Norwalk Virus

Phuong D. Nguyen

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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

Structure

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

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

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

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

Multiplication

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Current studies on the replication strategy of Norwalk virus

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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

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

  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

  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

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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

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)

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

Control

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

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 Toronto’s Sunnybrook Hospital closed March 13, 2002: 50% of 550 guests infected at an Oscar party

CDC’s 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