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Norwalk Virus

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Norwalk Virus
Phuong D. Nguyen
Introduction
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Family Caliciviridae
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Small, nonenveloped RNA viruses
Distinctive
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Cup-shaped depressions on a spherical
capsid surface
Size (27 to 35 nm)
Genome encodes a single major polypeptide
Cause of mucocutaneous and respiratory
tract lesions in animals such as swine and
cats
Norwalk virus is the best known of
the human caliciviruses
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Humans are the only known hosts
Has not been cultivated in vitro
Structure
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Small, round, 27 nm diameter, nonenveloped
Amorphous surface
(+) ssRNA, 7.5 kb
Single structural protein, 60 kD
Clinical attributes
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Acute viral gastroenteritis
Infectious dose
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Abdominal pain
Vomiting: non-bloody
Diarrhea: watery, non-bloody
Nausea
Headache
Low-grade fever
Malaise
Myalgia
Incubation period
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Unknown
Presumed to be low
Signs and symptoms
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Complications
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24 to 48 hours
Cases can occur within 12
hours of exposure
Dehydration among young and
elderly
Medical attention
Recovery
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24-60 hours
Complete
No long-term sequelae
Classification
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Norwalk and Norwalk-like viruses belong to the genus
of Noroviruses and are named after location of
outbreak
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Norwalk, Ohio
Snow Mountain, Colorado
Taunton and Southampton, England
Otofuke and Sapporo, Japan
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Sapovirus
Formerly known as Sapporo-like virus
Genetic homology yet distinct antigenicity among
Norwalk and Norwalk-like viruses by immune electron
microscopy studies
History
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In 1968, 50% of 232 students and teachers at a Norwalk
elementary school developed acute gastroenteritis
In 1972, the Norwalk virus was discovered by Dr. Al
Kapikan from the National Institute of Allergy and
Infectious Disease by immune EM of stool samples
from the 1968 outbreak
Multiplication
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Current studies on the replication strategy of
Norwalk virus
Cloning and sequencing of genome
Three open reading frames comprise a single,
positive strand of RNA
пЃ® A single capsid protein is encoded by one of the
reading frames
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Pathogenesis
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Fecal-oral route
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Eating contaminated food
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Drinking contaminated water
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Raw shellfish
Salad ingredients
Most common source
Municipal to recreational
Person to person
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Secondary transmission
Direct contact
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After ingestion, infection of the mucosa of the
proximal small intestine, damage of microvilli, and
malabsorption of D-xylose, lactose, and fat are caused
by the virus
No invasion of the colon and thus, no fecal leukocytes
in stool
Host defenses
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Most adults have serum antibodies
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No protection against infection
Markers or risk factors
Studies have shown short-term immunity persisting for about
12 weeks
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Strain-specific
Repeated infections likely
Further studies
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Genetic susceptibility
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High risk of severe infection among those with O blood type
Repetitive past exposures
Epidemiology
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50% of all outbreaks involving acute infectious,
nonbacterial gastroenteritis in the U.S. caused by the
Norwalk virus
Second most common cause of illness in U.S. after
influenza
Occurrence typically in older children and adults
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Serum antibodies acquired during early adulthood
Camps, schools, nursing homes, military populations, cruise ships
Infection is worldwide based on seroprevalence studies
Some evidence that Norwalk is spread by the respiratory
route
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Transmission through aerosolized virus-containing vomitus
Very rapid secondary spread of infection
Diagnosis
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Virus identified in stool specimens
Antibodies measured in serum samples
by immune EM or immunoassays
Few research laboratories with necessary
reagents from human volunteer studies
Limited diagnosis to community
outbreak settings
27 state public health laboratories can
test for virus in stool or contaminated
food and water by reverse transcriptase
polymerase chain reaction (RT-PCR)
Prevention
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Frequent hand-washing with soap and warm water
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After toilet visits
After changing diapers
Before eating or preparing food
Cook all shellfish thoroughly and wash fruits and vegetables
Clean and disinfect contaminated surfaces
Sanitary disposal of sewage
No food preparation by persons infected during symptoms and 3
days after
Control
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No specific antiviral therapy for Norwalk or Norwalklike viruses
Long-lasting immunity by development of a vaccine
not likely
Recent outbreaks
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September 2, 2003: 340 sick passengers and crew
on a cruise ship Regal Princess docked in Manhattan
July 7, 2003: 28 US Airways passengers coming
back to North Carolina from an Arizona Indian
reservation were ill before they boarded the plane
December 4, 2002: 60 ill passengers aboard the
Disney cruise ship Magic in Florida
November 28, 2002: ER at Toronto’s Sunnybrook
Hospital closed
March 13, 2002: 50% of 550 guests infected at an
Oscar party
CDC’s vessel sanitation program
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140 ships participate and given scores on a 100-point
scale
A clean bill of health given to a score of 86 and over
8 ships with a 100 point score
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Norwegian Star
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