Overview

Norovirus is a stomach bug causing vomiting and diarrhoea. Many of you must have experienced it first-hand! As you are aware, whilst it is going on, the symptoms are horrendous and you feel awful; however, these subside within 1-3 days. These symptoms are the result of gastroenteritis brought on by the virus. Whilst the mechanism is not fully understood, it is due to the activation of your body’s immune system once the virus has penetrated your gut wall. There is no vaccine to prevent norovirus infection. It is necessary to be vigilant over hand washing and hygiene practices.¹ 

What is norovirus?

Norovirus is a stomach bug that causes vomiting and diarrhoea.  It is the leading cause of acute gastroenteritis in the world, with approximately 685 million cases each year^. It produces symptoms such as feeling sick, diarrhoea, raised temperature, headaches, and body aches. While most people will feel better within 48 hours of feeling unwell, it can be dangerous for older people, young children, and immunocompromised persons.  Each year, there are an estimated 136,000 to 278,000 deaths due to norovirus infection.² 

What is the structure of norovirus? 

Norovirus is a non-enveloped virus. It doesn’t have a membrane coating like other viruses, such as the flu virus. Instead, it has a protein coat that protects its genetic material. Unfortunately, this means the virus can be hard to kill. Membrane-coated viruses are killed by alcohol gel and other detergents. Norovirus can also survive many ranges of temperatures, which is one reason why the virus spreads too quickly.

There are many diverse strains of norovirus, which are all quite genetically different and can infect various types of hosts including humans, mice, pigs, and bats. The human noroviruses will only infect humans and belong to one of three genotype groups (GI, GII, GIV). However, 75% of cases of human norovirus are caused by the genotype II virus.¹ 

How does norovirus cause gastroenteritis?

Norovirus is highly contagious and can be transmitted through contaminated food, water, surfaces, or airborne droplets from vomiting.

Firstly, it only takes a small amount of exposure to infect you; studies show a single particle has a 50% chance of infecting you. 

Secondly, whilst a person with norovirus is most infectious in the first 24 to 48 hours after starting symptoms, there is evidence that it remains active for up to several weeks. 

Thirdly, the virus can last on surfaces, in water, and in food for a long time, increasing the risk of contact. 

Finally, humans do not produce any long-term immunity to the virus. With different norovirus strains, we can repeatedly contract the virus in a lifetime.^2,3,4

Norovirus makes us unwell by causing swelling and inflammation within the gut and gastroenteritis. It occurs when the ingested virus attaches and penetrates your gut epithelial cells. How the virus manages to infiltrate the cells is not well known. The attachment of the virus onto the cell surface increases the concentration of virus particles on the cell surface, which then helps virus filtration. Once the virus has penetrated the epithelial wall, the exact method of how this causes gastroenteritis is poorly understood. It is thought that the virus causes the body to produce an immune response. This response involves the release of pro-inflammatory and anti-inflammatory cytokines (part of the immune system that tells the body what to do), which leads to gastroenteritis.^1,2,5

Why do we not gain immunity from the norovirus?

Most studies have shown that we do not gain long-term immunity to norovirus infection. The different strains of the virus also mean that getting infected by one strain does not protect you from another. The variety of strains and poor immune response makes a vaccine hard to develop so, currently, there is no vaccine.^4,5

What are the symptoms of norovirus?

The symptoms of norovirus start about 12-48 hours after exposure to the virus, and usually last for 1-3 days. The seriousness of the symptoms will usually depend on your health status.¹ 

• Nausea
• Vomiting
• Diarrhoea
• Stomach pains 
• Body aches 

Who is most at risk of norovirus?

Outbreaks of noroviruses are difficult to control because only a few virus particles can infect someone. It spreads by the fecal-oral route. Hospitals and care homes often suffer outbreaks of the disease. These settings also contain people who are more vulnerable to the disease, such as infants, the elderly, and the immunocompromised.^1,2  

Does your blood type affect whether you get norovirus?

Interestingly, not everyone exposed to a norovirus bug will get sick. There are suggestions that your blood type may affect your susceptibility to the virus. The virus binds to receptors on the intestinal cells, which are made up of different shapes of sugar molecules. The blood type in your cells determines the sugars in your cells. People with A, B, and AB blood types are less susceptible to contracting the virus than those with O type. However, other factors also have an effect. The bacteria in your gut can express molecules similar to the receptors on your intestinal cells, which means the virus can bind to these to reproduce.^4,6  

How is norovirus diagnosed?

Norovirus is usually diagnosed based on the symptoms, especially if someone close to you has similar symptoms. Laboratory testing is done by reverse transcription, a real-time polymerase chain reaction (RT-PCR). Enzyme immunoassays (EIAs) is also done but is less accurate.²

How is gastroenteritis from norovirus treated?

The good news is that most people with norovirus will feel better in 1-3 days without medical attention. However, it is important to keep hydrated during this time. Oral rehydration solutions prevent dehydration. In severe norovirus infections in immunocompromised, medical intervention may be needed to help with fluid loss, such as intravenous fluids.^1,4 

How to prevent the spread of norovirus?

Trying to stop norovirus outbreaks is challenging, given how infectious the virus is. Follow good hygiene techniques, such as handwashing regularly, using disinfectants, and preparing food safely, especially if someone near you has the virus. Handwashing with soap and water is the key. It is not killed by alcohol gel also. If you acquire a norovirus infection avoid contact with other people, wash your hands, and clean anything you touch regularly. If you are trying to prevent an outbreak in a hospital or cruise ship trace where the outbreak has come from and isolate any cases from other people.^3,4 

Summary

Norovirus produces symptoms such as diarrhoea, nausea, and vomiting, which will ease in 1-3 days. The elderly, young, and immunocompromised are at risk and require medical intervention. Symptoms are the consequences of the virus entering your gut and causing gastroenteritis. It is a highly infectious virus. Handwashing and hygiene protocols are preventive if someone around you has the virus.