Norovirus is the Cruise Ship's Worst Enemy

Noroviruses are members of a group of viruses called caliciviruses also known previously as Norwalk-like viruses. Infection with norovirus affects the stomach and intestines, causing an illness called gastroenteritis, or "stomach flu" This stomach flu is not related to the flu (or influenza), which is a respiratory illness caused by influenza virus. In addition, noroviruses are not related to bacteria and parasites that can cause gastrointestinal illnesses. Norovirus is not a new virus, but interest in it is growing as more is learned about how frequently noroviruses cause illness in people (CDC, 2003)

Noroviruses are a group of viruses that cause the “stomach flu,” or gastroenteritis (GAS-tro-en-ter-I-tis), in people. The term norovirus was recently approved as the official name for this group of viruses. Illness caused by norovirus infection has several names, including:(CDC, 2003)

• stomach flu – this “stomach flu” is not related to the flu (or influenza), which is a respiratory illness caused by influenza virus.
• viral gastroenteritis – the most common name for illness caused by norovirus. Gastroenteritis refers to an inflammation of the stomach and intestines.
• acute gastroenteritis
• non-bacterial gastroenteritis
• food poisoning (although there are other causes of food poisoning)
• calicivirus infection  (CDC, 2003)

Viruses are very different from bacteria and parasites, some of which can cause illnesses similar to norvirus infection. Viruses are much smaller, are not affected by treatment with antibiotics, and cannot grow outside of a person’s body. (CDC, 2003)

The Norwalk virus is the representative agent of a group of viruses, also called small round structured viruses (SRSVs) or the Norwalk-like family of agents. The agents are usually identified by the locale where an outbreak occurred (e.g., Hawaii, Snow Mountain, Montgomery County, Taunton, Amulree, Sapporo, and Otofuke).   (CDC, 1990)

Persons traveling, working in day-care centers or nursing homes should pay special attention to passengers, children or residents who have norovirus illness. This virus is very contagious and can spread rapidly throughout such environments.  (CDC, 2003)

 Symptoms of Illness Caused by the Norovirus

Norovirus infection causes gastroenteritis, which is an inflammation of the stomach and the small and large intestines. The symptoms of gastroenteritis are nausea, vomiting, and/or diarrhea accompanied by abdominal cramps. Some people also complain of headache, fever/chills, and muscle aches. Symptoms are usually brief and last only 1 or 2 days. However, during that brief period, people can feel very ill and vomit, often violently and without warning, many times a day. Symptoms usually begin 24 to 48 hours after ingestion of the virus, but can appear as early as 12 hours after exposure. There is no evidence that sick persons can become long-term carriers of the virus, but the virus can be in the stool and vomit of infected persons, from the day they start to feel ill to as long as 2 weeks after they feel better. (CDC, 2003)

Other infectious and non-infectious agents can cause symptoms similar to those of norovirus gastroenteritis; people who have these symptoms and have questions about the cause of their illness should consult a physician. (CDC, 2003)

Symptoms of norovirus illness usually begin about 24 to 48 hours after ingestion of the virus, but they can appear as early as 12 hours after exposure.  {CDC, 2003 37 /id}  Norovirus infection usually presents as acute-onset vomiting, watery non-bloody diarrhea with abdominal cramps, and nausea. Low-grade fever also occasionally occurs, and vomiting is more common in children. Dehydration is the most common complication, especially among the young and elderly, and may require medical attention. Symptoms usually last 24 to 60 hours. Recovery is usually complete and there is no evidence of any serious long-term sequelae.

Norovirus disease is usually not serious, although people may feel very sick and vomit many times a day. Most people get better within 1 or 2 days, and they have no long-term health effects related to their illness. However, sometimes people are unable to drink enough liquids to replace the liquids they lost because of vomiting and diarrhea. These persons can become dehydrated and may need special medical attention. This problem with dehydration is usually only seen among the very young, the elderly, and persons with weakened immune systems. There is no evidence to suggest that an infected person can become a long-term carrier of norovirus.  (CDC, 2003)

The symptoms of norovirus illness usually include nausea, vomiting, diarrhea, and some stomach cramping. Sometimes people additionally have a low-grade fever, chills, headache, muscle aches, and a general sense of tiredness. The illness often begins suddenly, and the infected person may feel very sick. The illness is usually brief, with symptoms lasting only about 1 or 2 days. In general, children experience more vomiting than adults. Most people with norovirus illness have both of these symptoms.  (CDC, 2003)

The incubation period is 24-48 hours, and the mean duration of illness is 12-60 hours. Nausea is prominent, with vomiting, non-bloody diarrhea, and abdominal cramps occurring in most cases. These symptoms are experienced by all age groups, but diarrhea is relatively more prevalent among adults, whereas a higher proportion of children experience vomiting. From 25%-50% of affected persons also report headache, fever, chills, and myalgias. Adults have died during illness caused by Norwalk-like viruses, presumably from electrolyte imbalance. Late sequelae have not been reported, but the elderly often report persistence of constitutional symptoms for up to several weeks.   (CDC, 1990)

 Mode of Transmission

Routes of transmission that have been documented include water, food (particularly shellfish and salads), aerosol, fomites, and person-to-person contact. Infectivity can last for as long as 2 days after resolution of symptoms. Presymptomatic shedding has been suspected on epidemiologic grounds but not proven in volunteer studies. (CDC, 1990)

Noroviruses are found in the stool or vomit of infected people. People can become infected with the virus in several ways, including: (CDC, 2003)

• eating food or drinking liquids that are contaminated with norovirus;
• touching surfaces or objects contaminated with norovirus, and then placing their hand in their mouth;
• having direct contact with another person who is infected and showing symptoms (for example, when caring for someone with illness, or sharing foods or eating utensils with someone who is ill).

Noroviruses are very contagious and can spread easily from person to person. Both stool and vomit are infectious. Particular care should be taken with young children in diapers who may have diarrhea.  How long are people contagious?  (CDC, 2003)

People infected with norovirus are contagious from the moment they begin feeling ill to at least 3 days after recovery. Some people may be contagious for as long as 2 weeks after recovery. Therefore, it is particularly important for people to use good handwashing and other hygienic practices after they have recently recovered from norovirus illness. (CDC, 2003)

Noroviruses are transmitted primarily through the fecal-oral route, either by consumption of fecally contaminated food or water or by direct person-to-person spread. Environmental and fomite contamination may also act as a source of infection. Good evidence exists for transmission due to aerosolization of vomitus that presumably results in droplets contaminating surfaces or entering the oral mucosa and being swallowed. No evidence suggests that infection occurs through the respiratory system. (CDC, 2003)

Noroviruses are highly contagious, and it is thought that an inoculum of as few as 10 viral particles may be sufficient to infect an individual. During outbreaks of norovirus gastroenteritis, several modes of transmission have been documented; for example, initial foodborne transmission in a restaurant, followed by secondary person-to-person transmission to household contacts. Although presymptomatic viral shedding may occur, shedding usually begins with onset of symptoms and may continue for 2 weeks after recovery. It is unclear to what extent viral shedding over 72 hours after recovery signifies continued infectivity.  (CDC, 2003)

Food and drinks can very easily become contaminated with norovirus because the virus is so small and because it probably takes fewer than 100 norovirus particles to make a person sick. Food can be contaminated either by direct contact with contaminated hands or work surfaces that are contaminated with stool or vomit, or by tiny droplets from nearby vomit that can travel through air to land on food. Although the virus cannot multiply outside of human bodies, once on food or in water, it can cause illness.(CDC, 2003)

Some foods can be contaminated with norovirus before being delivered to a restaurant or store. Several outbreaks have been caused by the consumption of oysters harvested from contaminated waters. Other produce such as salads and frozen fruit may also be contaminated at source.(CDC, 2003)

 Outbreak Characteristics

Anyone can become infected with these viruses. There are many different strains of norovirus, which makes it difficult for a person’s body to develop long-lasting immunity. Therefore, norovirus illness can recur throughout a person’s lifetime. In addition, because of differences in genetic factors, some people are more likely to become infected and develop more severe illness than others.  (CDC, 2003)

Numerous reports have described the course of outbreaks caused by Norwalk-like agents, usually involving adults and older children. The settings are diverse and include banquets, cruise ships, geriatric facilities, psychiatric wards, emergency rooms, cafeterias, recreational lakes, swimming pools, campgrounds, football teams, hotels, schools, dormitories, fast food restaurants, and others (52). Norwalk-like agents probably create a low background level of infection in a community until an infected individual contaminates a common source, and an explosive outbreak occurs. Although secondary cases can multiply the number of persons affected, outbreaks are generally limited to 1-2 weeks unless transmission is facilitated by a closed environment (e.g., a nursing home) or prolonged by renewal of the susceptible population (e.g., a new set of passengers on a cruise ship).   (CDC, 1990)

 Immunity

Mechanisms of immunity to norovirus are unclear. It appears that immunity may be strain-specific and lasts only a few months; therefore, given the genetic variability of noroviruses, individuals are likely to be repeatedly infected throughout their lifetimes. This may explain the high attack rates in all ages reported in outbreaks. Recent evidence also suggests that susceptibility to infection may be genetically determined, with people of O blood group being at greatest risk for severe infection. (CDC, 2003)

Studies of volunteers have documented the paradox that persons with the highest preexisting levels of Norwalk antibodies are at highest risk of developing symptomatic infection (49). Most persons' antibody levels against Norwalk virus rise after infection; these titers normally peak by the third week and persist until approximately the sixth week, after which they decline. Although preexisting antibody levels correlate with risk of symptomatic illness upon exposure to the virus, acutely elevated antibody levels appear to correlate with resistance to reinfection. The nature of resistance and susceptibility to the Norwalk-like agents is poorly understood.  (CDC, 1990)

 Treatment

Currently, there is no antiviral medication that works against norovirus and there is no vaccine to prevent infection. Norovirus infection cannot be treated with antibiotics. This is because antibiotics work to fight bacteria and not viruses.  (CDC, 2003)

Norovirus illness is usually brief in healthy individuals. When people are ill with vomiting and diarrhea, they should drink plenty of fluids to prevent dehydration. Dehydration among young children, the elderly, the sick, can be common, and it is the most serious health effect that can result from norovirus infection. By drinking oral rehydration fluids (ORF), juice, or water, people can reduce their chance of becoming dehydrated. Sports drinks do not replace the nutrients and minerals lost during this illness. (CDC, 2003)

 Prevention and Advoidance

Prevention of foodborne norovirus disease is based on the provision of safe food and water. Noroviruses are relatively resistant to environmental challenge: they are able to survive freezing, temperatures as high as 60°C, and have even been associated with illness after being steamed in shellfish. Moreover, noroviruses can survive in up to 10 ppm chlorine, well in excess of levels routinely present in public water systems. Despite these features, it is likely that relatively simple measures, such as correct handling of cold foods, frequent handwashing, and paid sick leave, may substantially reduce foodborne transmission of noroviruses.(CDC, 2003)

You can decrease your chance of coming in contact with noroviruses by following these preventive steps: (CDC, 2003)

• Frequently wash your hands, especially after toilet visits and changing diapers and before eating or preparing food.
• Carefully wash fruits and vegetables, and steam oysters before eating them.
• Thoroughly clean and disinfect contaminated surfaces immediately after an episode of illness by using a bleach-based household cleaner.
• Immediately remove and wash clothing or linens that may be contaminated with virus after an episode of illness (use hot water and soap).
• Flush or discard any vomitus and/or stool in the toilet and make sure that the surrounding area is kept clean.

  Disease Burden of Norovirus Gastroenteritis

CDC estimates that 23 million cases of acute gastroenteritis are due to Norovirus infection, and it is now thought that at least 50% of all foodborne outbreaks of gastroenteritis can be attributed to noroviruses.  (CDC, 2003)

Among the 232 outbreaks of norovirus illness reported to CDC from July 1997 to June 2000, 57% were foodborne, 16% were due to person-to-person spread, and 3% were waterborne; in 23% of outbreaks, the cause of transmission was not determined. In this study, common settings for outbreaks include restaurants and catered meals (36%), nursing homes (23%), schools (13%), and vacation settings or cruise ships (10%). (CDC, 2003)

Most foodborne outbreaks of norovirus illness are likely to arise though direct contamination of food by a food handler immediately before its consumption. Outbreaks have frequently been associated with consumption of cold foods, including various salads, sandwiches, and bakery products. Liquid items (e.g., salad dressing or cake icing) that allow virus to mix evenly are often implicated as a cause of outbreaks. Food can also be contaminated at its source, and oysters from contaminated waters have been associated with widespread outbreaks of gastroenteritis. Other foods, including raspberries and salads, have been contaminated before widespread distribution and subsequently caused extensive outbreaks. (CDC, 2003)

Waterborne outbreaks of norovirus disease in community settings have often been caused by sewage contamination of wells and recreational water. (CDC, 2003)

 References

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Cubitt WD, McSwiggan DA, Arstall S. An outbreak of calicivirus
infection in a mother and baby unit. J Clin Pathol 1980;33:1095-8.

Gary JJ, Wreghitt TG, Cubitt WD, Elliot PR. An outbreak of
gastroenteritis in a home for the elderly associated with astrovirus
type 1 and human calicivirus. J Med Virol 1987;23:377-81.