ID 01-10-17 9AM Viral Gastroenteritis Kevin Messacar, MD
Viral Gastroenteritis
Reading Assignments: Chapter 190 “Viral Gastroenteritis” in Harrison’s Principles of Internal Medicine, 18th Edition.
“Norovirus and Other Human Calicivirus Infections” J. Rosenberg, Pediatrics in Review Dec 2016, 37 (12) 548-549.
Learning Objectives:
1. List/Name viruses that can cause gastroenteritis.
2. Explain the hallmarks of viral gastroenteritis differentiating it from bacterial diarrhea. Interpret situations in which a viral diagnosis in gastroenteritis is useful/ not useful. Explain how diagnosis of gastroenteritis viruses can be made.
3. Describe the transmission routes of viruses causing gastroenteritis. Recognize the factors leading to high transmission rates in epidemics (low infectious dose, high viral load in stool, prolonged asymptomatic shedding, hardy on surfaces).
4. Describe the replication and pathogenesis of Rotaviruses and Noroviruses.
5. Explainreasons for thechanging epidemiology of the different gastroenteritis viruses. Which is most common in the US? Which is most common in developing countries? Which is the biggest contributor to mortality? Which is most likely to cause epidemics in a cruise ship or in a US hospital setting? Which are associated with common-source food-borne or water-borne outbreaks?
6. Describe the problems in eliciting immunity to gastroenteritis viruses. Explain why one can get viral gastroenteritis multiple times each year.
7. Compare the types of vaccines currently licensed and in development for gastroenteritis viruses. Explain which vaccines will need to contain multiple serotypes of virus.
I. Viral Gastroenteritis Overview
A. The clinical syndrome of viral gastroenteritis
1. Signs and symptoms:
a. acute onsetwaterydiarrhea (without blood or mucous) +/-vomiting (often precedes)
b. nausea, intestinal cramping, sometimes with muscle aches and low-grade fever.
B. Epidemiology of viral gastroenteritis
1. Shortincubation period
2.Short duration of symptoms
3. Prolonged asymptomatic shedding
4. Stable: can be transmitted on surfaces, foodborne, waterbone
5. Seasonal
6. Transmission:
a. Primarily person-to-person by fecal-oral spread
i. Nosocomial infections in hospitals/nursing homes, day care, cruise ships
b. Foodborne
c. Waterborne
7. Burden of Disease:
a. US: common infection (1-2 episodes/child/yr), common cause of hospitalization (1.8 million/yr)
b. Worldwide: common infection (~5 billion cases/yr, 6-7 episodes/child/yr in resource-poor countries), common cause of death (2.5 million/yr)
8. Viral Causes:
a. Viruses cause >75% of gastroenteritis of known etiology
b. Many different virus families:
i. Caliciviruses (includes norovirus, sapovirus)
ii. Rotavirus
iii. Enteric Adenoviruses
iv. Astroviruses
c. Multiple serotypes of each diarrhea virus
d. Same syndrome may be repeated frequently as immunity to viruses in GI tract may be short-lived and many viruses with many serotypes can cause the same syndrome.
C. Pathophysiology of viral gastroenteritis
1. Local infection of intestinal epithelial cells
2. Malabsorption due to virus killing mature enterocytes
3. Local villus ischemia leading to diarrhea
4. Viral enterotoxin changing transepithelial fluid balance
D. Diagnosis of viruses that cause gastroenteritis
1. A specific viral diagnosis is not needed/useful in most clinical situations (typically a clinical diagnosis)
2. A specific viral diagnosis is useful in:
a. Outbreak situations for epidemiologic purposes
b. Immune compromised hosts
c. Severe disease
3. Problems in identifying etiologic agents using traditional methods:
a. Most enteric viruses are difficult to grow in cell cultures.
b. Difficulty in correlating virus seen by electron microscopy in stools with disease because of many inapparent infections.
c. Serological screening by RIA or ELISA can tell exposure for epidemiologypurposes, but does not always signify active infection
d. Viral antigen tests only available for some viruses
4. RT-PCR detection of viral nucleic acids in stool is most sensitive: new multiplex PCR assays can detect many of the most common viral etiologies on a single panel. This allows for “syndromic testing” rather than “pathogen-specific testing”.
5. Research considerations of studying human gastroenteritis viruses
a. Immunoelectronmicroscopy (IEM)
b. Volunteer studies
c. No animal models are available for study of most human gasteroenteritis viruses, because the human viruses will only grow in differentiated human enteric epithelial cells.
d. Innovative methods for detecting and differentiating viral genomes or isolating infectious viruses
E. Treatment and Prevention of Viral Gastroenteritis
1. Rehydration and correction of electrolytes
a. Oral rehydration with fluids containing sodium, potassium, bicarbonate and glucose (WHO oral rehydration salts)
b. Intravenous fluids when severe (and where available)
2. No antibiotics available
3. Hygiene: handwashing, proper food preparation
4. Sanitation: toilets, water supply, diaper changing
5. Environmental cleaning
6. Isolation of symptomatic patients
7. Vaccines: rotavirus vaccines available, norovirus in development
II. Caliciviruses (Norovirus)
A. Phylogeny
1. FamilyCaliciviridae
Genera
Vesivirus infects marine animals and swine (vesicular fever)
Lagovirus infects rabbits (epidemic hemorrhagic fever)
Sapovirus infects humans (gastroenteritis)
Norovirus
GenogroupsGI infects humans
GII infects humans and swine
GIII infects cattle
GIV infects humans
GV infects mice
There are many human norovirus serotypes: Norwalk, Hawaii, Montgomery
County, Cockle agent, Snow Mountain agent, Marin County agent, etc.
2. New norovirus variants arise by mutation or recombination and are then selected in hostsimmune to previously circulating viruses, causing epidemics and pandemics.
3. A new antigenically different strain of norovirus called “GII.4 Sydney” was first detected in Sydney Australia in 2012 and has spread around the world. It is currently the principal cause of norovirus outbreaks in the US.
B. Virology
1. Small (27 nm) non-enveloped ssRNA virus with cup shaped (chalice-like) indentations (golf ball).
2. Genomes of noroviruses have been cloned. They are 7.7 kb, single stranded, positive (messenger) sense. The genomes include ORF1 at the 5’ end which encodes RNA-dependent RNA polymerase and non-structural proteins; ORF2 which encodes the capsid; and the major viral capsid protein, VP1; and at the 3’ end, ORF3 which encodes a minor, highly variable structural protein, VP2. A viral-encoded protease cleaves viral polyproteins and is necessary for viral replication.
3. Human noroviruses are difficult to grow in cell culture.
C. Clinical Characteristics
1. 1/3 asymptomatic but shedding virus
2. Most commonly vomiting, watery diarrhea, nausea, cramping
3. Associated malaise, headaches, myalgias, low-grade fevers in some
4. Can lead to dehydration, especially in young infants, elderly, immune compromised
5. Incubation: 15 hours to 2 days
6. Duration of symptoms: 1-2 days
D. Burden of Disease
1. 23 million cases/yr in US
2. Most common cause of outbreaks of diarrhea in older children and adults
3. 2nd most common cause of diarrhea in young children. Has replaced rotavirus as #1 cause of gastroenteritis in areas where rotavirus vaccine in use (such as the US).
E. Epidemiology
1. Transmission: fecal-oral person to person primarily. Also by contaminated surfaces, foodborne (esp. shellfish), waterborne.
2. Seasonality: Year-round outbreaks, winter predominance
3. Shedding, often asymptomatic, can last 4 days – 8 weeks with high viral load 10 million viral particles/mL
4. Persists on surfaces, water, food
5. Low infectious dose: 10-100 viruses
6. Common cause of outbreaks on cruise ships, in hospitals and nursing homes. Now most common cause of foodborne illness in US.
F. Immunology
1. Short-lived immunity: ~6 months
a. Virus factors: strain diversity, antigenic shift
b. Host factors: antibody confers short-term protection
2. Innate resistant hosts, depends on blood group antigens as receptor. Histo-blood group antigens and secretor status controlled by the FUT2 gene (encoding alpha 1,2 fucosyltransferase) are required for binding virions to intestinal cells. Individuals that are homozygous non-secretors of FUT2 (20% of Europeans) are highly resistant to norovirus infection.
G. Diagnosis and Treatment
1.Antigen tests newly available, but RT-PCR more sensitive. New multiplex PCR panels include norovirus.
2. Treatment is currently only rehydration and electrolyte correction
3. Potential drug target: viral-encoded protease necessary for cleavage of viral polyprotein.
4. Vaccine prospects: production of virus-like particles.
a. Multiple serotypes would likely have to be in effective vaccine.
b. May require frequent reformulations is response to viral evolution
III. Rotaviruses
A. Virology and Classification
1. Virion, genome structure, and virus replication
a. Rotavirus virion has11 genome segments of double stranded RNAand contains a double-stranded RNA-dependent RNA polymerase and other enzymes.Each genome segment encodes a single stranded mRNA which makes one protein.
b. Non-enveloped, but contains 3 protein shells (like “gobstopper candy”)
i. Outer capsid layer: composed of VP7 with VP4 spikes extruding. Confers acid stability. Induces neutralizing antibody.
ii. Inner capsid layer: contains VP6 (major rotavirus group antigen)
iii. Innermost core: VP2
2.Classified by three different systems
a. Serotype: depends on antigenicity of VP7, VP4 surface proteins
b. Genotype: 12 different types (1-4, 9 infect humans; 3, 5, 6, 8 infect animals)
c. Group (A-G): based on immunologic assay to VP6 inner capsid layer. A is most frequent cause of human infections. B causes swine infection and outbreaks in humans with regional endemic disease. C causes sporadic and epidemic human disease. D-G are rare in humans.
3. Reassortment of genome segments occurs whenever two different rotaviruses infect the same cell.
a. Reassortment permits immunological selection of virus variants in lab and in nature
b. Analysis of reassortants permits assignment of protein products to each genome segment
c. Reassortant viruses can be identified by different electrophoretic mobility of genome segments (electropherotyping), but are now most commonly identified by PCR of different genome segments.
4. Replication
a. Rotavirus virions assemble at RER, where inner cores are enveloped in a transient envelope, which loses its lipid and becomes the outer protein shell of the virion in the lumen of the RER.
b. Virions are NOT infectious unless activated by treatment with trypsin
i. Trypsin is available naturally in the gut, but must be supplied in cell cultures.
ii. Trypsin cleaves an outer capsid protein and aids virus
uncoating.
B. Pathogenesis
1. Primarily affects the small intestine where it replicates in villus epithelial cells causing mononuclear inflammation leading to villus shortening and stunting.
2. Mechanism causing diarrhea is unclear. Hypotheses include decreased absorptive properties of denuded villi and a secreted enterotoxin NSP4.
a. Diarrhea has been induced in research mice and rats by inoculation with NSP4. NSP4 has been found to:
-destabilize membranes leading to cell death.
-mobilize intracellular Ca2+ activating signal transduction pathways to transport from ER
-lead to Chloride secretion which leads to water following
-potentially activating the enteric nervous system
3. High viral titers are shed in stool (>1011 viral particles/mL)
C. Clinical Manifestations
1.Up to 50% of infections are asymptomatic
2. Symptomatic infections commonly consist of abrupt onset of fever and vomiting, followed by diarrhea. Stools are explosive, nonbloody, watery and can lead to dehydration.
3. Symptoms last 4-8 days and are self-limited
4. Incubation period: 1-3 days
5. Peak viral shedding occurs on day 3, and shedding can be prolonged (~3 weeks)
D. Burden of Disease and Epidemiology
1. The single most important cause of severe infantile gastroenteritis worldwide
a.Peaks at 6-24 months of age. A majority of children will be infected by 3 years of age, nearly all by 5 years of age
2. Can lead to life-threatening dehydration in infants
a. Prior to widespread vaccination: rotaviruses were the major cause of hospitalization of children <2 years of age for dehydration due to gastroenteritis. >180,000 infants were hospitalized in USA annually for rotavirus gastroenteritis, with 20-40 annual deaths.
b. Rotaviruses continue to be a major cause of infant deathand illness in developing countries with >500,000 deaths/yr.
i.Dehydration isFULLY TREATABLEwith oral rehydration therapy, distributed through the World Health Organization, and can be treated by intravenous fluids, if the dehydrated patients have access.
ii. Severe disease is preventable by vaccination
3. Transmission is primarily fecal-oral person to person transmission. Can remain infective for long periods on surfaces. Less frequently can be waterborne or foodborne. Rare respiratory transmission has been reported.
4. Seasonality is primarily winter in temperate climates such as the US, but is year-round in the tropics.
5. High-risk groups include immunocompromised, malnourished, and elderly.
E. Diagnosis and Treatment
1. Antigen detection by ELISA: Rotazyme (aka “the diaper dipstick”) can quickly identify rotavirus. RT-PCR allows genotyping. New multiplex PCR stool panels include rotavirus.
2. No effective antiviral treatments are available. Rehydration and electrolyte correction are paramount.
F. Immunology and Vaccinology
1. There is evidence of both serum and intestinal antibody response (IgM, IgG, IgA) as well as cellular immune response. Repeated infections may occur but are milder.
2. Less than 6 month old infants are less likely to have symptomatic infections. Breast feeding has been shown to be protective.
3. High risk populations for chronic infection include cellular immune deficient and transplant patients.
4. Rotashield (1998): Quadrivalent live Rhesus rotavirus vaccine.
a. Showed good efficacy (80-95% protective) against severe disease
b. Rare association with intussusception (1/10,000), so withdrawn in 1999.
5. RotaTeq (RV5): Pentavalent live bovine rotavirus vaccine.
a. Contains outer capsid proteins of 5 human rotavirus strains, made by reassortment with bovine viral genome segments.
b. 75% protective against RV disease and 98% protective against severe disease.
c. Given in 3 dose oral regimen (2, 4, 6 months).
6. Rotarix (RV1): Monovalent live human rotavirus vaccine.
a. Provides cross-protection against other strains.
b. 85% protective against severe disease.
c. Given in 2 dose oral regimen (2, 4 months).
7. Impact: In US the number of children hospitalized with rotavirus gastroenteritis was reduced by ~85%. A possible increased risk of intussusception has been noted (1/100,000) with newer vaccines but much lower than Rotashield.
IV. Enteric adenoviruses, especially types 40 and 41.
A. Virology: Non-enveloped, icosahedral dsDNA viruses.
B. Clinical Manifestations: Adenoviruses typically associated with URI symptoms, conjunctivitis, pharyngitis, pneumonia, hemorrhagic cystitis, but serotypes 40, 41 can cause gastroenteritis. Watery diarrhea, then vomiting lasting 5-12 days (longer than most viruses). Can cause persistent severe infection in immunocompromised
C. Epidemiology: Transmitted fecal-oral person-to-person with peak incidence at 2 years of age
D.Diagnosis and Treatment: Can be diagnosed by EM,Antigen tests, PCR assays, but difficult to grow in culture. No antiviral treatment, supportive care with rehydration. No vaccine.
H. Astroviruses
A. Virology
1. Small non-enveloped viruses with single stranded, plus sense RNA genomes, star shaped capsomers.
2. Astroviruses can be grown in cultures of human intestinal epithelial cells.
3. Trypsin treatment is necessary to activate virus infectivity
4. 7 astrovirus serotypes identified to date.
B. Epidemiology
1. Cause of 2-8% of diarrheal disease in children. Astroviruses were identified in >34% of diarrhea outbreaks in day care centers by EIA with group-specific monoclonal antibody and by RT-PCR.
2. Astrovirus can be secreted for prolonged periods by immunocompromised children.
3. Astrovirus outbreaks also identified in military recruits, school cafeterias and in nursing homes. Both food-borne, common source type outbreaks and person-to-person spread have been documented.
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