9. introduction to viruses Flashcards
LOs
- Discuss the factors which facilitate infection and clinical expression.
- Describe the structure and classification of viruses.
- Classify HIV, hepatitis B virus, varicella zoster virus and influenza viruses by family, nature of genome, nucleocapsid symmetry, and envelope status.
- Explain the concept of viral tropism.
- Outline how viruses replicate and spread within the host.
- Define the terms viral persistence & viral latency.
what happens during infection?
- organism encounters host (person)
- encounter results in exposure
- organism may then or not infect the host
- may give rise to a clinical syndrome
- and a host responds to the clinical syndrome
source and exposure routes of infection
what happens during infection?
- organism encounters host (person)
- encounter results in exposure
- organism may then or not infect the host
- may give rise to a clinical syndrome
- and a host responds to the clinical syndrome
virus factors
- These factors contribute to whether or not
there is an infection - Natural history/life cycle of the virus
- mode and efficiency of transmission
- Viral tropism
- Invasiveness
- Opportunism
- Virulence – do they cause disease
host factors
- Factors which make the host more
susceptible to viral infections - Age
- Genetic Predisposition – SNPs which make people more susceptible to certain viruses
- Immune system – Compromised or competent
- Specific immunity – active or passive
- Trauma/surgery/foreign body
basic virus structure characteristics
- DNA or RNA genome (only has ONE type of nucleic acid)
- protected by nucleic capsid made up of capsids
- may be a polymerase protein packaged with the virus
- may or may not be enveloped (characteristic of different virus families)
distinguishing characteristics of viruses (how are structural and functional proteins formed?)
-Obligate intracellular parasites (cannot replicate outside of host)(can survive for a little while outside of host tho)
- genetic material of virus encodes structural and functional proteins that are needed to make new viruses
- EG. structural genes encode capsid and glycoproteins on surface
- EG functional genes encode enzymes needed to replicate within cell
- the shorter the genome the fewer functional proteins it will have
- viruses with shorter genomes rely more on host cell for production of various enzymes needed
- New virus particles are formed by assembly of components synthesised using host cell proteins
-
characteristics used to classify viruses into families
- Type of nucleic acid (DNA or RNA)
- Symmetry of nucleocapsid
- Lipid envelope (presence or absence)
- Number of strands of nucleic acid (ss/ds) & their physical construction (e.g. segmented)
- Polarity of viral genome (e.g. positive or negative strand RNA) (pos RNA can be transcribes, neg cannot)
Cell Functions needed for Viral Propagation
- They require host cell machinery for translation of viral mRNA
- They need enzymes for replication of viral genome and assembly of new virions
- They need transport pathways to reach the sites of replication and viral assembly
- They also need an energy source
what is a nucleocapsid (core)?
The viral genome enclosed by the capsid protein coat
STRUCTURE OF VIRUSES: capsid structure
- Capsids protect the nucleic acid of the virus
- Capsids are constructed from a small number of virally-encoded protein subunits called
capsomeres - Viral particles show 3 types of symmetry:
- icosahedral
(EG. adeno, herpes) - helical
(EG. orthomyxo, paramyxo) - complex
(EG. poxviridae
STRUCTURE OF VIRUSES: icosahedral capsid (closed) structure
- This is a closed capsid
- This structure is extremely common amongst viruses
- It is composed of 20 solid equilateral triangles arranged around the face of a sphere
- The simplest capsid is made up by 3 identical subunits to form each face
- Each triangle is composed of 3 subunits therefore the minimum number of subunits in an
icosahedral capsid is 60 = (20x3) - The structural units may be composed of more than one viral protein
STRUCTURE OF VIRUSES: helical capsid structure
- This is an open capsid
- The capsomere proteins are bound to the viral
genome in a helical fashion - This is only found in RNA genomes
STRUCTURE OF VIRUSES: complex capsid structure
- Its neither purely helical nor purely icosahedral
- Poxviruses are the only viruses with
this type of symmetry - The capsid is a continuous cylinder
surrounded by a lipid layer and a
complex, proteinaceous core wall
(palisade layer)
(usually just pox viruses)
STRUCTURE OF VIRUSES: viral envelope structure
Not present in all viruses
Lipid bilayer derived from the host cell membrane
Contains viral glycoproteins (spikes) that project from the membrane.
what is the importance of viral surface proteins?
- These have significant importance to viruses
- They attach to membrane proteins (receptors) in the host
cell membrane and that determines viral tropism - They are targets for antibodies to neutralise them
- They are also used for serotype classifications
(antibody specificity)
how does the presence or absence of envelope influence whether a virus can survive outside a host ?
Basic Steps of Viral Replication
- Attachment to the cell
- Entry into the cell
- Uncoating of the capsid
- Production of viral proteins and
replication of viral genome - Assembly of new viral proteins
Gene expression and replication
- DNA genomes
- RNA genomes
- Retroviruses
MAKE MORE Q’s
more Qs
more Qs
more Qs
key cell functions required for viral propagation
EG of DNA viruses
HBV
- family?
- how is it spread?
- carriers?
- treatment?
chicken pox, caused by what virus family?
shingles, caused by what virus family?
EG of RNA viruses
How do Labs Diagnose Virus Infections
Detection of virus or viral components
- Nucleic acid amplification technology (NAT)(PCR)
- Antigen detection
- Culture
- Electron microscopy
Detection of virus-specific antibody responses by serological technique
what is viral tropism?
what affects attachment and entry of pathogen
- The need for a specific cell receptor narrows the species and the type of cells the virus can enter – viral tropism
Efficient virus infection of host cells depends on specificity interaction between proteins on the surface of the virus and “cognate” molecules expressed on the cell surface
EG of different virus receptors?
what happens in acute viral infections?
- Infect the host
- Establish a focus of infection (at site of infection or inoculation)
- Overcome the host defences
- Spread from initial focus
- Further replication
6 Exit from the host (in sufficiently large numbers to infect other hosts)
what are the sites of acute viral infection?
features of these?
Superficial Infections
* These viruses replicate locally at the site of infection within the epithelium
* Examples include influenza, rhinovirus (common cold) and noroviruses
* Short incubation period (2-4 days)
* Acute infection of short duration
Systemic Infections
* Involves a complicated trafficking of viruses for at least 2 weeks
* The virus replicates at multiple sites
* Natural infection leads to lifelong immunity (EG. measles, mumps, polio)
EG OF TRAIN of events for sars-cov-2 (DON’T NEED TO KNOW)
Outcome after viral infection of cell
- Clearance following acute phase of cell death - - e.g. influenza, viral gastroenteritis, poliovirus, measles
- Persistent infection with continued production of infectious virus and immune evasion - – e.g. hepatitis B, HIV
- Latent infection with virus persistence after initial clearance, and asymptomatic or symptomatic reactivation
- e.g. Herpes viruses – herpes simplex, varicella zoster
latent virus infection features
- Viral DNA persists but does not replicate to produce new infectious virus
- May never cause signs of disease
- May activate on one or more occasions, and be asymptomatic or cause disease
- Some latent infections may lead to malignant disease
Latent Virus Infections
what are viruses?
what must happens for them to replicate?
