Amebiasis

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Amebiasis

A parasitic infectious disorder caused by amoebas. The parasite may cause colitis which is manifested with bloody diarrhea, abdominal pain, nausea and fever. In rare cases it may spread to the liver, brain and lungs.

Causal Agent

Several protozoan species in the genus Entamoeba colonize humans, but not all of them are associated with disease. Entamoeba histolytica is well recognized as a pathogenic ameba, associated with intestinal and extraintestinal infections. The other species are important because they may be confused with E. histolytica in diagnostic investigations.

Life Cycle

Cysts and trophozoites are passed in feces. Cysts are typically found in formed stool, whereas trophozoites are typically found in diarrheal stool. Infection by Entamoeba histolytica occurs by ingestion of mature cysts in fecally contaminated food, water, or hands. Excystation occurs in the small intestine and trophozoites are released, which migrate to the large intestine. The trophozoites multiply by binary fission and produce cysts, and both stages are passed in the feces. Because of the protection conferred by their walls, the cysts can survive days to weeks in the external environment and are responsible for transmission. Trophozoites passed in the stool are rapidly destroyed once outside the body, and if ingested would not survive exposure to the gastric environment. In many cases, the trophozoites remain confined to the intestinal lumen (A: noninvasive infection) of individuals who are asymptomatic carriers, passing cysts in their stool. In some patients the trophozoites invade the intestinal mucosa (B: intestinal disease), or, through the bloodstream, extraintestinal sites such as the liver, brain, and lungs (C: extraintestinal disease), with resultant pathologic manifestations. It has been established that the invasive and noninvasive forms represent two separate species, respectively E. histolytica and E. dispar. These two species are morphologically indistinguishable unless E. histolytica is observed with ingested red blood cells (erythrophagocystosis). Transmission can also occur through exposure to fecal matter during sexual contact (in which case not only cysts, but also trophozoites could prove infective).

Who is at risk?

Although anyone can have this disease, it is more common in people who live in tropical areas with poor sanitary conditions. In the United States, amebiasis is most common in:

  • People who have traveled to tropical places that have poor sanitary conditions
  • Immigrants from tropical countries that have poor sanitary conditions
  • People who live in institutions that have poor sanitary conditions
  • Men who have sex with men

How does one get infected?

E. histolytica infection can occur when a person:

  • Puts anything into their mouth that has touched the feces (poop) of a person who is infected with E. histolytica.
  • Swallows something, such as water or food, that is contaminated with E. histolytica.
  • Swallows E. histolytica cysts (eggs) picked up from contaminated surfaces or fingers.

What are the symptoms?

Only about 10% to 20% of people who are infected with E. histolytica become sick from the infection. The symptoms are often quite mild and can include loose feces (poop), stomach pain, and stomach cramping. Amebic dysentery is a severe form of amebiasis associated with stomach pain, bloody stools (poop), and fever. Rarely, E. histolytica invades the liver and forms an abscess (a collection of pus). In a small number of instances, it has been shown to spread to other parts of the body, such as the lungs or brain, but this is very uncommon.

How is it diagnosed?

Your healthcare provider will ask you to submit fecal (poop) samples. Because E. histolytica is not always found in every stool sample, you may be asked to submit several stool samples from several different days.

Diagnosis of amebiasis can be very difficult. One problem is that other parasites and cells can look very similar to E. histolytica when seen under a microscope. Therefore, sometimes people are told that they are infected with E. histolytica even though they are not. Entamoeba histolytica and another ameba, Entamoeba dispar, which is about 10 times more common, look the same when seen under a microscope. Unlike infection with E. histolytica, which sometimes makes people sick, infection with E. dispar does not make people sick and therefore does not need to be treated.

If you have been told that you are infected with E. histolytica but you are feeling fine, you might be infected with E. dispar instead. Unfortunately, most laboratories do not yet have the tests that can tell whether a person is infected with E. histolytica or with E. dispar. Until these tests become more widely available, it usually is best to assume that the parasite is E. histolytica.

A blood test is also available but is only recommended when your healthcare provider thinks that your infection may have spread beyond the intestine (gut) to some other organ of your body, such as the liver. However, this blood test may not be helpful in diagnosing your current illness because the test can be positive if you had amebiasis in the past, even if you are not infected now.

How is it treated?

Several antibiotics are available to treat amebiasis. Treatment must be prescribed by a physician. You will be treated with only one antibiotic if your E. histolytica infection has not made you sick. You probably will be treated with two antibiotics (first one and then the other) if your infection has made you sick.

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