Amoebiasis is a form of gastroenteritis i.e., the inflammation of the gastrointestinal tract, and is generally associated with poor sanitation conditions. This disease is caused by an amoebozoa known as Entamoeba histolytica and is the third leading cause of death due to parasitic diseases around the world, following Malaria and Schistosomiasis. It accounts for as many as 100,000 deaths per year, globally, and more than 500.000 people are infected per year! Entamoeba spp. are infective amoebozoans, known to cause infections in all classes of vertebrates, along with some invertebrates. Although humans are hosts to about six species of Entamoeba, it's the Entamoeba histolytica species that is pathogenic and causes Amoebiasis. This article discusses the peculiarities involved with causes, development and treatment of E. histolytica infection and the disease caused by this amoebozoan.
Ingestion of the infective stage in the form of viable cysts via faecal contaminated food and water.
Cysts pass unharmed into the stomach and then into the small intestine. The alkaline oh of the small intestine promotes multiple fission of the cyst(s).
Excystation (i.e., escape of the parasite from the cyst).
Division of the metacyst into eight trophozoites (trophozoite refers to the growing stage of a parasite wherein it absorbs nutrition from the host and undergoes development to become the mature parasite).
The trophozoites make their way into the large intestine where they invade the host tissue. Lectins are involved in the binding of the parasite to the host cells.
From this point on, the parasite may live as a commensal in the intestinal lumen. It feeds on the bacteria and food in the gut. Individuals infected this way are known as asymptomatic carriers of the disease. The faecal matter of these persons carries cysts of the Entamoeba parasite.
The other route that the parasite can take is encystation and multiplication. Following multiplication, the parasite spreads laterally very quickly.
Eventually, the parasite causes destruction of the intestinal epithelium by forming lesions or ulcers on the mucosa and submucosa. This is done via special virulence factors called cysteine proteases and the Gal? Gal NAc lectins. This enzyme also circumvents the host's immune response by cleaving the secretory IgA, and IgG immunoglobulins and the complement factors.
The parasite can penetrate into the bloodstream, causing the infection to spread to the liver , the lungs and even the brain and cause lethal effects.
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The infection can be asymptomatic, going undetected and untreated for many years! The parasite survives on the bacterial cells and food particles of the gut.
In case of symptomatic amoebiasis, symptoms may begin with mild to unbearable abdominal pain. Intermittent fever and lethargy is also a common complaint. Weight loss may also occur.
The main symptoms of amoebiasis may range from mild to fulminant diarrhoea accompanied by blood due to rectal bleeding, along with mucus.
Appendicitis and extra-intestinal amoebiasis abscesses in the liver (hepatic amoebiasis), lung (pleuropulmonary amoebiasis), and brain (cerebral amoebiasis) may also result. Abscesses result from the digestion of the surrounding tissue by the parasite and consist of the trophozoites surrounded by dying tissue and cell debris.
Major complications may arise if the infection is left undetected or untreated, including.
Necrotizing colitis.
Perforated bowels.
Recto-vaginal fistula, which is the abnormal connection between the rectum and the vagina, causing bowel contents to leak through the fistula and pass through the vagina.
Empyema in the pleura.
Peritonitis, i.e., inflammation of the epithelium of the abdominal wall.
Amoebiasis is found to affect youngsters and middle-aged adults.
After infection, the incubation period can last upto 2-4 weeks or sometimes, even months or years! Moreover, most of the symptoms are nonspecific and identical to infections of Shigella, Salmonella and enteroinvasive E. coli. Hence, laboratory diagnosis is often quite difficult.
The diagnoses that may be done include.
Blood tests to detect leukocytosis, which is indicative of entamoebic infection.
Stool tests which include microscopic examination. The diagnosis depends on the presence of trophozoites in the fresh stool samples and cysts in the other samples.
Microscopic detection of erythrophagocytosis.
Serological tests based on antigen detection.
ELISA to detect the lectins associated with the parasite by observing anti-lectin antibodies in the serum.
PCR tests of sample isolated from stools.
Treatment involves the use of antiparasitic and antiamoebic agents.
Antiamoebic drugs include Iodoquinol, Paramomoycin and Diloxanide Furoate, which are often prescribed for asymptomatic infections.
Symptomatic luminal and extraluminal intestinal amoebiasis is treated by amebicides like metronidazole (marketed as Flagyl) and Iodoquinol (Yodoxin). Erythromycin may also be used in some cases.
In the case of hepatic amoebiasis, metronidazole, followed by diloxanide is the drug of choice.
When diagnosed with amoebiasis, the patient must adopt preventive eating habits as well, by totally avoiding spicy food and raw vegetables. Street foods are a major risk as well. Consumption of meat is not recommended. The affected individual must consume pasteurised dairy products only. Soft foods like rice, bananas and soups are recommended.
Amoebiasis is a common occurrence in developing and under-developed countries with improper and unsanitary hygiene standards. Good personal hygiene and avoiding contaminated food and water are the only ways how amoebiasis may be prevented.
Preventive measures for amoebiasis include.
Washing of hands regularly and thoroughly with soap and water.
Maintenance of clean toilets.
Thorough washing of hands when handling food.
Proper cleaning and washing of raw foodstuffs.
The viable cysts in water can be eliminated by hyperchlorination and/or iodination.
Education and awareness among general public is a must.
Disease management with regard to the rapid detection and proper treatment must be facilitated in the community.
When travelling, especially to developing and under-developed countries, one should take the following precautions.
Avoid consumption of raw food items.
Ensure consumption of only boiled or packaged drinking water or chlorinated water.
Preventive measures for amoebiasis include.
Amoebiasis is a gastrointestinal disease caused by the protozoan Entamoeba histolytica.
Third leading cause of deaths annually, worldwide.
Infective stage is the cyst, invasive stage is the trophozoite.
Causes ulcers in the intestine and can spread to the liver, lungs and brain.
Treatable by antiparasitic, antiamoebic drugs, and antibiotic drugs including iodoquinol, metronidazole, etc.
Preventive measures include maintenance of proper personal and social hygiene and proper sanitation standards in the community.
Q1. What species of Entamoeba is non-pathogenic but does infect humans?
Ans. The name of this species is Entamoeba dispar, which is morphologically identical to E. histolytica. Other infectious but non-pathogenic species include− E. coli, E. polecki, E. bangladeshi, and E. moshkovskii,.
Q2. What is amoeboma? Can it be cancerous?
Ans. Amoeboma or amoebic granuloma is a severe complication of amoebiasis which leads to granulation of the colon resulting in a large lesion in the bowel, which can be confused with cancer of the bowel.
Q3. How does iodoquinol work against amoebiasis?
Ans. Iodoquinol is a chelating agent that binds Fe ions, and reduces its availability fro the protozoans metabolism.
Q4. How does entamoeba histolytica reach the liver from the intestine?
Ans. The protozoan adopts the hepatic portal circulation route, wherein the hepatic portal vein connects the gut to the liver and hence, transports the parasites to the liver from the intestines.
Q5. What is Dientamoebiasis?
Ans. Dientamoebiasis, aka traveller’s diarrhoea is a disease of the lower gastrointestinal tract, caused by Dientamoeba fragilis. Its symptoms include diarrhoea, weight loss and fatigue.
Q6. How does temperature affect Entamoeba histolytica ?
Ans. It's been found that E. histolytica cysts can't tolerate temperatures above 40℃ and below -5℃. Hence, boiling of water and cooking of food is an important preventive measure against Amoebiasis.
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