Viruses and Viral pathogens of humans Flashcards
Define a pathogen
An organism that can cause disease (damage) in a host
Define pathogenicity
The ability to cause disease (Damage)
Define virulence
A relative measure of disease causing potential
Define Virulence factor
A pathogen derived substance or structure that contributes to virulence/damage
Why are Viruses not cells?
–They do not grow and divide
–They do not have cell walls, a nucleus or any other organelles seen in cells
–They are incapable of cellular metabolic activity
•They must hijack a host cells metabolic and synthetic machinery to make more virus
–Therefore, they cannot survive without a host and so are obligate parasites
Describe the general structure of a virus
–Genome (nucleic acid core)
•Contains the genetic information as ds/ss - RNA or DNA
– Capsid
•Protein shell for genome
- Gives protection during transmission
- Target for host immune response
- Made up of protein sub-units called CAPSOMERS
–Envelope (viral membrane)
•Not present in all viruses
•Contains host membrane components
–CSM or nuclear membrane
•Contains viral proteins
–Viral spikes proteins carrying receptors
•May have other proteins between envelope and capsid
–Tegument
–Matrix proteins
What is a completed virus?
This is known as Virion
Explain the known shapes of viruses
–Virus size: Typically 10 - 400nm
–Majority of viruses rod or sphere shaped
–Rods are said to have helical symmetry and spheres have icosohedral symmetry
–Can be a combination of both and have complex symmetry.
Explain the known structure of the Helical capsids
–These are assembled from rings of protomeres
–This type of symmetry enables very stable rods to be assembled
-Straight and flexible rods formed
What are complex viruses?
–Helical or icosohedral or neither
–May possess additional structures
•Pox viruses
–Small pox
–White pox
Explain the role of viral membranes in viruses
•May combat immune system
–May contain host or mimic host proteins
–Proteins for avoidance – protein A
•In enveloped viruses receptors are often on viral proteins = spikes
–Enable interactions with host cells
–Specific interactions = Tropism
•Allow entry of capsid and genome release
–Fusion with cell membrane
–Fusion with membrane of endosomes
What are the main aims for virus?
–To infect host cells
–Make more virus
–Infect more cell
–Be released
What must a virus do to infect the body?
the body virus must first reach a suitable entry site or portal of entry
List the different portals of entry for a virus
–Mucous membranes Respiratory tract Gastrointestinal tract(GI) Urinogenital tract Conjunctiva (Eye)
-Skin
Abrasions
Vectors
Iatrogenic infection.
How do viruses bind to permissive cells?
Using receptors
What is a permissive cell?
Allows replication when the virus binds to its receptors
Define a primary pathogen
They make you ill despite of your immune system
How do viruses enter the cell?
- Direct penetration – naked (non-enveloped) viruses
- Fusion with the cell membrane – enveloped viruses
- Entry via ENDOCYTOSIS or engulfment– enveloped viruses
Penetration and uncoating is essential in viral infection of cells as it allows the ?
release of the viral genome and starts the replication cycle
What is the main difference between the Lytic cycle and Lysogenic cycle?
The lytic cycle includes the virus using the cell to duplicate hundreds of times before bursting
While the Lysogenic cycle includes an integration of the virus DNA into the cells DNA and will stay this way until the environment is suitable.
Explain the two different ways the virus can exit the cell
–Direct lysis (bursting) of the infected cell
•Often naked viruses
•Most enteroviruses do this - Polio v
–Exit via enzymatic disruption inner cytoplasmic membrane
–Budding through/from the cell membrane or nuclear membrane
•Enveloped viruses
•HIV, HSV, influenza
–Both part of lytic life cycle
Explain infection and replication for viruses
–Replication in susceptible (permissive) cells at site of entry
–Overcome local innate defences
•Modulate immunity, colonise immune cells
–Spread from site of inoculation – viraemia
–Undergo further replication in target organ
–Escape from host in large enough numbers to infect new host
•Fecal, respiratory secretions, urine, semen
•Exit site not always linked to entry
What are the consequences of infection?
–Symptoms of viral infection are not caused by a secreted product - no metabolites!
Symptoms of disease often caused by
•Deregulation of cellular function
•Direct damage to cells from viral replication
•Host immune response to infection
–Stimulants of immune responses produced – cytokines
–Interferon - γ (gamma)
–A change in cellular characteristics may occur - CPE
Explain Recovery from viral infections
–Destruction of virus infected cells
•Innate and specific immune responses
–Immune response directed towards capsid/binding proteins – important viral antigens
–Immune response can prevent re-infection
- Often targets for vaccine production – influenza vaccine- formulated from strain specific H & N antigens
- If receptors blocked cant bind and infect cells–Neutralising antibodies
What causes Polio virus?
Poliomyelitis from the family Picornaviridae
Genus: Enterovirus
Containing RNA and naked
It is transmitted from human to human through faecal oral route
It takes 3 to 21 days to incubate
What does the Polio virus cause?
Polio multiplies in tonsins or Peyer’s patches then local lymph nodes
–Passes into blood
–Attacks CNS if no antibodies
–All enteroviruses are lytic, destroy host cells
–Can result in limb wasting, paralysis and death
What causes Yellow fever?
–Family: Alphavirus
–Genus: Faviviridae
–Is an example of an ARBOVIRUS
–RNA & spherical capsid
–Transmission: Human to human via mosquito vector.
–Incubation: 3 – 6 days
What does Yellow fever cause?
–Spreads from the lymph nodes and damages liver to cause jaundice (hence name).
–Also infects spleen kidney and heart
.–Symptoms include, fever chills, neuralgia, back pain and vomiting.
–Can cause in haemorrhaging in infected organs.
–Infection may lead to death
