Clinical syndromes associated with Entamoeba histolytica infection include noninvasive intestinal infection, intestinal amebiasis, ameboma, and liver abscess. Disease is more severe in the very young, the elderly, and pregnant women. Patients with noninvasive intestinal infection may be asymptomatic or may have nonspecific intestinal tract complaints. People with intestinal amebiasis (amebic colitis) generally have 1 to 3 weeks of increasingly severe diarrhea progressing to grossly bloody dysenteric stools with lower abdominal pain and tenesmus. Weight loss is common, but fever occurs only in a minority of patients (8%-38%). Symptoms may be chronic and may mimic inflammatory bowel disease. Progressive involvement of the colon may produce toxic megacolon, fulminant colitis, ulceration of the colon and perianal area, and rarely, perforation. Progression may occur in patients inappropriately treated with corticosteroids or antimotility drugs. An ameboma may occur as an annular lesion of the cecum or ascending colon that may be mistaken for colonic carcinoma or as a tender extrahepatic mass mimicking a pyogenic abscess. Amebomas usually resolve with antiamebic therapy and do not require surgery.
In a small proportion of patients, extraintestinal disease may occur. Although the liver is the most common extraintestinal site, the lungs, pleural space, pericardium, brain, skin, and genitourinary tract also may be involved. Liver abscess may be acute, with fever, abdominal pain, tachypnea, liver tenderness, and hepatomegaly; or chronic, with weight loss, vague abdominal symptoms, and irritability. Rupture of abscesses into the abdomen or chest may lead to death. Evidence of recent intestinal infection usually is absent.
The genus Entamoeba includes 6 species that live in human intestine. Three of these species are identical morphologically: E histolytica, Entamoeba dispar , and Entamoeba moshkovskii . The pathogenic E histolytica and the nonpathogenic E dispar and E moshkovskii are excreted as cysts or trophozoites in stools of infected people.
E histolytica can be found worldwide but is more prevalent in people of lower socioeconomic status who live in economically developing countries, where the prevalence of amebic infection may be as high as 50% in some communities. Groups at increased risk of infection in industrialized countries include immigrants from or long-term visitors to areas with endemic infection, institutionalized people, and men who have sex with men. E histolytica is transmitted via amebic cysts by the fecal-oral route. Ingested cysts, which are unaffected by gastric acid, undergo excystation in the alkaline small intestine and produce trophozoites that infect the colon. Cysts that develop subsequently are the source of transmission, especially from asymptomatic cyst excreters. Infected patients excrete cysts intermittently, sometimes for years if untreated. Transmission occasionally has been associated with contaminated food, water, and enema equipment. Sexual transmission also may occur.
The incubation period is variable, ranging from a few days to months or years but commonly is 2 to 4 weeks.
A presumptive diagnosis of intestinal infection depends on identifying trophozoites or cysts in stool specimens. Examination of serial specimens may be necessary. Specimens of stool, endoscopy scrapings (not swabs), and biopsies should be examined by wet mount within 30 minutes of collection and fixed in formalin or polyvinyl alcohol (available in kits) for concentration and permanent staining. E histolytica is not distinguished easily from the noninvasive and more prevalent E dispar and E moshkovskii , although trophozoites containing ingested red blood cells are more likely to be E histolytica . Polymerase chain reaction, isoenzyme analysis, and monoclonal antibody-based antigen detection assays can differentiate E histolytica, E dispar , and E moshkovskii .
The indirect hemagglutination (IHA) test has been replaced by commercially available enzyme immunoassay (EIA) test kits for routine serodiagnosis of amebiasis. The EIA test detects antibody specific for E histolytica in approximately 95% of patients with extraintestinal amebiasis, 70% of patients with active intestinal infection, and 10% of asymptomatic people who are passing cysts of E histolytica . Patients may continue to have positive test results even after adequate therapy.
Ultrasonography and computed tomography can identify effectively liver abscesses and other extraintestinal sites of infection. Aspirates from a liver abscess usually show neither trophozoites nor leukocytes.
Treatment involves elimination of the tissue-invading trophozoites as well as organisms in the intestinal lumen. E dispar and E moshkovskii infections do not require Treatment. Corticosteroids and antimotility drugs administered to people with amebiasis can worsen symptoms and the disease process. In settings where tests to distinguish species are not available, Treatment should be given to symptomatic people on the basis of positive results of microscopic examination. The following regimens are recommended:
- Asymptomatic cyst excreters (intraluminal infections): treat with a luminal amebicide, such as iodoquinol, paromomycin, or diloxanide.
- Patients with mild to moderate or severe intestinal symptoms or extraintestinal disease (including liver abscess): treat with metronidazole or tinidazole, followed by a therapeutic course of a luminal amebicide (iodoquinol or paromomycin). Nitazoxanide also may be effective for mild to moderate intestinal amebiasis, although it is only approved by the US Food and Drug Administration for Treatment of diarrhea caused by Giardia species or Cryptosporidium species.
Dehydroemetine followed by a therapeutic course of a luminal amebicide may be considered for patients for whom Treatment of invasive disease has failed or cannot be tolerated. However, dehydroemetine has significant toxicity and should be used with caution. An alternate Treatment for liver abscess is chloroquine phosphate concomitantly with metronidazole or tinidazole, followed by a therapeutic course of a luminal amebicide.
Surgical aspiration occasionally may be required when response of the abscess to medical therapy is unsatisfactory. To prevent spontaneous rupture of an abscess, patients with large liver abscesses may benefit from percutaneous or surgical aspiration.
Follow-up stool examination is recommended after completion of therapy, because no pharmacologic regimen is effective in eradicating intestinal infection completely. Household members and other suspected contacts also should have adequate stool examinations performed and be treated if results are positive for E histolytica
Isolation of the Hospitalized Patient
In addition to standard precautions, contact precautions are recommended for the duration of illness.
Careful hand hygiene after defecation, sanitary disposal of fecal material, and Treatment of drinking water will control the spread of infection. Sexual transmission may be controlled by use of condoms and avoidance of sexual practices that may permit fecal-oral transmission.
1 . For further information, see Drugs for Parasitic Infections.
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