Entamoeba histolytica is an anaerobic parasitic amoebozoan, part of the genus Entamoeba. Predominantly infecting humans and other primates causing amoebiasis, E. histolytica is estimated to infect about 35-50 million people worldwide. E. histolytica infection is estimated to kill more than 55,000 people each year. Previously, it was thought that 10% of the world population was infected, but these figures predate the recognition that at least 90% of these infections were due to a second species, E. dispar. Mammals such as dogs and cats can become infected transiently but are not thought to contribute significantly to transmission.
- Amoebas can be transmitted from person to person or through food and water.
- Infected people may be asymptomatic or may have diarrhea, constipation, spasmodic colic, epigastric tenderness, and fever.
- Doctors confirm the diagnosis by examining a feces sample and, if necessary, do other tests, such as colonoscopy, ultrasound, and blood tests.
- Treatment involves taking an amoeba-killing drug, followed by a drug that kills amoebic cysts in the large intestine.
Amoebiasis often occurs in areas with poor sanitation. Parasites are found all over the world, but most infections occur in Africa, the Indian subcontinent, and parts of Central and South America. Most cases in the United States have occurred in immigrants, and a few have occurred in people who have traveled to countries with poor sanitation.
The Entamoeba species exists in two forms:
- Active parasites (trophozoites)
- Dormant parasites (cysts)
Other types of amoebas do not infect people through the intestines, but directly infect the brain (primary amebic meningoencephalitis) or eyes (acanthamoeba keratitis).
Transmission of amoebiasis
When a person is infection after ingestion of a cyst, the cyst hatches and releases trophozoites, which invade the intestinal wall, causing ulcers in the inner lining of the intestinal wall, resulting in diarrhea. Occasionally, they can spread to the liver or other parts of the body. Some trophozoites form cysts and are excreted with trophozoites in the feces. In vitro, fragile trophozoites die. However, hard cysts can survive.
Cysts can be transmitted directly from person to person or indirectly through food or water. Amoebiasis can also be transmitted through oral-anal sex.
In unhygienic places, amoebiasis is spread by ingesting food or water contaminated with feces. Fruits and vegetables can be contaminated by fertilizing land with human manure, washing with contaminated water, or being handled by infected people. If an infected person is incontinent or has poor hygiene (for example, in a daycare center or psychiatric hospital), amoebiasis can occur and spread even in places with adequate sanitation.
Symptoms of amoebiasis
Most infected people have few or no symptoms. However, they excrete cysts from their feces, which can spread the infection.
Amoebiasis symptoms usually occur within one to three weeks and may include:
- Diarrhea and sometimes blood in the feces
- Spasmodic colic
- Weight loss and fever
In severe cases, the abdomen is tender to the touch, and people may have severe diarrhea with mucus and blood in the feces (called dysentery). Some patients have severe spasmodic colic and high fever. Diarrhea can lead to dehydration. People with chronic infection may lose weight (emaciation) and become anemic.
Sometimes a large lump (amoebiasis) may form in the large intestine (colon).
In some patients, amoeba spreads to the liver and causes a liver abscess. Symptoms include fever, sweating, chills, weakness, nausea, vomiting, weight loss, and discomfort in the right upper quadrant liver area.
Rarely, amoeba spreads to other organs, including the lungs or brain. The skin can also become infected, especially around the buttocks (an infection transmitted by contaminated feces), the genitals (such as penile ulcers caused by anal sex with an infected person), or wounds caused by abdominal surgery or injury.
Diagnosis of amoebiasis
- Feces examination
- Sometimes a test of the blood for amoebic antibodies is needed
- Sometimes a sample of large intestine tissue is taken for examination
To diagnose amoebiasis, doctors collect a feces sample for examination, and the best test is an amoebic antigen test. The best way to diagnose it is to test the protein released by the amoeba in the feces (antigen test) or to use polymerase chain reaction (PCR) technology to detect the amoebic genetic material in the feces. PCR amplifies the genetic material of amoeba in large quantities, making it easier to detect. Antigen or PCR are more useful than microscopic examination of feces specimens, which are often inconclusive. In addition, microscopic examination requires 3 to 6 feces samples to find amoebas, and even if they are seen, they cannot distinguish Entamoeba histolytica from some other related amoebas. For example, the E. dispar, which looks the same but is genetically different, does not cause disease.
A flexible viewing tube (endoscope) can be used to view the inside of the large intestine. If ulcers or other signs of infection are found, an endoscope may be used to obtain a sample of fluid or tissue from the abnormal area.
When the disease spreads outside the intestine (such as the liver), it is more difficult for doctors to diagnose amoebiasis because there is no amoeba in the feces. Ultrasound, computed tomography (CT), or magnetic resonance imaging (MRI) can confirm liver abscess, but these tests do not determine the cause. Amoebic antibodies in the blood are helpful in diagnosis. Or, if doctors suspect that a liver abscess is caused by an amoeba, they may start taking an amoeba-killing drug. If the patient improves, the diagnosis is confirmed.
Prevention of amoebiasis
Preventing contamination of food and water with human faeces is key to preventing amoebiasis. Improving health systems in areas where infection is endemic can help.
When travelling to endemic areas, avoid eating uncooked foods, including salads and vegetables, and avoid drinking water and ice that may be contaminated. Boiling water kills the cysts. It is important to wash your hands with soap and water. Filtering water through a 0.1 or 0.4 micron filter removes Entamoeba histolytica and other parasites and disease-causing bacteria. Dissolving iodine or chlorine in water may help. However, the effectiveness of iodine or chloride against Entamoeba histolytica depends on a number of factors, such as how turbid or muddy the water is (turbidity) and temperature.
Treatment of amoebiasis
- Antiamoebic drugs and/or Cysticidal drugs
If amoebiasis is suspected and symptoms are present, an antiamoebic drug – metronidazole or tinidazole – is needed. Metronidazole must be taken for 7 to 10 days. Tinidazole must be taken for 3 to 5 days. Tinidazole has fewer side effects than metronidazole. Patients should not drink alcohol while taking either drug or for several days after stopping the drug, as drinking can cause nausea, vomiting, flushing, and headache. Nitazoxanide has been proposed as an alternative treatment for amoebiasis. Metronidazole, tinidazole, or nitazoxanide should be used in pregnant women only if the benefits outweigh the risks.
Neither metronidazole, tinidazole, nor nitazoxanide can completely kill the cysts in the large intestine. A second drug (e.g., paromomycin, Diiodohydroxyquinoline, or furamide) is needed to kill cysts and prevent recurrence. One of these drugs can be used alone to treat people who don’t get sick but have Entamoeba histolytica in their feces.
People who are dehydrated need fluids.