Viral diarrhea: when a devious virus invites itself into our intestines
Infectious diarrhea can be caused by bacteria (bacterial diarrhea), parasites (parasitic diarrhea) or viruses (viral diarrhea). Five main viruses are implicated in viral diarrhea. The first of these is rotavirus, the main cause of diarrhea-associated deaths in children under 5 years old. And this in spite of the vaccination of infants against this terrible virus since 2006. The effectiveness of the vaccination appears to be lower in low-income countries, due undoubtedly to the composition of the gut microbiota.
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About this article
What is viral diarrhea?
Diarrhea is defined as the passage of at least three loose or liquid stools per day, and infectious diarrhea is diarrhea caused by infection by a pathogen (virus, bacteria, or parasite). 1-3 If the pathogen is a virus, the diarrhea is said to be “viral”.
Viral, bacterial and parasitic diarrhea are not the same thing
Under the broad umbrella of diarrhea, while there are some kinds that are non-infectious (e.g. in the case of a digestive disease such as Crohn’s disease), the great majority are caused by a pathogenic infection. Depending on the pathogen involved, we talk about:
- viral diarrhea if the culprit is a virus (e.g. rotavirus, which affects many infants);
- bacterial diarrhea if a bacterium is responsible (e.g. Vibrio cholerae, which is responsible for cholera epidemics);
- parasitic diarrhea if a parasite is the root cause (e.g. the mini-parasite composed of a single cell of Giardia intestinalis, which is responsible for the disease called giardiasis, dreaded by tourists, or the worm known as roundworm, dreaded by parents of young children).
Lastly, diarrhea can also be a common side effect (in up to 35% of patients) 4,5 of antibiotic treatment. This is known as antibiotic-associated diarrhea.
Which viruses are responsible?
While rotavirus is the deadliest (210,000 estimated deaths in 2016 among children under the age of 5), it is far from the only virus responsible for acute diarrhea or even death: adenovirus, norovirus, sapovirus and astrovirus each kill between 17,000 and 37,000 young children every year. 6
Note: SARS-Cov-2, the code name of the virus responsible for the COVID-19 pandemic, can also cause diarrhea. However, the frequency of these cases is difficult to quantify. Figures vary widely from one study to another, with some researchers reporting an incidence rate of diarrhea in 2% of cases, others in 50%! 7
How does a small virus cause acute viral diarrhea?
In viral diarrhea, as in infectious diarrhea in general, it all begins with a confrontation between a pathogen (in this case a virus) and the host. The outcome of this confrontation depends on complex equilibriums, which largely involve the gut microbiota: in mice, for example, certain bacteria seem to be able to prevent or even cure a rotavirus infection. 8 The same virus will thus have a different effect depending on the state of the gut microbiota of the infected person. A less diversified microbiota would be more susceptible to the virus. 12
When the virus gains the upper hand, the process is always essentially the same: the virus infects the cells that line the small intestine and causes lesions that hinder the absorption of fluids. 9 The result is very liquid diarrhea (but without the presence of blood) which may be accompanied by other symptoms such as vomiting, nausea, abdominal cramps and fever. 10 This is the famous viral gastroenteritis, which reappears every winter. Particularly severe in the case of infection by rotavirus, which causes more pronounced symptoms, it is usually accompanied by dysbiosis of the gut microbiota, unsettled by the invasion of this virus. 11,12
How to prevent viral diarrhea?
Given the danger of rotavirus for frail individuals, preventive vaccines have been developed to reduce its mortality rate in young children. It is estimated that the vaccination of infants against rotavirus prevented 139,000 deaths in children under the age of five in the period 2006 to 2019. This figure is considered to be both significant yet low considering the hopes pinned on the vaccination: the vaccine seems to be less effective in low- or moderate-income countries (which are paradoxically the most affected).
Among the factors that might explain this lower effectiveness is the composition of the body’s microbiota, which affects the immune response to the vaccination in the gut. 13-16 To this preventive measure, of course, are added the advice of hygiene and social distancing, which apply to all types of infectious diarrhea.
How to treat viral diarrhea?
Very common and usually not serious, viral diarrhea can nonetheless lead to severe dehydration in the most frail individuals (infants, malnourished children, immunocompromised adults, the elderly). Treatment focuses on fighting the loss of water and electrolytes (e.g. replenishing lost sodium, potassium and chloride ions):
- orally (with oral rehydration solutions, or ORS);
- or intravenously in the most serious cases.
Health professionals may also recommend certain probiotic strains to treat acute gastroenteritis in children. These good bacteria reduce the duration of diarrhea, and/or the duration of hospitalization, and/or the quantity of stools (ESPGHAN 2023).
Key points to remember
- Diarrhea is defined as the passage of at least three loose or liquid stools per day, and infectious diarrhea is diarrhea caused by infection by a pathogen (virus, bacteria, or parasite). Due to the resulting dehydration, diarrhea was responsible for 1.6 million deaths in 2016, mainly among malnourished or immunocompromised children or people living with HIV. 1-3
- In cases of infection by a virus, diarrhea is said to be “viral”. The rotavirus virus causes the most deaths, followed, far behind, by adenovirus, norovirus, sapovirus and astrovirus. 6
- Sometimes, the gut microbiota manage to counter the infection; at other times, the virus kills the gut microbiota and causes watery diarrhea, which may be combined with other symptoms such as vomiting, nausea, abdominal cramps and fever. This is viral gastroenteritis. 10
- Prevention of viral diarrhea rests on the vaccination of infants against rotavirus (less effective in low- or moderate-income countries), access to clean drinking water, and hygiene and social distancing practices which apply to all types of infectious diarrhea. 13-16
- The treatment of viral diarrhea focuses on fighting dehydration. Certain probiotic strains can be prescribed for the treatment of acute gastroenteritis in children (ESPGHAN 2023). Antibiotics, on the other hand, are ineffective against viruses and can also worsen the diarrhea.
