n All one word - Enterovirus
l Species
n Named for disease caused
l Poliovirus à polio
n Named for place found
l Marburg; Ebola
l Types
n Akin to subspecies, strains
n Some types should be separate species
l ______Binomial nomenclature
Informal Groupings of Animal Viruses
l Non-evolutionary groupings
l Based on ______between individuals
n Enteric viruses
n Respiratory viruses
n Zoonoses
n Sexually transmitted viruses
Enteric Viruses
l Ingested on ______-contaminated material
n “Fecal-oral route”
l Often cause gastroenteritis
n Inflammation of stomach and intestine
l Some cause systemic disease rather than gastroenteritis
n e.g., poliovirus
Respiratory Viruses
l Inhaled, ______in respiratory tract
l Remain ______in respiratory tract
l e.g., rhinovirus
n Not included
l Inhaled viruses causing systemic diseases
l (mumps virus, measles virus)
Zoonoses
l Transmitted from one animal species to another (including humans)
n e.g., rabies
l bat à Old Yeller à humans (humans cannot transmit rabies to other humans)
n e.g., canine distemper
l dogs à lions
n e.g., arboviruses Arthropod borne viruses
l Infect arthropods, replicate, transmitted to vertebrates
l e.g., West Nile Virus
Sexually Transmitted Viruses
l Transmitted during ______activity
l Many cause lesions in genital tract
n e.g., herpesviruses, papillomaviruses
l Some cause ______infections
n e.g., HIV, hepatitis viruses
Methods of Studying Viruses
l Cultivation of host cells
n Viruses multiply only ______host cells
l Living animals
l Embryonated chicken eggs
l Cell culture (tissue culture)
n Limited life span
n Immortal cells from tumor
l Quantitation
n Determining the number of virions present
Viral Quantitation
l Determining numbers
n Plaque assay
n Electron microscopic counting
n Quantal assays
n Hemagglutination
______Assay
l Used for virus that ______host cell
l Known volume of virus-containing solution added to tissue culture cells
l Each plaque represents one virion – Plaque Forming Units (PFUs)
n Similar to CFUs in bacteria
Viral Quantitation
l Electron microscopic ______
n Can often distinguish between infective and non-infective virions
n Helpful in identifying type of virus
l Quantal assays
n Several ______administered to cells
n ______= dilution at which 50% of host cells are infected or killed
l ID50 (infective dose)
l LD50 (lethal dose)
Hemagglutination
l Some viruses agglutinate (clump) red blood cells
n e.g., influenza virus
l Serial ______of virus added to RBCs
l Highest dilution showing maximal hemagglutination determined
Host-Virus Interactions
l Bacteriophage host organism is single cell
n Possess rudimentary defense mechanisms
l Animal virus host organism is multicellular
n Possesses various defense mechanisms
n Host can develop ______
Virus – Host Coevolution
l Host à more resistant
l Virus à less pathogenic
l “______”
n “Normal” host is often asymptomatic
n Disease results when transmitted to a susceptible host
l e.g., measles & smallpox in New World indigenous populations
Acute Infections
l Short duration
l Host may develop immunity
l Life cycle similar to that of virulent phage
n Attachment
n Entry
n Targeting
n Uncoating
n Replication
n Maturation
n Release
n Shedding
n Transmission
Characteristics of Acute Infections
l ______
n Similar to bacteriophages
l ______
n Entire virion enters cell
n Naked viruses by endocytosis
n Enveloped viruses by either endocytosis or membrane fusion
Characteristics of Acute Infections
l Targeting and uncoating
n ______
l Virion targeted to site where it will multiply
l e.g., most DNA viruses multiply in nucleus
n ______
l Nucleic acid separates from protein coat
Replication and Protein Production in Acute Infections
l Production of proteins
n Utilizes ______cell ______and other machinery
l Replication of nucleic acid
n Genetic material varies between families
n Methods of nucleic acid replication variable
n Often involves viral enzymes
Acute Infections
l Maturation
n ______of virions
n Multistep process
l Release
n May involve lysis or ______
l ______
n Virions exit host
n Generally use same opening or surfaces used to gain entry
l ______
n Transmitted to new host
Comparison of Replication Cycles
Persistent Infections
l Viruses continually present in the body
l Released by ______
l May or may not cause disease
n Can be transmitted to others
l Four categories
n Late complications following acute infection
n Latent infections
n Chronic infections
n Slow infections
n (Some overlap in these categories)
Persistent Infections
l Latent infections
n Acute infection à ______period à ______
n Infectious virions undetectable until reactivation
n Initial vs. reactivated symptoms may differ
n e.g., herpesviruses (HSV-1, HSV-II, varicella)
n Sometimes involves integration into host DNA
Persistent Infections
l Chronic infections
n Infectious virus ______at ______times
n Disease may be either present or absent over extended period of time
n e.g., Hepatitis B virus
l Slow infections
n Amount of virus gradually ______over long periods of time
n Asymptomatic over long periods of time
n e.g., lentiviruses, retroviruses (e.g. HIV)
Complex Infections
l Do not fit other categories well
l e.g. Retroviruses
n Single-stranded RNA viruses
n Some cause tumors or leukemia
n Best known is HIV
l HIV preferentially destroys helper T lymphocytes
n Cd4+ protein on surface
n Need T cells to fight off other diseases
n Result is immune deficiency - AIDS
HIV – a Retrovirus
l ______– backwards
l ______genome (HIV has two strands)
l RNA à DNA à RNA à protein
n Uses reverse transcriptase to go RNA à DNA
l Then makes dsDNA and integrates into host chromosome: now called provirus
HIV – Entry and Uncoating
l HIV virion binds to cell with ______
n Why are receptors there?
l Loses envelope to fusion with host membrane
l Naked nucleocapsid enters
l Uncoating to release ssRNA genome
HIV – Replication and Gene Expression
l ssRNA à ssDNA à dsDNA
l dsDNA ______into host chromosome
l dsDNA à ssRNA à more virions
l dsDNA à ssRNA message à long polypeptide
l à cut with ______to make smaller proteins
Reverse ______
l Critical to understanding HIV
l Copies info on RNA à DNA
l No ______à many mistakes
l Mistakes à ______à changes in proteins
l Host makes antibodies to coat proteins
l Coat proteins change before host can make enough antibodies
l Rapid ______– even faster than bacteria!
Tumors
l Tumors result from abnormal growth of cells
n “______cells”
n Most NOT caused by viruses
l Control of cell division / differentiation altered
n Tumor suppressor gene à inactivated form
n Proto-oncogene à mutation à oncogene
l Types of tumors
n Benign tumor remains in defined area
n Malignant tumor (cancer)
l Isolated
l Metastasizing – spread to other parts of body
l ______sometimes carried on viruses
Effects of Viruses on Animal Cell Hosts
Viral Host Range
l Includes all cell ______able to be infected
n e.g., species, strains, cell types within a species
n These cells possess receptors recognized by virus
l Host range can be altered
n Phenotypic mixing
n Genetic reassortment
______Mixing
l Animal cells sometimes simultaneously infected by two different viruses
n Host ranges overlap, but differ
l Viral genetic material and viral capsids ______
l Host range temporarily altered
n Can facilitate interspecies gene transfer
Genetic ______in Segmented Genomes
l Some viruses possess segmented genomes
n e.g., influenza virus
l Many strains of these viruses exist
n Host ranges overlap, but differ
l Two different strains can infect a single cell
n e.g., bird and human viruses can both infect pigs
l RNA segments mixed and matched
n “______”
n New strain avoids immunity already in place
Plant Viruses
l Many plant diseases are caused by viruses
n Especially prevalent in ______plants
n Yield can be severely reduced
n Without plants (food), we are dead
l Viruses occasionally produce desirable effects
n e.g., color variegation in tulips
n Eventually plant becomes excessively weak
Plant Viruses
l Enter via ______sites, not via receptors
l Source of virus
n Soil
n Vectors (insect, human, etc.)
n Tobacco
l Virus-resistant crops genetically engineered
Dodder, a parasitic plant that may vector viruses between its host plants
Effects of Plant Viruses
Viroids
l Infectious ______-stranded ______
n 246 – 375 nucleotides long
n ~10% of size of smallest known RNA virus
n Replicate autonomously
n ______, resistant to digestion by nucleases
n Cause disease
l Mechanism unknown
n All identified viroids infect plants
Prions
l Proteinaceous infectious agents
l ______only, no nucleic acid
l Linked to diseases of humans and other animals
n e.g., mad cow disease, kuru, scrapie, etc.
n “transmissible spongiform encephalopathies”
n Slow, always fatal
l Prion converts normal host protein into prion