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Amebiasis

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Biopsy from patient with amebic dysentery - inflammation of the intestine with ulcers in the colon due to infection with an amoeba.

Amebiasis (also known as Amebic dysentery or Amebic colitis) is an infectious disease caused by a single-celled parasite called Entamoeba histolytica (ent-a-ME-ba his-to-LI-ti-ka). The disease is most common in people who live in developing countries that have poor sanitary conditions. In the United States, Amebiasis is most often found in immigrants from developing countries. It also is found in people who have traveled to developing countries and in people who live in institutions that have poor sanitary conditions. Men who have sex with men can become infected and can get sick from the infection, but they often do not have symptoms.[1]

Symptoms and treatment

On average, about one in 10 people who are infected with E. histolytica becomes sick from the infection. The symptoms often are quite mild and can include loose stools, stomach pain, and stomach cramping. Amebic dysentery is a severe form of amebiasis associated with stomach pain, bloody stools, and fever. Rarely, E. histolytica invades the liver and forms an abscess. Even less commonly, it spreads to other parts of the body, such as the lungs or brain.

Several antibiotics are available to treat amebiasis. Treatment must be prescribed by a physician. You will be treated with only one antibiotic if your E. histolytica infection has not made you sick. You probably will be treated with two antibiotics (first one and then the other) if your infection has made you sick.[1]

For asymptomatic infections, iodoquinol, paromomycin, or diloxanide furoate (not commercially available in the U.S.) are the drugs of choice. For symptomatic intestinal disease, or extraintestinal, infections (e.g., hepatic abscess), the drugs of choice are metronidazole or tinidazole, immediately followed by treatment with iodoquinol, paromomycin, or diloxanide furoate.[2]

Entamoeba trophozoite (activated, feeding stage in the life cycle). Phylum Amoebozoa, Class Archamoebae.

Causal agent

Several protozoan species in the genus Entamoeba infect humans, but not all of them are associated with disease. Entamoeba histolytica is well recognized as a pathogenic ameba, associated with intestinal and extraintestinal infections. The other species are important because they may be confused with E. histolytica in diagnostic investigations.[2]

Infections are caused:

  • By putting anything into your mouth that has touched the stool of a person who is infected with E. histolytica.
  • By swallowing something, such as water or food, that is contaminated with E. histolytica.
  • By touching and bringing to your mouth cysts (eggs) picked up from surfaces that are contaminated with E. histolytica[1]

Infectious lifecycle

Amebiasis LifeCycle.png

Cysts and trophozoites are passed in feces(1). Cysts are typically found in formed stool, whereas trophozoites are typically found in diarrheal stool. Infection by Entamoeba histolytica occurs by ingestion of mature cysts (2) in fecally contaminated food, water, or hands. Excystation (3) occurs in the small intestine and trophozoites (4) are released, which migrate to the large intestine. The trophozoites multiply by binary fission and produce cysts (5), and both stages are passed in the feces(1). Because of the protection conferred by their walls, the cysts can survive days to weeks in the external environment and are responsible for transmission. Trophozoites passed in the stool are rapidly destroyed once outside the body, and if ingested would not survive exposure to the gastric environment. In many cases, the trophozoites remain confined to the intestinal lumen ((A): noninvasive infection) of individuals who are asymptomatic carriers, passing cysts in their stool. In some patients the trophozoites invade the intestinal mucosa ((B): intestinal disease), or, through the bloodstream, extraintestinal sites such as the liver, brain, and lungs ((C): extraintestinal disease), with resultant pathologic manifestations. It has been established that the invasive and noninvasive forms represent two separate species, respectively E. histolytica and E. dispar. These two species are morphologically indistinguishable unless E. histolytica is observed with ingested red blood cells (erythrophagocystosis). Transmission can also occur through exposure to fecal matter during sexual contact (in which case not only cysts, but also trophozoites could prove infective).[2]

References

  1. 1.0 1.1 1.2 Factsheet: Amebiasis by the Division of Parasitic Diseases. Center for Disease Control.
  2. 2.0 2.1 2.2 PDPx-Lab Assistance: Amebiasis by Division of Parasitic Diseases