Entamoeba histolytica
Sharmin Begum,1
Hayley Gorman,1
Attinder Chadha,1
and Kris Chadee1,
*
1
Departments of Microbiology, Immunology, and Infectious Diseases, Snyder Institute for Chronic Diseases, University of Calgary, Calgary, Alberta, Canada
TrendsTrendsininParasitologyParasitology
Mature
cyst
(ii)
(iii)
(iv)
(ii)
(iv) (iii)
(i)
(i) Ingestion through
contaminated food or water(ii)
Mature cysts excyst in the
small intestine
(iii) Trophozoites migrate to the
large intestine
Trophozoites colonize the colon
(asymptomatic infection) or can invade the
mucosa and disseminate to extraintestinal
sites (liver, lung, brain)
(iv) Newly generated cysts
are excreted in stool
Entamoeba histolytica
Life Cycle
Entamoeba histolytica is a human enteric protozoan parasite responsible for the disease amebiasis. Infection starts
through the ingestion of E. histolytica cysts contaminating food or water. The vast majority (90%) of infected individuals
are asymptomatic carriers in which the parasite resides firmly within the lumen of the colon and excrete cysts in stool to
complete the direct life cycle. In 10% of infected cases (symptomatic), E. histolytica breaches the intestinal mucosa and
invades the underlying lamina propria where it interacts with host immune cells, triggering a raging proinflammatory cytokine
response resulting in tissue damage and the hallmark amebic 'flask-shaped ulcer' formation. It is not known why a
large proportion of E. histolytica-colonized individuals do not progress to invasive disease. Several parasite virulence factors
along with host genetics, microbiota, and immune responses, are likely to drive the complex pathogenesis.
KEY FACTS:
Friedrick Lösch discovered E. histolytica
in 1873 and was the first to describe the
relationship between the parasite and
disease, although reports of bloody
diarrhea, consistent with amebiasis, date
back to 1000 BC.
E. histolytica is morphologically identical
to the noninfectious Entamoeba dispar
which lacks key cysteine proteinases
that are important for pathogenesis.
E. histolytica exists as an infectious cyst
with four nuclei. Upon ingestion, it
excysts in the terminal ileum to give rise
to disease-causing motile trophozoites
through its pseudopodium.
Virulence factors allow the parasite to
colonize colonic mucus and to overcome
innate epithelial barrier function.
DISEASE FACTS:
E. histolytica is prevalent in developing
countries due to poor sanitation and
hygiene but it has also emerged globally
from returning travelers and is spread by
sexual transmission through oral–anal
contact.
~100 million cases occur annually. In 2013,
11 300 deaths from amebiasis ranked it the
fourth leading cause of parasitic death.
Infection can lead to mild to bloody
diarrhea, cramping and fever, abdominal
pain, and weight loss. If left untreated, it
may contribute to extraintestinal
amebiasis, mostly liver abscess.
Stool sample analysis for cysts or
trophozoites, PCR, ELISA, and
serological evaluation for diagnosis.
Nitroimidazoles: metronidazole, are the
mainstay drug therapy for invasive
amebiasis.
TAXONOMY AND CLASSIFICATION:
PHYLUM: Amoebozoa
CLASS: Lobosea
ORDER: Amoebida
FAMILY: Entamoebidae
GENUS: Entamoeba
SPECIES: E. histolytica
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Inner mucus layer
NF-κB
IL-1β
NO
Tumor necrosis
factor (TNF)-α
IL-1β
TNF-α
NO
NO
IL-8
IL-8
Interleukin (IL)-8
Intestinal
epithelial
cell (IEC)
Lamina propria Macrophage
Dendritic cell
Cell death
Goblet
cell
Outer mucus layer
E. histolytica and microbiota
in contact with host cell
(iii) Invasion
Neutrophil recruitment
Inflammatory responses
Amebicidal
macrophage
(iv) In contact
with macrophage
(ii) Binding to epithelial cells
(i) Dissolution of the mucus layer
Am
ebic
*Correspondence:
kchadee@ucalgary.ca (K. Chadee).
676 Trends in Parasitology, July 2021, Vol. 37, No. 7 © 2021 Elsevier Ltd. All rights reserved. https://doi.org/10.1016/j.pt.2021.01.001
Trends in Parasitology | Parasite of the Month
Acknowledgments
K.C. is grateful for funding from the Natural Sciences and Engineering Research Council of Canada (RGPIN-2019-04136) and the Canadian Institutes of Health Research (PJT-407276)
for original research summarized here.
Declaration of Interests
The authors declare no competing interests.
Resources
https://amoebadb.org/
www.who.int/ith/diseases/amoebiasis/en/
www.cdc.gov/parasites/amebiasis/index.html
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