Amebiasis
Amebiasis is an infection of the large intestine or other organs caused by the single-celled parasite Entamoeba histolytica.
Amebiasis is relatively common in areas where sanitation is poor and fecal contamination of food and water occurs. It can also be acquired through certain sexual practices. Entamoeba histolytica initially infects the intestine, although it sometimes reaches other organs, such as the liver. Amebic infections are common in Latin America, Africa, and the Indian subcontinent.
Entamoeba histolytica exists in two forms: as an active parasite (trophozoite) and as a dormant parasite (cyst). Infection begins when cysts are swallowed. The cysts hatch, releasing trophozoites that multiply, cause ulcers on the lining of the intestine, and produce diarrhea. Some of the trophozoites form cysts, which are excreted in the feces along with trophozoites. Outside the body, the fragile trophozoites die, but the cysts are hardy. Cysts can be spread directly from person to person or indirectly through food or water.
In places with poor sanitation, transmission of amebiasis occurs through ingestion of fecally contaminated food or water. Fruits and vegetables may be contaminated when grown in soil fertilized by human stool, washed in polluted water, or prepared by someone who is infected. Amebiasis also may occur and spread in places with proper sanitation if incontinence and poor hygiene are present (for example, day care centers, or mental institutions). Amebiasis can be spread by sexual contact.
Symptoms
Only a few people infected with Entamoeba species develop symptoms. In the United States, most cases of symptomatic amebiasis occur among immigrants and, less commonly, people who have traveled to developing countries.
Infected people who develop symptoms typically have intermittent diarrhea, increased gas (flatulence), and cramping abdominal pain. In more severe cases, the abdomen is tender when touched, and the stool contains mucus and blood. The person may also have a fever. Wasting of the body (emaciation) and anemia can occur in people with chronic infection. Sometimes a large lump (ameboma) forms and blocks the intestine. Occasionally, the trophozoites perforate the intestinal wall and enter the abdominal cavity, causing severe abdominal pain and an abdominal infection (peritonitis) that requires immediate medical attention.
In some people, Entamoeba histolytica forms an abscess in the liver. Symptoms include fever, sweats, chills, weakness, weight loss, and pain or discomfort in the area over the liver.
Occasionally, Entamoeba histolytica spreads to other organs (including the lungs or brain). The skin may also become infected, especially around the buttocks, genitals, or wounds caused by abdominal surgery or injury.
Diagnosis
To diagnose amebiasis, a doctor collects stool samples for analysis. Three samples may be needed to make the diagnosis. A flexible tube (colonoscope) may be used to look inside the large intestine and to obtain a tissue sample if any ulcers are found there.
It can be difficult for a doctor to diagnose amebiasis when the disease spreads to sites outside the intestine (such as the liver), because the parasites may no longer be present in the stool. Ultrasound or computed tomography (CT) scans can be performed to confirm an abscess in the liver, but these tests do not indicate the cause. Blood tests for antibodies to the amebas may be helpful. Sometimes, doctors who suspect that a person has an amebic liver abscess simply try antiamebic drugs to see if they work. A positive response confirms the diagnosis.
Treatment
A person with symptomatic amebiasis involving the intestinal tract, or amebiasis that occurs in the liver or elsewhere outside the intestine, is given the antiamebic drug metronidazole. Nausea and vomiting develop if people taking metronidazole drink alcohol.
Metronidazole does not always kill cysts that are in the stool. Several drugs (such as iodoquinol, paromomycin, and diloxanide) can be used to kill the cysts in the stool.
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