37. Biological threats to the body - VIRUSES Flashcards
VIRUSES are OBLIGATE INTRACELLULAR PARASITES meaning
they MUST be able to get INSIDE the HOST CELL in order to REPLICATE and have an EFFECT
all VIRAL GENOMES/VIRUSES can ONLY FUNCTION when and how
after they are REPLICATED IN a CELL
must make mRNA that can be TRANSLATED by HOST RIBOSOMES
- HIJACK HOST PROTEIN SYNTHESIS MACHINERY to make viral proteins
FEATURES of a VIRION (VIRUS PARTICLE)
- VIRAL GENOME / NUCLEIC ACID
- viral genome surrounded and protected by PROTEIN SHELL - CAPSID
- some have Viral ENVELOPE
- envelope PROTEINS (glycoproteins)
- virus PROTEINS
not all VIRUSES have…
ENVELOPES
2 terms in grouping viruses together
- CLASSIFICATION : determining the evolutionary relationships between organisms
- TAXONOMY: assigning scientific names according to agreed international scientific rules
how are VIRUSES GROUPED (5)
ORDER
FAMILY (generally use family name)
SUB-FAMILY
GENUS
SPECIES (then talk about individual species)
Why is it important to be able to CLASSIFY viruses?
Classification makes possible PREDICTIONS
about the PROPERTIES of a virus.
e.g. replication, structure, pathogenesis & transmission
Particularly important when a NEW VIRUS is INDENTIFIED
What PROPERTIES are used for CLASSIFICATION
- SIZE of VIRION and CAPSID
- PRESENCE or ABSENCE of ENVELOPE
- SYMMETRY of CAPSID (protein shell)
- NATURE and SEQUENCE of VIRAL GENOME/NUCLEIC ACID
Another way of GROUPING viruses is based on
the… (2)
- SYMPTOMS
- ROUTE of TRANSMISSION
eg Respiratory viruses
Arboviruses (mosquitos)
Blood borne viruses
Enteric viruses
VIRUS INFECTION CYCLE - 6 STEPS
- ATTACHMENT of virus to receptors on host cell
- PENETRATION - releases capsid
- UNCOATING of CAPSID to release genetic material
- REPLICATION
synthesis of viral mRNA, viral protein for new capsids, viral nucleic acids (new genome)
- to form new viruses - ASSEMBLY of the new components
capsids form around nucleic acid - RELEASE of new viruses
by BUDDING or CYTOLYSIS
NEW VIRUSES can be RELEASED from host cell in 2 ways:
- BUDDING
takes membrane from cell and FORMS ENVELOPE - CYTOLYSIS
BREAKS CELLS OPEN
no envelope
how is BACTERIAL REPLICATION
by BINARY FISSION - DIVISION
(like human cells)
EXPONENTIAL GROWTH
2 - 4 - 8.. etc
how is VIRAL REPLICATION as opposed to bacterial
after entering cell, virion is DISASSEMBLED so NO /few infectious VIRUS DETECTED
- when all new components are made, NEW VIRUSES FORMED and RELEASED - can be DETECTED
EXPONENTIAL BURST
sudden increase from few to very many
VIRUS GENOMES can be either (5)
- DNA or RNA
- DOUBLE-STRANDED (ds) or SINGLE-STRANDED (ss)
- if SINGLE-STRANDED either POSITIVE or NEGATIVE sense
- LINEAR or CIRCULAR
- SINGLE genome or SEGMENTED (More than 1)
SINGLE-STRANDED VIRAL GENOME can be either…
POSITIVE or NEGATIVE
VIRUS GENOMES RANGE in SIZE
but relative to BACTEREIAL GENOMES and HUMAN GENOMES…
significantly SMALLER
largest viral: Herpesviruses 120-240 thousand bases
smallest viral: HIV 9750 bases
HUMAN CHROMOSOMES
smallest: 47 million
largest: 247 million basepairs
VIRAL GENOMES have to be able to MAKE
mRNA
– to be translated by ribosomes to make viral protein
– to make new virus particles (virions)
what is BALTIMORE CLASSIFICATION
– based on mRNA as the common theme
– viruses (genomes) classified depending on HOW
they MAKE mRNA (+ve sense )
how many CLASSES of VIRAL GENOMES
7
ds DNA
ss DNA
ds RNA
ss RNA (+)
ss RNA (-)
ss RNA (RT)
ds DNA (RT)
(retro-transcribing)
ALL MAKE mRNA in SPECIFIC WAY
modes of VIRAL TRANSMISSION (5)
- RESPIRATORY
- ORAL/FAECAL
- BLOOD BORNE viruses
- other BODY FLUIDS
- ARBOVIRUSES - via VECTOR eg mosquito
what are the 2 periods in VIRAL INFECTION
- INCUBATION PERIOD
- INFECTIOUS PERIOD
what is the INCUBATION PERIOD
the period BETWEEN when you are FIRST INFECTED with the virus, the virus is REPLICATING
what is the INFECTIOUS PERIOD
The period when you are RELEASING VIRUS which
can INFECT OTHERS
- CONTAGIOUS period
INCUBATION time, INFECTIOUS period, SYMPTOMS
example INFLUENZA
- INCUBATION time UPTO 7 DAYS(normally 2 -3)
- SYMPTOMS VARIES
- INFECTIOUS BEFORE SYMPTOMS show and AFTER
SYMPTOMS fade
INCUBATION time, INFECTIOUS period, SYMPTOMS
example ROTAVIRUS
- INCUBATION time around 2 DAYS
- SYMPTOMS UPTO 8 DAYS
- INFECTIOUS AFTER 2 DAYS, and can be for a DAY
OR TWO AFTER SYMPTOMS fade
how do you MEASURE HOW CONTAGIOUS viruses are
BASIC REPRODUCTIVE NUMBER R0
A means of expressing how many uninfected people can be infected by one infectious person
i.e. how well the virus SPREADS in a population
what does R0 <1 mean
the infection will generally DIE OUT in the population
(1 infected individual can infect less than 1 unaffected person)
what does R0 >1 mean
the infection will be MAINTAINED and SPREAD in the population
(1 infected individual can infect more than 1 unaffected person)
A high R0 means…
the virus will spread quickly
HOST DEFENCE MECHANISMS in immunological responses (4)
- ANATOMICAL and CHEMICAL - PHYSICAL BARRIERS
- Skin, mucous, tears,saliva, stomach acid - INTRINSIC
- IMMEDIATE, NON-SPECIFIC
- ALWAYS PRESENT in (un-infected) cells such as
autophagy/apoptosis/RNA silencing/anti-viral
proteins/recognition of invading pathogen - INNATE immunity
- IMMEDIATE (but needs to be switched on)
- key to recognising presence of virus
- Induced by the presence of infection
- e.g. NK cells/Complement/Cytokines - ADAPTIVE immunity
- SPECIFIC but slow
- T and B cells
- generating memory
what is PATHOGENESIS
mode of production or development of disease
HOW viruses CAUSE DISEASE
what is VIRULENCE
the CAPACITY of a virus to PRODUCE DISEASE in a host
you can think of VIRAL PATHOGENESIS in 2 ways:
- The EFFECT of the virus REPLICATING in the host
- The EFFECT of the IMMUNE RESPONSE
(The combined process occurring during viral infection - COMBINED EFFECT)
Do all viruses infections cause (noticeable)
disease?
NO
- subclinical infections (asymptomatic)
what is a SYSTEMIC INFECTION
SPREAD of virus THROUGHOUT the BODY
MULTIPLE ORGANS INFECTED
(Localised infection: replicate locally, doesnt spread)
define VIRAEMIA
the existence of VIRUSES/virus particles IN THE BLOOD STREAM
what can you get from SYSTEMIC INFECTION
Virus gains ACCESS into the BLOOD STREAM (lymphatic system).
- VIRAEMIA
Extremely RAPID virus DISSEMINATION (spread) THROUGHOUT the BODY
describe LYTIC and LATENT INFECTION
1ST : LYTIC
virus REPLICATING
SYMPTOMS of disease
virus production levels fall to 0 (immune response)
LATENT PERIOD
- virus still PRESENT but DORMANT
SILENT - no symptoms
some signal may cause REACTIVATION
LYTIC period again
- not always associated with symptoms
- virus production doesn’t often reach same levels
