Download an introduction to viruses

The Viruses
I. Discovery of Virues
A. Martinus Beijerinck
1. Considered the founder of virology
2. In 1898, he used filtration experiments to show tobacco mosaic
disease is caused by an agent smaller than bacteria
3. Coined the word virus
B. Wendell Stanley
1. Isolated the tobacco mosaic virus (TMV) when he crystallized sap
from tobacco leaves infected with tobacco mosaic disease (1935)
2. In 1939 TMV was observed with the electron microscope
II. Vectors
A. Arthropod Vectors
1. Vectors play a key role in the dissemination of disease
2. Arthropods vectors can be mosquitoes, flies, sand flies, lice, fleas, ticks and
mites
3. Many arthropod vectors are haematophagous, or blood feeders
4. A vector may not only be required for part of the parasite's developmental
cycle, but also transmits the parasite directly to the host
Arthropod vectors
Mosquito
Tick
Fly
Kissing bug
Lice
Flea
III. General Characteristics of Viruses
A. Are Viruses a Form of Life?
1. Nonliving characteristics
viruses lack:
a. plasma membrane
b. cell wall
c. cytoplasm
d. organelles
e. respiration
f. binary fission
g. food getting
h. locomotion
2. Living characteristics
a. Reproduce within a living host cell
B. The Virus Mutation Rate is Extraordinarily High
1. Many viruses, in particular RNA viruses , have short generation times and
relatively high mutation rates (on the order of one point mutation or
more per genome per round of replication for RNA viruses)
2. This elevated mutation rate allows viruses to quickly adapt to changes in
their host environment
3. Since viruses have such high mutation and reproductive rates, they can
adapt to changing environments quite well
C. Nucleic Acids of Viruses
1. Viruses have either DNA or RNA as their genetic material, but never both
-may be single or double stranded
-may be linear or circular
-may exist as a single piece of genetic material or multi-segmented pieces
D. Capsids
1. Composed of proteins called capsomeres
-capsomers may be composed of a single type of protein or several types
-the arrangement of capsomers is characteristic of a particular type of virus
-they account for most of the mass of the virus
-they determine the antigenicity of the virus
E. Envelopes
1. Some capsids are surrounded by an envelope
a. the envelope is composed of lipids, proteins and carbohydrates
b. the envelope is derived from
• host cell membranes (can evade host defense mechanism) and/or
• may contain some proteins synthesized from virus genes and/or
• may contain some material derived from normal host cell
components
c. spikes of the envelope are composed of carbohydrate and proteins
d. spikes are used for attachment to host cells and are used to identify
viruses
F. Types of Viruses
1. Polyhedral Viruses
a. capsids forming multiple (many) sides
b. icosahedron is a regular polyhedron with 20 triangular faces and 12 corners
2. Helical Viruses
a. their nucleic acids are surrounded by a capsid which form long hollow rods
3. Enveloped helical viruses
4. Enveloped polyhedral viruses
5. Complex viruses
Viruses must be grown in living animals or in embryonated eggs
IV. Bacterial Viruses- The Bacteriophages
A. Bacteriophage Virus Structure
B. The Lytic Cycle
Bacterial
cell wall
Bacterial
chromosome
Capsid
DNA
Capsid
Sheath
1 Attachment:
Phage attaches
to host cell.
Tail fiber
Base plate
Tail
Pin
Cell wall
Plasma membrane
2 Penetration:
Phage pentrates
host cell and
injects its DNA.
Sheath contracted
Tail core
3 Biosynthesis
Tail
DNA
4 Maturation:
Viral components
are assembled into
virions.
Capsid
5 Release:
Host cell lyses and
new virions are
released.
Tail fibers
Virual plaques- areas on a Petri plate where bacteria
have been killed by viruses
C. Viral Reproduction in Bacteria
D. The Lysogenic Cycle
E. Specialized Transduction
Prophage
gal gene
Galactose-positive
donor cell
Bacterial DNA
1
Prophage exists in galactose-using
host (containing the gal gene)
2
Phage genome excises,
carrying with it the adjacent
gal gene from the host
3
Phage matures and cell lyses,
releasing phage carrying gal gene
4
Phage infects a cell that cannot
utilize galactose (lacking gal
gene)
5
Along with the prophage, the
bacterial gal gene becomes
integrated into the new host’s DNA
6
Lysogenic cell can now metabolize
galactose
gal gene
gal gene
Galactose-negative
recipient cell
V. Human Viruses
A. Mode of Attack
1. Attachment to a host cell and endocytosis
2. Penetration
3. Uncoating to expose the genetic material
4. Replication of
. viral components using host-cell machinery
5. Assembly of viral components into complete viral particles
6. Release of viral particles to infect new host cells.
Figure 13.14
Budding viruses
C. Viral Infections
1. Acute infections are characterized by the short period of time between initial
infection and elimination by the host- ex. common flu
2. Latent infections invade host cells and remain dormant over a long period of
time, producing no symptoms- herpes simplex virus (cold sores)
3. Persistent infections are considered chronic infections that occurs over a long
period of time, often with fatal consequences- HIV/AIDS, cervical cancer, liver
cancer
VI. Human DNA Viruses
A. Human Papillomavirus (HPV)
-Polyhedral virus that infects the skin and mucous membranes
-The majority of the nearly 200 known types of HPV cause no symptoms in
most cases
-HPV can cause warts and cancer
1. Skin warts
a. some HPV infections cause warts which are noncancerous skin growths
b. common warts are warts on the hands and feet and more rarely on
elbows or knees
c. have a characteristic cauliflower morphology
d. planter warts are found on the soles of the feet that grow inward
2. HPV and Cancer
a. around 500,000 new cancers are attributable to HPV (worldwide)
b. about a dozen HPV types are called "high-risk" types because they can
lead to cervical, anal, vulvar, vaginal and penile, mouth and throat cancers
c. most common sexually transmitted infection
d. 40 HPV types infect the genital areas of both males and females
e. most people who become infected with HPV do not know they have it
3. Multiplication of Papovaviruses- DNA Virus
(Papilloma viruses )
Papovavirus
1 Virion attaches to host cell
7 Virions are released
Host cell
DNA
Capsid
DNA
2 Virion penetrates
cell and its DNA is
uncoated
Cytoplasm
6 Virions mature
Capsid proteins
mRNA
5 Late translation;
capsid proteins
are synthesized
4 Late transcription;
DNA is replicated
3 Early transcription and
translation; enzymes are
synthesized
B. Adenoviruses
1. Nonenveloped polyhedral viruses
2. 53 types in humans
3. Most common cause of illness of the respiratory
system- common cold, pneumonia, croup, and bronchitis
4. Can cause tonsillitis, stomach flu, conjunctivitis and
gastroenteritis
C. Orthopoxvirus
1. Smallpox is an infectious disease unique to humans
2. Smallpox is caused by two virus variants, Variola major and Variola minor
3. V. major produces a more serious disease and has an overall mortality rate
of 30–35%
4. V. minor causes a milder form of disease that kills about 1% of its victims
5. Smallpox localizes in small blood vessels of the skin and in the mouth
6. During the 20th century, it is estimated that smallpox was responsible for
300–500 million deaths and has killed more people throughout history
than any other infectious disease
7. After successful vaccination campaigns throughout the 19th and 20th
centuries, smallpox was eradicated from natural existence as of December
1979 through a global vaccination effort administered by the World Health
Organization (WHO)
8. Smallpox is the only human infectious disease to have been eradicated
9. It is the ultimate weapon of mass destruction
10. Currently, the only official stores of Variola are in freezers of two
research sites:
CDC (Atlanta)
VECTOR (Russia)
2. The smallpox vaccine was the first successful vaccine that was developed
by Edward Jenner in 1796
D. Herpes related virus
-enveloped viruses
1. Chicken pox is a highly contagious illness caused by varicella zoster virus (VZV)
-starts with a skin rash mainly on the body and head rather than at the
periphery and becomes itchy
2. Shingles
a. the initial infection with varicella causes the acute (short-lived) illness
chickenpox which generally occurs in children and young people
b. varicella can become latent in the nerve cell bodies without causing any
symptoms
c. years or decades after a chickenpox infection, the virus may break out of nerve
cell bodies and travel down nerve axons to cause viral infection of the skin in
the region of the nerve and cause shingles
3. Herpes simplex (HSV)
a. HSV is never removed from the body by the immune system
b. following a primary infection, the virus enters the nerves at the site of primary
infection, migrates to the cell body of the neuron, and becomes latent in the
ganglion
c. Oral herpes are sometimes called cold sores or fever blisters that infect the face
and lips.
Genital herpes
-Burkitt’s lymphoma-cancer of the lymphatic system
- Kaposi's sarcoma
VII. RNA Viruses RNA
A. Virus Reproduction
B. Orthomyxoviruses
-enveloped virus
-10 genes on 8 single strands of RNA molecules
- bird and pig share virus; pig and human share virus
-pigs provides a “mixing pot” able to be infected by the avian virus, thus
allowing passage to humans
-glycoprotein spikes of two types, H and N, are capable of “mix and match”
causing antigenic shift
-16 H types and 9 N types
-H5 stands for the fifth of several known types of the protein hemagglutinin
-N1 stands for the first of several known types of the protein neuraminidase
Spanish Flu Pandemic
-it is thought to be one of the most deadly pandemics in world history
-The Spanish flu was an unusually severe and deadly strain of avian influenza that
killed some 50 to 100 million people worldwide over about a year in 1918 and
1919.
-The 1918 flu caused an unusual number of deaths, possibly due to it causing a
cytokine storm the body
-The Spanish flu virus infected lung cells, leading to overstimulation of the
immune system via release of cytokines into the lungs
-This leads to extensive leukocyte migration towards the lungs, causing destruction
of lung tissue and secretion of liquid into the organ.
Influenza A Pandemics
* Spanish flu (H1N1) pandemic- 1918
-deadly strain of avian influenza that killed an estimated 50 million to
100 million people
-thought to be one of the most deadly pandemics in human
history
*Asian flu (H2N2)pandemic- 1957
-1.5 million deaths
* Hong Kong flu (H3N2) pandemic- 1968
-1 million deaths
* Avian flu (H5N1) 2007-2010 (human flu?)
-antigenic shift could make this a deadly human virus with a
mortality rate of 30-40%
*Swine flu (H1N1) 2009-2010
-18,000 deaths
Influenza virus B
1. This genus has one species, influenza B virus
2. Influenza B almost exclusively infects humans and is less common than
influenza A
3. mutates at a rate 2–3 times slower than type A and consequently is less
genetically diverse
Influenza virus C
1. This genus has one species, influenza C virus.
2. Infects humans, dogs and pigs, sometimes causing both severe illness and
local epidemics. However, influenza C is less common than the other
types and usually only causes mild disease in children
C. Hepatitis C virus
a. enveloped
b. causes liver damage- hepatitis, cirrhosis of the liver, and liver cancer
D. Poliovirus
-nonenveloped virus
-causes polio
-pandemic occurred during the 1950’s
-eliminated in North and South America
Poliomyelitis immunization days
E. Rhinovirus
-polyhedral
-Rhinoviruses are the most common viral infective agents in humans,
and a causative agent of the common cold
-there are 99 recognized types of rhinoviruses that differ based on their
varying surface proteins.
F. Ebola Virus
-enveloped
-Ebola hemorrhagic fever
-first emerged in 1976 in Zaire
G. Marburg Hemorrhagic Fever
-how the animal host first transmits Marburg virus to humans is unknown
-spread of the virus between humans has occurred in a setting of close
contact from droplets of body fluids, or direct contact with persons,
equipment, or other objects contaminated with infectious blood or tissues
Lungs
Liver
H. Rabies virus
-enveloped
-very long incubation period ranging from 10 days - 7 years
-Rabies is a preventable viral disease of mammals most often transmitted through
the bite of a rabid animal.
-The vast majority of rabies cases reported to the Centers for Disease Control and
Prevention (CDC) each year occur in wild animals like raccoons, skunks, bats, and
foxes.
-infects the central nervous system, ultimately causing disease in the brain and
death
I. West Nile Virus (WNV)
-enveloped mosquito-borne virus
-birds serve as the animal reservoir host
-approximately 80 percent of West Nile virus infections have no
symptoms
-symptoms may include fever, headaches, fatigue, nausea
-the most severe cases result in encephalitis
-Culex pipiens, Culex tarsalis, and Culex quinque, and Culex fasciatus are
the main vectors
J. Yellow Fever Virus
-enveloped virus transmitted by the mosquito Aedes aegypti
-yellow fever presents in most cases with fever, nausea, and pain, and it
generally subsides after several days
-sometimes a toxic phase follows, in which liver damage with jaundice can
occur and lead to death
-yellow fever belongs to the group of hemorrhagic fevers
Aedes aegypti the vector for yellow fever
K. Eastern Equine Encephalitis virus (EEEV)
-maintained in nature through a mosquito (Culiseta melanura) bird cycle
-other mosquitoes are called bridge vectors because they bring the virus
from avian populations to mammalian populations
-the fatality rate in humans is 35% and there is currently no cure for
human infections
Eastern Equine Encephalitis Virus
L. Hantaviruses
-contracted through rodent bites, urine, saliva or feces
-causes hantavirus pulmonary syndrome
-38% fatal
M. Measles Virus
-enveloped virus
-causes infection of the respiratory system
-symptoms include fever, cough, and rash
N. Human Immunodeficiency Virus (HIV)
-enveloped
-reverse transcriptase enzyme
-HIV infects vital cells in the human immune system such as helper T cells,
macrophages, and dendritic cells
- four major routes of transmission
*unprotected sexual contact
*contaminated needles
*breast milk
*transmission from an infected mother to her baby at birth
HIV budding from lymphocyte cell
Multiplication of a Retrovirus
Capsid
Reverse
transcriptase
Virus
Two identical + stands of RNA
1 Retrovirus penetrates
Host
cell
5 Mature
retrovirus
leaves host
cell, acquiring
an envelope as
it buds out.
host cell.
DNA of one of the host
cell’s chromosomes
Reverse
transcriptase
Viral RNA
Identical
strands of
RNA
Viral
proteins
2 Virion penetrates
cell and its DNA is
uncoated
4 Transcription of the
provirus may also occur,
producing RNA for new
retrovirus genomes and
RNA that codes for the
retrovirus capsid and
envelope proteins.
3 The new viral DNA is
tranported into the host cell’s
nucleus and integrated as a
provirus. The provirus may
divide indefinitely with the
host cell DNA.
Provirus
VIII. Prions
A. What are Prions?
-infectious proteins that cause disease
-transmissible by ingestion
-PrPC, normal cellular prion protein located on the cell surface
-PrPSc, scrapie protein, accumulate in brain cells forming plaques
B. Prion Diseases
-Creutzfeldt-Jakob disease
-mad cow disease
Creutzfeldt-Jacob Disease