Norovirus is one of the top causes of acute gastroenteritis (AGE) outbreaks and the leading cause of endemic AGE in children across the globe. This article underlines the causes, diagnosis and clinical management of this hazardous norovirus commonly known as winter vomiting sickness" or Stomach-flu.

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What is norovirus?

1. Norovirus is a genus under the family Caliciviridae, which also contains Sapovirus. The norovirus and sapovirus families include viruses that infect humans and other animals.
2. On the basis of sequence homology, noroviruses are classified into ten genogroups and 49 genotypes. Human pathogens are found in the GI, GII, and GIV genogroups; many genotypes are identified within each genogroup. Frequent recombination between strains and point mutations lead to fast changes in genetic diversity; many recombinants are just as infectious and virulent as the original strains.
3. GII (mostly GII.4, followed by other varieties such as GII.17) is the leading cause of human norovirus infection, followed by GI and GIV. GII.4 viruses are very prevalent and are linked to more severe results than other norovirus genotypes, including increased hospitalisation and mortality rates.

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What are the factors affecting pathogenesis?

1. Immunity 

Uncertainty surrounds the involvement of humoral and/or cellular immunity in infection and illness prevention. Reinfections are extremely common throughout a person's lifetime; however, symptomatic reinfection with the most prevalent genotype (GII.4) is uncommon during childhood, suggesting that the protective efficacy of a previous infection occurs against the same genotype and possibly against other genotypes within the same genogroup. However, the protective effect seems to be diminished when exposed to a norovirus from a different genogroup.

2. Vulnerable host 

1. Susceptibility of the host to norovirus infection has been connected to blood type antigens; these antigens may function as receptors or cofactors for infection.
2. They identified blood group antigens differ by viral genogroup. It has been shown that genogroup GI viruses preferentially attach to blood group A and O antigens, while genogroup GII viruses preferentially bind to blood group A and B antigens.
3. In contrast to these findings, a study of two norovirus outbreaks among Israeli military recruits found no connection between genogroup GII norovirus infection and ABO blood group antigens, indicating that GII viruses are capable of infecting persons of any blood type.

3. Intestinal physiology

1. Norovirus-induced diarrhoea is related to transitory malabsorption of D-xylose and lipids, as well as reduced activity of brush-border enzymes such as alkaline phosphatase and trehalase. The levels of absorption and brush-border enzymes revert to normal after two weeks of infection.
2. Uncertain are the processes behind vomiting and diarrhoea caused by norovirus. Normal individuals infected with norovirus have a substantial delay in gastric emptying.

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How the transmission occurs?

1. The faecal-oral route transmits norovirus, and the incubation period is between 24 and 48 hours. For transmission, a modest inoculum (100 virus particles) is needed. The peak of norovirus shedding in the stool occurs between 24 and 48 hours after sickness starts; the average length of viral shedding is four weeks. Following infection, viral shedding in the faeces of immunocompromised hosts may continue for months.
2. Airborne droplets of vomit carrying viral particles, fomite contamination, and contaminated food and drink intake may also spread norovirus infection. Commonly associated with breakouts include leafy greens, fresh fruits, and seafood, such as oysters.
3. Norovirus resists freezing temperatures, heating to 60 degrees Celsius, and disinfection with chlorine or alcohol.
4. Transmission of norovirus is more prevalent in symptomatic patients than in asymptomatic shedders, and nosocomial transmission is widespread.

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Outbreak prone areas 

Norovirus epidemics have occurred in several situations, including those listed below:

• Hospitals and long-term care institutions
• Restaurants and catered events
• Schools
• Daycare facilities, and community centres
• Contamination of municipal water supplies and recreational water exposure
• Cruise ships and resorts
• Military camps
• Prisons

Norovirus is the major cause of foodborne illness outbreaks; however, a minority of outbreaks may be attributed to particular dietary products.

• Shellfish
• Salads
• Sandwiches
• Vegetables like celery, melon, and raspberries are connected with transmission.

In one research, leafy vegetables (30 per cent), fruits (21 per cent), and molluscs were the most commonly detected items (19 per cent)

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What are the clinical presentations?

1. Symptomatic infection

Incubation period and duration

1. The incubation period is normally between 24 and 48 hours (range: 12 to 72 hours), and the onset of symptoms is frequently sudden. The typical duration of symptoms is 48 to 72 hours, with fast recovery.
2. Norovirus shedding in faeces peaks during the first 24 to 48 hours following sickness start; the average length of viral shedding is four weeks.

Typical clinical features  

Following symptoms can be seen,

• Non-bloody, nonbilious nausea and vomiting- Vomiting is more prevalent in norovirus infection than in gastroenteritis caused by the vast majority of other viruses. 
• Non-bloody diarrhoea, and abdominal discomfort- If diarrhoea is present, it is usually mild in severity (approximately four to eight stools over a period of 24 hours). Lack of mucus and absence of faecal leukocytes in stools.
• Prevalent symptoms- myalgias, malaise, and headache.
• Fever develops in almost fifty per cent of patients.
• Dehydration- Patients are often uncomfortable but do not seem to be gravely unwell, although severe dehydration may result. Clinically, norovirus infection cannot be separated from other causes of acute gastroenteritis, notably rotavirus.

Severe disease and complications

1. There have been reports of severe symptoms in elderly persons, infants less than 12 months, and immunocompromised patients.
2. In patients with severe disease, fever is more prevalent and the duration of the sickness may be several days longer than in healthy persons.
3. Infrequently, neurologic problems like benign convulsions and encephalitis have been documented in children.

Chronic sequelae

The most often documented chronic complication of norovirus infection in immunocompromised patients is chronic diarrhoea, which may linger for several months and result in wasting or failure to thrive.

2. Asymptomatic infection

1. Norovirus infection is often shed in the stool of asymptomatic patients, particularly youngsters.
2. Asymptomatic norovirus shedding has diagnostic consequences, since diarrhoea attributable to another cause in an asymptomatic carrier may be misdiagnosed as norovirus infection.
3. Moreover, asymptomatic shedding has epidemiologic consequences; for instance, asymptomatic food handlers may be able to transmit illness to others, since the virus loads on the hands of asymptomatic and symptomatic food handlers during outbreaks are comparable.

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How is it diagnosed?

Presumptive diagnosis based on clinical suspicions

All patients with acute onset vomiting and/or diarrhoea should be evaluated for norovirus infection, particularly if they live in middle- or high-income countries where rotavirus vaccinations are frequently administered. In such cases, the diagnosis of norovirus is often made on the basis of circumstantial evidence; the probability of norovirus is greater in the context of an epidemic or during the winter months in temperate countries.

1. Stool testing- Confirming the diagnosis with stool testing is normally unnecessary, while it may be beneficial in immunocompromised individuals with severe or prolonged symptoms; identifying norovirus as the origin of symptoms might aid in discontinuing treatments for other diseases. Identifying the cause of gastroenteritis outbreaks may also be important for public health objectives.
2. A stool molecular test- norovirus acute gastroenteritis is a plausible diagnosis in a patient with watery diarrhoea and a stool molecular test that is positive solely for norovirus. Due to the prevalence of asymptomatic norovirus shedding, a molecular diagnosis of norovirus does not definitely establish that the symptoms are caused by norovirus, especially if other organisms are also found during testing.
3. Stool RT-PCR- noroviruses and other viral causes of gastroenteritis are easily detected using stool RT-PCR, which is the backbone of laboratory diagnosis. Several antigen-detection enzyme immunoassays have been developed, however, their sensitivity and specificity are inferior to those of RT-PCR.

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How is norovirus treated?

There is no known cure for norovirus. However, you can alleviate symptoms while the infection takes its course. These consist of:

• Consuming a lot of liquids
• Resting extensively
• Consuming bland food

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What is the prognosis of a norovirus infection?

In most individuals, norovirus infection is a self-limiting disease that lasts two to three days. As stated before, problems are often caused by dehydration or underlying diseases. If IV rehydration is unavailable in nations where children may already be dehydrated, many children may perish from dehydration.

In 2006, the World Health Organization proposed a novel prepackaged oral rehydration salts (ORS) mix that can be supplied to developing nations and easily placed into clean water to avoid dehydration in many patients. This strategy has improved the outlook for a great number of youngsters in poor nations.

Is prevention of norovirus infections possible? Is there a vaccination for norovirus?

1. Once a sick individual has been identified, it is essential for caregivers and household contacts to practise proper hand hygiene. This involves washing hands following interaction with the individual or his surroundings. Additionally, hands should be cleansed before handling the face or making meals. The Centers for Disease Control and Prevention (CDC) advocates washing hands with soap and water over using hand sanitisers; alcohol-based hand sanitisers are not highly efficient but have been found to lessen the transmission rate in some contexts. Sharing silverware and plates is inappropriate. Solid surfaces may be cleaned using diluted chlorine bleach (five to twenty-five teaspoons per gallon of water). Other disinfectants, such as Lysol, may assist in decontaminating some surfaces.
2. Norovirus infections may be avoided by practising proper hand cleanliness with soap and water (rather than alcohol solutions) and avoiding contact with ill persons and their surroundings. This is more difficult than it sounds. On a cruise liner, one ill staff member may contaminate the food given to hundreds of passengers. Contamination during the harvesting of fresh fruits and vegetables may result in widespread epidemics when the product is distributed throughout the nation. Strict cleanliness requirements for food workers may aid in reducing the likelihood of outbreaks. Many researchers believe that routinely washing fruits and vegetables before consumption may also help decrease or avoid illnesses.
3. Noroviruses may also be transmitted in hospitals and nursing homes. The Centers for Disease Control and Prevention (CDC) has developed suggestions for institutions to minimise the infection rate. Hand cleanliness is stressed as the most essential aspect of these infection-prevention methods. Whenever an epidemic is detected at a hospital or institution, epidemiologists and anyone with expertise in infection control should be called. When a community-based epidemic is suspected, public health authorities should be contacted immediately. The primary risk factor for norovirus infection is close contact with an infected person or with objects they may have touched or coughed on. The danger is higher if an infected individual prepares your food or lives in close proximity to you and others (dorm, cruise ship, school).
4. Those who get norovirus do not develop immunity to subsequent illnesses. Even if the body produces antibodies against the infecting strain, there are several infectious strains. The virus is continually modifying its RNA to produce new strains that are resistant to the human immune system.
5. Due to the diversity of norovirus strains, it has proven challenging to develop a vaccine. However, this is an important field of study, and many vaccines have shown promise in experimental models. Vaccine studies are underway in people, although there is yet no commercially available vaccine.

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Disclaimer- The views and opinions expressed in this article are those of the author and do not necessarily reflect the official policy or position of M3 India.

The author is a practising super specialist from New Delhi.