Parasitic diarrhea: microbiota, a key prognostic element?
Infectious diarrhea can be caused by bacteria (bacterial diarrhea), parasites (parasitic diarrhea) or viruses (viral diarrhea). In parasitic diarrhea, the culprit may be small microorganisms such as Giardia intestinalis (responsible for giardiasis) or Entamoeba histolytica (which cause amebiasis), or worms, the most well-known of which is undeniably Ascaris lumbricoides. Not all individuals will react the same way to a parasitic infection. While most of us will remain asymptomatic, others will suffer from diarrhea that can be severe, or even fatal. Gut microbiota are increasingly cited as a key factor to explain this variability.
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About this article
What is parasitic 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 parasite, the diarrhea is said to be “parasitic”.
They 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 parasites are responsible?
Intestinal parasites can be classified into two main categories : 6-8
- protozoa, organisms made up of a single cell and measuring less than one millimeter, among which the best-known are Giardia intestinalis (responsible for giardiasis), Entamoeba histolytica (which causes the dreaded amebiasis), Cyclospora cayetanensis (cyclosporiasis) and Cryptosporidium spp (cryptosporidiosis);
- helminths, organisms made up of multiple cells, better known under the name worms, of which the most well-known is undeniably Ascaris lumbricoides.
Infections by these 2 types of parasites are common: 350 million people are estimated to be infected by the 3 most common types of protozoa, and 895 million by helminths transmitted through soil. Due to the globalization of food production, international travel and waves of migration, protozoa infections are now more common in the West than worm infections. 6
How do parasites cause acute parasitic diarrhea?
In parasitic diarrhea, as in infectious diarrhea in general, it all begins with a confrontation between a pathogen (in this case a parasite), usually transmitted by contaminated food or polluted water, and the host (our body). However, parasitic infection does not necessarily lead to symptoms such as diarrhea. Infections by the protozoan Entamoeba histolytica are usually asymptomatic, but can provoke an invasive disease of the large intestine in immunocompromised patients.7 The same applies in the case of worms: a mild infection often goes unnoticed, while the presence of a large number of worms can cause a whole series of symptoms such as growth disorders and problems with physical development. This is because the worms feed on the tissues of the host, leading to intestinal blood loss and hindering absorption of nutrients. 8,9
Each parasite has its own habits : 7
- among protozoa, Giardia intestinalis infects the upper part of the small intestine and can cause, 6 to 15 days after infection, acute diarrhea that is very liquid, while Entamoeba histolytica attacks the large intestine and can cause bloody diarrhea with mucus ;
- the worm Ascaris lumbricoides infects us through the ingestion of its eggs, which may be present on improperly washed food (fruits and vegetables) or in polluted water. Its eggs release larvae into the digestive tract. These in turn pass through the wall of the digestive tract and reach the lungs via the bloodstream, where they may cause their first symptoms (early pulmonary phase). This is just the start. The larvae climb the trachea up to the esophagus and slide down the esophagus and then the digestive tract, where they turn into adults (late intestinal phase, with diarrhea, mild abdominal pain, weight loss, nausea and vomiting), which lay eggs that are eliminated in stools.
How to prevent parasitic diarrhea?
Prevention rests above all on hygiene advice which applies to all types of infectious diarrhea.
Another avenue inspired by an observation: different people react differently to parasites; some are asymptomatic while others suffer from acute diarrhea. The gut microbiota can explain this clinical variation: for example, in Bangladesh, low abundance of the bacterium Megasphaera before and at the time of detection of the parasite Cryptosporidium has been associated with parasitic diarrhea in infants, which suggests that the gut microbiota may play a role in the severity of cryptosporidiosis. 10
If we look at helminths, the connections between these worms and gut microbiota are also the subject of multiple studies. Complex interactions exist between the two, but also with the host's immune system. However, intestinal worms should not necessarily be avoided: in small doses, they could have beneficial health effects, such as resistance to other digestive system pathogens or even the prevention of allergic ailments.11
How to treat parasitic diarrhea?
As with all types of diarrhea, parasitic diarrhea exposes the very young, the elderly and immunocompromised patients in particular to the risk of dehydration. Treatment thus focuses primarily on treating the symptoms, namely, fighting the loss of water and electrolytes (sodium, potassium and chloride ions):
- orally (with oral rehydration solutions, or ORS);
- or intravenously in the most serious cases.
When a health care professional suspects that a parasitic infection could be the cause of diarrhea (e.g. if a patient has recently returned from the tropics), they will usually request examination of a stool sample in order to identify the possible parasite. However, it is not always so simple. The parasite may not be present in all stools (which would require multiple samples), and the presence of a small quantity of a parasite does not necessarily mean that the culprit has been found. This makes parasitic infections such as giardiasis potentially difficult to diagnose, and the treatment decision also depends on the history and symptoms of the patient. 12
When a worm infection is proven, health care professionals may prescribe safe and effective medications such as albendazole or mebendazole.9 These medications are often distributed as part of deworming programs, targeting in particular children and women of reproductive age (ages 15-49) in endemic areas.8
Key points to remember about parasitic infectious 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). 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 parasite, diarrhea is said to be “parasitic”. There are 2 types of intestinal parasites: protozoa, measuring less than one millimeter, among which the best-known are Giardia intestinalis, responsible for giardiasis, and Entamoeba histolytica, which causes the dreaded amebiasis; and helminths, better known by the name worms, of which the most well-known is undeniably ascaris (roundworm). 6-8
- In the great majority of cases, the presence of digestive parasites is asymptomatic and not severe, possibly even beneficial (immunity). Sometimes, however, they cause diarrhea, of which the characteristics (presence of blood, presence of mucus, significant amount of liquid, etc.) depend on the parasite. 7
- Prevention of parasitic diarrhea rests above all on good hygiene practices (washing and cooking of food, hand-washing) and on the development of access to clean drinking water. Microbiota are also the subject of multiple studies, since they may explain the variation in symptoms and severity from one person to another. 10
- The treatment of parasitic diarrhea focuses on fighting dehydration. If the parasite is identified, the prescription of medication can be considered. 9 Systematic deworming programs are also proposed. 8
