Entamoeba histolytica

Disease / Pathogenesis

Entamoeba histolytica is the main cause of amoebiasis. Infections can be very serious as the amoeba has the ability to penetrate the intestinal wall and become extra intestinal causing abscesses and destruction in all types of tissue ranging from skin, liver, brain, lungs to name but just a few. It does seem to have a predilection for liver. Extra intestinal infection can be serious and life threatening. Greater than 50,000 people a year worldwide die from infections with Entamoeba histolytica.

The disease / pathogenesis are dependent on the location of the infection. As an example, liver involvement will produce abnormal liver enzyme tests; jaundice; abdominal pain in the right upper quadrant that is intense; fever and chills; loss of appetite; and weight loss.

Location in the Host

Generally the location in the host is lumen of the colon and the cecum. But if the organism becomes extra intestinal it can be found in the liver, brain, lung, etc.. where abscess formation will result. Radiologic scans of the suspected locations will detect the presence of these abscesses.

Geographic Distribution


Life Cycle

Morphology & Diagnosis

Trophozoites and cyst forms of the ameba can be detected in feces when intestinal. As with most enteric parasites the shedding of these forms may be intermittent and therefore at least three specimens for ova and parasite examination are optimal. Endoscopic aspirates and biopsies can be examined as well.

Extra intestinal infections may require the use of other methods to make the diagnosis. These include serological methods as well as imaging studies (scans of the liver). Many times the diagnosis is done through a thorough clinical history.

Feces examinations can lack sensitivity and many times are detection is dependent on the expertise of thuds the technologist performing the ova & parasite examinations. Recent advances in testing have enhanced the detection of the organism in suspected cases of amebic dysentery. EIA methods have shown promise but from this authors experience lack some sensitivity. Most promising are PCR or multiplex PCR testing. Sensitivity and specificity are great but their cost is prohibitive in most hospital laboratory settings. The multiplex PCR tests however provide results for a number of enteric pathogens that include parasites, bacteria, and viruses. Though costly, these multiplex PCR tests can prove to be money saving costs down stream when accounting for the elimination of other more costly tests if the offending enteric pathogen is identified. Most laboratories are still finding it difficult though to find the backing from the hospital administration that is needed to offer multiplex PCR testing.

Morphologically, when the cyst is detected in the feces in a ova & parasite examination, the cysts are 10-20u in size. They characteristically will have two to four nuclei (never more than four); they will have a small compact karyosome that usually is centrally located. A finely granular chromatin can be seen.

Trophozoites have a single nucleus. The organisms are very active and therefore the pseudopods may appear stretched out more than most other amoeba. The nucleus has a compact centrally located karyosome … though at times it may appear eccentric. The cytoplasm may contain ingested bacteria. Pathogenic forms will have ingested red blood cells and when seen they are considered diagnostic.


Entamoeba histolytica cyst in formalin / Lugol’s stain – CDC-Dr. L.L.C. Moore, Jr
Entamoeba histolytica cyst formalin / Lugol’s stain – CDC-Dr. Mae
Entamoeba histolytica and Entamoeba coli in trichrome stain CDC-Dr. Mae Melvin
Entamoeba histolyica trophozoite stained with trichrome – CDC-Dr. Mae Melvin; Dr. Greene
Entamoeba histolytica trophozoite stained with trichrome – CDC-DPDx – Melanie Moser