E. histolytica is a unicellular single-nucleus protozoan that is said to infect 10%-12% of the world’s population, the majority of these being in the tropics. In the United States, the population at greatest risk are travelers to third-world countries, immigrants or migrants from these areas, immunocompromised persons, and about 20%-32% of male homosexuals. The organism life cycle consists of a trophozoite free-living stage that occurs in the human colon. There are two subgroups (strains or possibly species) of E. histolytica (different by enzyme analysis). One strain can invade tissue (10% of E. histolytica infections) and causes diarrhea with or without mucosal ulceration, sometimes entering the bloodstream and producing abscesses in the liver (or occasionally other organs). The much more common, relatively nonpathogenic strain can be asymptomatic, cause mild nonspecific gastrointestinal (GI) symptoms, or cause bloodless diarrhea that usually is not severe. Both types have the same basic life cycle in which the trophozoite forms a cyst, develops four nuclei, and passes outside the body in the feces. The cyst is said to be fairly resistant to the environment and can survive up to 3 months under the right conditions. The cyst is the infective stage and produces infection after being ingested in contaminated water or on food that has been in contact with either contaminated water or soil or fertilizer contaminated with human feces. Once the cyst reaches the colon the trophozoite inside emerges and divides, eventually forming eight trophozoites. About 10% of persons infected have symptoms; of these, about 10% (range 2%-20%) develop extraintestinal amebic infection and the remainder have diarrhea or the more severe colon mucosal inflammation known as amebic dysentery. Interestingly, male homosexuals with or without HIV-1 virus infection usually do not develop the invasive type of amebiasis. Because of the two subtypes of E. histolytica, there is a wide clinical spectrum of infection: a severe acute colitis (amebic dysentery) that may resemble severe ulcerative colitis (sometimes with blood and mucus in the stool) or shigellosis; chronic diarrhea similar to milder ulcerative colitis; intermittent mild diarrhea; asymptomatic carriers; and even a group with constipation. Acutely ill patients are usually afebrile and have normal white blood cell counts and hemoglobin values; although patients with severe amebic colitis or hepatic abscess frequently have low-grade fever, leukocytosis between 10,000 and 20,000/cu mm, and mild anemia. E. histolytica usually does not produce eosinophilia.

Stool examination. E. histolytica is more difficult to diagnose than most of the common intestinal parasites and requires special precautions. If the stool specimen is soft or liquid, it may contain motile trophozoites and should be sent to the laboratory immediately (with the time of collection noted) or placed into a special fixative, because trophozoites are fragile and quickly degenerate. Wet mounts should be done within 30 minutes after the specimen is obtained. Well-formed stools usually contain protozoan cysts rather than trophozoites and may be either temporarily refrigerated or placed into fixative. For collection procedure, three specimens, one specimen collected every other day, are more reliable than a single specimen. If results of the stool specimens on three alternate days are all negative, and if strong clinical suspicion is still present, a saline purge should be used. After a saline purge (e.g., Fleet Phosphosoda), the patient should pass liquid stools within a few hours. Oily laxatives (e.g., mineral oil or magnesia) make the stools useless for examination. Enema specimens are not advisable because they are too dilute to be of much value, and, in addition, the trophozoites may be destroyed. Barium, if present, also makes the preparation unfit to read. If stool specimens for amebae must be sent by mail, they should be placed in a preservative (one part specimen to three parts of 10% formalin). If possible, a second portion preserved in a special polyvinyl alcohol fixative (PVA) (in the same proportions) should be included along with the formalin-fixed portion. Formalin preserves ameba cysts and also eggs and larvae of other parasites. Polyvinyl alcohol fixative is used to make permanent stained preparations of protozoan trophozoites and cysts, which is not possible after formalin fixation (there is some disagreement about preservation of cysts with PVA). Stained slides considerably increase the chances of finding protozoan trophozoites and cysts and provide better cytologic detail for identification purposes.

Serologic tests. Serologic tests for amebiasis are available in reference laboratories. The most widely used procedures are gel diffusion, IHA, and slide LA. Various ELISA methods have been reported, based on several purified or recombinant antigens from E. histolytica. In patients with intestinal amebiasis, these tests detect about 10% of those who are asymptomatic E. histolytica carriers, less than 50% of those with mild amebic diarrhea, and about 85%-90% of those with invasive amebiasis. The IHA, LA, and ELISA tests are slightly more sensitive than gel diffusion. Thus, results of the more severe cases are more likely to be positive. The IHA and LA antibody levels persist for several years, so a positive test result does not necessarily mean active infection. Gel diffusion antibodies may become undetectable in 6 months, although some have reported persistent elevation for 1-2 years. Nucleic acid probes for antigen in stools have been reported and may be available in some university medical centers or large reference laboratories.

Extraintestinal amebiasis

The preceding discussion was concerned with the usual type of amebiasis—amebiasis localized to the colon. Visceral amebiasis is not common. Liver involvement with abscess formation is seen in a majority of these cases. Clinical hepatic amebiasis is always associated with chronic rather than acute ameba infestation. Only 30%-50% of patients provide a history of diarrhea. Only about 25% of patients have amebae detectable in the stool. Patients with classic hepatic amebiasis have hepatomegaly, right upper quadrant pain, elevation of the right hemidiaphragm, leukocytosis as high as 20,000/mm3 (20 x 109 L), and fever. Surprisingly, the alkaline phosphatase level is normal in more than one half of patients. Liver scan is often very helpful, both for detection and localization of a lesion. Results of the various serologic tests for amebiasis are positive in 90%-95% of cases.

Amebic encephalitis is a rare condition produced by free-living amebae of the Acanthamoeba and Naegleria species. Naegleria infection takes place in normal individuals, usually with a recent history of swimming in rivers, fresh-water lakes, or fresh-water swimming pools. Acanthamoeba infections occur in persons with decreased immunologic defenses, frequently without any history of swimming. Acanthamoeba keratitis is also being reported due to contamination of contact lenses. Diagnosis can be made through phase contrast examination of spinal fluid or permanent slides of centrifuged spinal fluid stained by Wright’s stain or trichrome stain. Centrifugation or refrigeration decreases motility of the organisms, which would hinder phase contrast examination but not permanent stained slide examination. Gram-stained smears are not recommended. Calcofluor white stain used for detection of fungus is reported to detect Acanthamoeba cysts as well (but not trophozoites).