Entamoeba histolytica infection affects up to 10% of the world’s population and is the third leading cause of parasitic deaths, trailing only malaria, the clinical manifestation of Plasmodium species parasite infection, and schistosomiasis, the umbrella term for the disease caused by Schistosoma spp. infection. This parasite can be found in colder climates such as Alaska, Russia, and Canada, in addition to subtropical and tropical areas of the world. E. histolytica is commonly found in areas where human waste is used as fertilizer, areas with poor sanitation, hospitals for the mentally ill, prisons, and day care centers. Because it causes frequent asymptomatic infections in homosexual men, particularly in Western countries, this organism has historically been prevalent in homosexual communities.
Common associated disease or condition names
Intestinal amebiasis, amebic colitis, amebic dysentery, extraintestinal amebiasis
Morphology of Entamoeba histolytica
- E. histolytica trophozoites (trophs) range in size from 8 to 65 m, with an average size of 12 to 25 m.
- The trophozoite moves in a rapid, unidirectional, progressive manner, aided by finger-like hyaline pseudopods.
- A karyosome is a small central mass of chromatin found in a single nucleus (also referred to as karyosomal chromatin). Eccentric or fragmented karyosomal material is an example of a karyosomal variant.
- This amebic parasite’s karyosome is surrounded by chromatin material, a morphologic structure known as peripheral chromatin. This peripheral chromatin is typically fine and evenly distributed in a perfect circle around the nucleus. Variations, such as uneven peripheral chromatin, are possible. Although the appearance of the karyosome and peripheral chromatin may vary, most trophozoites retain the more typical features described.
- When unstained preparations are stained, the nucleus becomes visible. Lightly stained fibrils located between the karyosome and peripheral chromatin may be visible in stained preparations.
- The cytoplasm of the E. histolytica trophozoite is finely granular and has the appearance of ground glass.
- Because E. histolytica is the only intestinal ameba with this characteristic, red blood cells (RBCs) in the cytoplasm are considered diagnostic. Bacteria, yeast, and other debris may be present in the cytoplasm, but their presence is not diagnostic.
- E. histolytica cysts are typically smaller than trophs, measuring 8 to 22 m in length, with an average range of 12 to 18 m.
- The presence of a hyaline cyst wall aids in identifying this morphologic form. Young cysts have unorganized chromatin material that transforms into squared or roundended structures called chromatoid bars, which are defined as structures containing condensed RNA material.
- In young cysts, a diffuse glycogen mass, a cytoplasmic area with no defined borders thought to represent stored food, is also visible.
- The glycogen mass usually disappears as the cyst matures, a process that most likely represents the use of the stored food.
- Typically, one to four nuclei are present. In every way, these nuclei resemble those of the trophozoite, but they are usually smaller. Nuclear variations do occur, with the most common being eccentric (rather than central) karyosomes, thin plaques of peripheral chromatin, or a crescent-shaped clump of peripheral chromatin on one side of the nucleus.
- The nuclei have been enlarged to reveal more nuclear detail. The mature infective cyst has four nuclei (containing four nuclei).
- The cytoplasm continues to be fine and granular. The cyst stage contains no RBCs, bacteria, yeast, or other debris.
Method of Transmission Amebiasis
There are several ways to spread Entamoeba histolytica. The infective stage, the cyst, is ingested through hand-to-mouth contact and food or water contamination. Furthermore, E. histolytica can be transmitted through unprotected sex. By depositing infective cysts on unprotected food, flies and cockroaches may also serve as vectors (living carriers responsible for transmitting parasites from infected hosts to uninfected hosts) of E. histolytica. Inadequately treated water supplies are another source of infection.
Life Cycle of Entamoeba histolytica
- After ingesting the infective cyst, excystation occurs in the small intestine.
- A single cyst produces eight motile trophozoites as a result of nuclear division.
- These motile amebas settle in the large intestine lumen, where they replicate via binary fission and feed on living host cells.
- Trophozoites can migrate to other organs in the body, such as the liver, and cause an abscess to form. If these trophozoites do not return to the lumen of the large intestine, their life cycle ends, and diagnosis will be based on serologic testing.
- When four nuclei are present, encystation occurs in the intestinal lumen, and cyst formation is complete.
- These infective cysts are transmitted to the environment through human feces and are resistant to a variety of physical conditions.
- It is common to survive in a feces-contaminated environment for up to a month.
- It is important to note that, in addition to cysts, trophozoites may be present in the stool under certain conditions.
- If the intestinal motility is fast, liquid or semiformed samples may contain trophozoites. Cysts, on the other hand, will form if the intestinal motility is normal.
Clinical Symptoms of Amebiasis
Entamoeba histolytica is the only pathogenic intestinal ameba that has been identified. The severity of symptoms varies and is determined by two major factors:
- The parasite’s location(s) in the host
- The extent of tissue invasion.
Asymptomatic Carrier State
The asymptomatic carrier state of a patient infected with E. histolytica is caused by three factors:
- The parasite is a low-virulence strain
- The inoculation into the host is low
- The patient’s immune system is intact.
Amebas may reproduce in these cases, but the infected patient exhibits no clinical symptoms.
Symptomatic Intestinal Amebiasis.
Patients infected with E. histolytica who exhibit symptoms frequently develop amebic colitis, which is defined as an intestinal infection caused by the presence of amebas displaying symptoms. In some cases, these patients may progress from amebic colitis to amebic dysentery, a condition characterized by blood and mucus in the stool. Individuals suffering from amebic colitis may experience nonspecific abdominal symptoms or more specific symptoms such as diarrhea, abdominal pain and cramping, chronic weight loss, anorexia, chronic fatigue, and flatulence. Secondary bacterial infections can occur after flask-shaped amebic ulcers form in the colon, cecum, appendix, or rectosigmoid area of the intestine. As previously stated, stools recovered from patients suffering from amebic dysentery are distinguished by the presence of blood, pus, and mucus.
Symptomatic Extraintestinal Amebiasis
E. histolytica Trophozoites that enter the bloodstream are removed and housed in the liver. An abscess in the right lobe of the liver and trophozoite extension through the diaphragm, resulting in amebic pneumonitis, are possible complications. Patients in this state frequently exhibit symptoms similar to a liver infection plus a cough, with upper right abdominal pain and fever being the most common. Weakness, weight loss, sweating, severe nausea and vomiting, as well as constipation with or without alternating diarrhea, may occur.
E. histolytica has been shown to migrate to and infect other organs besides the liver, including the lung, pericardium, spleen, skin, and brain. Venereal amebiasis is also possible. Men contract penile amebiasis after having unprotected sex with a woman who has vaginal amebiasis. During anal intercourse, the disease can also be transmitted. It’s worth noting that the trophozoite form of E. histolytica is the most commonly found in these genital areas.
Laboratory Diagnosis of Entamoeba histolytica
Standard and alternative methods can be used to diagnose E. histolytica infection. In addition to traditional wet preparation and permanent staining techniques on a suspected stool sample, material collected during a sigmoidoscopy procedure, as well as hepatic abscess material, can be processed and examined in the same way. In culture, E. histolytica is supported by a special medium known as TYI-S-33. Other laboratory tests, including immunologically based procedures, may be used when E. histolytica is suspected but not recovered in stool samples. Antigen tests, enzyme-linked immunosorbent assay (ELISA), indirect hemagglutination (IHA), gel diffusion precipitin (GDP), and indirect immunofluorescence are currently available methods (IIF). There are serologic tests available to detect E. histolytica, but they are typically only useful in cases of extraintestinal infections.
Prevention and Control
Several precautions can be taken to avoid E. histolytica infections. Uncontaminated water is required, which can be obtained by boiling or treating with iodine crystals. It’s worth noting that the infective (quadrinucleated) cyst is resistant to chlorination. To ensure a safe water supply, a water treatment regimen that includes filtration and chemical treatment is required. Proper food washing, avoiding the use of human feces as fertilizer, good personal hygiene and sanitation practices, protecting food from flies and cockroaches, and avoiding unprotected sexual practices all help to break the transmission cycle.
Treatment of Amebiasis
Treatment regimens for patients infected with E. histolytica differ depending on the type of infection. Asymptomatic individuals may be treated with paromomycin, diloxanide furoate (Furamide), or metronidazole because there is concern that an infection with E. histolytica may become symptomatic only in the intestinal tract or with subsequent extraintestinal invasion (Flagyl). Iodoquinol, paromomycin, or diloxanide furoate are commonly used to treat patients with symptomatic intestinal amebiasis. Patients with extraintestinal amebiasis should be treated with metronidazole or tinidazole in conjunction with symptomatic intestinal amebiasis treatment.