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VIRUSES
Describe some of the
characteristics of
viruses.
•
•
•
•
Tiny, non-living particles
Do not respire, move & grow
Cannot reproduce on their own
Must reproduce inside of a host
cell
• Typical infection lasts 7-10 days
• Not given Latin names: named for
disease caused, organ infected,
or region detected in
Viral structure
• All viruses consist of 2 basic
parts:
– nucleic acid
– protein coat
• Capsid: protein coat containing
inner core of DNA or RNA
• May have tail fibers and
projections to aid in attachment
• Some have extra outer layer
called envelope
Animation: http://www.johnkyrk.com/virus.html
Viral recognition and attachment
• Virus must recognize specific
receptor site on host cell
• Lands on host and “locks in”
• Attachment is specific: virus can
enter and reproduce in only a few
types of cells
• Ex: tobacco mosaic virus cannot
affect human cells even though we
are exposed to it daily
Describe viral reproduction.
• Viruses must reproduce inside of a host
cell
• They are classified as intracellular
parasites
• The method of entry into the host cell
depends on the virus
• There are two main types of viral
reproduction:
– 1. Lytic cycle
– 2. Lysogenic cycle
Lytic Cycle
• Lytic cycle: viral DNA
takes over host
nucleus, produces
new viruses, the cell
fills with new viruses,
at the end the cell
bursts and the cell
dies
Steps of the Lytic Cycle
1. ATTACHMENT:
virus attaches to
the cell surface
2. ENTRY:
injects DNA,
takes over host
nucleus
3. REPLICATION:
reproduces virus
parts
4. ASSEMBLY:
puts virus parts
together into mature
viruses
5. RELEASE/LYSES:
cell splits open
releasing viruses
that can infect
others
Describe the lysogenic cycle.
•
•
•
•
•
virus attacks cells, but
does not kill immediately
Viral DNA is mixed with
host DNA like lytic cycle
Does not immediately
begin to reproduce, host
may function normally
while virus stays dormant
Provirus can pop out at
any time and begin
reproducing
Example: Herpes (cold
sores, fever blisters)
Lytic Cycle
Viral Multiplication - Basic
Steps
• Virus attaches to host cell
• Whole virus or genetic material enters host
• Viral DNA or RNA directs host to make viral
genetic material and protein
• Viral nucleic acids and proteins are
assembled
• New viral particles are released from cell
Lysogenic
Pathway
Stimulus may cause cell
to enter lytic pathway
Latent period
extends the
cycle
Viral DNA
becomes
part of host
chromosome
for a time
Viral material
integrated
Viral material
passed on
Replication
of an
Enveloped
Virus
Transcription
of viral genes
DNA replication
Assembly
Translation
Proteins
Lysogeni
c Cycle
Viroids
• Smaller than viruses
• Strands or circles of RNA
• No protein-coding genes
• No protein coat
• Cause many plant diseases
Prions
• Small proteins
• Linked to human diseases
– Kuru
– Creutzfeldt-Jakob disease (CJD)
• Animal diseases
– Scrapie in sheep
– Bovine spongiform encephalopathy
(mad cow disease)
Enveloped, Unsegmented
Negative ssRNA Viruses
• Hemorrhagic Fevers
– Marburg virus and Ebola virus are the causative
agents
– Natural reservoir and mode of transmission to
humans unknown
– Spread person to person by contaminated body
fluids and syringes
– Virions attack many cells of the body
Enveloped, Unsegmented
Negative ssRNA Viruses
• Especially macrophages and liver cells
– Uncontrolled bleeding under the skin and from
every body opening
• Viral glycoprotein prevents neighboring cells from
adhering
– Allows blood to leak out of the vessels
– The only treatment involves fluid replacement
– Up to 90% of human victims die
Filamentous Ebola viruses
Figure 25.33
Sites of known human cases
of Marburg and Ebola virus
Figure 25.34
Laboratory personnel working
in level four biocontainment
Figure 25.35
Ebola
https://www.youtube.com/watch?v=_ZC27I
Q037I
• Ebola facts: Fox News
• http://www.foxnews.com/health/2014/10/
22/separating-fact-from-fiction-aboutebola/
• https://www.youtube.com/watch?v=X0Jp
n22HJys
• BBC update on Ebola 2014
• https://www.youtube.com/watch?v=8kKL
KpB1EHA
ssRNA
Viruses
• Naked,
CommonPositive
Colds Caused
by Rhinoviruses
– Cause most cases of the common cold
– Infections limited to the upper respiratory tract
– A single virus is often sufficient to cause a cold
– Virus transmitted by aerosols, by fomites, or via
hand-to-hand contact
• Most commonly transmitted by direct person-toperson contact
– Some immunity can be acquired against
serotypes infected with in the past
• The number of infections tends to decrease with age
Rhinoviruses, the most
common cause of colds
Figure 25.1
ssRNA
Viruses
• Naked,
CommonPositive
Colds Caused
by Rhinoviruses
– Manifestations of rhinoviruses are usually
characteristic
– Medications can help relieve the symptoms
• Do not reduce the duration of the disease
– Handwashing is the most important
preventative measure
ssRNA Viruses
• Naked,
DiseasesPositive
of the Enteroviruses
– Transmitted via the fecal-oral route
• Ingestion of contaminated food or water
• Via fomites
• Oral contact with infected hands or fomites
– Infect the pharynx and intestine
• Spread via the blood infecting various targets in the
body
– Three main enteroviruses
• Polioviruses, coxsackieviruses, and echoviruses
ssRNA Viruses
• Naked,
DiseasesPositive
of the Enteroviruses
– Poliomyelitis
• Polio is caused by one of three serotypes of
poliovirus
• Last case of wild-type poliomyelitis in the Americas
was in 1979
• Four conditions caused by polioviruses
–
–
–
–
Asymptomatic infections
Minor polio
Nonparalytic polio
Paralytic polio
» Can result in bulbar poliomyelitis
A hospital ward full of
mechanical respirators
Figure 25.2
Reports of naturally occurring
polio in 2009
Figure 25.3
Violinist Itzhak Perlman
Figure 25.4
ssRNA Viruses
• Naked,
DiseasesPositive
of the Enteroviruses
– Poliomyelitis
• Postpolio syndrome
– Crippling deterioration in the function of polio-affected
muscles
• Near elimination of polio due to development of two
vaccines
– Inactivated polio vaccine (IPV)
» Developed by Jonas Salk
– Oral polio vaccine (OPV)
» Developed by Albert Sabin
ssRNA Viruses
• Naked,
DiseasesPositive
of the Enteroviruses
– Other diseases of enteroviruses
• Coxsackieviruses and echoviruses also cause
human disease
• Infection occurs via the fecal-oral route
• Most infections are subclinical or produce mild
symptoms
ssRNA Viruses
• Naked,
DiseasesPositive
of the Enteroviruses
– Poliomyelitis
• Postpolio syndrome
– Crippling deterioration in the function of polio-affected
muscles
• Near elimination of polio due to development of two
vaccines
– Inactivated polio vaccine (IPV)
» Developed by Jonas Salk
– Oral polio vaccine (OPV)
» Developed by Albert Sabin
Positive ssRNA Viruses
• Naked,
Hepatitis A
– Hepatitis A virus is the causative agent
• Survives on various surfaces and is resistant to
common household disinfectants
– Transmitted through the fecal-oral route
– Signs and symptoms are due to the patient’s
immune response
– Infection does not cause chronic liver disease
– Complete recovery occurs most of the time
Comparison of Hepatitis
Viruses
Figure 25.3
Positive ssRNA Viruses
• Naked,
Acute Gastroenteritis
– Caliciviruses and astroviruses can cause acute
gastroenteritis
– Can cause outbreaks in day care centers,
schools, and hospitals
– Caliciviruses cause diarrhea, nausea, and
vomiting
– Astroviruses cause diarrhea but no vomiting
– No specific treatment except replacement of
fluid and electrolytes
Viruses of the families
Caliciviridae and Astroviridae
Figure 25.6
ssRNA Viruses
• Naked,
Hepatitis Positive
E
– Also known as enteric hepatitis
– Caused by hepatitis E virus
• Formerly classified as a calicivirus
– Fatal in 20% of pregnant women
– No treatment
– Prevent by interrupting the fecal-oral route of
transmission
Enveloped, Positive ssRNA
• Viruses
Togaviridae and Flaviviridae
– Enveloped, icosahedral +ssRNA viruses
– Arboviruses
• Often transmitted by arthropods
• Coronaviridae
– Enveloped, helical +ssRNA viruses
Togaviruses
Figure 25.7
Enveloped +ssRNA coronavirus
Figure 25.8
Enveloped, Positive ssRNA
• Viruses
Diseases of Positive RNA Arboviruses
– Zoonoses
• Animal diseases that spread to humans
– Mosquitoes and ticks transmit arboviruses
among animal hosts
– Arthropod vectors remain infected
• Are a continual source of new infections
– Most infections result in mild, flu-like symptoms
– Arboviruses can occasionally result in secondstage infections
• Encephalitis, dengue fever, and yellow fever
Enveloped, Positive ssRNA
• Viruses
Other Diseases of Enveloped +ssRNA
Viruses
– Rubella
• Also called “German measles”
• Rubella virus is the causative agent
• One of the five childhood diseases that produces
skin lesions
• Infection begins in respiratory system but spreads
throughout the body
• Characterized by a rash of flat, pink to red spots
• Infections in children are usually not serious
• Adults can develop arthritis or encephalitis
• Infection of pregnant women can cause congenital
Hosts and transmission of
viruses of WEE and EEE
Figure 25.9