Entamoeba histolytica

style="background:#Template:Taxobox colour;"\|Entamoeba histolytica
	; Entamoeba histolytica cyst
style="background:#Template:Taxobox colour;" \| Scientific classification
Domain:	Eukaryota;
Phylum:	Amoebozoa;
Class:	Archamoebae;
Genus:	Entamoeba;
Species:	E. histolytica;

	Amoebiasis Microchapters
Home
Patient Information
Overview
Historical Perspective
Classification
Pathophysiology
Causes
Differentiating Amoebiasis from other Diseases
Epidemiology and Demographics
Risk Factors
Natural History, Complications and Prognosis
Diagnosis
History and Symptoms
Physical Examination
Laboratory Findings
Imaging
Treatment
Medical Therapy
Surgery
Prevention
Cost-Effectiveness of Therapy
Future or Investigational Therapies
Case Studies
Case #1
Entamoeba histolytica On the Web
Most recent articles
cited articles
Review articles
CME Programs
Powerpoint slides
Images
American Roentgen Ray Society Images of Entamoeba histolytica All Images X-rays Echo & Ultrasound CT Images MRI
Ongoing Trials at Clinical Trials.gov
US National Guidelines Clearinghouse
NICE Guidance
FDA on Entamoeba histolytica
CDC onEntamoeba histolytica
Entamoeba histolytica in the news
Blogs on Entamoeba histolytica
to Hospitals Treating Amoebiasis
Risk calculators and risk factors for Entamoeba histolytica

Error creating thumbnail: File missing

Life-cycle of the Entamoeba histolytica

This page is about microbiologic aspects of the organism(s). For clinical aspects of the disease, see Amoebiasis.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Jesus Rosario Hernandez, M.D. [2]

Overview

Entamoeba histolytica is a an anaerobic parasitic protozoan that is responsible for the development of amoebiasis.

Higher Order Classification

Cellular organisms; Eukaryota; Protista; Amoebozoa; Archamoebae; Entamoeba

Natural Reservoir

Usually humans (only)
Reports of animals as natural reservoirs of E. histolytica have been described.

Structure and Genome

The exact number of chromosomes in E. histolytica is still unknown.
The cysts of E. histolytica contain 4 nuclei with even distribution of chromatin between the nuclei.
The trophozoites are spherical/oval shaped with a thin cell membrane and a single nucleus.
E. histolytica is able to move using pseudopods.

Life Cycle

Cysts and trophozoites are passed in human feces. Cysts are typically found in formed stool, whereas trophozoites are typically found in diarrheal stool.
Infection by Entamoeba histolytica occurs by ingestion of mature cysts in fecally contaminated food, water, or hands.
Excystation occurs in the small intestine and trophozoites are released, which migrate to the large intestine.
The trophozoites multiply by binary fission and produce cysts, and both stages are passed in the feces.
Because of the protection conferred by their walls, the cysts can survive days to weeks in the external environment and are responsible for transmission.
Trophozoites passed in the stool are rapidly destroyed once outside the body, and if ingested would not survive exposure to the gastric environment.
In many cases, the trophozoites remain confined to the intestinal lumen (A: noninvasive infection) of individuals who are asymptomatic carriers, passing cysts in their stool.
In some patients the trophozoites invade the intestinal mucosa (B: intestinal disease), or, through the bloodstream, extraintestinal sites such as the liver, brain, and lungs (C: extraintestinal disease), with resultant pathologic manifestations.
It has been established that the invasive and noninvasive forms represent two separate species, respectively E. histolytica and E. dispar. These two species are morphologically indistinguishable unless E. histolytica is observed with ingested red blood cells (erythrophagocystosis).
Transmission can also occur through exposure to fecal matter during sexual contact (in which case not only cysts, but also trophozoites could prove infective).

Microbiological Characteristics

Genus and Species	Entamoeba histolytica
Etiologic Agent of:	Amoebiasis; Amoebic dysentery; Extraintestinal Amoebiasis, usually Amoebic Liver Abscess = “anchovy sauce”); Amoeba Cutis; Amoebic Lung Abscess (“liver-colored sputum”)
Infective stage	Cyst
Definitive Host	Human
Portal of Entry	Mouth
Mode of Transmission	Ingestion of mature cyst through contaminated food or water
Habitat	Colon and Cecum
Pathogenic Stage	Trophozoite
Locomotive apparatus	Pseudopodia (“False Foot”)
Motility	Active, Progressive and Directional
Nucleus	'Ring and dot' appearance: peripheral chromatin and central karyosome
Mode of Reproduction	Binary Fission
Pathogenesis	Lytic necrosis (it looks like “flask-shaped” holes in Gastrointestinal tract sections (GIT)
Type of Encystment	Protective and Reproductive
Lab Diagnosis	Most common is Direct Fecal Smear (DFS) and staining (but does not allow identification to species level); Enzyme immunoassay (EIA); Indirect Hemagglutination (IHA); Antigen detection – monoclonal antibody; PCR for species identification. Culture: From faecal samples - Robinson's medium, Jones' medium
Treatment	Metronidazole for the invasive trophozoites PLUS a luminal amoebicide for those still in the intestine (Paromomycin is the most widely used)

Trophozoite Stage
Pathognomonic/Diagnostic Feature	Ingested RBC; distinctive nucleus

Cyst Stage
Chromatoidal Body	'Cigar' shaped bodies (made up of crystalline ribosomes)
Number of Nuclei	1 in early stages, 4 when mature
Pathognomonic/Diagnostic Feature	'Ring and dot' nucleus and chromatoid bodies

Differential diagnosis

Entamoeba histolytica must be differentiated from other causes of viral, bacterial, and parasitic gastroentritis.

Organism			Age predilection	Travel History	Incubation Size (cell)	Incubation Time	History and Symptoms			Diarrhea type8				Food source	Specific consideration
Organism			Age predilection	Travel History	Incubation Size (cell)	Incubation Time	Fever	N/V	Cramping Abd Pain	Small Bowel	Large Bowel	Inflammatory	Non-inflammatory	Food source	Specific consideration
Viral	Rotavirus		<2 y	-	<10²	<48 h	+	+	-	+			+	-	Mostly in day cares, most common in winter.
	Norovirus		Any age	-	10 -10³	24-48 h	+	+	+	+			+	-	Most common cause of gastroenteritis, abdominal tenderness,
	Adenovirus		<2 y	-	10⁵ -10⁶	8-10 d	+	+	+	+			+	-	No seasonality
	Astrovirus		<5 y	-		72-96 h	+	+	+	+			+	Seafood	Mostly during winter
Bacterial	Escherichia coli	ETEC	Any age	+	10⁸ -10¹⁰	24 h	-	+	+	+			+	-	Causes travelers diarrhea, contains heat-labile toxins (LT) and heat-stable toxins (ST)
		EPEC	<1 y	-	10^†	6-12 h	-	+	+	+			+	Raw beef and chicken	-
		EIEC	Any ages	-	10^†	24 h	+	+	+		+		+	Hamburger meat and unpasteurized milk	Similar to shigellosis, can cause bloody diarrhea
		EHEC	Any ages	-	10	3-4 d	-	+	+		+	+		Undercooked or raw hamburger (ground beef)	Known as E. coli O157:H7, can cause HUS/TTP.
		EAEC	Any ages	+	10¹⁰	8-18 h	-	-	+		+		+	-	May cause prolonged or persistent diarrhea in children
	Salmonella sp.		Any ages	+	1	6 to 72 h	+	+	+		+	+		Meats, poultry, eggs, milk and dairy products, fish, shrimp, spices, yeast, coconut, sauces, freshly prepared salad.	Can cause salmonellosis or typhoid fever.
	Shigella sp.		Any ages	-	10 - 200	8-48 h	+	+	+		+	+		Raw foods, for example, lettuce, salads (potato, tuna, shrimp, macaroni, and chicken)	Some strains produce enterotoxin and Shiga toxin similar to those produced by E. coli O157:H7
	Campylobacter sp.		<5 y, 15-29 y	-	10⁴	2-5 d	+	+	+		+	+		Undercooked poultry products, unpasteurized milk and cheeses made from unpasteurized milk, vegetables, seafood and contaminated water.	May cause bacteremia, Guillain-Barré syndrome (GBS), hemolytic uremic syndrome (HUS) and recurrent colitis
	Yersinia enterocolitica		<10 y	-	10⁴ -10⁶	1-11 d	+	+	+		+	+		Meats (pork, beef, lamb, etc.), oysters, fish, crabs, and raw milk.	May cause reactive arthritis; glomerulonephritis; endocarditis; erythema nodosum. can mimic appendicitis and mesenteric lymphadenitis.
	Clostridium perfringens		Any ages		> 10⁶	16 h	-	-	+		+		+	Meats (especially beef and poultry), meat-containing products (e.g., gravies and stews), and Mexican foods.	Can survive high heat,
	Vibrio cholerae		Any ages	-	10⁶-10¹⁰	24-48 h	-	+	+	+			+	Seafoods, including molluscan shellfish (oysters, mussels, and clams), crab, lobster, shrimp, squid, and finfish.	Hypotension, tachycardia, decreased skin turgor. Rice-water stools
Parasites	Protozoa	Giardia lamblia	2-5 y	+	1 cyst	1-2 we	-	-	+	+			+	Contaminated water	May cause malabsorption syndrome and severe weight loss
		Entamoeba histolytica	4-11 y	+	<10 cysts	2-4 we	-	+	+		+	+		Contaminated water and raw foods	May cause intestinal amebiasis and amebic liver abscess
		Cryptosporidium parvum	Any ages	-	10-100 oocysts	7-10 d	+	+	+	+			+	Juices and milk	May cause copious diarrhea and dehydration in patients with AIDS especially with 180 > CD₄
		Cyclospora cayetanensis	Any ages	+	10-100 oocysts	7-10 d	-	+	+	+			+	Fresh produce, such as raspberries, basil, and several varieties of lettuce.	More common in rainy areas
	Helminths	Trichinella spp	Any ages	-	Two viable larvae (male and female)	1-4 we	-	+	+		+		+	Undercooked meats	More common in hunters or people who eat traditionally uncooked meats
		Taenia spp	Any ages	-	1 larva or egg	2-4 m	-	+	+	+			+	Undercooked beef and pork	Neurocysticercosis: Cysts located in the brain may be asymptomatic or seizures, increased intracranial pressure, headache.
		Diphyllobothrium latum	Any ages	-	1 larva	15 d	-	-	-	+			+	Raw or undercooked fish.	May cause vitamin B₁₂ deficiency

8Small bowel diarrhea: watery, voluminous with less than 5 WBC/high power field

Large bowel diarrhea: Mucousy and/or bloody with less volume and more than 10 WBC/high power field
† It could be as high as 1000 based on patient's immunity system.

The table below summarizes the findings that differentiate inflammatory causes of chronic diarrhea^[1]^[2]^[3]^[4]^[4]

Cause	History	Laboratory findings	Diagnosis	Treatment
Diverticulitis	Bloody diarrhea Left lower quadrant abdominal pain Abdominal tenderness on physical examination Low grade fever	Leukocytosis Elevated serum amylase and lipase Sterile pyuria on urinalysis	Abdominal CT scan with oral and intravenous (IV) contrast	bowel rest, IV fluid resuscitation, and broad-spectrum antimicrobial therapy which covers anaerobic bacteria and gram-negative rods
Ulcerative colitis	Diarrhea mixed with blood and mucus, of gradual onset. Signs of weight loss Rectal urgency Tenesmus Blood is often noticed on underwear Different degrees of abdominal pain	Anemia Thrombocytosis A high platelet count Elevated ESR (>30mm/hr) Low albumin	Endoscopy	Induction of remission with mesalamine and corticosteroids followed by the administration of sulfasalazine and 6-Mercaptopurine depending on the severity of the disease.
Entamoeba histolytica	Abdominal cramps Diarrhea Passage of 3 - 8 semiformed stools per day Passage of soft stools with mucus and occasional blood Fatigue Intestinal gas (excessive flatus) Rectal pain while having a bowel movement (tenesmus) Unintentional weight loss	cysts shed with the stool	detects ameba DNA in feces	Amebic dysentery Metronidazole 500-750mg three times a day for 5-10 days Tinidazole 2g once a day for 3 days is an alternative to metronidazole Luminal amebicides for E. histolytica in the colon: Paromomycin 500mg three times a day for 10 days Diloxanide furoate 500mg three times a day for 10 days Iodoquinol 650mg three times a day for 20 days For amebic liver abscess: Metronidazole 400mg three times a day for 10 days Tinidazole 2g once a day for 6 days is an alternative to metronidazole Diloxanide furoate 500mg three times a day for 10 days must always be given afterwards.