Amebiasis is a parasitic infection of the intestines caused by the amoeba Entamoeba histolytica, or E. histolytica.

The symptoms of amebiasis include loose stool, abdominal cramping, and stomach pain. However, most people with amebiasis won’t experience significant symptoms.

Amebiasis is common in tropical countries with underdeveloped sanitation. It’s most common in the Indian subcontinent, parts of Central and South America, Mexico, and parts of Africa. It’s relatively rare in the United States.

People with the greatest risk for amebiasis include:

  • people who have traveled to tropical locations where there’s underdeveloped sanitation
  • immigrants from tropical countries with underdeveloped sanitary conditions
  • people who live in institutions with underdeveloped sanitary conditions, such as prisons
  • men who have sex with other men
  • people with suppressed immune systems and other health conditions

E. histolytica is a single-celled protozoan that usually enters the human body when a person ingests cysts through food or water. It can also enter the body through direct contact with fecal matter.

The cysts are a relatively inactive form of the parasite that can live for several months in the soil or environment where they were deposited in feces. The microscopic cysts are present in soil, fertilizer, or water that’s been contaminated with infected feces.

Food handlers may transmit the cysts while preparing or handling food. Transmission is also possible during anal sex, oral-anal sex, and colonic irrigation.

When cysts enter the body, they lodge in the digestive tract. They then release an invasive, active form of the parasite called a trophozoite. The parasites reproduce in the digestive tract and migrate to the large intestine. There, they can burrow into the intestinal wall or the colon.

According to the Centers for Disease Control and Prevention (CDC), only about 10 to 20 percent of people who have amebiasis become ill from it.

While most people have no symptoms, amebiasis can cause bloody diarrhea, colitis, and tissue destruction. The person can then spread the disease by releasing new cysts into the environment through infected feces.

When symptoms do occur, they tend to appear 1 to 4 weeks after ingestion of the cysts. Symptoms at this stage tend to be mild and include loose stools and stomach cramping.

In a rare complication of the disease, the trophozoites may breach the intestinal walls, enter the bloodstream, and travel to various internal organs. They most commonly end up in the liver, but may also infect the heart, lungs, brain, or other organs.

If trophozoites invade an internal organ, they can potentially cause:

  • abscesses
  • infections
  • severe illness
  • death

If the parasite invades the lining of the intestine, it can cause amebic dysentery. Amebic dysentery is a more dangerous form of amebiasis with frequent watery and bloody stools and severe stomach cramping.

Another very rare complication is fulminant necrotizing amoebic colitis, which can destroy bowel tissue and lead to bowel perforation and peritonitis.

The liver is a frequent destination for the parasite, where it can cause a collection of pus called an amebic liver abscess. Symptoms include fever and tenderness in the upper-right part of the abdomen.

Your doctor may suspect amebiasis after asking about your recent health and travel history.

It can be difficult to diagnose amebiasis because E. histolytica looks a lot like other parasites, such as E. dispar, which is occasionally seen with E. histolytica but is generally considered nonpathogenic, meaning it’s not associated with disease.

To detect E. histolytica and rule out other possible infections, your doctor may order tests such as stool samples and antigen testing.

The following tests may be performed to check for the presence of E. histolytica:

  • Most commonly, you may have to provide stool samples for several days that will be screened for the parasite. This is because the number of amoebas may vary from day to day and may be too low to detect from just one stool sample.
  • A stool test called enzyme-linked immunosorbent assay (ELISA) is often performed to detect E. histolytica antigens.
  • Using a blood sample or nasal swab, a molecular polymerase chain reaction (PCR) test may be performed to distinguish E. histolytica from other infections.
  • Your doctor may also order blood tests to help determine if the infection has spread beyond your intestines to another organ, such as your liver.
  • When the parasites spread outside the intestine, they may no longer show up in your stool. Your doctor may order an ultrasound or CT scan to check for lesions on your liver.
  • If lesions appear, your doctor may need to perform a needle aspiration to see if the liver has any abscesses. An abscess in the liver is a serious consequence of amebiasis.
  • Finally, a colonoscopy may be necessary to check for the presence of the parasite in your large intestine (colon).

If tests detect the presence of E. histolytica, amebiasis needs to be treated regardless of whether you’re experiencing symptoms or not.

If tests only detect E. dispar, another amoeba that may cause amebiasis, treatment is generally not warranted since it’s nonpathogenic.

The treatment generally consists of the following:

  • If you have symptoms, you’ll follow a 10-day course of the antiamoebic drug metronidazole (Flagyl) that you’ll take as a capsule, followed by an antibiotic such as diloxanide furoate or paromomycin.
  • Your doctor may also prescribe medication to control nausea if you need it.
  • If you do not have symptoms, you may be treated with antibiotics.
  • If the parasite is present in your intestinal tissues, the treatment must address the organism as well as any damage to your infected organs.
  • Surgery may be necessary if the colon or peritoneal tissues have perforations.

Proper sanitation is the key to avoiding amebiasis. As a general rule, thoroughly wash your hands with soap and water after using the bathroom and before handling food.

If you’re traveling to places where the infection is common, follow this regimen when preparing and eating food:

  • Thoroughly wash fruits and vegetables before eating.
  • Avoid eating fruits or vegetables unless you wash and peel them yourself.
  • Use bottled water and soft drinks from sealed containers.
  • If you must drink tap water, boil it for at least 1 minute, or use a store-bought “absolute 1 micron” filter and add disinfecting chlorine, chlorine dioxide, or iodine tablets to the filtered water.
  • Avoid ice cubes or fountain drinks.
  • Avoid peeled fresh fruit or vegetables.
  • Avoid milk, cheese, or other unpasteurized dairy products.
  • Avoid food sold by street vendors.

Amebiasis generally responds well to treatment and should clear up in about 2 weeks.

If you have a more serious case where the parasite appears in your internal tissues or organs, your outlook is still good as long as you get appropriate medical treatment.

However, if amebiasis is left untreated, it can be deadly.