Entamoeba Histolytica


Entamoeba histolytica is one of the more serious of the parasitic infections of humans, affecting an estimated 50 million persons worldwide. It is an amoeba, meaning a single cell organism that divides by binary fission and can replicate in the intestinal tract, primarily in the large intestine or colon.  In the colon the organisms actually bore their way into the bowel wall, and cause ulcerations, severe bloody diarrhea called dysentery, and abdominal pain.  Some of the amoeba can inter the blood stream where they can spread to other tissues.  The liver is where all the blood from the intestine goes first, and liver abscesses caused by the amoeba are the most common distant site of disease.  Abscesses can also occur in other organs, especially the lung and the brain.

E. histolytica trophozoite

Amebic dysentery is one of the classic causes of bloody severe diarrhea seen more in developing countries without clean water supplies.   The typical symptoms include bloody diarrhea, abdominal pain, weight loss, fatigue, and dehydration.  This can be a very serious problem and can be fatal especially in infants and children.  It can lead to chronic malnutrition and illness in adults.  Rarely in the colon wall an inflammatory process due to the amoeba called an ameboma can develop that leads to bowel obstruction that can mimic cancer, Crohn’s disease, and other colon disorders.  It requires a biopsy for diagnosis, but is a relatively rare complication.  Usually the bowel symptoms are primarily related to the dysentery.  Fever is present in over 85% or cases, bloody diarrhea in most and more chronic symptoms are noted in longer duration cases like weight loss and fatigue.

Amebic liver abscess on CT scan

Amoebic abscesses are the most serious common complications of Entamoeba histolytica infection.  When the organism invades the blood stream through the colon wall it often lodges in the liver where it continues to multiply.  The liver reacts and creates an inflammatory wall around the organisms, similar to the process for abscess formation with any other bacterial infection.  The abscess can become very large, but unlike most large bacterial abscesses these can usually be treated with medication and don’t usually require drainage procedures or surgical excision.  Rupture of an amoebic liver abscess can lead to shock and death in a high percentage of cases.

Abscesses in other organs can also often be treated with medications.  Metronidazole is the medication usually used to treat amebic abscesses as well as to treat amoebic dysentery.  It is inexpensive, can be effectively dosed orally, and has a high success rate in eradication of the amebiasis.

The live cycle of Entamoeba histolytica is fairly simple as parasitic life cycles go.  It exists in two stages, the infective cyst and the trophozoite that causes disease and symptoms in humans.  Humans are the primary host of E. histolytica, although other animals can be transient or accidental hosts.  The cysts can survive for weeks or months in wet environments, though are easily killed by heat or freezing temperatures.  When the cysts are ingested the cyst extrudes a trophozoite with four nuclei.  This divides into four trophozoites, and the nuclei or each of these divides to lead to eight initial trophozoites.   These organisms migrate to the large intestine where they grow and multiply by binary fission to increase their numbers.  After some time they undergo encystation to become infective cysts that are passed in the stool and can lead to infection of anyone who ingests them.

When the trophozoites invade the wall of the colon, they can migrate to other tissues through the blood stream and lead to abscess formation as discussed above.

Factors that lead make a community prone to high prevalence of Entamoeba histolytica infection include poor sanitation, crowding, and poverty.  In addition to these factors infection is more common in men who have sex with men, in institutionalized persons and developmentally disabled persons, and in those with immunodeficiency like AIDS.

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