Lecture 32 - Norovirus Flashcards
Why are gastroenteric viruses hard to control?
1) By the time you feel ill or mount an immune response, most acute infections are complete.
2) Virus has spread to another host by the time immune response is mounted
Proportion of infective diarrhoeas that are viral
75%
First and second most common types of viral illness
1st - upper respiratory
2nd - gastroenteritis
Viruses that invade GAT to cause systemic illness
1)
2)
1) Hep A
2) Enterovirus
Type of cells in the intestinal epithelial monolayer
Columnar vilious epithelial cells
Common site of viral pathogen entry in GIT
M cells
Most important immune response to gastroenteric viruses
Primary immune response.
Takes 7-10 days for adaptive to mount, but viral infection often controlled after three days.
What is the adaptive immune response important for in gastroenteric viruses?
Final clearance of the virus, memory
Viral family of norovirus
Caliciviridae (ssRNA, non-enveloped)
Size of norovirus virion
25-35nm
Proportion of serious gastroenteritis cases attributable to norovirus
~20%
When is norovirus more common?
Winter
Norovirus demographic most at risk
Infects adults and children at equal rate
Norovirus incubation period
Normal is under 24 hours.
Range is 12-48 hours.
Duration of norovirus symptoms
Around 3 days
Norovirus symptoms 1 2 3 4 5 6
1) Nausea
2) Fever
3) Headache
4) Abdominal cramping
5) Vomiting
6) Watery diarrhoea
Norovirus epidemiology 1 2 3 4
1) Worldwide distribution
2) ~2 million cases/year in Australia
3) Major cause of foodborne gastro outbreaks
4) Most people are seropositive at four years
Norovirus genotype that often causes pandemics
Norovirus GII.4
Number of pandemics associated with norovirus GII.4
Six.
Proportion of norovirus outbreaks caused by GII.4 genotype
62-80%
Number of breakpoints identified in norovirus GII.4 lineage
11
Three recombination hotspots in norovirus GII.4 lineage
1) Open reading frame 1/2 overlap
2) Open reading frame 2/3 overlap
3) Within open reading frame 2 (which encodes VP1 viral capsid)
Number of capsid proteins in norovirus
Only one - VP1
Why is there such a high norovirus mutation rate?
RDRP mistakes.
Recombination
NIAID class of norovirus
Class II bioterrorism agent
How long after cessation of symptoms can norovirus be secreted?
4-6 weeks
Infective dose of norovirus
10-100 pfu
Type of water treatment that norovirus can survive
Chlorination at normal levels (EG: in a swimming pool)
Primary norovirus transmission
1
2
1) Contamination of foods at raw stage (EG: infect shellfish that filter feed infected water)
2) Application of contaminated water to crops
Secondary norovirus transmission
Infected workers contaminate food
Name of Perth cruise ship with repeated norovirus outbreaks
Sea Princess
Norovirus detection
1)
2)
3)
1) Specimen detection (EM) on stool, vomitus (can’t detect on food)
2) RT-PCR
3) Serology
Is there a human tissue culture for norovirus?
No
Type of beverage that worsens norovirus symptoms
Sugary
Norovirus vaccine
Doesn’t exist
Why is little understood of norovirus replication and pathology?
No human tissue culture that can grow norovirus.
Can only be grown in murine models, which don’t display the same symptoms as in humans
Initial observation that led to development of murine norovirus model
RAG-/-, IFNabR-/- or IFNgR-/- survived murine norovirus infection
IFNabgR-/- or STAT1-/- succumbed to norovirus infection
Which cells does murine norovirus replicate in?
Macrophages, monocytes, DCs
Size of norovirus genome
7.5kb
Genes in norovirus ORF1
Non-structural genes
Key enzymes NS6 (protease) and NS7 (RDRP)
How are norovirus capsid VP1 and small basic protein VP2 transcribed?
Subgenomic RNA
Contents of norovirus ORF2
VP1 capsid
Contents of norovirus ORF3
VP2 small basic protein
Contents of norovirus ORF4
VF1 virulence factor
How are norovirus nonstructural proteins initially translated?
As a polyprotein
How are norovirus non-structural proteins expressed?
As a polyprotein, which is cleaved by NS6 protease
Effects of norovirus on host cell architecture
Forms membranous structures in cytoplasm.
Murine norovirus replicates on these structures
Marker for norovirus replication
dsRNA
Innate immune response to MNV 1) 2) 3) 4)
1) MDA5 detects dsRNA, binds IPS1 MAV on mitochondrial surface
2) IRF3/IRF7/NFkB stimulated, enter nucleus
3) IFNab stimulates IFNabR
4) STAT1/STAT2, IRF9 bind, enter nucleus
What happens to cells deficient in MDA5 in norovirus infection?
Can’t produce cytokines to combat norovirus infection
Part of GIT that norovirus might infect
Peyers patches
