Search results for: Norovirus

What Is Norovirus?

When someone talks about having “the stomach flu,” they are probably describing acute-onset gastroenteritis caused by one of the noroviruses, which are members of the “calcivirus” family (Caliciviridae).[1] Noroviruses are entirely unrelated to influenza viruses.[2]

The Centers for Disease Control and Prevention (CDC) estimates that noroviruses cause nearly 21 million cases of acute gastroenteritis annually, making noroviruses the leading cause of gastroenteritis in adults in the United States. Norovirus is highly contagious and transmitted by infected individuals at an enormous rate. It is simple genetically and evolves quickly, and exposure does not lead to lasting immunity. It is estimated that each individual experiences norovirus five times during the course of their life.[3]

Nature has created an ingenious bug in norovirus. The round blue ball structure of norovirus is actually a protein surrounding the virus’s genetic material. [4]  The virus attaches to the outside of cells lining the intestine, and then transfers its genetic material into those cells. Once the genetic material has been transferred, norovirus reproduces, finally killing the human cells and releasing new copies of itself that attach to more cells of the intestine’s lining. 

The virus, due to its structure, is also very stable in the environment and is resistant to many sanitizers/disinfectants. It is a major concern for several sectors including health care, education, and tourism, and in food, shellfish, and produce. But most foodborne norovirus outbreaks occur in restaurants or institutional food service settings.

How Common is Norovirus?

Currently, norovirus is the most common cause of acute gastroenteritis in the United States.[5] While bacterial causes of foodborne illness, such as pathogenic Escherichia coli and Salmonella, are often cited as commonly reported sources of food poisoning, noroviruses cause 58% of foodborne illnesses acquired in the U.S. Thus, norovirus is the leading cause of outbreaks from contaminated food in this country. According to CDC estimates, this translates into about 2,500 reported norovirus outbreaks in the United States each year. Norovirus outbreaks occur throughout the year but are most common from November to April. About 1 in every 15 individuals in the U.S. will get norovirus illness annually. By five years of age, one out of every 14 children will visit an emergency room, and one out of 278 children will be hospitalized due to norovirus.

Norovirus outbreaks have been reported in many settings, including healthcare facilities, restaurants and catered events, schools, and childcare centers.[6] Cruise ships account for a small percentage (1%) of reported norovirus outbreaks overall.

Sources of Norovirus and Challenges Associated with the Virus

In addition to humans, norovirus can infect a broad range of hosts including livestock, pets, and wild animals (e.g., marine mammals and bats). Little is known about norovirus infections in most non-human hosts, but the close genetic relatedness between some animal and human noroviruses, coupled with a lack of understanding of where newly appearing human norovirus genotypes and variants are emerging from, has led to the hypothesis that norovirus may not be host restricted and might be able to jump the species barrier. To date, no animal noroviruses have been detected in human stool, but some serological evidence hints to possible transmission from animals to humans.

In humans, norovirus is transmitted primarily through the fecal-oral route, with as few as 10 to 100 virion particles needed to initiate infection. Transmission occurs either person-to-person or through contamination of food or water. It can also transmit through several other means such as droplets of vomit and fomites (surfaces contaminated with either feces or vomit) contaminated with norovirus. The infection spreads mainly among the people in crowded and enclosed places such as schools, shelters, hotels, resorts, nursing homes, cruise ships, and airline flights, even those of short duration. The probability of norovirus infections is increased among the people with compromised immune systems, as well as people following unhygienic practices. 

Historically, food is the most common vehicle of transmission for noroviruses; of 232 outbreaks of norovirus between July 1997 and June 2000, 57% were foodborne, 16% were spread from person-to-person, and 3% were waterborne. When food is the vehicle of transmission, contamination occurs most often through a food handler improperly handling a food directly before it is eaten.

How is norovirus transmitted?

Norovirus outbreaks can result from the evolution of one strain due to the pressure of population immunity. Typically, norovirus outbreaks are dominated by one strain, but they can also involve more than one strain. For example, some outbreaks associated with shellfish have been found to contain up to seven different norovirus strains. Swedish outbreak studies also reveal a high degree of genetic variability, indicating a need for wide detection methods when studying these outbreaks.[7]

Once infected, individuals begin viral shedding in stool and vomitus as soon as eight hours after exposure, with viral shedding peaking four days after exposure and, in some instances, continuing for up to 56 days. Given the high titers produced during viral shedding and the low number of norovirus particles that are needed to establish infection, it is possible that one individual could potentially infect thousands.[8]

Although norovirus-infected individuals shed the highest number of viral particles while they are ill, asymptomatic shedding is also possible for a short period of time before the onset of symptoms. Concentrations of shed norovirus particles are high both for symptomatic and asymptomatic infected individuals, and peak titers of shedding usually occur during the first five days after infection.

Food handlers and health-care workers are considered important contributing factors in the spread of norovirus both in foodborne outbreaks and in person-to-person transmitted outbreaks occurring in healthcare institutions. Several case reports have confirmed the role of infected food handlers in causing outbreaks, and the percentage of outbreaks in which food handlers were involved has been reported to be as high as 34% to 70%. In healthcare settings, nosocomial (hospital-acquired) norovirus transmission is mainly caused by symptomatic shedders, but asymptomatic transmission is not uncommon.[9]

Role of Restaurants

Food service establishments are routinely linked to foodborne illness outbreaks, and out of the over 9,000 foodborne illness outbreaks reported to the CDC between 1998 to 2004 (a vast underestimation of disease burden), over half (52%) were associated with restaurants or delicatessens.[10] Not surprisingly, noroviruses were identified in almost half of the foodborne outbreaks when a specific cause was determined, and restaurants were by far the most common setting/source for these outbreaks (64%). These findings have led the CDC and others to call upon the food service industry to instigate preventive measures and policies to curtail norovirus transmission in their facilities.[11]

Yet noroviruses are especially difficult to manage once introduced into a retail foodservice or grocery facility. Sometimes described as the “perfect human pathogen,” these viruses are able to last for up to two weeks on surfaces, are resistant to most commercially available disinfectants, and are easily spread by touch. Only a few virus particles are needed to make someone sick. In the case of a retail food setting, the presence of a single norovirus-infected staff member or customer can, within hours, lead to numerous cases of disease in the local population. Logistically, this results in staff absences, temporary closures of facilities for disinfection, and the need to dispose of potentially contaminated food, although there are also broader impacts such as legal expenses and damage to brand.[12]

The history of norovirus outbreaks has led the CDC and others to call upon the restaurant industry to instigate preventive measures and policies to curtail norovirus transmission in their facilities including handwashing, cleaning, and sanitizing, and implementing employee health policies.

Symptoms & Risks of Norovirus Infection

The most common symptoms are sudden onset of vomiting and watery diarrhea, although stomach cramps and pain also often occur. Some people experience fever and body aches. Symptoms usually start 12 to 48 hours after being exposed and typically last about 1 to 3 days.[13]

Although symptoms usually only last 1 to 2 days in healthy individuals, norovirus infection can become quite serious in children, the elderly, and immune-compromised individuals. In some cases, severe dehydration, malnutrition, and even death can result from norovirus infection, especially among children, and among older and immune-compromised adults in hospitals and nursing homes. 

Diagnosing a Norovirus Infection

Diagnosis of norovirus illness is based on the combination of symptoms, particularly the prominence of vomiting, fever, and short duration of illness. If a known norovirus outbreak is in progress, public health officials may obtain specimens from ill individuals for testing in a lab.[14] Lab tests consist of identifying norovirus under an electron microscope. A reverse transcriptase polymerase chain reaction test (RT-PCR assay) can also detect norovirus in food, water, stool samples, and on surfaces. These tests isolate and replicate the suspected virus’s genetic material for analysis. An ELISA can also be performed, which detects antigens; they are easier to perform than RT-PCR, but less sensitive and can also result in many false negatives.

Treating a Norovirus Infection

Collecting a stool sample and using molecular methods to find viral RNA is the preferred method to test for norovirus in the public health world. In many cases, however, diagnosis is made based on symptoms. Most people who become ill recover within 24 to 48 hours after symptoms with just rest and fluids to prevent dehydration. Antibiotics are not effective, as the illness is caused by a viral pathogen.[15]

There is no specific treatment available for norovirus. In most healthy people, the illness is self-limiting; however, outbreaks among infants, children, elderly, and immune-compromised populations may result in severe complications among those affected. Death may result without prompt measures. A vital part of therapy for norovirus is the replacement of fluids and minerals such as sodium, potassium, and calcium—otherwise known as electrolytes—lost due to persistent vomiting and diarrhea. This rehydration can be done either by drinking large amounts of liquids, or intravenously.

Preventing Norovirus Infection

The role of food handlers has been documented substantially, highlighting that keeping ill food handlers out of the kitchen coupled with proper handwashing is the best strategy to prevent norovirus. Outbreaks linked to infected restaurant staff in the past have led to multiple meals served from the same kitchen causing illness. In a 2006 outbreak in Michigan, the investigation revealed that several foodservice workers had been ill and continued to work, ultimately leading to environmental contamination and persistence when a line cook had vomited in the restaurant. At least 364 restaurant patrons became ill with gastroenteritis consistent with norovirus. The investigation also identified deficiencies with employee handwashing practices, cleaning, and sanitizing of food and nonfood contact surfaces.

The good news about norovirus is that it does not multiply in foods as many bacteria do. In addition, thorough cooking destroys the virus. To avoid norovirus, make sure the food you eat is cooked completely. While traveling in areas that have polluted water sources, raw vegetables should be washed thoroughly before being served, and travelers should drink only boiled drinks or carbonated bottled beverages without ice.[16]

Shellfish (i.e., oysters, clams, mussels) pose the greatest risk, and any particular serving may be contaminated with norovirus; there is no way to detect a contaminated oyster, clam, or mussel from a safe one. Shellfish become contaminated when their waters become contaminated—e.g., when raw sewage is dumped overboard by recreational or commercial boaters. Shellfish are filter feeders and will concentrate virus particles present in their environment. With shellfish, only complete cooking offers reliable protection; steaming does not kill the virus or prevent its transmission. Some researchers suggest that norovirus monitoring in shellfish areas could be a good preventive strategy. Waterborne norovirus outbreaks are ubiquitous, but difficult to recognize. Improved analysis of environmental samples would have the potential to significantly improve the detection of norovirus in shellfish waters.[17]

[1]           Tripathi, M., & Kumar, S. (2019). Developments of an Emerging Infectious Agent: Norovirus. Annual Research & Review in Biology, 31(4), 1-6. https://doi.org/10.9734/arrb/2019/v31i430054

[2]           Desai AN. (2019). What Is Norovirus? JAMA. 322(20):2032.

[3]           “Updated Norovirus Outbreak Management and Disease Prevention Guideline.” Centers for Disease Control and Prevention, 4 Mar 2011. Available at: https://www.cdc.gov/mmwr/preview/mmwrhtml/rr6003a1.htm.

[4]           Kallem, S., & Bigback, K. (2005, September 1). Caliciviridae. Retrieved September 07, 2020, from https://web.stanford.edu/group/virus/calici/2005kallem/caliciviridae.html

[5]           Chikara Ogimi, Yae Jean Kim, Emily T Martin, Hee Jae Huh, Cheng-Hsun Chiu, Janet A Englund, What’s New With the Old Coronaviruses?, Journal of the Pediatric Infectious Diseases Society, Volume 9, Issue 2, June 2020, Pages 210–217, https://doi.org/10.1093/jpids/piaa037

[6]           “Common Settings of Norovirus Outbreaks.” Centers for Disease Control and Prevention, 8 Oct 2020. Available at: https://www.cdc.gov/norovirus/trends-outbreaks/outbreaks.html.

[7]           Lopman B, et al. (2012). Environmental transmission of norovirus gastroenteritis. Curr Opin Virol. 2(1):96-102.

[8]           Marsh Z, et al. (2018). Epidemiology of Foodborne Norovirus Outbreaks – United States, 2009-2015. Food Safety. 6(2):58-66.

[9]           Lopman B, Simmons K, Gambhir M, Vinjé J, Parashar U. (2014). Epidemiologic implications of asymptomatic reinfection: a mathematical modeling study of norovirus. Am J Epidemiol. 179(4):507-12.

[10]         Hall AJ, Wikswo ME, Pringle K, Gould LH, Parashar UD. (2014). Vital Signs: Foodborne Norovirus Outbreaks – United States, 2009-2012. MMWR. 63(22):491-5.

[11]         Sabrià A, et al. (2016). Norovirus shedding among food and healthcare workers exposed to the virus in outbreak settings. J Clin Virol. 82:119-25.

[12]         Kosa KM, Cates SC, Hall AJ, Brophy JE, Fraser A. (2014) Gaps in Food Safety Professionals’ Knowledge about Noroviruses. J Food Prot. 77(8):1336-41.

[13]         “Facts About Noroviruses.” Centers for Disease Control and Prevention, 15 Oct 2018. Available at: https://www.cdc.gov/nceh/vsp/pub/Norovirus/Norovirus.htm.

[14]         Hall AJ, et al. (2011). Updated Norovirus Outbreak Management and Disease Prevention Guidelines. MMWR. 60(RR03):1-15.

[15]         “Norovirus infection.” Mayo Clinic, 5 Feb 2020. Available at: https://www.mayoclinic.org/diseases-conditions/norovirus/symptoms-causes/syc-20355296.

[16]         Lopman B, et al. (2012). Environmental transmission of norovirus gastroenteritis. Curr Opin Virol. 2(1):96-102

[17]         “Facts About Noroviruses.” Centers for Disease Control and Prevention, 15 Oct 2018. Available at: https://www.cdc.gov/nceh/vsp/pub/Norovirus/Norovirus.htm.

A restaurant in Tacoma, WA, has been temporarily closed because of an outbreak of norovirus illnesses.

The Tacoma-Pierce County Health Department reported that the Harvest Buffet at 3121 S. 38th St. was closed on Thursday morning. Staff is cleaning and sanitizing the restaurant and all ready-to-eat food is being discarded.

The outbreak was discovered when multiple people became sick after they ate at the restaurant on Oct. 6. The meal was their only common source of food, according to the health department.

Anyone who ate at the restaurant and became ill is asked to contact the health department at food@tpchd.org, report online, or 253-649-1696.

Symptoms of norovirus infection may include vomiting and/or diarrhea, nausea, muscle aches, fever, and headache. Symptoms typically start 12 to 48 hours after exposure and can last for one to three days. Most people recover without treatment, however some may need medical attention for dehydration.

People with norovirus infections can spread the infection easily to others. The virus can live on surfaces for long periods of time.

People who became ill or anyone with an ill person in their household should take steps to keep the virus from spreading. The health department recommends the following steps.

Clean vomit or diarrhea accidents immediately.

Step 1. Remove vomit or poop.

  • Pick up the chunks with paper towels or other disposable material. Wear disposable gloves if available.
  • Soak up liquids with absorbent materials. Use kitty litter or dry oatmeal for carpeted areas.
  • Double bag and discard.
  • Do not use a vacuum cleaner.

Step 2. Sanitize areas where vomit or poop was present, as well as bathroom surfaces and other things you frequently touch.

  • Disinfect hard surfaces using 1 2/3 cups of household bleach per gallon of water. Allow for 1 minute of contact time.
  • Sanitize all handles and knobs in your house with the bleach solution.
  • Linens (including clothing, towels, napkins): Wash separately in hot water and dry on high.
  • Steam clean carpets using the highest setting for heat.
  • Avoid cross-contamination (use separate sanitation cloths for bathroom and other surfaces).
  • Clean and disinfect all containers used (e.g., buckets).

Wash hands often with soap and warm water for at least 20 seconds. Especially after cleaning, restroom use, and before eating.

(To sign up for a free subscription to Food Safety News,click here)

A study in China has provided estimates on the incidence of five pathogens, with norovirus coming at the top.

By examining rates across gender and age groups, scientists can identify priority populations for targeted preventive interventions aimed at reducing disease burden. These insights help in the development of public health policies and the management of food safety risks.

The traditional passive surveillance system often misses specific pathogens and is plagued by under-reporting and under-diagnosis, complicating an understanding of the disease burden. However, a study published in China CDC Weekly found that the establishment of a laboratory-based foodborne disease surveillance platform in 2011 has enhanced disease tracking.

The study developed a pyramid model to estimate the incidence of five pathogens. The model considers steps such as patients seeking care, hospitals collecting stool specimens, labs analyzing these samples, and the subsequent identification of pathogens.

Incidence by age and gender
Scientists merged data from hospital surveillance and previous community surveys in China with data from the population census to estimate the incidence of diarrheal diseases. Pathogens included Salmonella, Vibrio parahaemolyticus, Shigella, diarrheagenic E. coli (DEC), and norovirus.

The prevalence of infection by five pathogens varied significantly between genders, with males showing a higher overall prevalence. Salmonella, DEC, and norovirus were more common in men than in women. In contrast, Vibrio parahaemolyticus was more prevalent among women.

Southern China had the highest prevalence of these pathogens, while northeastern China had the lowest. 

Norovirus showed the highest incidence rate, predominantly affecting those aged 1 to 4. However, only a fraction of these cases are attributable to foodborne transmission. Salmonella was most prevalent in infants under 1 year old, followed by the 1 to 4-year-old age group.

The highest incidence of Vibrio parahaemolyticus was among people aged 35 to 44, likely due to a preference for raw or undercooked seafood, including shellfish, said scientists. DEC was most frequent in the 20 to 24-year-old group. Shigella had the lowest incidence, with the highest rates among children aged 5 to 9.

“This study provides the first estimates of the incidence of five pathogens, classified by age and gender, derived from active surveillance data on foodborne diseases in China. These findings serve as a crucial foundation for informed decision-making and regulation in assessing food safety risks based on disease burden,” said researchers.

Mushroom poisoning
In a different outbreak report, four patients in Yunnan Province were affected by Cordierites frondosus poisoning in two incidents. Cordierites frondosus is a type of toxic mushroom.

The condition of two patients deteriorated after they were re-exposed to sunlight on the seventh day following initial poisoning. In the other incident, two patients reported a mild, needle-like sensation on areas of their skin exposed to the sun on the twelfth day after poisoning.

Scientists said it was advisable to avoid sunlight for at least two weeks following poisoning.

In May and June 2023, four cases of suspected mushroom poisoning were treated at a hospital—patients presented with photosensitive dermatitis 21 to 45 hours after ingesting wild wood ear mushrooms. The China CDC partnered with the local hospital to investigate.

Findings suggest that poisonings were triggered by the consumption of Cordierite fronds, a wild mushroom species that resembles the traditionally edible Auricularia fungi.  

Symptoms of photosensitive dermatitis include redness, swelling, itching, blister formation, and sharp pain in the facial region and hands. These symptoms are amplified upon exposure to sunlight.

The four patients were three males and one female aged 45 to 57. Two patients had gastrointestinal symptoms three hours after mushroom consumption. Development of photosensitive dermatitis in all patients occurred 21 to 45 hours after consuming 30 to 50 grams of mushrooms.

Local officials have installed signs in villages about poisonous mushrooms indigenous to Yunnan. The signs aim to educate residents on identifying toxic mushrooms and understand the risks of mushroom poisoning. Since taking this action, no other instances of similar poisoning have been reported.

(To sign up for a free subscription to Food Safety News, click here.)

Officials have revealed findings from an investigation after hundreds of people reported feeling sick after going to a celebration in the town of Berkel en Rodenrijs.

GGD Rotterdam-Rijnmond and NVWA looked into the source of illnesses. The biggest cause of gastrointestinal complaints appeared to be norovirus. A total of 918 people reported being sick after attending the event but no serious illnesses were recorded.

GGD Rotterdam-Rijnmond visited the site where King’s Day was celebrated in Berkel en Rodenrijs several times and stool samples from 11 people were examined in the laboratory.

Based on a digital questionnaire, which was partially completed by 2,680 respondents, 900 people reported being sick. Analysis showed that people who had drunk something from the stall of one café or in the café itself became ill more often. This applied to alcoholic and non-alcoholic drinks and there was no link with food.

An employee at the café became ill while working on King’s Day and was likely able to infect other people. The café followed the catering code by sending the sick person home as soon as symptoms became clear, but the virus was able to spread in the meantime. An NVWA inspection found no violations.

Officials said there are indications that norovirus was circulating in Berkel en Rodenrijs before King’s Day. Three of the 11 people with a positive norovirus result had the same type of the virus as the employee through genetic testing. They added it is likely that not everyone who became ill was infected from the same source.

(To sign up for a free subscription to Food Safety News,click orovirushere)

The FDA is advising consumers not to eat and restaurants and food retailers not to serve or sell and to dispose of Lots B231126, B240103, and B240214 of frozen, raw, half-shell oysters processed by JBR (KR 15 SP) in Tongyeongsi, Republic of Korea on Nov. 27, 2023, Jan. 4, 2024, and Feb. 15, 2024, and shipped to distributors in California because they may be contaminated with norovirus.

Frozen oysters have a long shelf life, so the FDA is recommending that restaurants, other businesses and consumers check their freezers for the implicated oysters.

Also, JJBR (KR-15-SP) Tongyeong, Republic of Korea (ROK) is recalling frozen half-shell oysters with lot number B231227 because they may be contaminated with norovirus. 

The recalled oysters were harvested from ROK Designated Area No. 1 on Dec. 27, 2023. They were processed by JBR (KR-15-SP) in Tongyeong, ROK on Dec. 28, 2023, as frozen half-shell oysters, with lot number B231227. 

These oysters were distributed by Khee Trading, Inc. (Compton, CA), which is also recalling these oysters. It is believed that these frozen oysters were further distributed to numerous locations in other states.

On May 15, 2024, JC Foods Inc., of Salt Lake City, UT initiated a voluntary recall of frozen half shell oysters, lot number B231227, that had been distributed to their retail customers in Utah.

Retailers should not serve or sell, and consumers should not eat frozen half-shell oysters with lot number B231227 that were processed by JBR of ROK on 12/28/2023, and distributed to restaurants and retailers in California, Colorado, Nevada, New Jersey and Utah. Additional distribution may have occurred, and the states are continuing to conduct trace forward investigations. The FDA is assisting with and coordinating interstate notification efforts as a result of the states’ investigations.

Symptoms of norovirus infection may include vomiting and/or diarrhea, nausea, muscle aches, fever, and headache, according to the Centers for Disease Control and Prevention. Symptoms typically start 12 to 48 hours after exposure and can last for one to three days. Most people recover without treatment, however some may need medical attention for dehydration.

People with norovirus infections can spread the infection easily to others. The virus can live o n surfaces for long periods of time.

To prevent others from getting sick always wash hands carefully with soap and warm water after using the bathroom or changing diapers. Use soap and water to clean toilets or other areas that may be soiled with stool or vomit. Hard surfaces can be disinfected with 1/3 cup household bleach mixed with one gallon of water – always wear gloves when handling bleach-based cleaners. Wash soiled clothing and bedding in hot water and detergent. Soft surfaces that cannot be laundered can be steam cleaned.

(To sign up for a free subscription to Food Safety News,click here)

What Is Norovirus?

When someone talks about having “the stomach flu,” they are probably describing acute-onset gastroenteritis caused by one of the noroviruses, which are members of the “calcivirus” family (Caliciviridae).[1] Noroviruses are entirely unrelated to influenza viruses.[2]

The Centers for Disease Control and Prevention (CDC) estimates that noroviruses cause nearly 21 million cases of acute gastroenteritis annually, making noroviruses the leading cause of gastroenteritis in adults in the United States. Norovirus is highly contagious and transmitted by infected individuals at an enormous rate. It is simple genetically and evolves quickly, and exposure does not lead to lasting immunity. It is estimated that each individual experiences norovirus five times during the course of their life.[3]

Nature has created an ingenious bug in norovirus. The round blue ball structure of norovirus is actually a protein surrounding the virus’s genetic material. [4]  The virus attaches to the outside of cells lining the intestine, and then transfers its genetic material into those cells. Once the genetic material has been transferred, norovirus reproduces, finally killing the human cells and releasing new copies of itself that attach to more cells of the intestine’s lining. 

The virus, due to its structure, is also very stable in the environment and is resistant to many sanitizers/disinfectants. It is a major concern for several sectors including health care, education, and tourism, and in food, shellfish, and produce. But most foodborne norovirus outbreaks occur in restaurants or institutional food service settings.

How Common is Norovirus?

Currently, norovirus is the most common cause of acute gastroenteritis in the United States.[5] While bacterial causes of foodborne illness, such as pathogenic Escherichia coli and Salmonella, are often cited as commonly reported sources of food poisoning, noroviruses cause 58% of foodborne illnesses acquired in the U.S. Thus, norovirus is the leading cause of outbreaks from contaminated food in this country. According to CDC estimates, this translates into about 2,500 reported norovirus outbreaks in the United States each year. Norovirus outbreaks occur throughout the year but are most common from November to April. About 1 in every 15 individuals in the U.S. will get norovirus illness annually. By five years of age, one out of every 14 children will visit an emergency room, and one out of 278 children will be hospitalized due to norovirus.

Norovirus outbreaks have been reported in many settings, including healthcare facilities, restaurants and catered events, schools, and childcare centers.[6] Cruise ships account for a small percentage (1%) of reported norovirus outbreaks overall.

Sources of Norovirus and Challenges Associated with the Virus

In addition to humans, norovirus can infect a broad range of hosts including livestock, pets, and wild animals (e.g., marine mammals and bats). Little is known about norovirus infections in most non-human hosts, but the close genetic relatedness between some animal and human noroviruses, coupled with a lack of understanding of where newly appearing human norovirus genotypes and variants are emerging from, has led to the hypothesis that norovirus may not be host restricted and might be able to jump the species barrier. To date, no animal noroviruses have been detected in human stool, but some serological evidence hints to possible transmission from animals to humans.

In humans, norovirus is transmitted primarily through the fecal-oral route, with as few as 10 to 100 virion particles needed to initiate infection. Transmission occurs either person-to-person or through contamination of food or water. It can also transmit through several other means such as droplets of vomit and fomites (surfaces contaminated with either feces or vomit) contaminated with norovirus. The infection spreads mainly among the people in crowded and enclosed places such as schools, shelters, hotels, resorts, nursing homes, cruise ships, and airline flights, even those of short duration. The probability of norovirus infections is increased among the people with compromised immune systems, as well as people following unhygienic practices. 

Historically, food is the most common vehicle of transmission for noroviruses; of 232 outbreaks of norovirus between July 1997 and June 2000, 57% were foodborne, 16% were spread from person-to-person, and 3% were waterborne. When food is the vehicle of transmission, contamination occurs most often through a food handler improperly handling a food directly before it is eaten.

How is norovirus transmitted?

Norovirus outbreaks can result from the evolution of one strain due to the pressure of population immunity. Typically, norovirus outbreaks are dominated by one strain, but they can also involve more than one strain. For example, some outbreaks associated with shellfish have been found to contain up to seven different norovirus strains. Swedish outbreak studies also reveal a high degree of genetic variability, indicating a need for wide detection methods when studying these outbreaks.[7]

Once infected, individuals begin viral shedding in stool and vomitus as soon as eight hours after exposure, with viral shedding peaking four days after exposure and, in some instances, continuing for up to 56 days. Given the high titers produced during viral shedding and the low number of norovirus particles that are needed to establish infection, it is possible that one individual could potentially infect thousands.[8]

Although norovirus-infected individuals shed the highest number of viral particles while they are ill, asymptomatic shedding is also possible for a short period of time before the onset of symptoms. Concentrations of shed norovirus particles are high both for symptomatic and asymptomatic infected individuals, and peak titers of shedding usually occur during the first five days after infection.

Food handlers and health-care workers are considered important contributing factors in the spread of norovirus both in foodborne outbreaks and in person-to-person transmitted outbreaks occurring in healthcare institutions. Several case reports have confirmed the role of infected food handlers in causing outbreaks, and the percentage of outbreaks in which food handlers were involved has been reported to be as high as 34% to 70%. In healthcare settings, nosocomial (hospital-acquired) norovirus transmission is mainly caused by symptomatic shedders, but asymptomatic transmission is not uncommon.[9]

Role of Restaurants

Food service establishments are routinely linked to foodborne illness outbreaks, and out of the over 9,000 foodborne illness outbreaks reported to the CDC between 1998 to 2004 (a vast underestimation of disease burden), over half (52%) were associated with restaurants or delicatessens.[10] Not surprisingly, noroviruses were identified in almost half of the foodborne outbreaks when a specific cause was determined, and restaurants were by far the most common setting/source for these outbreaks (64%). These findings have led the CDC and others to call upon the food service industry to instigate preventive measures and policies to curtail norovirus transmission in their facilities.[11]

Yet noroviruses are especially difficult to manage once introduced into a retail foodservice or grocery facility. Sometimes described as the “perfect human pathogen,” these viruses are able to last for up to two weeks on surfaces, are resistant to most commercially available disinfectants, and are easily spread by touch. Only a few virus particles are needed to make someone sick. In the case of a retail food setting, the presence of a single norovirus-infected staff member or customer can, within hours, lead to numerous cases of disease in the local population. Logistically, this results in staff absences, temporary closures of facilities for disinfection, and the need to dispose of potentially contaminated food, although there are also broader impacts such as legal expenses and damage to brand.[12]

The history of norovirus outbreaks has led the CDC and others to call upon the restaurant industry to instigate preventive measures and policies to curtail norovirus transmission in their facilities including handwashing, cleaning, and sanitizing, and implementing employee health policies.

Symptoms & Risks of Norovirus Infection

The most common symptoms are sudden onset of vomiting and watery diarrhea, although stomach cramps and pain also often occur. Some people experience fever and body aches. Symptoms usually start 12 to 48 hours after being exposed and typically last about 1 to 3 days.[13]

Although symptoms usually only last 1 to 2 days in healthy individuals, norovirus infection can become quite serious in children, the elderly, and immune-compromised individuals. In some cases, severe dehydration, malnutrition, and even death can result from norovirus infection, especially among children, and among older and immune-compromised adults in hospitals and nursing homes. 

Diagnosing a Norovirus Infection

Diagnosis of norovirus illness is based on the combination of symptoms, particularly the prominence of vomiting, fever, and short duration of illness. If a known norovirus outbreak is in progress, public health officials may obtain specimens from ill individuals for testing in a lab.[14] Lab tests consist of identifying norovirus under an electron microscope. A reverse transcriptase polymerase chain reaction test (RT-PCR assay) can also detect norovirus in food, water, stool samples, and on surfaces. These tests isolate and replicate the suspected virus’s genetic material for analysis. An ELISA can also be performed, which detects antigens; they are easier to perform than RT-PCR, but less sensitive and can also result in many false negatives.

Treating a Norovirus Infection

Collecting a stool sample and using molecular methods to find viral RNA is the preferred method to test for norovirus in the public health world. In many cases, however, diagnosis is made based on symptoms. Most people who become ill recover within 24 to 48 hours after symptoms with just rest and fluids to prevent dehydration. Antibiotics are not effective, as the illness is caused by a viral pathogen.[15]

There is no specific treatment available for norovirus. In most healthy people, the illness is self-limiting; however, outbreaks among infants, children, elderly, and immune-compromised populations may result in severe complications among those affected. Death may result without prompt measures. A vital part of therapy for norovirus is the replacement of fluids and minerals such as sodium, potassium, and calcium—otherwise known as electrolytes—lost due to persistent vomiting and diarrhea. This rehydration can be done either by drinking large amounts of liquids, or intravenously.

Preventing Norovirus Infection

The role of food handlers has been documented substantially, highlighting that keeping ill food handlers out of the kitchen coupled with proper handwashing is the best strategy to prevent norovirus. Outbreaks linked to infected restaurant staff in the past have led to multiple meals served from the same kitchen causing illness. In a 2006 outbreak in Michigan, the investigation revealed that several foodservice workers had been ill and continued to work, ultimately leading to environmental contamination and persistence when a line cook had vomited in the restaurant. At least 364 restaurant patrons became ill with gastroenteritis consistent with norovirus. The investigation also identified deficiencies with employee handwashing practices, cleaning, and sanitizing of food and nonfood contact surfaces.

The good news about norovirus is that it does not multiply in foods as many bacteria do. In addition, thorough cooking destroys the virus. To avoid norovirus, make sure the food you eat is cooked completely. While traveling in areas that have polluted water sources, raw vegetables should be washed thoroughly before being served, and travelers should drink only boiled drinks or carbonated bottled beverages without ice.[16]

Shellfish (i.e., oysters, clams, mussels) pose the greatest risk, and any particular serving may be contaminated with norovirus; there is no way to detect a contaminated oyster, clam, or mussel from a safe one. Shellfish become contaminated when their waters become contaminated—e.g., when raw sewage is dumped overboard by recreational or commercial boaters. Shellfish are filter feeders and will concentrate virus particles present in their environment. With shellfish, only complete cooking offers reliable protection; steaming does not kill the virus or prevent its transmission. Some researchers suggest that norovirus monitoring in shellfish areas could be a good preventive strategy. Waterborne norovirus outbreaks are ubiquitous, but difficult to recognize. Improved analysis of environmental samples would have the potential to significantly improve the detection of norovirus in shellfish waters.[17]

[1]           Tripathi, M., & Kumar, S. (2019). Developments of an Emerging Infectious Agent: Norovirus. Annual Research & Review in Biology, 31(4), 1-6. https://doi.org/10.9734/arrb/2019/v31i430054

[2]           Desai AN. (2019). What Is Norovirus? JAMA. 322(20):2032.

[3]           “Updated Norovirus Outbreak Management and Disease Prevention Guideline.” Centers for Disease Control and Prevention, 4 Mar 2011. Available at: https://www.cdc.gov/mmwr/preview/mmwrhtml/rr6003a1.htm.

[4]           Kallem, S., & Bigback, K. (2005, September 1). Caliciviridae. Retrieved September 07, 2020, from https://web.stanford.edu/group/virus/calici/2005kallem/caliciviridae.html

[5]           Chikara Ogimi, Yae Jean Kim, Emily T Martin, Hee Jae Huh, Cheng-Hsun Chiu, Janet A Englund, What’s New With the Old Coronaviruses?, Journal of the Pediatric Infectious Diseases Society, Volume 9, Issue 2, June 2020, Pages 210–217, https://doi.org/10.1093/jpids/piaa037

[6]           “Common Settings of Norovirus Outbreaks.” Centers for Disease Control and Prevention, 8 Oct 2020. Available at: https://www.cdc.gov/norovirus/trends-outbreaks/outbreaks.html.

[7]           Lopman B, et al. (2012). Environmental transmission of norovirus gastroenteritis. Curr Opin Virol. 2(1):96-102.

[8]           Marsh Z, et al. (2018). Epidemiology of Foodborne Norovirus Outbreaks – United States, 2009-2015. Food Safety. 6(2):58-66.

[9]           Lopman B, Simmons K, Gambhir M, Vinjé J, Parashar U. (2014). Epidemiologic implications of asymptomatic reinfection: a mathematical modeling study of norovirus. Am J Epidemiol. 179(4):507-12.

[10]         Hall AJ, Wikswo ME, Pringle K, Gould LH, Parashar UD. (2014). Vital Signs: Foodborne Norovirus Outbreaks – United States, 2009-2012. MMWR. 63(22):491-5.

[11]         Sabrià A, et al. (2016). Norovirus shedding among food and healthcare workers exposed to the virus in outbreak settings. J Clin Virol. 82:119-25.

[12]         Kosa KM, Cates SC, Hall AJ, Brophy JE, Fraser A. (2014) Gaps in Food Safety Professionals’ Knowledge about Noroviruses. J Food Prot. 77(8):1336-41.

[13]         “Facts About Noroviruses.” Centers for Disease Control and Prevention, 15 Oct 2018. Available at: https://www.cdc.gov/nceh/vsp/pub/Norovirus/Norovirus.htm.

[14]         Hall AJ, et al. (2011). Updated Norovirus Outbreak Management and Disease Prevention Guidelines. MMWR. 60(RR03):1-15.

[15]         “Norovirus infection.” Mayo Clinic, 5 Feb 2020. Available at: https://www.mayoclinic.org/diseases-conditions/norovirus/symptoms-causes/syc-20355296.

[16]         Lopman B, et al. (2012). Environmental transmission of norovirus gastroenteritis. Curr Opin Virol. 2(1):96-102

[17]         “Facts About Noroviruses.” Centers for Disease Control and Prevention, 15 Oct 2018. Available at: https://www.cdc.gov/nceh/vsp/pub/Norovirus/Norovirus.htm.

According to the Danish food agency, there is still uncertainty among chefs and kitchen workers about the management of norovirus.

A Danish Veterinary and Food Administration (Fødevarestyrelsen) control and guidance campaign targeted restaurants, kitchens, and shops that supply ready-to-eat (RTE) food. Work involved 829 visits from June to November 2023.

The agency asked kitchen managers what steps they should take to prevent outbreaks. Some said they were unsure how far they could go concerning an employee’s illness and health.

Scale of issue
Norovirus is the most common cause of foodborne outbreaks in Denmark. The campaign focused on companies’ training of employees regarding personal hygiene, guidance on staff notification, and employer obligations to avoid contamination with foodborne viruses.

Danish officials said that if outbreaks are to be prevented and reduced, kitchen staff and company managers need to have the necessary knowledge to operate in a hygienically correct manner and in a way that minimizes the risk of food contamination with norovirus, as well as enabling them to act responsibly if an incident occurs, by informing management.

“Several kitchen managers have replied that they have not yet introduced procedures for handling norovirus and have not instructed employees in what to do if they are affected by norovirus,” said Lene Mølsted Jensen from the Danish Veterinary and Food Administration.

“Because a sick employee can be a danger to food safety, the kitchen manager can inquire about the symptoms if the employee reports being sick with an upset stomach, just as the employee also must inform the boss about an upset stomach.”

Results showed that all visited companies had sufficient procedures around personal hygiene, including the behavior of employees in connection with symptoms of norovirus, so no sanctions were applied.

However, the fact that there were 20 outbreaks with 790 cases in 2021 and 2022 points to a difference between these findings and the companies’ actions in daily operations. The Danish Veterinary and Food Administration said detecting issues with a single pre-announced inspection can be difficult.

Main control findings
On two occasions, inspectors found sites did not have the necessary facilities for employees to maintain a high level of personal hygiene. In both cases, there was a lack of handwashing facilities.

Over half of the questionnaire respondents were unaware that General Data Protection Regulation (GDPR) rules do not prevent them from asking about an employee’s possible symptoms.

Less than half of the participants knew about a guide from the Danish Veterinary and Food Administration with advice on avoiding infecting consumers with norovirus.

More than 60 percent of companies use the Danish Veterinary and Food Administration as their primary source of information when looking for advice in the event of a suspected incident. Other sources include industry organizations and consultants.

In 2019, 2020, and 2021, outbreaks resulting from infection with norovirus represented 37 percent, 17 percent, and 22 percent, respectively, of all registered foodborne outbreaks. The number of confirmed norovirus outbreaks increased to 14 in 2021 from six in 2020.

The Danish Veterinary and Food Administration repeated bits of a control campaign from 2020/2021. The initiative is part of efforts against foodborne viruses from January 2019 to December 2023, alongside the agency’s strategy and goal of fewer people getting sick from food.

Similar work in 2022 revealed kitchen staff and food companies lacked knowledge about how foodborne viruses such as norovirus are transmitted and how to avoid passing the infection on to customers and colleagues.

Norovirus can spread through sick people, contaminated surfaces, or contaminated food and water. A person usually develops symptoms 12 to 48 hours after being exposed. Most people with norovirus illness get better within 1 to 3 days, but they can still spread the virus for another few days. Kitchen workers should stay away from work for at least 48 hours after symptoms have passed.

(To sign up for a free subscription to Food Safety News, click here.)

The FDA is warning consumers not to eat and restaurants and retailers not to sell certain raw, frozen oysters from Korea because of an outbreak of norovirus infections.

The California Department of Health notified the U.S. Food and Drug Administration of the outbreak on April 9. The FDA issued its warning on April 16.

The implicated fresh frozen half-shell oysters are from lots B231126, B240103, and B240214 processed by JBR (KR 15 SP) in Tongyeongsi, Republic of Korea. They were processed on Nov. 27, 2023, Jan. 4, 2024, and . Feb. 15, 2024, and have long shelf life when kept frozen.

On April 15, authorities at the Ministry of Oceans and Fisheries in the Republic of Korea advised the processing firm to initiate a recall of the distributed lots of frozen, raw, half-shell oysters because of possible norovirus contamination.

The oysters were shipped to distributors in California and may have been further distributed.

“Shellfish, such as oysters, contaminated with norovirus can cause illness if eaten, and potentially severe illness in people with compromised immune systems. Food containing norovirus may look, smell, and taste normal,” according to the FDA warning. 

Restaurants and retailers should also be aware that shellfish, such as oysters, may be a source of pathogens and should control the potential for cross-contamination of food processing equipment and the food processing environment. They should follow the steps below:

  • Wash hands with warm water and soap following the cleaning and sanitation process.
  • Retailers, restaurants, and other food service operators who have processed and packaged any potentially contaminated products need to be concerned about cross-contamination of cutting surfaces and utensils through contact with the potentially contaminated products.
  • Retailers that have sold bulk product should clean and sanitize the containers used to hold the product.
  • Regular frequent cleaning and sanitizing of food contact surfaces and utensils used in food preparation may help to minimize the likelihood of cross-contamination.

About norovirus
Symptoms of norovirus infection may include vomiting and/or diarrhea, nausea, muscle aches, fever, and headache, according to the Centers for Disease Control and Prevention. Symptoms typically start 12 to 48 hours after exposure and can last for one to three days. Most people recover without treatment, however some may need medical attention for dehydration.

People with norovirus infections can spread the infection easily to others. The virus can live o n surfaces for long periods of time.

To prevent others from getting sick always wash hands carefully with soap and warm water after using the bathroom or changing diapers. 

It is very difficult to kill norovirus, but its spread can be contained by using soap and water to clean toilets or other areas that may be soiled with stool or vomit. Even microscopic amounts of the virus are highly infectious.

Hard surfaces can be disinfected with 1/3 cup household bleach mixed with one gallon of water – always wear gloves when handling bleach-based cleaners. Wash soiled clothing and bedding in hot water and detergent. Soft surfaces that cannot be laundered can be steam cleaned.

(To sign up for a free subscription to Food Safety News,click here)

An agency in Ireland has published a notice to try to reduce the incidence of contaminated oysters in the market and minimize norovirus-related illnesses.

The Sea-Fisheries Protection Authority (SFPA) Food Safety Information Notice covers strategies to manage norovirus risks in oysters.

It coincides with the SFPA and Food Safety Authority of Ireland’s (FSAI) annual food safety workshop in late February, which focused on norovirus and food incidents.

Norovirus is transmitted through the consumption of food or water contaminated with fecal matter or through contact with infected individuals or surfaces. The virus can be present in discharges from wastewater treatment systems.

Recent illnesses reported

Bivalve mollusks such as oysters are known to accumulate and concentrate norovirus. This poses a health risk as these shellfish are often consumed raw or lightly cooked, making them a potential source of infection. There is no regulatory limit for norovirus relating to shellfish.

In March, two people in Spain and three in Finland fell sick with norovirus after eating oysters from France. Oysters from Ireland were linked to two norovirus cases in Finland. Norovirus in a seaweed salad from China sickened three people in Italy. Norovirus in oysters from the Netherlands affected two people in Belgium.

In separate incidents in February, three people were ill in France after eating oysters, and five cases were reported in Spain. Also in February, the Netherlands issued a Rapid Alert System for Food and Feed (RASFF) alert for rotavirus in oysters from Ireland. In March, France reported E. coli in Irish mussels.

SFPA said businesses involved in oyster production must be aware of the risk and implement appropriate management actions, especially during the high-risk winter period. These measures should be part of their food safety management system, adhering to Hazard Analysis and Critical Control Points (HACCP) principles.

Related advice and workshop

Guidance to help mitigate the risk of norovirus contamination in oysters has been developed by the SFPA, FSAI, the Marine Institute, and Bord Iascaigh Mhara (BIM).

Risk factors for shellfish-related norovirus include cold weather, low water temperatures, and high rainfall, potentially leading to sewage system overflows.

The only laboratory in Ireland currently doing norovirus testing in shellfish is the Marine Institute. Still, due to resource and scheduling issues, only a restricted number of samples can be tested. It is expected that, given demand, private commercial labs will begin to provide norovirus testing in the future. Private labs in the UK and elsewhere can provide such testing.

SFPA, FSAI, and the Marine Institute also hosted a workshop for industry in February on the requirements relating to sanitary surveys for shellfish harvesting areas and Ireland’s implementation of the related legislation.

SFPA presented data management and shellfish classification and gave an update on the sanitary survey program in Ireland. The keynote speaker was Michelle Price-Howard from the Centre for Environment, Fisheries, Aquaculture and Science (CEFAS).

(To sign up for a free subscription to Food Safety News, click here.)

Norovirus, the highly contagious illness that causes vomiting and diarrhea, has surged in the Northeast region of the United States in the past few weeks, according to recent data from the Centers for Disease Control and Prevention (CDC). CDC statistics show that food is the most common transmission vehicle for noroviruses.

While surveillance data show an increase in positive test results nationwide—as is typical for norovirus during the winter months—northeastern states have been hit especially hard, with a three-week average of 13.7 percent for positive norovirus tests. Those tests have held above a 10 percent positive rate since December 2023.

Other U.S. regions have also seen an uptick in norovirus illnesses at lower rates. For the past three weeks, southern states have seen a positive test rate of about 9.4 percent, the Midwest is at 10 percent, and western states have seen a 12.6 percent positive rate.

What is norovirus?

The Centers for Disease Control and Prevention (CDC) estimates that noroviruses cause nearly 21 million cases of acute gastroenteritis annually, making noroviruses the leading cause of gastroenteritis in adults in the United States. 

Nature has created an ingenious bug in norovirus. Norovirus’s round blue ball structure is a protein surrounding the virus’s genetic material. The virus attaches to the outside of cells lining the intestine and then transfers its genetic material into those cells. Once the genetic material has been transferred, norovirus reproduces, finally killing the human cells and releasing new copies of itself that attach to more cells of the intestine’s lining.

Humans are the only host of norovirus, which has several mechanisms that allow it to spread quickly and easily. Norovirus infects humans in a pathway similar to the influenza virus’ mode of infection. In addition to their similar infective pathways, norovirus and influenza also evolve to avoid the immune system similarly. Both viruses are driven by heavy immune selection pressure and antigenic drift, allowing evasion of the immune system, which results in outbreaks. Norovirus can survive a wide range of temperatures and in many different environments. Moreover, the viruses can spread quickly, especially in places where people are close, such as cruise ships and airline flights, even those of short duration. 

How is norovirus transmitted?

Norovirus causes nearly 60 percent of all foodborne illness outbreaks. Norovirus is transmitted primarily through the fecal-oral route, with fewer than 100 microscopic norovirus particles needed to cause infection. Transmission occurs either person-to-person or through contamination of food or water. Of 232 norovirus outbreaks between July 1997 and June 2000, 57 percent were foodborne, 16 percent were spread from person to person, and 3 percent were waterborne. When food is the vehicle of transmission, contamination occurs most often through a food handler improperly handling food directly before it is eaten.

Infected individuals shed the virus in large numbers in their vomit and stool, shedding the most viral particles while ill. Aerosolized vomit has also been implicated as a mode of norovirus transmission. Previously, it was thought that viral shedding ceased approximately 100 hours after infection; however, some individuals continue to shed norovirus long after they have recovered from it, in some cases up to 28 days after experiencing symptoms. Viral shedding can also precede symptoms, which occur in approximately 30 percent of cases. Often, an infected food handler may not even show symptoms.

What are the symptoms and risks of a norovirus infection?

Norovirus illness usually develops 24 to 48 hours after ingesting contaminated food or water. Symptoms typically last a relatively short time, approximately 24 to 48 hours. These symptoms include nausea, vomiting, diarrhea, and abdominal pain.  Headache and low-grade fever may also accompany this illness. People infected with norovirus usually recover in two to three days without serious or long-term health effects. 

Although symptoms usually only last one to two days in healthy individuals, norovirus infection can become serious in children, the elderly, and immune-compromised individuals. In some cases, severe dehydration, malnutrition, and even death can result from norovirus infection, especially among children and among older and immune-compromised adults in hospitals and nursing homes.

How do you diagnose a norovirus Infection?

Diagnosis of norovirus illness is based on the combination of symptoms, particularly the prominence of vomiting, little fever, and the short duration of illness.  If a known norovirus outbreak is in progress, public health officials may obtain specimens from ill individuals for testing in a lab. These lab tests consist of identifying norovirus under an electron microscope. 

How do you treat a norovirus infection?

There is no specific treatment available for norovirus. In most healthy people, the illness is self-limiting and resolves in a few days; however, outbreaks among infants, children, elderly, and immune-compromised populations may result in severe complications among those affected. Death may result without prompt measures. Replacing fluids and minerals such as sodium, potassium, and calcium – otherwise known as electrolytes – lost due to persistent diarrhea is vital. This can be done either by drinking large amounts of liquids or intravenously.

How do you prevent a norovirus infection?

Common settings for norovirus outbreaks include restaurants and events with catered meals (36 percent), nursing homes (23 percent), schools (13 percent), and vacation settings or cruise ships (10 percent). Proper hand washing is the best way to prevent the spread of norovirus. 

Shellfish (oysters, clams, mussels) pose the most significant risk. Any serving may be contaminated with norovirus; there is no way to detect a contaminated oyster, clam, or mussel from a safe one.  Shellfish become contaminated when their waters become contaminated—e.g. when raw sewage is dumped overboard by recreational or commercial boaters). Shellfish are filter feeders and will concentrate virus particles present in their environment. With shellfish, only complete cooking offers reliable protection; steaming does not kill the virus or prevent its transmission. 

(To sign up for a free subscription to Food Safety News,click here)