Viruses as causes of disease Flashcards
What is a virus?
- An infectious, obligate, intracellular parasite
- Comprising genetic material (DNA or RNA) surrounded by a protein coat and/or a membrane
How do viruses replicate?
Viruses can only replicate by exploiting the energy and reproductive machinery of cells of higher organisms
What are all the characteristics of viruses?
● Non-cellular structure – do not have membranes or any cell organelles
● Consist of an outer protein coat and a strand of nucleic acid, either DNA or RNA
● Come in a variety of shapes
● Do not carry out metabolic reactions on their own – require the organelles and
enzymes of a host to carry out such reactions
Do viruses or bacteria have a cell wall?
Bacteria
Do viruses or bacteria have organelles?
Bacteria
Do viruses or bacteria have DNA AND RNA?
Bacteria (viruses have 1 or the other)
Are viruses or bacteria dependent on host cell?
Virus
Are viruses or bacteria alive?
Bacteria
Give an example of a non-enveloped virus
adenovirus, parvovirus
Give an example of an enveloped virus
influenza, HIV
What is a virus envelope made of?
Coat of lipid
Describe the steps of virus replication
- Attachment: virion (virus outside cell), attaches to viral and cell receptors with surface proteins e.g. HIV
- Cell entry: uncoating of virion within cell
- loss of surface proteinsonly
central viral core carrying the nucleic acid and some associated
proteins enter host cell - Interaction with host cells: genetic material migrates to cell nucleus to genome of host cell, use cell materials (enzymes, amino acids, nucleotides) for
their replication - Replication: may localize in nucleus, cytoplasm or both
- Assembly: occurs in nucleus, in cytoplasm or at cell membrane
- Release: bursting open of cell, or by leaking from the cell over a period of time via budding/exocytosis
What products are made by viral mRNA in translation?
- Structural proteins
- Viral genome
- Non-structural proteins eg. enzymes
How do viruses cause disease?
- Direct destruction of host cells
- Modification of host cell
- Over-reactivity of immune system
- Damage through cell proliferation
- Evasion of host defences
What is an example of a virus that directly destroys host cells?
- poliovirus → host cell lysis and death after viral replication period of 4 hours
- Neurone cells → paralysis
What is an example of a virus that modifies host cells?
- rotavirus → atrophies villi and flattens epithelial cells
- Rotavirus causes diarrhoea in young children
- decreases small intestine SA
- nutrients including sugar not absorbed
- hyperosmotic state
- profuse diarrhoea
- Rotavirus causes diarrhoea in young children
What is an example of a virus that triggers an over reaction of the immune system?
- hepatitis B
- infects liver cells by binding to MHC class 1 receptor which triggers cytotoxic T lymphocytes to kill hepatocyte
- Jaundice
- Pale stool
- Dark urine
- RUQ pain
- Fever + malaise
- Itching
- infects liver cells by binding to MHC class 1 receptor which triggers cytotoxic T lymphocytes to kill hepatocyte
What is an example of a virus that damages through cell proliferation?
human papillomavirus → cervical cancer
- causes dysplasia and neoplasia → metastatic and local spread
What are 2 categories of evasion of host defences?
Cellular level
Molecular level
What are cellular level evasion mechanisms?
Latency
Cell-cell spread
What is latency?
virus able to lie dormant in certain host cells and react in the future
What is varicella zoster virus’ primary and secondary infection?
Chickenpox
Shingles (herpes zoster)
Where do HSV (herpes simplex virus) 1+2 and VZV (varicella zoster virus) lay dormant?
In nerve root ganglion
Where do EBV and HHV-8 (human herpes virus) lay dormant?
Lymphoid cells
Where do HHV-6, HHV-7 (human herpes virus) and CMV (Cytomegalovirus) lay dormant?
Myeloid cells
Give an example of a virus that spreads cell-cell
measles, HIV
What are molecular level evasion mechanisms?
- Antigenic variability/drift
- Prevention of host cell apoptosis
- Downregulation of interferon and other intracellular host defence proteins
- Interference with host cell antigen processing pathways
What does antigenic variability mean?
Virus changes structure eg. Flu virus changes every year
Give an example of a virus that prevents host cell apoptosis
eg. hepesviridae, EBV
EBV can infect lymphoid cells and stop lymphoid cells to die → lymphoma
What does downregulation of interferon and other intracellular host defence proteins lead to?
Neighbouring cells become susceptible to infection
What does Interference with host cell antigen processing pathways lead to?
Can’t present abnormal antigen to immune system
eg. herpesviridae, measles, HIV
