Virions, Prions, and Viroids:
Infectious Agents of Animals
and Plants
Topics
Structure
Classification
Methods of study
Viral infections
Viral tumors
Human tumors
Viral host range
Prions
Structure
Analogous to bacteriophages
• Capsid (protein coat) made up of capsomeres
• Naked vs. enveloped viruses
• Attachment proteins or spikes in enveloped viruses
Distinct from bacteriophages
• Segmented viruses (contain more than one RNA
molecule)
• Plant viruses do not enter via receptor attachment
Shapes:
Isometric
Helical
Pleomorphic
Classification
Taxonomy in constant flux
Most common taxonomic criteria for animal viruses:
1. Genome structure (DNA or RNA, ss vs. ds,
segmented or not)
2. Virus particle structure (isometric, helical,
pleomorphic)
3. Presence or absence of viral envelope
Viruses infecting vertebrates are divided into 14 RNA
families and 7 DNA families (Ending: -viridae)
Coronavirus
• Classification:
– Coronaviridae (Family)
– Coronavirus (Genus)
– Common cold virus (Species) (together
with rhinoviruses)
• Structure:
– non-seg., lin., ssRNA, helical, env.
Herpesvirus
• Classification:
– Herpesviridae (Family)
– Herpesvirus (Genus)
– Herpes simplex type 1 / type 2 (Species)
• Structure:
– non-seg., lin., dsDNA, helical, env.
http://darwin.bio.uci.edu/~faculty/wagner/movieindex.html
Cells Infected with a Herpes Virus
Groupings Based on Routes of
Transmission
Not a taxonomic grouping – more than one family may be
included in one transmission grouping
Methods of Study
Much more expensive and difficult
to study animal viruses than
bacteriophages
• Cultivation in host cells
– Living animal
– Embryonated chicken eggs
– Cell or tissue culture (= in
vitro)
Methods of Study cont: Quantitation
• Plaque assay (useful for infective and lytic viruses)
• Virion counting with EM
• Quantal assay
(ID50 or LD50)
• Hemagglutination
(e.g.influenza virus)
Acute Viral Infections
Productive infection of relative short duration
Naked viruses usually cause cell lysis, while
enveloped viruses do not
Symptoms due to localized or widespread tissue
damage
Host defense mechanisms gradually eliminate virus
Examples of acute infections
Time course
Mumps, Measles, Influenza, and
Poliomyelitis
Time Course of Acute Viral Infection
HEV
Reproductive Cycle
1. Attachment
2. Entry
3. Uncoating
4. Replication of NA and
protein
5. Maturation of viral particles
6. Cell lysis
7. Spreading and shedding
8. Transmission to next host
Transcription Strategy
Mechanisms of Release
1. Cytopathic effect: Unlike virulent
phages most animal viruses do not
encode for cell lytic enzymes.
Instead degenerative changes
associated with the virus lead to
cell death.
2. Budding: from plasma membrane
(most common) or from Golgi
apparatus. May or may not kill cell.
Enveloped viruses
Persistent Viral Infections
Virus continually present in body. Released by
budding.
May or may not cause disease
Carrier able to spread disease
Four categories (if more than one applies = complex
infections):
1. Late complications from acute infections
2. Latent infections
3. Chronic infections
4. Slow infections
Late Complications from Acute
Infections
1. Subactue sclerosing panencephalitis (SSPE) years following measles in 1 in 300,000. Virus
persistent in brain tissue with decreased
transcription of viral genome. Lack of envelope
protein production prevents immune response to
eliminate infected cells. High Ab, low CMI. Exact
mechanism unknown.
2. Progressive (pan)encephalitis following rubella
Latent Infections
• Acute infection  symptomless period 
reactivation of disease
• Symptoms of reactivation may differ from
original disease
• No measurable viral particles during
symptomless period
• Examples: HSV-1 and HSV-2; varicella
Chickenpox
- Varicella
Blister-like rash on surface
of skin and mucous
membranes. Blisters
usually appear first on
trunk and face, then
spread to almost
everywhere else.
Shingles or
Herpes Zoster
About 20 % of those people
who have had chicken pox
will get zoster at some time
during their lives. Most
people will get zoster only
once.
“Shingles” comes from
latin cingulum, which
means girdle or belt.
It occurs in an area of
the skin that is supplied
by the sensory fibers of
a single nervedermatome. Rash
appears as well-defined
band on one side of
body, or on one side of
face, arms or legs.
Chronic Infections
• Virus can be demonstrated at all times
• Disease may or may not be present for
extended time periods or show up late
(carriers!)
Slow Infections
• Gradual increase of infectious agent over long
time period – often no apparent symptoms for long
time (=preclinical phase)
• Usually slowly progressive lethal diseases
• Examples
– AIDS
– Lentivirus
– prions
HIV: Example of Complex Infection
• Retrovirus – ssRNA, envelope
• RNA  DNA (with the help of reverse
transcriptase)  permanent integration into host
genome (=provirus)
• Polyprotein is cleaved into individual proteins with
viral protease  assembly of virions  budding
Viral Tumors (Neoplasms)
• Benign
• Malignant  cancer, metastasizes
• Proto-oncogenes and oncogenes are regulatory
genes
• Properties of normal and transformed cells
• Only about 15% of human tumors are due to viruses
• Examples of human tumors:
– Kaposi’s sarcoma (herpes virus)
– Squamous cell carcinomas (HPV)
– Hepatocellular carcinoma (HBV and HCV)
Kaposi’s Sarcoma
Purplish lesions of a
skin cancer not usually
seen in young men
Viral Host Range
• Mostly species – and even cell type – specific
• Exception: Zoonotic viruses are transmissible
from animals (arthropods, vertebrates) to man
(zoonosis)
– Arboviruses (West Nile virus), rabies etc.
• Modification of host range due to
– Phenotypic mixing
– Genetic reassortment
Genetic Reassortent
• In segmented viruses
• Simultaneous infection of one
cell with 2 different types of
viruses leads to exchange of
genetic information
• Creation of major new influenza
strains  resulting in pandemics
• Antigenic shift vs. antigenic drift
Other Infectious Agents: Prions
• Small proteinaceous infectious particles (resist
inactivation by procedures that modify nucleic acids)
• Prion diseases are often called spongiform
encephalopathies because of the post mortem
appearance of the brain with large vacuoles in the
cortex and cerebellum
• Human prion diseases
– CJD: Creutzfeld-Jacob Disease
– BSE: Mad cow disease (BSE)
– GSS: Gerstmann-Straussler-Scheinker syndrome
– Kuru