What are Noroviruses?

Noroviruses are a group of related viruses that cause acute gastroenteritis in humans. These viruses are characterized by their non-enveloped structure and single-stranded RNA genome. They belong to the Caliciviridae family and are highly contagious, with as few as 10 virus particles being sufficient to infect a person.


  • Genome: Single-Stranded RNA
  • Capsid: Non-enveloped
  • Strains: Multiple, with GII.4 being the most predominant
  • Stability: Resilient to environmental stresses

Classification and Strains

Noroviruses are classified into seven genogroups (GI to GVII), of which GI, GII, and GIV affect humans. The GII.4 strain has been associated with the majority of norovirus outbreaks, and its ability to frequently mutate makes it particularly concerning from a public health standpoint.

What Foods Can Be Contaminated?

Commonly Affected Foods

Noroviruses have the potential to contaminate a wide variety of foods. However, certain types of food are more susceptible to contamination:

  • Shellfish: Especially oysters and clams, filter large volumes of water, concentrating pathogens like noroviruses.
  • Fresh Produce: Leafy greens, fruits, and root vegetables can be contaminated during growth or post-harvest processing.
  • Ready-to-Eat Foods: Foods that require minimal to no additional cooking or preparation, such as deli meats and salads.
  • Water: Contaminated water can transmit noroviruses, affecting all foods prepared with it.

Modes of Contamination

Direct Contamination

Occurs when the food comes into direct contact with fecal matter or vomit. This is common in the case of shellfish harvested from contaminated waters.


Occurs during the handling or preparation of food. An infected food handler can easily spread the virus to the food items they are in contact with.

Environmental Contamination

In some cases, noroviruses can be airborne, especially if there is vomiting or diarrhea in the surrounding environment, leading to contamination of exposed foods.

How Do They Affect Human Health?

Clinical Manifestations

Once ingested, noroviruses target the gastrointestinal system, primarily infecting the epithelial cells lining the small intestine. The symptoms usually appear 12 to 48 hours after exposure and may include:

  • Nausea: A feeling of unease in the stomach that often precedes vomiting.
  • Vomiting: Expulsion of stomach contents.
  • Diarrhea: Frequent loose or watery stools.
  • Stomach Cramps: Pain or discomfort in the abdominal area.
  • Fever and Malaise: Less common but may occur in some cases.

Duration and Complications

The symptoms typically last for 24–72 hours and are self-limiting. However, in certain vulnerable populations, such as infants, elderly people, and immunocompromised individuals, norovirus infection can lead to severe dehydration, requiring hospitalization.

Long-Term Effects

Generally, norovirus infection does not have long-term health implications for otherwise healthy individuals. However, repeated bouts of infection can contribute to malnutrition and developmental delays in children.

Risk Factors

  • Age: Infants and elderly people are more susceptible to complications.
  • Immunity: Those with weakened immune systems are at higher risk.
  • Pre-existing conditions: Gastrointestinal disorders can exacerbate the impact of norovirus infection.

How Common is Illness?

Prevalence and Incidence

Norovirus is considered the leading cause of acute gastroenteritis across age groups worldwide. In the United States alone, noroviruses are responsible for approximately 21 million illnesses annually. Globally, noroviruses cause an estimated 200,000 deaths each year, with a higher mortality rate observed in developing countries.

Outbreak Scenarios

  • Healthcare Facilities: Noroviruses are commonly reported in healthcare settings like hospitals and nursing homes.
  • Cruise Ships: Closed environments with shared facilities are particularly conducive to the rapid spread of norovirus.
  • Educational Institutions: Schools and universities often experience outbreaks due to the high-density population.
  • Community Gatherings: Events where food is shared can become sites of outbreaks if contamination occurs.

Economic Impact

The economic burden of norovirus illnesses is significant. In the United States, the total estimated annual cost, including healthcare expenditures and productivity loss, is approximately $2 billion.

Where Do They Come From?

Primary Sources

Human Carriers

Noroviruses are primarily spread from person to person, often through fecal-oral routes. Infected individuals can shed billions of virus particles but only a few are needed for infection.

Contaminated Water Sources

Water sources contaminated with sewage are another common origin. This is particularly relevant for shellfish that may filter and concentrate the virus.

Animals and Pets

There is currently no definitive evidence to suggest that animals play a significant role in transmitting noroviruses to humans.

Reservoirs and Persistence

  • Human Intestine: The primary reservoir for noroviruses is the human intestine.
  • Environment: Noroviruses can survive on surfaces for days to weeks, making environmental contamination a concern.

Zoonotic Potential

While some noroviruses have been detected in animals, their zoonotic potential, i.e., their ability to transmit from animals to humans, is still not clearly established.

How Are They Affected by Environmental Factors?


Noroviruses are notably resilient and can survive a wide range of temperatures. They have been found to be stable at temperatures as low as -20°C and as high as 60°C.

pH Levels

Noroviruses can survive in acidic conditions, remaining stable in pH levels as low as 3. This contributes to their persistence in acidic foods and beverages.

Disinfectants and Sanitizers

Traditional sanitizers like alcohol-based hand rubs are less effective against noroviruses. Chlorine-based disinfectants are generally more effective but require higher concentrations for full efficacy.

Moisture and Humidity

Noroviruses can survive for extended periods in both wet and dry conditions, although they tend to be more stable in moist environments.

Ultraviolet (UV) Radiation

Exposure to UV radiation can inactivate noroviruses, but it requires specific doses and exposure times to be fully effective.

Food Processing Methods

  • Cooking: High temperatures can inactivate noroviruses, but some foods like shellfish may require extended cooking times.
  • Freezing: Freezing does not effectively inactivate noroviruses.
  • Pasteurization: Effective for liquid foods but not always practical for solid foods.

How Can They Be Controlled?

Personal Hygiene Measures

  • Handwashing: Soap and water are more effective than alcohol-based hand sanitizers for removing noroviruses.
  • Avoid Food Handling: Infected individuals should not handle food for at least two to three days after symptoms subside.

Food Preparation and Storage

  • Thorough Cooking: Cooking food to a minimum internal temperature of 75°C can effectively inactivate noroviruses.
  • Safe Water: Use water that has been properly treated and is free from contamination.
  • Cross-Contamination: Use separate cutting boards and utensils for different food types to prevent cross-contamination.

Environmental Measures

  • Disinfection: Use chlorine-based disinfectants for surface cleaning.
  • Air Filtration: In enclosed spaces, use HEPA filters to reduce the risk of airborne transmission.

Surveillance and Monitoring

  • Rapid Detection: Utilize molecular diagnostic methods like RT-PCR for prompt identification of norovirus outbreaks.
  • Public Reporting: Timely reporting of cases to health authorities can help in the initiation of appropriate control measures.

Vaccines and Therapeutics

As of now, there is no commercially available vaccine or antiviral treatment for norovirus. However, several candidates are in clinical trials.

Are There Rules and Regulations?

United States

Food and Drug Administration (FDA)

The FDA has established guidelines for the safe harvesting and processing of seafood, including testing protocols for noroviruses.

Centers for Disease Control and Prevention (CDC)

The CDC provides guidelines for controlling and preventing norovirus outbreaks, especially in healthcare settings.

United States Department of Agriculture (USDA)

The USDA sets standards for the safe handling and preparation of meats and poultry but does not specifically address noroviruses.

European Union

European Food Safety Authority (EFSA)

EFSA provides scientific advice on noroviruses in food and has issued guidelines for the monitoring and control of noroviruses in shellfish and fresh produce.

World Health Organization (WHO)

WHO offers guidelines on the safe preparation and handling of food to prevent norovirus outbreaks. These guidelines are globally recognized and adopted by many countries.

Local Legislation

Some states and local governments may have additional regulations or guidelines specific to noroviruses, particularly related to local food markets and public gatherings.

Compliance and Enforcement

Non-compliance with established guidelines can result in fines, penalties, and even closure of food establishments.

Resources and Further Readings

PDF: Norovirus Illness: Key Facts – CDC

How Norovirus Spreads – CDC

Pediatric norovirus infection – Springer

Norovirus Burden and Trends – CDC

Gaps in Food Safety Professionals’ Knowledge about Noroviruses

Norovirus pathogenesis: mechanisms of persistence and immune evasion in human populations

Global prevalence of norovirus in cases of gastroenteritis – TheLancet

Persistence and Elimination of Human Norovirus in Food and on Food Contact Surfaces – ScienceDirect