GI viruses Flashcards
H
What are the general features of viruses causing acute gastroenteritis?
and list some examples
General features
- Small, but very tough
- Non-enveloped
- Resistant to drying
- Hard to disinfect
- Rapid disease course
- Incubation down to 24 hr
- Symptoms from 24 hr to 7 days
- Spectacular replication
- Low minimal infectious dose, ≤ 10 infectious units
- Massive yield, up to 1010 infectious doses excreted
Viruses that cause gastroenteritis
- Rotavirus
- Pediatric diarrhoea
- Norovirus
- Epidemic gastro
- Adenovirus
- Serotypes 40 and 41 (less frequent)
- Astrovirus
- Not discussed
- Many not identified
Describe the features of rotavirus
- Characteristic wheel shape → ‘Rota’
- Seven groups (A-G) defined based on broad serology
- Group A are the major cause of infantile diarrhoea
- A complex three-layered virion
- Complete particles
- Rough particles
- Cores
Reoviruses
- Respiratory, Enteric, Orphan = Reo
- Found in healthy adults by cell culture in the ’50s
- Orphan virus = not associated with disease
- Characteristics
- dsRNA, ~20 kb
- Segmented genome
- Non-enveloped virion with 3 capsid layers
- Replicates in the cytoplasm
- Other reoviruses
- Most remain orphans
- Infect mammals, birds and reptiles
H
What is the public health significance of rotavirus?
- Significance: 17% of child mortality attributable to diarrhoea, 50% of which is rota related
- similar contribution to paediatric diarrhoea in developed/ing
What are the consequences fo the rotavirus lifestyle?
- Segmented dsRNA genome
- Encodes and packs a RNA dependent RNA polymerase
- Genome cannot act as mRNA
- Sequesters dsRNA within cores
- dsRNA triggers innate immune responses
- Segmentation allows easy shuffling of genes between viruses in co-infections
- Leads to increased genetic diversity, quickly
- Encodes and packs a RNA dependent RNA polymerase
- Replication entirely in the cytoplasm
- A ‘viroplasm’ or virus factory set up
- Non-enveloped viron
- Virus released by cell lysis or secretion, can’t ‘bud’ out
Describe rotavirus disease and pathogenesis
- Typical disease course
- 3 days of vomiting that starts first
- 5 days of diarrhoea that overlaps the vomiting
- Most emphasis is on the cause of diarrhoea
- Direct viral damage to enterocytes at the tips of villi
- Shortening of villi, malabsorption and secretion before, after, during
- BUT diarrhoea does not entirely correlate with damage
- Work in animal models suggests it’s more complex
Pathogenesis is complicated:
- malabsorption leading to enterocyte damage –> diarrhoea
- secretion and increased motility stimulated by ENS —> diarrhoea
- note that the virus protein NSP4 has suspected roles in all 3 mechanisms of diarrhoea - unclear if has a role in humans and in active infections
Describe rotavirus transmission and epidemiology
Transmission
- Fecal-oral
- Environmental contamination
- Not generally associated with food
- Hospital-acquired infection not uncommon
Epidemiology
- Up to 90% of children have anti-rotavirus antibody at 3
- Adult disease frequently subclinical
- Seasonal
- Serotype appearance and dominance can vary
Describe the genome and proteins, serotypes of rotavirus
- The rotaviruses genome is in 11 segments
- 1 segment per gene
- Two genes encode the major virion surface proteins
- VP7 = G and VP4 = P
- G and P are variable giving rise to serotypes
- VP4 (P)
- VP7 (G)
- Other genes encode:
- Other virion proteins
- Polymerase
- Virulence factors
- unknown proteins
Describe change in sertotypes
- G1 to G4 were most prevalent worldwide into the 1990s
- G9 and G12 now important in several countries
- Data below were collated from across Australia
H
Describe rotavrisuses and discuss intussception risk
- Multiple infections with rotavirus are possible
- BUT these are often with different serotype
- AND are generally much less severe
These suggest that a vaccine might work - Live attenuated vaccines best explored
- Single genotype (attenuated human rotavirus)
- Rotarix (GlaxoSmithKline) G1P1[8]
- Mixtures of reassortant viruses
- RotaShield (withdrawn after a single year in 1999, long story…)
- RotaTeq (CSL/Merck) G1, 2, 3, 4 P1[8]
- Single genotype (attenuated human rotavirus)
- Rotarix and RotaTeq in the National Immunisation Program as of 2007
- Given at 2 and 4 (R’rix) or 2, 4 and 6 months (R’Teq)
Multivalent rotavirus vaccines (e.g. RotaTeq)
- Reassortants have different gene segments from different viruses; on attenuated bovine rotavirus backbone
- rotateq has 4 G-types and one human P-type in its collecton
Rotavirus in Australia 2008 vs 2014
- Hospitalisations and Emergency department visits reduced by 71% for under 5s
- 7000 admissions avoided
Source: Immunise Australia website
Impact on mortality
- Data on mortality from Mexico
- decreased significantly
Richardson V et al. N Engl J Med 2011;365:772-773.
Does the vaccine drive serotype change?
- In Australia different states use different vaccines
- Rotarix (G1, P8) states (inc NSW and ACT)
- RotaTeq (G1,2,3,4, P8) states (inc Vic, Qld, SA)
- short answer: yes, selecting for those with different serotype, but does not seem to confer resistance
- no data suggesting reduced efficacy of vaccines
- may be down to steady P serotype
Roczo-Farkas et al. Communicable Diseases Intelligence Volume 40 No 4 – December 2016
Rotavirus vaccine and intussusception
- There is a small increase in risk of intussusception, especially within a week of the first dose - data is suspect
- TGA, 2011
- Minimise risk:
- Stick to schedule (esp. upper age limit)
- Don’t vaccinate risk groups (immunocompromised or ill)
- Vigilance, advise parents (severe colic, fever, bloody stool, drawing legs up)
H
Discuss norovirus and challenges with culture
- An outbreak of gastro occurred at a Norwalk school
- A rectal swab taken and a bacteria-free sample made
- Fed to adult volunteers, their stool collected
- Serially passaged to other volunteers
- Virus identified by immune electron microscopy
- Volunteers needed for serum and virus samples
- Similar viruses were called Norwalk-like viruses
- AKA small round structured viruses
Studying noroviruses is very hard
- Human norovirus cannot be grown in culture
- Only host is human
- Only source of norovirus is human poo
- Only way to test virus viability is to feed it to humans
Volunteers have contributed greatly…
- A mouse norovirus is the only one that grows well in culture; one other calicivirus genus can be grown
Lots of extrapolation required…
- With enough virus, the genome can be sequenced
- Individual proteins can be expressed and studied
Describe norovirus molecular biology
- Norovirus is a member of the calicivirus family
- Calici from calyx, Latin for cup
- Discovered in ’60s by EM
- Characteristics
- sense, linear, ssRNA genome, ~7.5 kb -> straight to ribisome
- or positive = genome same sense as mRNA
- Non-enveloped virion: bleach and water/soap
- Replicates in the cytoplasm
- Other caliciviruses
- Saporovirus (GI disease in humans)
- Rabbit calicivirus used as biological control agent
Noroviruses are relatively simple
- Capsid composed of one major and one minor protein
- Major capsid protein (VP1) forms virus-like particles
- The genome is a single strand of + sense RNA
- has three open reading frames
ORF1 makes a polyprotein
- A common trick used by viruses
- Cleaved into individual proteins by the viral proteinase
- A total of 8 proteins including the 2 capsid proteins
- Approximately 2500 amino acids in total
- Many mammalian proteins are bigger!
Discuss consequences of norovirus lifestyle
- Positive (+ve) sense ssRNA genome
- Encodes an RNA dependent RNA polymerase
- Genome is also mRNA
- Doesn’t need to pack the polymerase
- Encodes an RNA dependent RNA polymerase
- ORF1 is a polyprotein
- Encodes the proteinase to cleave this into active units
- Non-enveloped viron
- Virus released by cell lysis or secretion, can’t ‘bud’ out
- ‘Hit and run’ strategy for dealing with immune system
- Massive replication before even innate immunity kicks in
- note high viral load in faeces/vomit
Discuss norovirus pathogenesis
- Poorly understood
- Primary cellular tropism not established
- Villi of small intestine shortened and blunted
- Some evidence of mononuclear infiltration
- Virus released in faeces and vomit
Discuss norovirus lfiestyle
- Incubation from ½ to 2 days
- Disease course
- Acute symptoms generally last ½ to 2½ days
- Rapid disease almost diagnostic:
- Kaplan’s criteria for identifying norovirus outbreaks
Discuss norovirus routes of transmission
- Transmission
- Fecal-oral
- Food borne
- Contaminated water
- Contaminated by food preparers
- Environmental
- Long-lived on surfaces
- Aerosols?
- Explosive vomiting
