An Introduction to the An Introduction to the VirusesViruses

Chapter 6

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The Search for the Elusive The Search for the Elusive VirusVirus

• ______________ postulated that rabies was ______________ postulated that rabies was caused by a virus (1884)caused by a virus (1884)

• Ivanovski and Beijerinck showed a Ivanovski and Beijerinck showed a disease in tobacco was caused by a virus disease in tobacco was caused by a virus (1890s)(1890s)

• 1950s virology was a multifaceted 1950s virology was a multifaceted disciplinediscipline– Viruses: noncellular particles with a definite Viruses: noncellular particles with a definite

size, shape, and chemical composition size, shape, and chemical composition

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The Position of Viruses in the The Position of Viruses in the Biological SpectrumBiological Spectrum

• There is no universal agreement on There is no universal agreement on how and when viruses originatedhow and when viruses originated

• Viruses are considered the most Viruses are considered the most abundant microbes on earthabundant microbes on earth

• Viruses played a role in evolution of Viruses played a role in evolution of Bacteria, Archaea, and EukaryaBacteria, Archaea, and Eukarya

• Viruses are obligate intracellular Viruses are obligate intracellular parasitesparasites

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Size of virusesSize of viruses

Naming virusesNaming virusesClassified based on structures, size, nucleic acids, Classified based on structures, size, nucleic acids,

host species, target cells.host species, target cells.• 3 orders, 63 families, and 263 genera of viruses3 orders, 63 families, and 263 genera of viruses• Family name ends in -viridae Family name ends in -viridae • Genus name ends in -virus, SimplexGenus name ends in -virus, Simplexvirus, virus,

HantaHantavirusvirus, Entero, Enterovirusvirus• Name of genus or family Name of genus or family beginsbegins with with

description of virus description of virus – appearance: appearance: togatogavirus, virus, coronacoronavirusvirus– place collected: place collected: adenoadenovirus, virus, hantahantavirusvirus– effect on host: effect on host: lentilentivirusvirus– acronymns: acronymns: picornapicornavirus; (virus; (arboarbovirus)virus)

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ExamplesExamples

• Family – HerpesviridaeFamily – Herpesviridae

• Genus – VaricellovirusGenus – Varicellovirus

• Common name – chickenpox virusCommon name – chickenpox virus

• Disease - chickenpoxDisease - chickenpox

• Family – HerpesviridaeFamily – Herpesviridae

• Genus – SimplexvirusGenus – Simplexvirus

• Common name – herpes virus Common name – herpes virus (Herpes simplex virus I (HSV-I)(Herpes simplex virus I (HSV-I)

• Disease – fever blisters, cold soresDisease – fever blisters, cold sores

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terminologyterminology

• Virus = virus particleVirus = virus particle•___________– fully formed, virulent, fully formed, virulent,

extracellularextracellular•___________– host cell is lysed – host cell is lysed

(broken) to release virions(broken) to release virions•___________– viruses are inactive – viruses are inactive

(latent stage) and host cell isn’t lysed (latent stage) and host cell isn’t lysed (virus may be integrated within host (virus may be integrated within host genome)genome)

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Viral structureViral structure

•___________ (with spikes/receptors; (with spikes/receptors; 13/20 – derived from host cell) – 13/20 – derived from host cell) – may may not be presentnot be present

•___________ (morphologies: helical, (morphologies: helical, icosohedral, complex)icosohedral, complex)

• Nucleic acidNucleic acid (DNA/RNA; ds or ss) (DNA/RNA; ds or ss)

• Matrix proteinsMatrix proteins (some enzymes) (some enzymes)

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capsidscapsids• Nearly all viruses have Nearly all viruses have capsids-capsids- protein protein

coats that enclose & protect their nucleic coats that enclose & protect their nucleic acidacid

• Each Each capsidcapsid is constructed from identical is constructed from identical subunits called subunits called capsomerscapsomers made of protein – made of protein – can self-assemblecan self-assemble

•___________= capsid & nucleic acid= capsid & nucleic acid

• 2 types (based on shape): 2 types (based on shape): – helical helical – icosahedralicosahedral

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EnvelopeEnvelope

• If present, made primarily of lipidsIf present, made primarily of lipids

•______________________ ; ; (ER, nuclear (ER, nuclear envelope, cell membrane-budding)envelope, cell membrane-budding)

• May be embedded with spikes for May be embedded with spikes for host recognition/bindinghost recognition/binding

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4 morphological types4 morphological types

• Due to capsule shape and/or Due to capsule shape and/or whether it’s envelopedwhether it’s enveloped

1.1. HelicalHelical

2.2. IcosahedralIcosahedral

3.3. Enveloped (with Hel. or Icos. Enveloped (with Hel. or Icos. capsid)capsid)

4.4. complexcomplex

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1. Helical capsid1. Helical capsid• Rod-shaped Rod-shaped

capsomerscapsomers

• Coil around hollow Coil around hollow centercenter

• Nucleic acid is Nucleic acid is kept inside – kept inside – wound-up within wound-up within tube tube (nucleocapsid)(nucleocapsid)

Morphological types

Helical – capsid surrounds DNA like hollow tubeEx: TMV, influenza, measles, rabies (last 3 are enveloped)

Morphological types

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Tobacco Mosiac Virus

Influenza virus

Morphological types

2. icosahedral2. icosahedral

• 20-sided with 12 20-sided with 12 cornerscorners

• Vary in the number Vary in the number of capsomersof capsomers

• Each capsomer may Each capsomer may be made of 1 or be made of 1 or several proteinsseveral proteins

• Some are envelopedSome are enveloped

Morphological types

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Figure 6.7 Figure 6.8

Icosahedral – 3D, 20 sided

Human papilloma virusAdenovirus Herpesvirus

Morphological types

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3. envelope3. envelope• Naked virusesNaked viruses

– Capsid proteins exposed to environmentCapsid proteins exposed to environment– Released from cells as cells Released from cells as cells ____________________– Not a good pathogenic feature should keep host Not a good pathogenic feature should keep host

alivealive

• Enveloped virusesEnveloped viruses– Covered with a lipid bilayerCovered with a lipid bilayer– ______________________– Host cell keeps producing virus much longer Host cell keeps producing virus much longer

sometimes for extended periodssometimes for extended periods– Can bud off cellCan bud off cell – thereby not destroying (lysing) it – thereby not destroying (lysing) it

Morphological types

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Enveloped vs. naked virusesEnveloped vs. naked viruses

Morphological types

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4. Complex viruses4. Complex viruses

• Have additional or special structuresHave additional or special structures• Examples: Examples: •___________– lack normal capsid – – lack normal capsid –

instead, layers of lipoprotiens and instead, layers of lipoprotiens and fibrils on surfacefibrils on surface

•___________viruses have a polyhedral viruses have a polyhedral head, helical tail and fibers for head, helical tail and fibers for attachment. attachment.

Morphological types

Fig 6.9a,c

Morphological types

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Nucleic AcidsNucleic Acids• Viral genomeViral genome – either DNA or – either DNA or

RNA but never bothRNA but never both

• Carries genes necessary to invade Carries genes necessary to invade host cell and redirect cell’s activity host cell and redirect cell’s activity to make new virusesto make new viruses

• Number of genes varies for each Number of genes varies for each type of virus – few to hundredstype of virus – few to hundreds

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Nucleic AcidsNucleic Acids• DNA viruses DNA viruses

– Usually double stranded (ds) but may be Usually double stranded (ds) but may be single stranded (ss)single stranded (ss)

– Circular or linearCircular or linear

• RNA viruses RNA viruses – Usually single stranded, may be double Usually single stranded, may be double

stranded, may be segmented into separate stranded, may be segmented into separate RNA piecesRNA pieces

– ssRNA genomes ready for immediate ssRNA genomes ready for immediate translation are translation are positive-sense RNApositive-sense RNA

– ssRNA genomes that must be converted into ssRNA genomes that must be converted into proper form are proper form are negative-sense RNAnegative-sense RNA

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Enzymes important to Enzymes important to replicationreplication

•Pre-formed enzymes may be Pre-formed enzymes may be presentpresent– ___________– DNA or RNA– DNA or RNA– ___________– copy RNA– copy RNA– ______________________– synthesis of – synthesis of

DNA from RNA (AIDS virus)DNA from RNA (AIDS virus)

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Viral life cyclesViral life cycles

Host rangeHost range• Spectrum of Spectrum of cellscells a virus can infect a virus can infect

– cell has to have a specific structure (receptor) on cell has to have a specific structure (receptor) on its surface for viral attachmentits surface for viral attachment

– cell has to contain all of the enzymes and cell has to contain all of the enzymes and materials needed to produce new virionsmaterials needed to produce new virions

• May be one May be one speciesspecies or many or many– HIV (only humans) vs rabies (many animals)HIV (only humans) vs rabies (many animals)– Bacteriophages – tend to be very specificBacteriophages – tend to be very specific

• May be one May be one tissuetissue or many within a host or many within a host– Hepatitis (liver) vs polio (intestinal & nerve cells)Hepatitis (liver) vs polio (intestinal & nerve cells)

Life cycle

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DNA DNA replication in replication in virusesviruses

* Uses host machinery!*

animal virus replication (life animal virus replication (life cycle)cycle)

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Viral Multiplication – animal Viral Multiplication – animal virusesviruses

1. ___________– binding of virus to specific – binding of virus to specific molecule on host cellmolecule on host cell

2. ___________– genome enters host cell– genome enters host cell

3. ___________– the viral nucleic acid is – the viral nucleic acid is released from the capsidreleased from the capsid

4. ___________– viral components are produced– viral components are produced

5. ___________– new viral particles are – new viral particles are constructedconstructed

6. ___________– assembled viruses are released – assembled viruses are released by budding (exocytosis) or cell lysisby budding (exocytosis) or cell lysis

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1. Adsorption and Host 1. Adsorption and Host RangeRange

• Virus coincidentally collides with a Virus coincidentally collides with a susceptible host cell and adsorbs susceptible host cell and adsorbs specifically to receptor sites on the cell specifically to receptor sites on the cell membranemembrane

• Spectrum of cells a virus can infect – Spectrum of cells a virus can infect – host host rangerange– ___________– human liver cells– human liver cells– ___________– primate intestinal and nerve cells– primate intestinal and nerve cells– ___________– various cells of many mammals– various cells of many mammals

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1. Adsorption to host cell membrane(a) Coronavirus with envelope spikes (b) adenovirus (naked capsid) with surface capsid

spikes

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Entry: (2. penetration & 3. Entry: (2. penetration & 3. uncoating)uncoating)Entry (viruses with envelopes)• first must attach to cell membrane using

receptors -- usually specific for host AND cell type• After adsorption _________ of membranes – (like

two soap bubbles) • leaves the capsid free in the cytoplasm

Entry (viruses without envelopes)• first must attach using receptors (specific) • if a match, then engulfment -- ___________• capsid then in the cytoplasm but within a

vacuole • virus must uncoat without the genes being

degraded

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Figure 6.13

Entry of viruses into cells (penetration and uncoating)• Endocytosis and engulfment of herpesvirus; • Fusion of cell membrane with viral envelope

(mumps)

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Synthesis: Replication and Protein Synthesis: Replication and Protein ProductionProduction

• Varies depending on whether the Varies depending on whether the virus is a DNA or RNA virusvirus is a DNA or RNA virus

• DNA virusesDNA viruses generally are replicated generally are replicated and assembled in the and assembled in the ___________

• RNA virusesRNA viruses generally are replicated generally are replicated and assembled in the and assembled in the ___________– Positive-sense RNA contain the message Positive-sense RNA contain the message

for translationfor translation– Negative-sense RNA must be converted Negative-sense RNA must be converted

into positive-sense messageinto positive-sense message

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Duplication and synthesis: viral Duplication and synthesis: viral replication (more details in Ch. replication (more details in Ch. 9)9)

Transport of DNA/RNA to nucleus• most viruses must get genes to nucleus to survive • some integrate into host DNA (leads to latency)

– may require conversion of RNA copy to DNA

• some remain independent chromosomes

Viral replication• some do direct copies of DNA->DNA or RNA->RNA • others use "reverse transcriptase" to do RNA-

>DNA conversion – then DNA->RNA using HOST enzymes – timing is important -- some replicate only late in the

cycle, after all proteins are made

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5. Assembly: Filling the capsid

•Capsid proteins made in cytoplasm

•DNA or RNA gets fills empty capsids

•final modifications to capsid – to plug any holes from DNA/RNA entry – to mature the outer proteins

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6. Release6. Release

• Two ways assembled viruses leave host Two ways assembled viruses leave host cell:cell:

1. ___________– exocytosis; nucleocapsid binds to – exocytosis; nucleocapsid binds to membrane which pinches off and sheds the membrane which pinches off and sheds the viruses gradually; cell is not immediately viruses gradually; cell is not immediately destroyeddestroyed

2. ___________– nonenveloped and complex – nonenveloped and complex viruses released when cell dies and rupturesviruses released when cell dies and ruptures

• Number of viruses released is variableNumber of viruses released is variable– 3,000-4,000 released by poxvirus3,000-4,000 released by poxvirus– >100,000 released by poliovirus>100,000 released by poliovirus

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Budding of parainfluenza virus

Budding of HIV

Viral budding – picking up cell membrane upon exit

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Damage to Host CellDamage to Host Cell

Cytopathic effectsCytopathic effects - virus-induced - virus-induced damage to cellsdamage to cells

1.1. Changes in size and shapeChanges in size and shape2.2. Cytoplasmic inclusion bodiesCytoplasmic inclusion bodies3.3. Inclusion bodiesInclusion bodies4.4. Cells fuse to form multinucleated cellsCells fuse to form multinucleated cells5.5. Cell lysisCell lysis6.6. Alter DNAAlter DNA7.7. Transform cells into cancerous cellsTransform cells into cancerous cells

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Figure 6.16

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Persistent infections with Persistent infections with virusesviruses

• Persistent infectionsPersistent infections - cell harbors the - cell harbors the virus and is not immediately lysedvirus and is not immediately lysed

• Can last weeks or host’s lifetime; several Can last weeks or host’s lifetime; several can periodically reactivate – can periodically reactivate – ______________________– Measles virusMeasles virus – may remain hidden in brain – may remain hidden in brain

cells for many yearscells for many years– Herpes simplex virusHerpes simplex virus – cold sores and genital – cold sores and genital

herpesherpes– Herpes zoster virusHerpes zoster virus – chickenpox and shingles – chickenpox and shingles

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• Some animal viruses permanently alter genetic Some animal viruses permanently alter genetic material material ________________

• Transformed cells have increased rate of Transformed cells have increased rate of growth, altered chromosomes, and can growth, altered chromosomes, and can become “immortal” become “immortal” tumors (cancerous or tumors (cancerous or noncancerous growths)noncancerous growths)

• Mammalian viruses capable of initiating Mammalian viruses capable of initiating tumors are called tumors are called _________________– PapillomavirusPapillomavirus – genital warts/cervical cancer – genital warts/cervical cancer– Epstein-Barr virusEpstein-Barr virus – Burkitt’s lymphoma – Burkitt’s lymphoma– HBVHBV and liver cancer and liver cancer

Persistent infections with Persistent infections with virusesviruses

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Multiplication Cycle in Multiplication Cycle in BacteriophagesBacteriophages

• ___________– bacterial viruses (phages)– bacterial viruses (phages)

• Most widely studied are those that infect Most widely studied are those that infect Escherichia coliEscherichia coli – complex structure, DNA – complex structure, DNA

• Multiplication goes through similar stages as Multiplication goes through similar stages as animal virusesanimal viruses

• Only the nucleic acid enters the cytoplasm - - uncoating is not necessaryuncoating is not necessary

• Release is a result of Release is a result of cell lysiscell lysis induced by induced by viral enzymes and accumulation of viruses - viral enzymes and accumulation of viruses - lytic cyclelytic cycle

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6 Steps in Phage 6 Steps in Phage ReplicationReplication

1. Adsorption – binding of virus to specific molecule on host cell

2. Penetration – genome enters host cell3. Replication – viral components produced4. Assembly – viral components assembled5. Maturation – completion of viral

formation6. Release – viruses leave cell to infect

other cells

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Figure 6.17

525252

Figure 6.18

Figure 6.19

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Lysogeny: The Silent Virus Lysogeny: The Silent Virus InfectionInfection• Not all phages complete the lytic cycleNot all phages complete the lytic cycle• Some DNA phages (Some DNA phages (temperate phagestemperate phages)), ,

undergo adsorption and penetration but don’t undergo adsorption and penetration but don’t replicatereplicate

• The viral genome The viral genome insertsinserts into bacterial genome into bacterial genome and becomes an and becomes an inactive inactive ___________– – the cell is the cell is not lysednot lysed

• Prophage is retained and copied during normal Prophage is retained and copied during normal cell division cell division transfer of temperate phage transfer of temperate phage genome to all host cell progeny – genome to all host cell progeny – ___________

• InductionInduction can occur resulting in can occur resulting in activationactivation of of lysogenic prophage followed by viral replication lysogenic prophage followed by viral replication and cell and cell lysislysis

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Figure 6.17

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Lysogeny Lysogeny

• Lysogeny results in the Lysogeny results in the spread of the spread of the virus without virus without ______________________

• Phage genes in the bacterial Phage genes in the bacterial chromosome can cause the production chromosome can cause the production of toxins or enzymes that cause of toxins or enzymes that cause pathology – pathology – lysogenic conversionlysogenic conversion– Corynebacterium diphtheriaeCorynebacterium diphtheriae – Vibrio choleraeVibrio cholerae– Clostridium botulinumClostridium botulinum

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Growing animal virusesGrowing animal virusesObligate intracellular parasites that require Obligate intracellular parasites that require appropriate cells to replicateappropriate cells to replicate

1.1. live animalslive animals (dependent on host (dependent on host specificity) specificity)

2.2. bird embryosbird embryos – chicken, duck; – chicken, duck; intact, self-supporting unit, sterile, intact, self-supporting unit, sterile, self-nourishedself-nourished

3.3. cell culturecell culture – can be used to – can be used to cultivate but also to test toxic (ex. cultivate but also to test toxic (ex. Hemolytic) effectsHemolytic) effects

Viral Culture

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Figure 6.20

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Figure 6.21

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Medical Importance of Medical Importance of VirusesViruses

• Viruses are the most common cause of Viruses are the most common cause of acute infections acute infections

• Several billion viral infections per yearSeveral billion viral infections per year

• Some viruses have high mortality ratesSome viruses have high mortality rates

• Possible connection of viruses to chronic Possible connection of viruses to chronic afflictions of unknown causeafflictions of unknown cause

• Viruses are major participants in the Viruses are major participants in the earth’s ecosystemearth’s ecosystem

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Detection of Animal Viral Detection of Animal Viral InfectionsInfections

• More difficult than other agentsMore difficult than other agents

• Consider overall clinical pictureConsider overall clinical picture

• Take appropriate sample Take appropriate sample – Infect cell culture – look for characteristic Infect cell culture – look for characteristic

cytopathic effectscytopathic effects– Screen for parts of the virusScreen for parts of the virus– Screen for immune response to virus Screen for immune response to virus

(antibodies)(antibodies)

diagnosis

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Treatment of viral infections

•Antiviral drugs

•Target viral replication – but usually lots of side effects (affects host cells too)

•Some drugs target virus-specific enzymes

Best bet…get ___________and avoid

infection

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Clinically-important viruses (just a few examples!)

HepC

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Important viruses you should Important viruses you should know…know…

• Smallpox (variola major, minor) – complex virus; inclusions

• Herpesviridae – (herpes; chicken pox – varicella zoster); chronic latent state reactivated; nuclear inclusions

• HPV – can transform cells; warts cervical cancer• Hepatovirus (A, B, C) – see table• Polio – enterovirus – enteric (oral vaccine)• Cold (Rhinovirus) – antibiotics ineffective! It’s a virus!!• West Nile – is a flavivirus; spread by moquitoes; bird is

reservoir• SARS – coronavirus (like the virus that causes

bronchitis); prominent spikes on envelope• influenza – Flu; Type A is the one you’ve had;• Rotavirus – viral food poisoning; vomiting and diarrhea

– sometimes concurrently!!• HIV – retrovirus; latency; (see slide)

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HerpesviridaeHerpesviridae

• Examples:– _______________ (cold sores); 2 (genital herpes)– ______________________ (chicken pox)

• Have chronic latent phase in nerve cells

• Reactivated by various stimuli:– Foods– Sunlight– Stress

• Chicken pox reappears as “___________”

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Hepatoviruses and HepatitisHepatoviruses and HepatitisOrganismOrganism Hep AHep A Hep BHep B Hep CHep C

VirusVirus Non-Non-enveloped; enveloped; ssRNAssRNA

Enveloped; ss and Enveloped; ss and dsDNAdsDNA

RNA (flaviviridae)RNA (flaviviridae)

DiseaseDisease Short-term Short-term hepatitishepatitis

Chronic infection Chronic infection (6%), liver cancer, (6%), liver cancer, deathdeath

Chronic infection Chronic infection (75-85%) liver (75-85%) liver disease very disease very common, cancer, common, cancer, deathdeath

TransmissioTransmissionn

Fecal/oralFecal/oral Blood/fluid contact Blood/fluid contact (IV, sex, tattoos); (IV, sex, tattoos); verticalvertical

Blood/fluid contact Blood/fluid contact (IV, sex, tattoos); (IV, sex, tattoos); verticalvertical

TreatmentTreatment Immune Immune globulinglobulin

Interferon, nucleoside Interferon, nucleoside analogsanalogs

Interferon or Interferon or ribavirinribavirin

PreventionPrevention VaccineVaccine VaccineVaccine No vaccine; avoid No vaccine; avoid exposureexposure

Special Special notesnotes

Resolves (self-Resolves (self-limiting); avoid limiting); avoid aspirin, alcoholaspirin, alcohol

Survives in infected Survives in infected blood for days, blood for days, freezingfreezing

““silent epidemic; silent epidemic; 4M people, 80% 4M people, 80% asymptomatic!asymptomatic!

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InfluenzaInfluenza• Type A = Hong Kong, Asian, Swine flu; Type A = Hong Kong, Asian, Swine flu;

AvianAvian InfluenzaInfluenza (bird flu – emerging (bird flu – emerging disease)disease)

• Bird Bird human (right now) human (right now) Pandemic: Pandemic: human human human human; antigenic ; antigenic

shiftshift Influenza pandemic of Influenza pandemic of ________ killed 20+ killed 20+

million people! (more than plague?) million people! (more than plague?) The viruses of the last three global The viruses of the last three global

influenza pandemics were first found in influenza pandemics were first found in China. China.

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HIVHIV• Retrovirus – ssRNA genome – depends on Reverse

Transcriptase to make DNA (will be covered later)• Latent phase – inserts in host genome – 2 wks. - 20

years? – can’t see virus particles in bloodstream• Infects cells – CD4+ T-cells (lymphocytes),

macrophages• Destroys immune system AIDS person frequently

dies from 2ndary infection and/or cancer• HIV also attacks organs directly (kidney, heart)

2007 data:• number of people living with HIV = ___________

• Number of people who died of AIDS = __________

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HIV – enveloped virusHIV – enveloped virus

• HIV does not survive well outside the HIV does not survive well outside the human body or in the outside the human body or in the outside the environment. (environment. (Enveloped – fragile – does Enveloped – fragile – does not survive drying)not survive drying)

• The studies also shown that drying HIV The studies also shown that drying HIV reduces the amount of viral by nearly 95 reduces the amount of viral by nearly 95 percent within several hours. percent within several hours.

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HIV/AIDS – stages HIV/AIDS – stages

• Stage I: asymptomatic, not categorized as AIDS

• Stage II: include minor mucocutaneous manifestations (ex. recurrent yeast infections) and recurrent upper respiratory tract infections

• Stage III: includes unexplained ______________ for longer than a month, severe bacterial infections and pulmonary tuberculosis. Wasting begins.

• Stage IV includes ___________of the brain, candidiasis of the esophagus or repiratory, Pnemocystis carinii pneumonia, and Kaposi's sarcoma (cancer); these diseases are used as indicators of AIDS.

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___________- misfolded proteins, contain no nucleic acid

– Cause transmissible spongiform encephalopathies – fatal neurodegenerative diseases

– Common in animals:• Scrapie in sheep and goats

• Bovine spongiform encephalopathies (BSE), a.k.a. mad cow disease

• Wasting disease in elk

• Humans – Creutzfeldt-Jakob Syndrome (CJS)

– Extremely resistant to usual sterilization techniques

Other Other noncellularnoncellular infectious infectious particlesparticles

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Figure 6.22

747474

Other Noncellular Infectious Other Noncellular Infectious AgentsAgents

• Satellite virusesSatellite viruses – dependent on – dependent on other viruses for replicationother viruses for replication– Adeno-associated virus – replicates Adeno-associated virus – replicates

only in cells infected with adenovirusonly in cells infected with adenovirus– Delta agent – naked strand of RNA Delta agent – naked strand of RNA

expressed only in the presence of expressed only in the presence of hepatitis B virushepatitis B virus

• ___________ – short pieces of RNA, no – short pieces of RNA, no protein coat; only been identified in protein coat; only been identified in plantsplants