Amebiasis is caused by Entamoeba histolytica, a protozoan found worldwide. Although most cases of amebiasis are asymptomatic, dysentery and invasive extraintestinal disease can occur. Amebic liver abscess is the most common manifestation of invasive amebiasis, but other organs can also be involved, including pleuropulmonary, cardiac, cerebral, renal, genitourinary, and cutaneous sites. In developed countries, amebiasis primarily affects migrants from and travelers to endemic regions, men who have sex with men, and immunosuppressed or institutionalized individuals.
E histolytica is transmitted via ingestion of the cystic form (infective stage) of the protozoa. Viable in the environment for weeks to months, cysts can be found in fecally contaminated soil, fertilizer, or water or on the contaminated hands of food handlers. Fecal-oral transmission can also occur in the setting of anal sexual practices or direct rectal inoculation through colonic irrigation devices. Excystation then occurs in the terminal ileum or colon, resulting in trophozoites (invasive form). The trophozoites can penetrate and invade the colonic mucosal barrier, leading to tissue destruction, secretory bloody diarrhea, and colitis resembling inflammatory bowel disease. In addition, the trophozoites can spread hematogenously via the portal circulation to the liver or even to more distant organs.
Amebic infection was first described by Fedor Losch in 1875 in St. Petersburg, Russia. In 1890, Sir William Osler reported the first North American case of amebiasis, when he observed amebae in stool and abscess fluid from a physician who previously resided in Panama. The species name E histolytica was first coined by Fritz Schaudin in 1903. In 1913, in the Philippines, Walker and Sellards documented the cyst as the infective form of E histolytica. The life cycle was then established by Dobell in 1925.
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