How Viruses Cause Disease Flashcards
simply, what is a virus?
an infective, simple micro-organism, not capable of independent existence, therefore requiring a host cell to reproduce
basic structure of a virus
DNA or RNA
capsid protein
lipoprotein envelope
how are viruses classified?
- the type of nucleic acid they carry
and - the presence/absence of an envelope
give the basic principles of the virus life cycle in human cells (5)
- attachment
- entry
- replication
- assembly
- release
describe the attachment phase for viruses.
- virus enters body
- randomly collides with host cell
- VIRAL TROPISM - determines if virus can attach and enter via appropriate receptors
what are the different types of tropism?
host tropism
- the difference between human and dog
tissue tropism
- difference between lung and kidney
cellular tropism
- difference between the receptors on cell
describe the entry phase for virus’.
enveloped viruses
- enter via membrane fusion (bind with receptors) or endocytosis
non-enveloped viruses
- enter via endocytosis or penetration
what is endocytosis?
the substance is brought into the cell and internalised
in normal human cells, how do cells replicate?
- proteins produced by transcription of DNA into mRNA
- mRNA translated into amino acids
describe the replication phase of virus’
- virus removes capsid
- exposes genome
- viral mRNA mimics host mRNA
- host cell makes viral proteins
- viral proteins make virions, structural proteins, enzymes - proteases and ones needed for replication
what does the replication process depend on?
RNA or DNA
single or double stranded
HAS TO BE VIRAL mRNA
what virus can immediately use host cell processes for translation of their proteins and why?
hep C
- single stranded and their genome is already mRNA, 5’ to 3’
how do other virus’ create mRNA in the replication phase?
- single stranded negative sense RNA virus
- uses RNA dependent RNA polymerase
- creates positive sense mRNA via complementary base pairing
- uses positive sense mRNA for translation
describe the assembly phase of virus’
- all viral components transported to a site and put together, forming a bud
describe the 3 ways in which a virus can be released.
lysis
- the cell splits apart, dies, virus bye
exocytosis
- host cells transport system releases virions at surface
budding
- virions enclose themselves in host cell membrane and leave
describe 4 major mechanisms by which viruses cause disease in humans
disruptive cell function
inhibit host cell protein manufacture
- reduced cell function and cell death
forming inclusion bodies inside the cell
- accumulation of toxic proteins and metabolites that damage the cell
damage caused by the host immune response
what are the possible routes of entry for virus into a human body?
- mucosal contact
- parental - injection
- respiratory droplet/aerosol
- faecal-oral
- direct contact
- vertical - mother to child
what must there be in the entry phase, for a virus to establish effective entry?
- sufficient dosage of the virus
- host cell must be: acceptable, susceptible and permissive
- a host immune system which is inadequate or absent
how can the host immune response damage and cause further disease?
may be excessive release of antibodies, interferons and pro inflam cytokines
the extent of infection may be localised or systemic, expand on these.
localised
- shorter incubation period
- virus remains at the site of entry
- may have focal lesions and affect large areas of the same mucous membrane
generalised
- longer incubation period
- initial replication
- generalised spread via blood to lymph notes or via nerves
- virus can reach distant organs
give examples of viral infections in the skin, respiratory tract, liver and CNS.
skin rashes
- varicella zoster
respiratory tract
- localised infection - influenza
- generalised infection - measles
liver
- hepatitis virus’ have tropism for hepatocytes in the liver
CNS
- rabies via nerves
- polio via bloodstream
illness may be acute or chronic, describe both.
acute
- present quickly
- mild with spontaneous recovery OR rapid decline and death
- either easily transmitted or very infectious
chronic
- infection persists
- immune system cant fully control
- damages host cells over time
e.g. hep c and HIV
what is latency? where is it established?
when the virus infects the cell but doesn’t replicate or make progeny
- it periodically replicates
e.g. herpes simplex cold sores
established in the neurone of the dorsal root ganglia
what is viral transformation?
some viruses have potential to induce malignant change by disrupting normal expression of host cell response
SO, what are the clinical stages of viral infection?
- incubation period - no symptoms
- virus attaching - prodromal period
- symptoms caused by early immune response
- non-specific illness - specific-illness period
- specific effects of virus on target cells
- immune response immunologically attack - recovery period
give examples of therapeutic interventions that counteract viral pathogenic mechanisms
- antiviral drugs - interrupt virus life cycle
- vaccination
