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)


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


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)


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!


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


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


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


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