LECTURE 15 (Virology) Flashcards
How are viruses different from other microorganisms?
- Viruses are not cells
- Not capable of independent replication (cannot synthesise their own energy or their own proteins)
- Too small to be seen in a light microscope
- Particles composed of an internal core containing either DNA or RNA (not both) covered by a protective protein coat (CAPSID)
- ONLY VIRUSES have genetic material composed of single-stranded DNA or of single-stranded or double-stranded RNA
Describe the virus structure
- Nucleic acid genome (DNA or RNA) surrounded by protein shell “CAPSID”
- Some human viruses packaged into a LIPID MEMBRANE/ENVELOPE (usually acquired from cytoplasmic membrane of infected during release from cell)
- Genomes of enveloped viruses form protein complex “NUCLEOCAPSID” + surrounded by MATRIX which serves as bridge between nucleocapsid and inside of viral membrane
- “SPIKES” protrude from surface of virus + involved in initial contact with host receptors
What is the function of the Capsid/Envelope?
- To protect nucleic acid genome from damage
- To aid in process of entry into cell
- To package enzymes for early steps of infection process
What are some examples of Enveloped and Non-enveloped viruses?
Enveloped virus = HIV, HSV, HBV & Influenza virus
Non-enveloped virus = Norovirus, Parvovirus, HAV, HEV
Describe the envelope structure
- Outer lipid bilayer membrane called “ENVELOPE”
- Viral envelope contains “spikes”, “peplomers” or “viral envelope proteins” -> bind to host receptors + antigens for immune response
- “MATRIX PROTEIN” serves as a bridge between nucleocapsid and inner membrane of envelope
- MORE SENSITIVE to detergents (solvents, ethanol, ether, heat) compared to non-enveloped viruses
What are capsids/nucleocapsids?
Capsids/Nucleocapsids = virus-encoded specific proteins that protect the genome and confer shapes to viruses
PROPERTIES:
- composed of many copies of one/different kinds of protein subunits
- presence of many identical protein subunits in viral capsids + existence of identical spikes in envelope membrane -> important in adsorption, haemagglutination + recognition of viruses by neutralising antibodies
Describe the genome structure
- Either DNA or RNA
- Either single-stranded DNA/RNA or double-stranded RNA
- Either POSITIVE SENSE (polarity of mRNA) or NEGATIVE SENSE (complementary to/antisense of mRNA) or AMBISENSE (++ –)
What are the steps of Virus replication?
1) ADSORPTION of the virus specific to receptors on the cell surface
2) PENETRATION by the virus + intracellular release of nucleic acid
[cell engulfs virus by endocytosis]
3) PROLIFERATION of viral components
[virus-coded synthesis of capsid and non-capsid proteins + replication of nucleic acid by viral and cellular enzymes]
4) ASSEMBLY of replicated nucleic acid + new capsid protein
5) RELEASE of virus from cell into ECF
What are the different virus replication effects that can happen?
- Non-productive response = when some viruses enter a host cell but no new virus is produced -> the cell survives and divides and the viral genetic material persists indefinitely in a latent-state
- Persistent infection = A “chronic infection” where a low level of virus is produced with little or no damage to target tissue
- Abortive infection = early viral proteins that cause cell death
- Lytic/Virulent viruses = Viruses that enter only into a productive relationship
- Temperate viruses = viruses that establish either a productive/non-productive relationship with host cell
What are the possible consequences of viral infection for the host cell?
- CYTOCIDAL INFECTION (NECROSIS) = cell destruction
- APOPTOSIS = cell suicide -> interrupts viral replication cycle
- NONCYTOCIDAL INFECTION = does not destroy host cell (but may be destroyed by secondary immunological reactions)
- LATENT INFECTION = neither replicates or destroys
- TUMOUR TRANSFORMATION = viral infection transforms into a cancer cell
What is the pathogenesis of viruses?
1) Transmission of virus + entry into the host
2) Replication of virus + damage to cells
3) Spread of virus to other cells + organs
4) Immune response
5) Persistence of virus in some instances
Describe how viruses are transmitted + portal of entry
- Main portals are respiratory, GI and genital tracts, skin, across placenta, blood
- Transmission from mother to offspring is “VERTICAL TRANSMISSION” + all other transmissions (faecal-oral, respiratory aerosol, insect bite) are “HORIZONTAL TRANSMISSION”
What are symptoms of viral disease usually caused by?
Death of infected cells and a consequent loss of function
What is Immunopathogenesis?
The process by which the symptoms of viral disease are caused by the immune system rather than the killing of cells by the virus
TYPES:
- Killing of virus-infected cell by cytotoxic T cells
- Formation of virus-antibody complexes deposited in the tissues
Describe the Innate immune response
- Largely mediated by IFNs
[induce expression of enzymes that block viral replication -> antiviral state] - Able to recognise viruses as “foreign”
- Presence of cytokines is one of the earliest indications of viral infection
Describe the adaptive immune response
- Second-line of defence if innate defences are overwhelmed
- Comprised of antibodies + immune cells (B cells and T cells)
- Key feature: MEMORY
What are the several ways of immune evasion?
- Some viruses encode the receptors for various mediators of immunity (e.g IL-1 and TNF) -> when released from virus-infected cells, proteins bind to immune mediators + block their functions -> “CYTOKINE DECOYS”
- Reduce the expression of MHC class I proteins -> reducing ability of cytotoxic T cells to kill virus-infected cells
- Synthesis of RNAs that block phosphorylation of an initiation factor (eIF-2) -> reduces ability of interferon to block viral replication
- Have multiple antigenic types (serotypes)
How do viruses affect the immune system?
- CYTOTOXIC T CELLS = reduces MHC class I proteins -> decreases killing by cytotoxic T cells
- HELPTER T CELLS (Th-1) = Block IL-12 which reduces formation of Th-1 cells -> decreases cell-mediated immunity
- INTERFERON = blocks synthesis of IF by virus-infected cells + blocks synthesis of kinase that phosphorylates initiation factor-2
- INTERLEUKINS = encode receptors for immune mediators (secreted by infected cells) -> bind mediators + inactivate them
- CHEMOKINES = encode chemokine-binding protein -> inhibits migration of inflammatory cells to sites of infection
- COMPLEMENT = encodes protein that binds to C3b -> blocks opsonisation + ability to participate in forming the membrane attack complex
What are the different states that viruses can persist in the host?
- Carrier state = people who produce virus for long periods of time + serve as a source of infection for others
(e.g hepatitis C) - Latent infections = do not produce virus at present time but can be reactivated at a subsequent time
(e.g herpes) - Slow virus infections = diseases with a long incubation period
What are the direct and indirect methods in which viruses can be diagnosed?
DIRECT METHODS
- virus isolation
- genome detection
- NS1 detection
INDIRECT METHODS
- serology IgM
- serology IgG
What are the different lab procedures that can detect viruses?
- Identification in Cell culture
[“cytopathic effect” (CPE) in cell culture + antibody-based test e.g fluorescent antibody, complement fixation, ELISA] - Microscopic identification
[Inclusion bodies seen in nucleus/cytoplasm of infected cells + Multinucleated giant cells + Electron microscopy] - Serologic procedures
[presence of IgM = diagnose current infection, presence of IgG = not used to diagnose current infection] - Detection of viral antigens & nucleic acids
What is Selective toxicity?
The ability of a drug to inhibit viral replication without significantly damaging the host cell
ADDITIONAL INFO:
difficult to achieve a high degree of selective toxicity with antiviral drugs since virus can only replicate within cells + uses many cellular functions
What are the different types of Antiviral drugs and their MOA?
- Nucleoside/Nucleotide analogs
- Reverse transcriptase inhibitors
- Protease inhibitors
[inhibit viral protease required at late stage of replicative cycle to cleave the viral polypeptide] - Integrase inhibitors
[lock activity of “viral integrase” -> without integration of virally encoded DNA into host chromosome, life cycle cannot continue] - Fusion inhibitors = disrupt fusion of viral envelope with cell membrane
What are the two classes and the different types of vaccines?
CLASSES:
- those that contain live virus whose pathogenicity has been attenuated
- those that contain killed virus
DIFFERENT TYPES:
- live attenuated vaccine
- chimeric live attenuated vaccine
- inactivated vaccine
- subunit vaccine
- nucleic acid-based vaccine
EXPLANATION: the purpose of viral vaccines is to use the adaptive immune response of the host to prevent viral disease
Why are live viral vaccines preferable to killed vaccines?
- Induce a higher titer of antibody -> longer-lasting protection
- Induce a broader range of antibody (both IgA and IgG, not just IgG)
- Activate cytotoxic T cells -> kill virus-infected cells
What is the disadvantage of live-viral vaccines?
- Reversion to virulence -> should not be given to immunocompromised individuals or to pregnant women
- Vaccines grown in chick embryos should not be given to those with an anaphylactic reaction to eggs
Describe the structure of a bacteriophage
- Capsid head
- Nucleic acid (DNA)
- Tail (composed of collar + sheath)
- Baseplate
- Spikes/Tail fiber
Which viruses are Orthomyxoviruses and Paramyxoviruses?
ORTHOMYXOVIRUSES
- influenza virus
PARAMYXOVIRUSES
- parainfluenza viruses
- measles virus
- mumps virus
What are the different types of Influenza?
- Influenza A
[causes worldwide epidemics (pandemics) of influenza + main host are humans, swine, avians, equines, marine animals, bats] - Influenza B
[causes major outbreaks of influenza + main host are humans and seals] - Influenza C
[causes mild respiratory tract infections but does not cause outbreaks of influenza]
EXPLANATION: Pandemics occur when a variant of influenza A virus that contains a new haemagglutinin against which people do not have pre-existing antibodies is introduced into the human population
Describe the structure of Influenza
- Composed of a segmented single-stranded RNA genome + a helical nucleocapsid + an outer lipoprotein envelope
- Contains an RNA-dependent RNA polymerase -> transcribes the NEGATIVE-POLARITY genome into mRNA
- Envelope covered with spikes [HEMAGGLUTININ + NEURAMINIDASE]
What is the function of Hemagglutinin and Neuraminidase?
Hemagglutinin = binds to cell surface receptor to initiate infection of cell
Neuraminidase = cleaves neuraminic acid (sialic acid) to release progeny virus from infected cell
Influenza viruses show changes in antigenicity of their hemagglutinin and neuraminidase proteins, what are the two types of antigenic changes?
- Antigenic shift = major change based on reassortment of segments of the genome RNA
[appear infrequently] - Antigenic drift = minor change based on mutations in the genome RNA
[appear virtually every year]
What is a Non-structural protein called “NS-1”?
An important determinant of the virulence of the virus encoded by the genome
FUNCTION:
Inhibits the production of Interferon mRNA -> innate defences are reduced + viral virulence is correspondingly enhanced
Describe the antigenic drift/shift in influenza A and B
- Influenza A animal viruses are the SOURCE OF RNA SEGMENTS that encode the antigenic shift variants that cause epidemics among humans
- Influenza B has no animal source of new RNA segments -> does NOT undergo antigenic shifts -> but undergoes enough antigenic drift that current strain must be included in new influenza vaccine each year
What is Influenza?
An infectious disease that is caused by influenza viruses
TRANSMISSION & EPIDEMIOLOGY:
- Transmitted by airborne respiratory droplets
- Occurs in winter months in northern hemisphere but year-round in tropics
- Bacterial pneumonia secondary to influenza cause a significant number of deaths (especially in elderly)
PATHOGENESIS:
- After virus in inhaled, NEURAMINIDASE degrades the protective mucous layer, allowing virus to gain access to cells of upper + lower respiratory tract
SYMPTOMS:
- Fever
- Myalgia
- Headache
- Sore throat
- Cough
COMPLICATIONS:
- Pneumonia caused by Staphylococcus aureus
- Reye’s syndrome
[encephalopathy + liver degeneration -> life-threatening complication in children after influenza B exposure]
LAB DIAGNOSIS:
- FLU OIA and QuickVue tests -> detect viral antigen using monoclonal antibodies
- ZSTATFLU tests -> detect viral neuraminidase
PREVENTION:
- immunisation
What do Parainfluenza viruses cause?
- “Croup” (acute laryngotracheobronchitis), laryngitis, bronchiolitis and pneumonia is CHILDREN
- A disease resembling the common cold in ADULTS
Describe the structure of Paramyxoviruses
- Composed of one piece of single-stranded RNA, a helical nucleocapsid and an outer lipoprotein envelope
- Contains RNA dependent RNA Polymerase -> transcribes NEGATIVE-POLARITY genome into mRNA
- Contain a hemagglutinin + neuraminidase on the same spike
What are the different types of Parainfluenzas?
- Parainfluenza 1 = major cause of “acute croup” (laryngotracheitis) in infants/young children + URI, pharyngitis and tracheobronchitis in all ages + outbreaks occur in fall months
- Parainfluenza 2 = associated with croup, mild URI and acute lower respiratory disease in children + outbreaks occur in fall months
- Parainfluenza 3 = major cause of severe lower respiratory disease in infants and young children + URI or tracheobronchitis in adults + happens in all seasons
- Parainfluenza 4 = least common + associated with mild upper respiratory illness only
What is the progression of Parainfluenza viruses?
Onset begins as a mild upper respiratory infection with variable progression over 1-3 days to involvement of middle or lower respiratory tract
Duration varies from 4-21 days but is usually 7-10 days
What is Respiratory Syncytial Virus?
The most important cause of pneumonia and bronchiolitis in infants and causes otitis media in children and pneumonia in elderly + chronic cardiopulmonary patients
PATHOGENESIS:
infects the bronchi, bronchioles and alveoli of lung -> acute phase of cough, wheezing and respiratory distress lasts 1-3 weeks
TRANSMISSION & EPIDEMIOLOGY:
- Occurs via respiratory droplets + direct contact of contaminated hands with nose or mouth
- Causes outbreaks of respiratory infections every winter + in hospitalised infants
SYMPTOMS:
- Fever
- Dry or wet cough + difficulty breathing/wheezing
- Sneezing + runny nose
- Loss of appetite/refuse breastfeed
DIAGNOSIS:
- Virus isolation (usually in monkey kidney cell cultures)
- PCR
- Serology
- Immunofluorescence or immunoenzyme assays
TREATMENT:
- Adequate oxygenation + ventilatory support
- Close observation for bacterial super-infection and right-sided heart failure
- Ribavirin aerosol
PREVENTION:
- Monoclonal antibody + immune globulin for prophylaxis
- No vaccine
- Breastfeeding (to increase immunity naturally)
- Avoid crowded areas
Describe the Respiratory Syncytial Virus (RSV)
- Causes syncytium formation in cell cultures
- Enveloped RNA virus has a linear (unsegmented) genome
- Two glycoproteins (G and F) mediate attachment and syncytium formation
- Most important respiratory virus that causes severe infection in infants
What are the common epidemiological and clinical characteristics of Measles and Mumps?
- Worldwide distribution with a high incidence of infection in non-immune individuals
- Humans are sole reservoir
- Person-to-person spread by respiratory (aerosol) route
What is Measles?
A highly contagious virus caused by measles virus that is characterised by a maculopapular rash. It’s a Paramxyovirus (Morbillivirus), envelope, single-stranded RNA.
Incubation period is 10-14 days + duration is 3-5 days
TRANSMISSION:
- via respiratory droplets produced by coughing + sneezing (during prodromal period + few days after rash appears)
PATHOGENESIS:
Inoculation of respiratory tract + local replication in respiratory tract -> Lymphatic spread + Viremia -> Wide dissemination (conjunctivae, respiratory tract, urinary tract, small blood vessels, lymphatic system, CNS) -> Virus-infected endothelial cells + immune T cells -> Rash -> Recovery (have life-long immunity)
COMPLICATIONS:
- Post-infectious encephalitis
- Subacute sclerosing panencephalitis (defective measles virus infection of CNS)
- No resolution of acute infection due to defective CMI
SYMPTOMS:
- Fever
- Cough
- Conjunctivitis
- Koplik spots
[bright red lesions with a white , central dot located on buccal mucosa]
DIAGNOSIS:
- Usually diagnosed with symptoms
- Cell culture
- PCR
TREATMENT:
- No antiviral therapy
PREVENTION:
- Live, attenuated vaccine given at 15 months
[vaccine should not be given before 15 months since maternal antibody in child can neutralise the virus + reduce immune response]
What is Mumps?
A disease characterised by parotid gland swelling that occurs primarily in childhood and is caused by the Mumps virus.
Incubation period of 18-21 days and duration is 1 week
TRANSMISSION & EPIDEMIOLOGY:
- via respiratory droplets
- occurs world-widee with peak incidence in winter
- spreads airborne (coughing, sneezing), saliva (kissing, sharing drinks) and touching contaminated surface
- occurs in 5-15 years, rarely <1 years
PATHOGENESIS:
- infects upper respiratory tract and then spreads through blood to infect parotid glands, testes, ovaries, pancreas and sometimes meninges
- Lifelong immunity occurs in those who have had it
SYMPTOMS:
- Fever + malaise
- Tender swelling of parotid glands
- Loss of appetite
- Pain when eating, in ears, jaw and chin and on both sides of face
DIAGNOSIS:
- Clinically (symptoms)
- Laboratory tests
TREATMENT:
- No antiviral therapy
PREVENTION:
- Live, attenuated vaccine
- Two immunisations: at 15 months then at 4-6 years
[vaccine should not be given to immunocompromised people or pregnant women]
What are the differences between Orthomyxoviruses and Paramyxoviruses?
ORTHOMYXOVIRUSES:
- No giant cell formation
- Hemagglutinin and neuraminidase on DIFFERENT spikes
PARAMYXOVIRUSES:
- Giant cell formation
- Hemagglutinin and neuraminidase on SAME spike
