Amebiasis

Entamoeba histolytica under the microscope, with ingested red blood cells. Source: CDC via Wikimedia Commons.

Other Names

• Amebic dysentery
• Amoebiasis (British spelling)
• Amoebic dysentery

Causes

The life cycle of Entamoeba histolytica. Source: CDC via Wikimedia Commons.

Amebiasis is caused by the single-celled parasite Entamoeba histolytica. The parasite lives primarily in humans, though some animals can be infected. The organism has two forms, a cyst (egg) and a trophozoite. The onset of the infection occurs when a person swallows a cyst. The cyst travels to the small intestine, where it produces trophozoites. These trophozoites are capable of movement, and they move about the intestine, eating food particles and blood cells. On occasion, the trophozoites penetrate the walls of the intestine and migrate to other parts of the body where they settle, multiply, and produce abscesses. Eventually the intestinal trophozoites produce more cysts that leave the body in the stool. These cysts can then be spread to other people.

People can carry cysts for a long time with no symptoms, but still be able to spread the disease to others. These people are called carriers

The cysts can last for months outside the body, although temperatures of greater than 40°Celsius or less than -5°C destroys them.^[1]

Signs and Symptoms

Most infections do not cause symptoms. About one in 10 people who are infected with E. histolytica becomes sick from the infection. The symptoms often are mild and can include loose stools, stomach pain, and stomach cramping. Amebiasis, however, does have some severe forms. These include amebic dysentery and amebic abscesses.

Amebic dysentery

Amebic dysentery or colitis is a severe form of amebiasis that causes abdominal pain, bloody stools, weight loss, generalized weakness, and fever. Patients may need to evacuate the bowels 10-12 times per day, and stools may consist solely of mucus and blood. Pain can be severe enough to be confused with appendicitis. Fever is common, occurring in 6 out of every 10 sufferers. Amebic colitis may be misdiagnosed as inflammatory bowel disease and treated with steroids, which can worsen the condition and lead to toxic megacolon (a severely enlarged bowel that can rupture). Infection usually begins 1 to 4 weeks after swallowing an ameba, but sometimes it starts earlier or later.

Abscesses

Rarely, E. histolytica invades the liver and forms an abscess. An abscess is a pocket of infection that expands and destroys or compresses surrounding tissues and organs. Symptoms of an amebic liver abscess include fever and abdominal pain on the right, just under the ribs where the liver is located. Most patients do not have, and never had, any intestinal symptoms, such as diarrhea, and diagnosis begins with a suggestive history. For example, if the patient recently traveled to a foreign country and has these symptoms, amebic abscess should be considered. Almost all liver abscesses develop within 6 months of traveling to an endemic area (place where amebiasis is common). Imaging studies, including ultrasound, computed tomography (CT) or magnetic resonance imaging (MRI), can show the abscess. However, imaging cannot distinguish an amebic abscess from other types of abscess.

Less commonly, the parasites spread to other parts of the body, such as the lungs or brain. These abscesses can rupture and cause severe abdominal pain and peritonitis or spread into the lung cavities. If the infection spreads to the sac around the heart, causing pericarditis, it can be fatal.

Diagnosis

A diagnosis can be made based on the following tests:^[1]

1. Microscopy. This is the traditional means of diagnosing the disease—one simply looks at a sample of stool under a microscope. Because E. histolytica is not always found in every stool sample, several samples from different days may be needed. Sometimes red blood cells that have been ingested by the parasite are visible. Unfortunately, the parasites do not always show up, and when they do, they often look like other parasites. Entamoeba histolytica and another ameba, Entamoeba dispar, which is about 10 times more common, look similar under the microscope, but E. dispar is completely harmless.^[2] The confusion can cause a misdiagnosis.
2. Culture. Trying to get the ameba to grow outside the body is very difficult and unreliable, and is therefore not generally done.
3. Antibody (serological) testing. When the body is exposed to an infection, the immune system creates antibodies to fight it off. These can be detected with a blood test, and provide evidence that the person has been infected with E. histolytica. Unfortunately, this test does not distinguish between past and present infection. If the test is performed on someone from a part of the world where E. histolytica is common, they are quite likely to have antibodies from an old infection that is not present anymore. The test will make it seem that they are infected when they aren't. The test is more useful when performed on someone who isn't likely to have been infected in the past—for example, in someone who has never previously traveled to an endemic area.
4. Antigen tests. These are considered the most useful tests for detecting E. histolytica. They test directly for the parasite itself by exposing some stool to a strip of paper coated with antibodies. The parasites will stick to the antibodies on the paper. The test distinguishes E. histolytica from other parasites. Some antigen tests are experimental and not all labs have access to them.
5. CT scan (computerized tomography). CT, as well as ultrasound and magnetic resonance imaging (MRI) can also aid in the diagnosis of E. histolytica.^[3] Most patients with intestinal amebiasis for more than ten days have at least one liver abscess that is visible on a CT scan.

Treatment

Several antibiotics are available to treat amebiasis. A single antibiotic is used if the E. histolytica infection has not made the patient sick. Two antibiotics are used in sequence when symptoms do appear. Frequently, either metronidazole (Flagyl) or tinidazole (Fasigyn) are used to treat amebiasis. If this does not work, chloroquine, emetine, and dehydroemetine can be used. Eliminating cysts in carriers who do not have symptoms is accomplished with diloxanide furoate (Furamide), iodoquinol (Yodoxin), and [[paromomycin]] (Humatin). Nitazoxanide is a newer drug that shows promise against not only E. histolytica but many other parasites as well.^[4]

Amebic abscess is treated similarly to dysentery, with antibiotics. Sometimes surgical drainage may be performed, but this is usually to rule out other (bacterial) causes of abscess. It is also performed if an abscess is about to, or has already ruptured. Most abscesses, though, don't need to be drained, as they can usually be cured with antibiotics.^[5]

Prevention

Several precautions reduce the risk of infection in people traveling to or living in developing countries. People should:

• Drink only bottled or boiled (for 1 minute) water, or carbonated (bubbly) drinks in cans or bottles. Fountain drinks and any drinks with ice cubes are not safe. Water can be made safe by filtering it through an "absolute 1 micron or less" filter and dissolving iodine tablets in the filtered water.
• Avoid fresh fruit or vegetables that were peeled by someone else.
• Avoid milk, cheese, or dairy products that may not have been pasteurized.
• Avoid anything sold by street vendors.

Chances of Developing Amebiasis

Risk Factors

Although anyone can have this disease, some people are more susceptible than others:

• People in developing countries that have poor sanitary conditions
• Immigrants from developing countries
• Travelers to developing countries
• People who live in institutions that have poor sanitary conditions
• HIV-positive patients
• Men who have sex with men

How Amebiasis is Spread

The transmission of amebiasis is feco-oral. This means that a person can become infected when the cyst is transferred from feces (stool) to mouth. This transfer occurs in several ways:

• Putting anything into the mouth that has touched the stool of a person who is infected
• Swallowing something, such as water or food, that is contaminated
• Touching and bringing to the mouth cysts picked up from contaminated surfaces

The infection can be spread from person to person. However, the risk of spreading infection is reduced if an infected person practices careful personal hygiene and his infection is treated with antibiotics. Good hygiene includes thorough hand washing with soap and water after using the toilet, after changing diapers, and before handling food.

Research

Vaccines are being developed and tested for the treatment of amebiasis. The vaccine is a modified version of the proteins expressed on the surface of E. histolytica. A study in rodents found that the vaccine prevented the formation of liver abscesses, but much more research is needed to determine if these vaccines are useful and safe in humans.^[6]

References

1. ↑ ^{1.0 1.1} Tanyuksel M, Petri WA Jr. Laboratory diagnosis of amebiasis. Clin Microbiol Rev. 2003 Oct;16(4):713-29. Abstract | Full Text
2. ↑ Diamond LS, Clark CG. A redescription of Entamoeba histolytica Schaudinn, 1903 (Emended Walker, 1911) separating it from Entamoeba dispar Brumpt, 1925. J Eukaryot Microbiol. 1993 May-Jun;40(3):340-4. Abstract
3. ↑ Ralls PW, Henley DS, Colletti PM et al. Amebic liver abscess: MR imaging. Radiology. 1987 Dec;165(3):801-4. Abstract | Full Text
4. ↑ Ali V, Nozaki T. Current therapeutics, their problems, and sulfur-containing-amino-acid metabolism as a novel target against infections by "amitochondriate" protozoan parasites. Clin Microbiol Rev. 2007 Jan;20(1):164-87. Abstract | Full Text
5. ↑ Ralls PW, Barnes PF, Johnson MB et al. Medical treatment of hepatic amebic abscess: rare need for percutaneous drainage. Radiology. 1987 Dec;165(3):805-7. Abstract | Full Text
6. ↑ Lotter H, Rüssmann H, Heesemann J, et al. Attenuated recombinant Yersinia as live oral vaccine carrier to protect against amoebiasis. Int J Med Microbiol. 2008;298:79-86. Abstract

External Links

US FDA: Entamoeba histolytica