Harvard Health Publications

Gastrointestinal Amebiasis

What Is It?

Gastrointestinal amebiasis is an infection of the large intestine caused by microscopic one-celled parasites commonly known as amoebas (Entamoeba histolytica). Because these parasites live in the large intestine, they travel in the feces of infected people, and can contaminate water supplies in places where sanitation is poor. The parasite can contaminate fruits and vegetables grown in areas where human feces are used as fertilizer. They can be transferred on the dirty hands of infected people who don't wash their hands often or correctly.

Once amoebas enter the mouth, they travel through the digestive system and settle in the large intestine. Harmless strains of the parasite (Entamoeba dispar) live there without causing damage. E. histolytica can live in the intestine without causing symptoms, but it also can cause severe disease. These amoebas may invade the wall of the intestine, leasing to amoebic dysentery, an illness that causes intestinal ulcers, bleeding, increased mucus production and diarrhea. These amoebas also may pass into the bloodstream and travel to the liver or, infrequently, to the brain, where they form pockets of infection (abscesses).

About 10% of the world's population is infected with amoebas, particularly people who live in Mexico, India, Central America, South America, Africa and the tropical areas of Asia. In industrialized countries, amebiasis is most common in recent immigrants and travelers who visit countries where amoebas are prevalent.

Symptoms

In more than 90% of cases, the infecting strain of amoeba does not cause any symptoms. When symptoms do occur, they usually begin within months after amoebas first enter the body. In some people, the symptoms are mild, consisting of mild pain and gurgling sounds in the lower abdomen, along with two or three loose stools daily. In other people, however, there can be full-blown symptoms of amoebic dysentery, including high fever, severe abdominal pain and 10 or more episodes of diarrhea daily. Typically, this diarrhea is watery or contains blood and mucus.

When amoebas spread to the liver and cause liver abscess, symptoms can include fever, nausea, vomiting, and pain in the upper right portion of the abdomen, weight loss and an enlarged liver. It is possible for a person to develop symptoms of an amoebic liver abscess without ever having had the diarrhea typical of infection.

Diagnosis

Your doctor will ask about your exposure to amoebas, especially about any recent trips to areas where amoebas are common. Your doctor will look for specific symptoms of amoebic infection, especially frequent diarrhea or loose stools and the presence of blood and mucus in your bowel movements. Because other conditions can cause bloody diarrhea, and because there are other types of infectious diarrhea, information about bowel disease in your family and details of your travels are particularly important.

Usually, a series of three stool samples will be checked in the laboratory for the presence of E. histolytica. In many cases, this stool test can confirm the diagnosis. Several different blood tests that can make a diagnosis with a high degree of accuracy are also available. In special cases, when the diagnosis is not clear after stool and blood tests, proctosigmoidoscopy or colonoscopy may be needed to allow your doctor to examine the intestinal wall directly and to take tissue samples for laboratory examination. In these tests, a doctor inserts a thin, lighted instrument into the rectum and colon to view them directly.

When symptoms of fever and abdominal pain, especially on the right upper side, suggest a possible liver abscess, you may need an ultrasound or CT scan of the liver. Because people who have a liver abscess but no longer have parasites in the intestines, stool tests may be less helpful. In these situations, doctors confirm the diagnosis with a blood test or a needle aspiration or biopsy. In a needle aspiration or biopsy, a small piece of tissue from the abscess is removed and examined in a laboratory.

Expected Duration

Harmless amoebas can live in the intestines for years without causing symptoms. When invasive amoebas cause symptoms of amoebic dysentery, attacks can last from a few days to several weeks. Unless you are treated, you can have another attack.

Prevention

There is no immunization to protect against gastrointestinal amebiasis. If you travel to areas where amebiasis is common, you can decrease your risk of infection by drinking only canned or bottled drinks or water that has been boiled. Eat only foods that have been cooked thoroughly. Drink only pasteurized milk and dairy products. If you eat raw fruit, eat only those that you have freshly peeled.

Treatment

Gastrointestinal amebiasis is treated with nitroimidazole drugs, which kill amoebas in the blood, in the wall of the intestine and in liver abscesses. These drugs include metronidazole (Flagyl) and tinidazole (Tindamax, Fasigyn). Metronidazole usually is given for 10 days, either by mouth or directly into the veins (intravenously). To kill amoebas and cysts confined to the intestine, three drugs called luminal drugs are available: iodoquinol (Diquinol and others), paromomycin (Humatin) and diloxanide furoate (Furamide). One of these drugs is used along with metronidazole when gastrointestinal symptoms are present. In people who pass amoebas in their stools without having symptoms of amebiasis, the luminal drugs alone can clear the amoebas from the bowel.

When To Call a Professional

Call your doctor whenever you have continuing diarrhea, especially if it contains blood and mucus. If you have severe abdominal pain, especially in the right upper side, and a fever, visit a doctor the same day. When you have non-bloody diarrhea that is frequent and symptoms of dehydration such as dizziness, call your doctor.

Prognosis

Drug treatment can cure amebiasis within a few weeks. However, because medication cannot keep you from getting infected again, repeat episodes of amebiasis may occur if you continue to live in or travel to areas where amoebas are found. Among children in developing countries, especially infants and those younger than 5, gastrointestinal amebiasis can be fatal. Worldwide, amebiasis is the third most common cause of death from parasitic infections.

External resources

Centers for Disease Control and Prevention 1600 Clifton Road Atlanta, GA 30333 Toll-Free: 1-888-232-3228 TTY: 1-888-232-6348 http://www.cdc.gov


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