Amebiasis

BASIC INFORMATION
Also called amoebic dysentery, amebiasis can take the form of either an acute or a chronic protozoal infection. Extraintestinal amebiasis can induce hepatic abscess and infection of the lungs, pleural cavity, pericardium, peritoneum and, rarely, the brain.
Amebiasis occurs worldwide; about 90% of infections are asymptomatic and the rest produce symptoms ranging from amoebic dysentery to abscesses of the liver and other organs. It's the third most common cause of death from paralytic disease, after schistosomiasis and malaria. It's most common in the tropics, subtropics, and other areas with poor sanitation and health practices.

SYNONYMS
Amebic dysentery (when severe intestinal infection)
EPIDEMIOLOGY & DEMOGRAPHICS
INCIDENCE (IN U.S.): Highest in institutionalized patients, sexually active homosexual men
PREVALENCE (IN U.S.): 4% (80% of infections asymptomatic)
PREDOMINANT SEX:
• Equal sex distribution in general
• Striking male predominance of liver abscess
PREDOMINANT AGE: Second through sixth decades
PEAK INCIDENCE: Peaks at age 2 to 3 yr and >40 yr
GENETICS: Infection more likely to be fulminant in young infants
PHYSICAL FINDINGS & CLINICAL PRESENTATION
• Often nonspecific
• Approximately 20% of cases symptomatic
1. Diarrhea, which may be bloody
2. Abdominal and back pain
• Abdominal tenderness in 83% of severe cases
• Fever in 38% of severe cases
• Hepatomegaly, RUQ tenderness, and fever in almost all patients with liver abscess (may be absent in fulminant cases)

ETIOLOGY
• Caused by the protozoal parasite E. histolytica
• Transmission by the fecal-oral route
• Infection usually localized to the large bowel, particularly the cecum where a localized mass lesion (ameboma) may form
• Extraintestinal infection in which the organism invades the bowel mucosa and gains access to the portal circulation
DIAGNOSIS
DIFFERENTIAL DIAGNOSIS
• Severe intestinal infection possibly confused with ulcerative colitis or other infectious enterocolitis syndromes, such as those caused by Shigella, Salmonella, Campylobacter, or invasive Escherichia coli
• In elderly patients: ischemic bowel possibly producing a similar picture
WORKUP
• Three stool specimens over a period of 7 to 10 days to exclude the diagnosis (sensitivity 50% to 80%)
• Concentration and staining the specimen with Lugol’s iodine or methylene blue to increase the diagnostic yield
• Available culture (rarely necessary in routine cases)
LABORATORY TESTS
• Stool examination is generally reliable.
• Mucosal biopsy is occasionally necessary.
• Serum antibody may be detected and is particularly sensitive and specific for extraintestinal infection or severe intestinal disease.
• Aspiration of abscess fluid is used to distinguish amebic from bacterial abscesses.
IMAGING STUDIES
Abdominal imaging studies (sonography or CT scan) to diagnose liver abscess
TREATMENT
The patient with amebiasis is treated with metronidazole, the preferred amebicide for intestinal and extraintestinal infection that causes symptoms of acute colitis or liver abscess. Other agents for asymptomatic carriers include diloxamine furoate, iodoquinol, or paromomysin. For liver abscess without intestinal infections, the patient receives metronidazole, tinidazole, imidazole, plus the other agents as listed above for the asymptomatic carrier. Exploratory surgery is hazardous because it can lead to peritonitis, perforation, and pericecal abscess. Percutaneous drainage of abscesses of the liver may be necessary, but surgery is usually reserved for patients with a perforation or rupture.
ACUTE GENERAL Rx
• Metronidazole (750 mg PO tid for 10 days) is used in the treatment of mild to severe intestinal infection and amebic liver abscess; it may be administered intravenously when necessary.
• Follow with iodoquinol (650 mg PO tid for 20 days) to eradicate persistent cysts.
• For asymptomatic patients with amebic cysts on stool examination, use iodoquinol or paromomycin (500 mg PO tid for 7 days).
• Avoid antiperistaltic agents in severe intestinal infections to avoid risk of toxic megacolon.
• Liver abscess is generally responsive to medical management but surgical intervention indicated for extension of liver abscess into pericardium or, occasionally, for toxic megacolon.
DISPOSITION
Host immunity incomplete and reinfection rate high for patients remaining at risk
REFERRAL
• For consultation with infectious diseases specialist for extraintestinal infection or persistent or relapsing intestinal infection
• For surgical consultation:
1. For toxic megacolon
2. For impending rupture of or extension of liver abscess into adjacent structures

CAUSES
Amebiasis is caused by Entamoeba histolytica. This protozoan has two stages: during the cystic stage, it can survive outside the body; during the trophozoite stage, it can't. Transmission occurs through ingesting stool-contaminated food or water or through oral-anal sexual practices. The ingested cysts pass through the intestine, where digestive secretions break them down, freeing the motile trophozoites within. The trophozoites then multiply and either invade and ulcerate the mucosa of the large intestine or simply feed on intestinal bacteria. As the trophozoites are carried slowly toward the rectum, they are encysted and then excreted in stool. Humans are the principal carriers.