Amebiasis (Entamoeba Histolytica) Infection

Amebiasis is a parasitic disease caused by infection with Entamoeba histolytica or another amoeba (E. dispar).

• Most individuals with the disease may have no symptoms.
• E. histolytica is the species that produces symptoms only in about 10% of those infected.
• The single or one-celled organism usually produces dysentery and occasionally invasive extra-intestinal problems (invasive amebiasis), the most common of which are liver abscesses, although other organs can be involved.
• The disease is most common in people who live in tropical areas with poor sanitary conditions.
• Amebiasis causes 50,000-100,000 deaths worldwide each year.

What causes amebiasis?

It is caused by infection with the protozoan parasite Entamoeba histolytica. It begins when a person drinks contaminated water or eats foods contaminated with the cystic form (infective stage), comes in contact with contaminated colonic irrigation devices or the fecally contaminated hands of food handlers, or by oral-anal sexual practices. The cystic form changes into trophozoites (invasive form) in the ilium or colon and invades the mucosal barrier, leading to tissue destruction and diarrhea. These trophozoites can reach the portal blood circulation to the liver and eventually go to other organs. It only infects humans, and the CDC does not classify it as a free-living organism.

Risk factors include:

• drinking contaminated water,
• eating contaminated foods,
• association with food handlers whose hands are contaminated,
• anal sexual practices,
• contaminated medical devices such as colonic irrigation devices,
• malnourishment,
• recipients of corticosteroids,
• pregnancy,
• very young patients, and
• travelers to endemic areas such as Southeast Asia or Central America.

Amebiasis is contagious from person to person. It is spread by the fecal-oral route by an infected person. The contagious period lasts as long as the infected patient excretes cysts in their stools. Consequently, the contagious period may last for weeks to many years if untreated.

The incubation period for amebiasis is variable. Symptoms begin to appear about one to four weeks after ingestion of the cysts; however, the range may be from a few days to years.

What are the symptoms of amebiasis?

Although only about 10%-20% of people infected with the parasites become ill, those individuals may have the following symptoms and signs:

• Early symptoms (in about 1-4 weeks) include loose stools and mild abdominal cramping.
• If the disease progresses, frequent, watery, and/or bloody stools with severe abdominal cramping (termed amoebic dysentery) may occur.
• If the trophozoites reach the intestinal walls and go through them, symptoms of liver infection such as liver tenderness and fever are the initial signs and symptoms of liver abscess formation (hepatic amebiasis).
• Other organs (heart, lungs, brain [meningoencephalitis], for example) may produce symptoms specific to the organ and produce severe illness and/or death.
• Abdominal tenderness and/or stomach pain
• Tenesmus
• Flatulence
• Appetite loss
• Weight loss
• Fatigue
• Anemia
• Occasionally cause skin lesions (cutaneous amebiasis)

Diagnosis of amebiasis

In addition to your primary care physician, the following specialists may be consulted:

• Gastroenterologist
• Infectious disease specialist
• General surgeon (especially if the patient develops severe disease like fulminant colitis)
• Occasionally, dermatologist

If your recent health history and travel history suggest a possibility of amebiasis, your doctor may ask you to provide several stool samples to screen for the presence of E. histolytica cysts in your stools.

In addition, some routine blood tests as well as other tests to determine if parasites spread to other organs may be initiated for laboratory diagnosis. These tests may include the following:

• Liver function tests
• Serological tests
• Enzyme-linked immunosorbent assay (ELISA)
• Ultrasound of the liver
• CT scan of the liver and perhaps other organs
• Colonoscopy of the large intestine to search for parasites
• Your doctor may run other serological tests to rule out other infectious diseases like giardiasis, paragonimiasis, and arboviral brain infections, for example.

What is the treatment for amebiasis?

Asymptomatic infections are not treated unless they are occurring in non-endemic areas. If patients are shedding E. histolytica cysts, the following luminal agents (drugs that work on cysts that are not invading the gastrointestinal epithelium) are recommended:

• Paromomycin (Humatin)
• Iodoquinol (Yodoxin)
• Diloxanide furoate

To treat invasive amebiasis, metronidazole (Flagyl, MetroGel, Noritate) is recommended even for amoebic liver abscesses (up to 10 cm in size). Tinidazole (Tindamax) is FDA approved for the treatment of both intestinal or extraintestinal (invasive) amebiasis. Other countries have similar drugs for treatments, but they are not available in the United States.

Amoebic colitis can be treated with nitroimidazoles, but they should be followed up by the use of a luminal agent.

Treatment of hepatic amebiasis has been successful in some patients with chloroquine (Aralen) or dehydroemetine (which is only available from the CDC and is not a preferred treatment because of its potential for heart toxicity).

If the gastrointestinal tract is suspected to be perforated (perforation can occur with fulminant amoebic colitis), broad-spectrum antibiotics may be used to prevent peritonitis.

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What are surgical treatment options for amebiasis?

Surgical treatments are required or indicated for amebiasis treatment due to the following:

• Gastrointestinal bleeding (massive or uncontrolled)
• Perforated amoebic colitis
• Toxic megacolon
• Failure to respond to metronidazole after four days of treatment
• Amoebic liver abscesses greater than 10 cm in size
• Empyema after the liver abscess rupture
• Amoebic liver abscess represents a risk of rupture to the pericardium
• Impending abscess rupture (no medical response in about 3-5 days to expanding abscess)
• Percutaneous drainage by catheter can be lifesaving in patients with amoebic pericarditis

What are complications of amebiasis?

Although infrequent, there can be serious complications of the disease, such as:

• liver abscesses (it is possible to develop these abscesses without the typical diarrhea stage),
• lung abscesses,
• brain abscesses,
• ameboma (a large local lesion of the bowel caused by the response to the infecting parasite),
• fulminant or necrotizing colitis,
• rectal rational fistula,
• bloody dysentery,
• toxic megacolon,
• increased risk for cancer, and
• death.

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What is the prognosis for amebiasis?

In general, the prognosis of amebiasis is good since the vast majority of infected individuals showed little or no symptoms. However, if complications develop such as abscesses, peritonitis, or toxic megacolon, the prognosis may vary from fair to poor depending on the availability of medical support services.

Recovery time for amebiasis is related to the severity of the disease.

• If a person has no symptoms, there is no recovery time.
• Recovery time after medical treatment varies from about 1-2 weeks to as many as four weeks or more after surgery.
• You and your doctor need to discuss your estimated recovery time once treatment begins.

Is it possible to prevent amebiasis?

Yes, amebiasis can be prevented by:

• stopping the fecal contamination of food and water by the Centers for Disease Control personnel by correcting poor sanitation.
• Identification and treatment of food handlers or other carriers of the parasite can reduce the chance of getting food-borne amebiasis.
• Avoiding sexual practices that involve fecal-oral contact also may reduce the chance of getting the disease.
• Avoiding malnutrition and alcohol use can reduce the risk of the disease.

Gal-lectin, an antigen from the parasite, has been used as a vaccine to protect animals against intestinal amebiasis and amoebic liver abscesses. Other parasitic components are being tried as possible vaccine components to use in humans. Unfortunately, amebiasis doesn’t result in any long-term immunity so individuals can be reinfected multiple times.

Gal-lectin, an antigen from the parasite, has been used as a vaccine to protect animals against intestinal amebiasis and amoebic liver abscesses. Other parasitic components are being tried as possible vaccine components to use in humans. Unfortunately, amebiasis doesn’t result in any long-term immunity so individuals can be reinfected multiple times.