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Amebiasis

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Amebiasis is a disease which is caused by amoeba. An amoeba is a single-celled microscopic organism that has no solid body structure. Amebiasis is contracted by consuming contaminated food or water containing the cyst stage of the parasite. It can also be spread by person to person contact. Simply said, Amebiasis is an intestinal illness caused by a microscopic parasite called Entamoeba histolytica. The other names of Amebiasis are Amebic dysentery and Intestinal amebiasis. 24

Fedor Aleksandrovich Lošch, in St. Petersburg, Russia, first described amebiasis in 1875. He originally named the organism Amoeba coli and documented its pathogenicity in a dog fed with dysenteric stools from a patient. In 1886 in Egypt, Kartulis proved amebae to be the cause of intestinal and hepatic lesions in patients with diarrhea. Later, Councilman and Lafleur, at Johns Hopkins University Hospital, distinguished between bacillary and amebic dysentery in 1891. Walker and Sellards, in the Philippines, described the pathogenic role of amebae in extensive studies in 1913. 1

The parasite exists in 2 forms: a motile form, called the trophozoite, and a cyst form, responsible for the person-to-person transmission of infection. The trophozoite of E histolytica inhabits the large intestine to produce lesions of amebic colitis. Invasion of the colonic mucosa leads to dissemination of the organism to extracolonic sites, predominantly the liver. Faced with an adverse colonic environment, the trophozoite changes to the cystic form, better adapted to survival. 1

Amebiasis is an infection caused by the protozoal organism E histolytica, which causes colitis and liver abscess. Amebiasis occurs when a person swallows microscopic cysts containing the parasites. The cysts may be in contaminated food or water. Transmission generally occurs through ingestion of cysts from food or water contaminated by feces. All household members should have their stools examined because person to person transmission can occur. 31

The cyst of E histolytica averages 12 mm, ranging from 5-20 mm (see Image 2). It has 1-4 nuclei that are morphologically similar to the nuclei of the trophozoite. The cyst may have iodine-stainable glycogen clumps and chromatoid bodies with smooth rounded edges. 1

Amebiasis is caused by the parasite Entamoeba histolytica. To get Amebiasis general one needs to come in direct contact with an infected stool, but it can also be received through infected water. Once the cyst form of the amoeba makes it was in to the small intestine an infection begins. 25

Entamoeba dispar is a nonpathogenic protozoon morphologically identical to E histolytica. The previously reported asymptomatic infections due to the so-called nonpathogenic strains of E histolytica now are recognized to be due to E dispar. These 2 species of Entameba can be distinguished by the monoclonal antibodies. Other morphologically distinct organisms, such as Entamoeba coli and Entamoeba hartmanni, are also nonpathogenic. Infections due to E histolytica cause a spectrum of illnesses, listed below. 1

In severe cases of intestinal amebiasis, the organism invades the lining of the intestine, producing sores (ulcers), bloody diarrhea, severe abdominal cramps, vomiting, chills, and fevers as high as 104-105�F (40-40.6�C). In addition, a case of acute amebic dysentery may cause complications, including inflammation of the appendix (appendicitis), a tear in the intestinal wall (perforation), or a sudden, severe inflammation of the colon (fulminating colitis). 3

An ameboma is a mass of tissue in the bowel that is formed by the amebiasis organism. It can result from either chronic intestinal infection or acute amebic dysentery. Amebomas may produce symptoms that mimic cancer or other intestinal diseases. 3

After adherence, trophozoites invade the colonic epithelium to produce the ulcerative lesions typical of intestinal amebiasis (see Image 4). The trophozoites of E histolytica lyse the target cells by using lectin to bind to the target cells’ membranes and using the parasite’s ionophorelike protein to induce a leak of ions (ie, Na+, K+, Ca+) from the target cell cytoplasm. A number of hemolysins, encoded by plasmid (ribosomal deoxyribonucleic acid [rDNA]) and cytotoxic to the intestinal mucosal cells, have been described in E histolytica. An extracellular cysteine kinase causes proteolytic destruction of the tissue, producing flask-shaped ulcers (see Image 5). Phorbol esters and protein kinase C activators augment the cytolytic activity of the parasite. 1

Even though Entamoeba histolytica must be considered a major intestinal pathogen most infections are asymptomatic with no invasion of the mucosa. E. histolytica is widely distributed infecting approximately l0% of the world’s population but invasive amebiasis is a major clinical problem only in certain regions. 46

The most common form of extraintestinal amebiasis is amebic abscess of the liver. In the United States, amebic liver abscesses occur most frequently in young Hispanic adults. An amebic liver abscess can result from direct infection of the liver by E. histolytica or as a complication of intestinal amebiasis. Patients with an amebic abscess of the liver complain of pain in the chest or abdomen, fever, nausea, and tenderness on the right side directly above the liver. 3

Cell-mediated immunity is important in limiting the disease and preventing recurrences. Antigen-specific blastogenic responses occur, leading to production of lymphokines, including interferon-d (IFN-d), which activates the killing of E histolytica trophozoites by the macrophages. This killing depends on contact, oxidative pathways, nonoxidative pathways, and nitric oxide (NO). Lymphokines, such as tumor necrosis factor-alpha (TNF-a), are capable of activating the amebicidal activity of neutrophils. Incubation of CD8+ lymphocytes with E histolytica antigens in vitro elicits cytotoxic T-cell activity against the trophozoites. During acute invasive amebiasis, T-lymphocyte response to E histolytica antigens is depressed by a parasite-induced serum factor. 1

Prevalence rate of amebiasis in the United States is about 4%. E dispar infection, which is always asymptomatic, is 10 times more common than E histolytica infection. Moreover, only 10% of E histolytica infections cause invasive disease. Therefore, only 1% of persons with stool microscopy showing Entamoeba develop symptomatic amebiasis. Increased prevalence of amebiasis in the United States is noted in those who have emigrated from endemic areas, are of lower socioeconomic status, are institutionalized (especially individuals with mental retardation), are male homosexuals, and who live in communal situations. 1

It is vital to distinguish between amebiasis and another disease, inflammatory bowel disease (IBD) that produces similar symptoms because, if diagnosed incorrectly, drugs that are given to treat IBD can encourage the growth and spread of the amebiasis organism. Because of the serious consequences of misdiagnosis, potential cases of IBD must be confirmed with multiple stool samples and blood tests, and a procedure involving a visual inspection of the intestinal wall using a thin lighted, tubular instrument (sigmoidoscopy) to rule out amebiasis. 3

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One Response to “Amebiasis”

  1. Spencer on April 28th, 2008

    Just read your post and thought you would like to know about this site dedicated to alleviating misdiagnosis and unnecessary lab testing called ARUP Consult. It is a physician resource, however, anyone can access it. It includes algorithms and diagnostic advice per condition and disease as well as the most up-to-date lab test information. Hope this helps.
    http://www.arupconsult.com

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