Norwalk Virus

1
Norwalk Virus

• Phuong D. Nguyen

2
Introduction

• Family Caliciviridae
• Small, nonenveloped RNA viruses
• Distinctive
• 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
• Humans are the only known hosts
• Has not been cultivated in vitro

3
Structure

• Small, round, 27 nm diameter, nonenveloped
• Amorphous surface
• () ssRNA, 7.5 kb
• Single structural protein, 60 kD

4
Clinical attributes

• Acute viral gastroenteritis
• Infectious dose
• Unknown
• Presumed to be low
• Signs and symptoms
• Abdominal pain
• Vomiting non-bloody
• Diarrhea watery, non-bloody
• Nausea
• Headache
• Low-grade fever
• Malaise
• Myalgia

• Incubation period
• 24 to 48 hours
• Cases can occur within 12 hours of exposure
• Complications
• Dehydration among young and elderly
• Medical attention
• Recovery
• 24-60 hours
• Complete
• No long-term sequelae

5
Classification

• Norwalk and Norwalk-like viruses belong to the
  genus of Noroviruses and are named after location
  of outbreak
• Norwalk, Ohio
• Snow Mountain, Colorado
• Taunton and Southampton, England
• Otofuke and Sapporo, Japan
• Sapovirus
• Formerly known as Sapporo-like virus
• Genetic homology yet distinct antigenicity among
  Norwalk and Norwalk-like viruses by immune
  electron microscopy studies

6
History

• 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

7
Multiplication

• 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
• A single capsid protein is encoded by one of the
  reading frames

8
Pathogenesis

• Fecal-oral route
• Eating contaminated food
• Raw shellfish
• Salad ingredients
• Drinking contaminated water
• Most common source
• Municipal to recreational
• Person to person
• Secondary transmission
• Direct contact

9

• 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

10
Host defenses

• Most adults have serum antibodies
• No protection against infection
• Markers or risk factors
• Studies have shown short-term immunity persisting
  for about 12 weeks
• Strain-specific
• Repeated infections likely
• Further studies
• Genetic susceptibility
• High risk of severe infection among those with O
  blood type
• Repetitive past exposures

11
Epidemiology

• 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
• 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
• Transmission through aerosolized virus-containing
  vomitus
• Very rapid secondary spread of infection

12
Diagnosis

• 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)

13
Prevention

• Frequent hand-washing with soap and warm water
• 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

14
Control

• No specific antiviral therapy for Norwalk or
  Norwalk-like viruses
• Long-lasting immunity by development of a vaccine
  not likely

15
Recent outbreaks

• 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 Torontos Sunnybrook
  Hospital closed
• March 13, 2002 50 of 550 guests infected at an
  Oscar party

16
CDCs vessel sanitation program

• 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
• Norwegian Star
• Carnival Legend
• Brilliance of the Seas
• Aurora
• Star Princess
• Seven Seas Navigator
• The World
• Wind Surf