Headlines like "Norovirus Hits N.J. Campus," "Norovirus Closes Washington-Area Hotel," and "Sick Ship Docked in Miami, Second Norovirus Outbreak" are frequent these days. Noroviruses, a common cause of gastroenteritis, generate havoc as the nation’s No.1 cause of foodborne disease and epidemic diarrhea.^1,2

Almost every American has an attack of acute viral gastroenteritis (also known as stomach flu or infectious diarrhea) at least once a year. In fact, acute gastroenteritis is the second most frequent cause of illness in the United States, with only the common cold occurring more frequently. Luckily, gastroenteritis is seldom serious, lasting only a few days.

Outbreaks of norovirus gastroenteritis are especially common in closed settings and have been occurring with increasing frequency in hospitals, nursing homes, day-care centers, schools, military bases, prisons, and hotels. The viruses have also sickened thousands of passengers on cruise ships and hundreds of restaurant patrons.

The sharp increase in norovirus outbreaks has been attributed to a new, more aggressive viral strain designated GII.4 that began circulating throughout Europe in 2005 and 2006. In the United States, this strain was first detected in January 2006 from stool samples of passengers infected with a particularly virulent GI bug while aboard the cruise ship Minerva II. Within months, the viral strain spread across the United States, and between October and December 2006, it was found in 60% of the tested stool specimens from other acute gastroenteritis outbreaks. Recent genetic studies of the Minerva strain confirm that it is genetically similar to the norovirus strain, GII.4 2006(b) variant, that has been causing the outbreaks in Europe.^3,4

To help limit the spread of this virus, nurses in all settings need to be knowledgeable about the extreme infectivity of noroviruses and recommendations on preventing infection and transmission. 

Noroviruses fall into five distinct genogroups based on their genetic makeup, designated GI to GV. Genogroups I, II, and IV are associated with human infections while genogroup III contains pig and cattle strains, and genogroup V contains mouse strains. Human genogroups are further divided into more than 25 genetic clusters, and each cluster contains many genetically diverse strains. As an example, Norwalk virus, the prototype norovirus strain, belongs to genogroup I, cluster 1 (GI.1).⁵

Strains belonging to genogroup II, cluster 4 (GII.4) are the current predominant strains worldwide.³ Compared to other strains, genogroup II.4 viruses seem to be more frequently associated with outbreaks in closed settings, suggesting that there may be something unique about the strains that makes them more virulent or more persistent in the environment than previous strains. This uniqueness and other factors related to modern life — such as eating more food prepared by potentially infected workers and spending more time in closed settings — have enabled GII.4 strains to spread rapidly, first across Europe, then throughout Canada and the United States.⁴

From Norwalk to Noro

Noroviruses (genus Norovirus, family Caliciviridae) were formerly called "Norwalk-like viruses" after a 1968 outbreak of "winter vomiting disease" in Norwalk, Ohio. The viruses are small (27 nm to 35 nm in diameter), round, nonenveloped structures with a single-stranded, positive-sense RNA genome.⁵ They are relatively heat and acid stable and can survive freezing and heating to 140 F for 30 minutes as well as chlorine disinfection that kills most bacteria. In fact, noroviruses can survive in 10 parts per million (ppm) chlorine, well in excess of levels routinely present in public water systems.⁶ Because of their environmental stability, the viruses can persist for weeks on environmental surfaces and in contaminated water. Furthermore, the viruses are resistant to most of the common cleaning solutions, such as quaternary ammonium compounds ("quats"), used in food establishments and hospitals. Quats work by disrupting the viral envelope and are ineffective on nonenveloped viruses, such as noroviruses.^5,7

The numbers

Each year, noroviruses cause an estimated 23 million infections, 50,000 hospitalizations, and 300 deaths in the United States.¹ The major route of transmission is fecal-oral, usually through ingestion of contaminated food and water. Infection also can occur from direct contact with infected people or by touching contaminated environmental surfaces and fomites (contamination from hands to mouth). Noroviruses are highly transmissible, requiring as few as 10 viral particles to cause infection.⁶ This low infectious dose facilitates fecal-oral transmission of the virus from environmental surfaces that have been contaminated with stool and vomitus.^5,6

Airborne transmission of norovirus also occurs when the act of vomiting aerosolizes viral particles. People in the immediate area can inhale and swallow droplets of vomitus containing viral particles (air-oral/mucous membrane transmission). Norovirus transmission has been documented in people walking through an ED in which a vomiting patient was being evaluated. Another documented outbreak occurred in a hotel restaurant where a norovirus-infected diner vomited. Patrons at adjacent tables developed gastroenteritis one or two days afterward.^2,5,6

Noroviruses cause about two-thirds of all food-related illnesses in the United States, and almost any type of food can serve as a vehicle for outbreaks.^1,2 Outbreaks can start at the food source, for example, with raw shellfish (particularly oysters) harvested from contaminated water. Outbreaks can also start via direct contamination of food by an ill or recently recovered food handler immediately before the food’s consumption. Public health guidelines recommend that food handlers not return to work for 48 to 72 hours after recovering from gastroenteritis.⁵ However, recent studies have demonstrated that with noroviruses, viral shedding can persist in the stool for up to three weeks after acute illness.^2,8 This obviously increases the risk of secondary spread from food handlers.

Of equal concern, especially with food-related outbreaks, is that viral shedding can occur for prolonged periods of time in the absence of clinical illness. For example, over 30% of healthy volunteers given bacteria-free stool filtrate of noroviruses in experimental studies did not develop clinical illness. Yet viral shedding was detected in their stool for more than two weeks after administration of the virus.⁸

Contaminated environmental surfaces are another frequent cause of norovirus infections. The viruses are transmitted, hand to mouth, after a person touches surfaces contaminated with feces or vomitus. The viruses can contaminate surfaces and survive in the inanimate environment on a computer mouse, door handles, telephone buttons, telephone receivers, keyboards, and faucets for extended periods of time. For example, tests found that feline caliciviruses (a surrogate for human noroviruses, which cannot be grown in the laboratory) can survive for up to three days on telephone buttons and receivers and for one or two days on a computer mouse. In stool samples, they survive for more than four days and on carpets for more than two days.^9,10

Viruses at sea

Cruise ships are particularly vulnerable to norovirus outbreaks, and secondary transmission of the virus often leads to amplification of an outbreak. Once an outbreak occurs, decontamination is extremely difficult, and the viruses can persist in the environment as a continuing source of infection. In 2002, serial outbreaks of a genetically sequenced strain of norovirus affected passengers on six consecutive voyages despite a one-week aggressive sanitation of the ship after the second cruise. Epidemiological analysis of the outbreaks indicated an initial foodborne source of infection with subsequent person-to-person transmission. Environmental contamination probably perpetuated the outbreaks.¹¹

Noroviruses are also efficiently spread from person-to-person (direct exposure to ill people) during recreational activities (especially contact sports) and through contaminated swimming pools, water parks, or lakes. Since relatively high levels of chlorine do not kill the virus, swimming pools can serve as vehicles for transmission when infected or recently recovered swimmers shed the virus into the water.^5,8,12 People on average carry 0.14 grams of feces on the anal area, and the fecal matter can rinse off in swimming pools. If a person is shedding norovirus, he or she could contaminate an entire pool.¹³

The immune response to noroviruses is not clearly understood. However, because of genetic factors, some people are known to be more susceptible to norovirus infections than others. Researchers have found that susceptibility to norovirus infections correlates with a person’s blood group antigens (A, B, and O) and Lewis blood group secretor— a minor blood group that determines a person’s ability to secrete ABO blood group substances in saliva and other body fluids. People’s disease susceptibility varies by secretor status (Se+, Se-).

Eighty percent of the U.S. population are ABO secretors. People with blood type O, and especially blood type O secretors, are at greatest risk for norovirus infections while people who express blood type B antigens (B and AB blood types) are less susceptible to infection. Interestingly, secretor-negative people (20% of population) of all blood types (ABO) seem to be uniformly resistant to norovirus infection even when challenged with large amounts of norovirus.^5,8,12

Noroviruses have an average incubation period of 12 to 48 hours (median in outbreaks is 33 to 36 hours). Illness occurs abruptly, often without warning, and is characterized by a sudden onset of explosive vomiting and watery, nonbloody diarrhea. Other symptoms (abdominal cramps, nausea, muscle pain, headache, and sporadic low-grade fever with temperatures of 101 F to 102 F) can be present, but the frequency and the intensity of vomiting distinguish noroviruses from other enteric pathogens.

People of all ages experience these symptoms, but diarrhea is more common among adults, as is vomiting among children.⁵ Symptoms generally last 24 to 60 hours, and recovery is usually complete without any serious long-term sequelae. After recovery, norovirus can be shed in the stool for two to three weeks. However, immunocompromised patients (transplant recipients, for example) can develop chronic diarrhea and shed the virus for months or even years.⁵ Resistance to reinfection (acquired immunity) is strain-specific and persists for only a few months; thus, recurrent infections can occur during one’s life.

Given that human noroviruses cannot be grown in the laboratory, specific diagnosis requires detection of viral ribonucleic acid (RNA) in the stools of affected people using reverse transcription polymerase chain reaction assays, enzyme-linked immunoassays, or direct visualization of small round structured viruses by electron microscopy. However, since testing is usually done only in specialized laboratories, it is not always readily available; clinicians usually rely on clinical symptoms and epidemiological characteristics to determine whether the cause of an outbreak is norovirus infections.^1,8 The following clinical criteria, developed 25 years ago, can be used to differentiate outbreaks of norovirus gastroenteritis from bacterial and parasitic gastroenteritis:^1,14

• Vomiting in more than half of affected people
• A short incubation period, 24 to 48 hours
• A short duration of illness, 12 to 60 hours
• No bacterial pathogen isolated from stool culture  

As a note: Among patients with acute infectious diarrhea who do not need to seek medical care, a high probability (92%) exists that norovirus is the cause of the illness.²

What to do

No specific treatment exists for infections with norovirus since infections are generally mild and self-limited. Patients can take analgesics for headaches and muscle aches and isotonic liquid to replace fluid losses. Bismuth subsalicylate (Pepto-Bismol) can reduce GI symptoms although it has no effect on the number of stools or viral shedding.^8,12 More aggressive treatment may be necessary for patients who are elderly or immunosupressed, have chronic diseases (such as diabetes), or are receiving cancer chemotherapy or steroid therapy. Patients with severe vomiting and diarrhea may be at risk for dehydration and electrolyte abnormalities.² But in general, norovirus gastroenteritis is of minimal risk to healthy people and rarely causes death.¹⁴

Vulnerable hospitals

An outbreak of norovirus can be a formidable problem in healthcare facilities, leading to nursing unit closures and a major disruption in activities. The spread of the virus is facilitated by its low infectious dose and by its persistence in the environment as a source of continuing infection.¹⁴

Once an environment is contaminated with noroviruses, decontamination is exceptionally difficult, and the most common disinfectants used for sanitizing or disinfecting large surfaces (quats) are not reliable. In the absence of hospital disinfectants that have specific claims for activity against noroviruses, the CDC recommends the use of freshly prepared dilute chlorine bleach for hard, nonporous surfaces. A minimum concentration of 1,000 ppm (generally a dilution of 1 part household bleach solution to 50 parts water) has been found effective when tested on feline caliciviruses.⁷ In highly contaminated areas, up to 5,000 ppm chlorine bleach (1 cup bleach to 9 cups water) is recommended with a contact time of up to 10 minutes for maximum effect.

For maximum performance, chlorine bleach should be used only on surfaces that have first been thoroughly cleaned since organic matter (diarrhea and vomitus) rapidly inactivates bleach. Particular attention should be paid to areas with frequent hand contact, such as door handles, push plates, elevator buttons, telephones, computer keyboards, railings, light switches, and curtain pull rods.

Contaminated restroom surfaces are often implicated in the transmission of infection. Toilet seats and handles, stall doors and latches, faucet handles, sinks, and soap and towel dispensers should be rigorously and frequently disinfected. When cleaning areas heavily contaminated with vomitus or feces, healthcare workers should wear surgical masks (not respirators) to prevent spattering of infectious material and ingestion of viral particles. In addition, healthcare workers using bleach should wear appropriate personal protective equipment (PPE), e.g., gloves and goggles.^7,15

Carpets are particularly difficult to clean. They should be steam cleaned at 158 F for five minutes or 212 F for one minute to inactive the virus.¹⁵ To prevent aerosolization of norovirus, dry vacuuming of carpets should be avoided unless the vacuum has a high-efficiency particulate filter.¹⁴

As another precaution, air currents should be minimized. Staff and patients should be careful about opening windows or turning on a fan, which can disperse viral particles through the air.¹⁴

Other infection control measures during an outbreak include restricting ill patients to private rooms when possible, excluding ill staff from work, paying strict attention to hand hygiene using soap and water, and stressing the importance of infection control measures to caregivers and visitors.

Nurses and other healthcare providers can generally use standard precautions when caring for patients with suspected norovirus infection. However, contact precautions are advised when caring for patients who are incontinent or diapered, during outbreaks in a facility, and when there is the possibility of splashes that could contaminate clothing.⁷ Droplet precautions are recommended if the patient is vomiting. During an outbreak, it is advisable to place patients with suspected norovirus in private rooms or to cohort such patients. All staff must be informed of the patient’s norovirus infection and use PPE. Healthcare workers should wear gowns and gloves when in contact with an infected patient or when they suspect an environment may be contaminated.⁷ Nonessential staff should not enter affected clinical areas.

Stay home!

All infected healthcare workers should remain off work until vomiting and diarrhea ceases. In addition, food handlers and healthcare workers who provide direct patient care should be excluded from duty for an additional three days after their last symptoms stop since there is a risk of transmitting the virus to others during that time.¹⁵

Hospitals should emphasize hand washing to prevent fecal-oral transmission. Staff should wash their hands after contact with an infected patient and the patient’s environment and before leaving a clinical area that has infected patients.

Lastly, healthcare facilities should post notices about infection-control measures to remind both healthcare workers and visitors about how to reduce the risk of transmission. They should be told to:

• Wash their hands frequently, especially after using the toilet and before preparing or eating food.
• Keep their fingers away from their mouths.
• Avoid putting objects that may be contaminated into their mouths (e.g., "storing" chewed gum on a surface and later putting it back into the mouth).
• Avoid direct contact (e.g., shaking hands) with infected people

The risk of infection with norovirus is increasing for everyone, and in nearly all cases, transmission is from transient organisms found on the hands. Transmission can be significantly reduced if infected people and those exposed to them practice good personal hygiene. Nurses can limit the transmission of norovirus by paying meticulous attention to handwashing and by encouraging coworkers and visitors to do the same.