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