Amebiasis
The most important causative agent of amebiasis is Entamoeba histolytica*
- Many individuals are asymptomatic reservoirs.
- Intestinal amebiasis, sometimes called amebic dysentery.
– Characterized by abdominal pain, colitis, and diarrhea; blood may be present in the stools.
- Extra-intestinal amebiasis: If the parasites reach the bloodstream, the parasites can travel to other organs.
– Because the trophozoites can get “caught” in the
liver as they move through the
hepatic portal system, liver involvement is the most common extra-intestinal manifestation.
– The liver abscesses and pain, especially in the right upper quadrant, as well as hepatomegaly and leukocytosis are associated with hepatic amebiasis.
The course of Entamoeba histolytica in the human host
- First, cysts are ingested in contaminated water.
– The cyst has multiple nuclei and elongated chromatoid body.
- Then, in the small intestine, the trophozoites are released from the cysts, by a process called excystation.
- The trophozoites migrate to the large intestine.
- The trophozoites multiply.
– Some transform to the inactive cyst form via a process called encystation.
This prepares them for survival in the external environment.
- Trophozoites release cytotoxins that destroy host tissues; they also ingest red blood cells.
– In the histological image of the large intestine, we highlight an example of the characteristic flask-shaped lesion produced by invasive amebiasis.
– This helps us make sense of the species name “histolytica,” which combines Greek words for
“tissue” and “dissolving.”
Trophozoites can invade deeper tissues; once they reach the bloodstream, they can be disseminated and cause extra-intestinal disease.*
- The liver is most commonly affected; other important targets include the lungs and brain.
Trophozoites and cysts are excreted in the feces; recall that trophozoites cannot survive in the external environment, but the cysts, with their protective coverings, are adapted to survive until ingested by the next host.*
In fecal samples, one must be careful not to confuse Entamoeba histolytica
with a morphologically similar species, Entamoeba dispar,
which is a non-pathological commensal. A helpful distinguisher is that only Entamoeba histolytica engulfs red blood cells (though these may not always be visible).*
- Carriers of Entamoeba histolytica can be prescribed paramomycin or iodoquinol; as we denoted in our table, treatment for active amebiasis typically involves metronidazole.
Images:
“Details - Public Health Image Library(PHIL).” Accessed November 5, 2018. https://phil.cdc.gov/details_linked.aspx?pid=416.
CDC/Dr. Mae Melvin; Dr. E. West. Gross pathology of liver containing amebic abscess Creation Date: 1962.
