Amebiasis is an intestinal infection caused by the single-celled, parasitic ameba, Entamoeba histolytica. Amebiasis is also called amebic dysentery.
Humans and possibly non-human primates are the only natural hosts for E. histolytica, so amebiasis has probably been common throughout human history. The causative organism was first described in 1875, in St. Petersburg, Russia, by Fedor Aleksandrovich Losch. He named the microbe Amoeba coli and demonstrated its pathogenicity by feeding a dog feces from an infected patient. In Egypt in 1886, S. Kartulis proved that the pathogen caused intestinal and liver lesions in patients suffering from diarrhea. The first case of amebiasis in North America was described in 1890 by Sir William Osler, who found the amoeba in stool from a physician who had lived in Panama. In 1891, researchers at Johns Hopkins University Hospital first distinguished between amebic dysentery caused by this ameba and dysentery caused by bacteria. A. coli was renamed E. histolytica by Fritz Schaudin in 1903. In 1913, Walker and Sellards carried out extensive studies on the parasite in the Philippines and demonstrated that its cyst, or encapsulated form, was the infective agent.
The complete life cycle of E. histolytica was documented in 1925. Ingestion of even a very small number of E. histolytica cysts can result in illness. In the ileum—the lowermost portion of the small intestine—or the colon (large intestine), the cysts develop into very motile trophozoites. The trophozoites are more easily destroyed than the cysts but, unlike the cysts, they are potentially pathogenic. The trophozoites can colonize the mucous layer of the colon and produce lesions that cause amebic colitis. From the colon, the trophozoites may invade the intestinal lining. From there, they sometimes enter the bloodstream and are carried to the liver, where they cause amebic liver abscess—an accumulation of pus. Rarely, trophozoites travel through the bloodstream to the lung or other organs. Alternatively, trophozoites in the colon may encyst—transform into new cysts—that are excreted in the feces, from which they can invade new hosts.
Although infection with E. histolytica is very common, it does not cause amebiasis in most people. Furthermore, there are many closely related species of Entamoeba, some of which can also live in the human gastrointestinal tract. Although these other species are not known to cause disease, they can be difficult to distinguish from E. histolytica.
Amebiasis is common in developing countries, especially tropical regions with conditions of overcrowding and poor sanitation. Amebiasis can spread rapidly through refugee camps and in the wake of natural disorders or warfare. In the United States, however, amebiasis is rare. It is most common among immigrants, people living in institutions, those who engage in anal intercourse, and people with HIV/AIDS.
Severe amebiasis is most common in the very young, the elderly, and pregnant women. Risk factors for severe amebiasis include:
Amebiasis occurs worldwide. It is estimated that about 10% of the world's population is infected with some species of Entamoeba, and the prevalence of infection is as high as 50% in parts of Asia, Africa, and Central and South America. However, only in recent years has it become possible to distinguish between pathogenic and nonpathogenic Entamoeba species. Amebiasis is a significant public health problem in Africa, India, Mexico, and parts of South America. A study at an Egyptian clinic found that 38% of patients suffering from acute diarrhea had amebic colitis. Studies have found that more than 8% of the population of Mexico test positive for E. histolytica, and it has been estimated that E. histolytica infection without symptoms may be as high as 11% in Brazil.
Worldwide, it is estimated that there are 40–50 million cases of amebic colitis and amebic liver abscess annually, causing 40,000–100,000 deaths. Amebiasis is second only to malaria as a cause of death from infection by a protozoan, and third to malaria and schistosomiasis as a cause of death from any parasite.
In the United States, the overall prevalence of amebiasis is about 4%, with the majority of cases occurring in Hispanic, Asian, and Pacific Islander immigrants and travelers to regions where the disease is endemic. Between 1990 and 2007, there were 134 deaths in the United States that were attributed to amebiasis, with over 40% occurring in California and Texas.
Invasive amebiasis is primarily a disease of young adults and is rare in children under age five. Although amebic colitis affects both sexes equally, amebic liver abscess is 7–10 times more common in men than in women, primarily affecting men between the ages of 18 and 50. It is also more common among postmenopausal women.
People with acute amebiasis do not usually transmit the disease to others, since they are not shedding cysts during this phase of the illness. However, people with untreated amebiasis can remain infective for years, as they periodically pass the cysts in their feces. E. histolytica cysts are spread through water or food that has been contaminated with human feces. The cysts can survive in the environment for weeks or months, so transmission is common in places where human waste is used for fertilizer or where produce is washed with sewage-contaminated water. E. histolytica can also be spread through person-to-person contract, including contact with the mouth or rectal area of an infected person. Major sources of transmission include inadequate hand washing by an infected person, which can result in the transfer of the cysts to food or objects, and sexual contact, especially anal sexual practices. Transmission also can occur by direct rectal inoculation via contaminated colonic irrigation devices. Flies can potentially transmit cysts in feces.
Only 10–20% of those infected with E. histolytica develop amebiasis. Symptoms most often develop 7–10 days after exposure; however, symptoms can take months or even years to appear. Symptoms are often very mild, and may include:
Symptoms of severe amebiasis, usually called amebic dysentery, include:
Once E. histolytica cysts become trophozoites in the colon or terminal ileum, they can invade the wall of the colon and destroy tissue, leading to colitis, acute dysentery, and bloody or chronic diarrhea. The parasites can also spread through the bloodstream to the liver and, rarely, to the lungs, brain, or other organs.
If the E. histolytica infection has spread, an abdominal examination may indicate tenderness or enlargement of the liver. Ultrasound or computed tomography (CT or CAT) scans can reveal amebic liver abscess. In combination with specific antibody tests for E. histolytica, amebic liver abscess is considered diagnostic for the disease.
Treatment for amebiasis is usually ten days of oral metronidazole or a similar drug. Patients who are vomiting may have to be given the drug intravenously. Anti-diarrheal medications can worsen the condition. Following treatment, stool should be reexamined to ensure that the infection has been cured.
E. histolytica is endemic in many parts of the developing world, including Latin America, Africa, India, and Southeast Asia. It is typically a problem in crowded, unsanitary living conditions, which increase the opportunities for person-to-person transmission. However, when food or drinking water supplies become contaminated with infected feces, the disease can spread to many people simultaneously and can become a serious public health problem. Furthermore, because most infected people do not develop symptoms, amebiasis outbreaks can easily go unrecognized. Estimates of the prevalence of E. histolytica infection in people without symptoms living in developing countries range from 1–20%. In regions where E. histolytica is endemic, as many as 25% of people have antibodies to the parasite due to prior infection. Nevertheless, amebiasis is an uncommon cause of traveler's diarrhea, usually occurring only in people who have stayed in endemic areas for more than one month.
In the United States and Canada, physicians and laboratories are required to report cases of amebiasis to public health officials. Once laboratories have definitively diagnosed an outbreak of E. histolytica by distinguishing it from E. dispar, the source of the infection and its mode of transmission must be determined. If a source of contaminated food or water can be identified, appropriate measures must be taken to remove it. Prophylactic antimicrobial medications are not normally used in outbreaks.
With treatment, amebiasis usually lasts about two weeks and has a good prognosis. Without treatment, the illness can recur. Mortality rates from amebic colitis range from 1.9–9.1%. In about 0.5% of cases of amebic colitis, the parasite begins to destroy the colon or can even rupture it, with a resulting mortality rate of 40%. If the parasite spreads through the blood to the liver, it can cause amebic liver abscess, with a mortality rate of 1–3%. Mortality rates appear to be highest among men and those aged 75 and older.
There is no vaccine against E. histolytica, so prevention is largely dependent on good sanitation measures. The most important preventative measures are sanitary disposal of human feces and protecting water supplies from fecal contamination with public health measures, especially water purification and chlorination and sewage treatment. However, chlorination of water, as generally used for municipal water supplies, does not always kill E. histolytica cysts. Sand filtration of the water removes almost all cysts, and diatomaceous earth filters remove any remaining cysts.
When visiting tropical countries with poor sanitation, travelers should drink only purified or boiled water and should not eat uncooked vegetables or unpeeled fruit. Safer sex methods, such as the use of condoms and dental dams for oral or anal contact, can help prevent the spread of amebiasis.
Proper, thorough hand washing is essential for preventing the spread of amebiasis. Hands should be washed before and after handling food, using the toilet, or changing diapers. Special care must be taken when handling the feces or contaminated clothing or bed linens of amebiasis patients, so as to prevent the spread of the pathogen. Infected people should not prepare food for others or directly care for hospitalized or institutionalized people until they are declared free of infection.
See also Dysentery ; Parasites ; Sanitation .
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U.S. Centers for Disease Control and Prevention, 1600 Clifton Road, Atlanta, GA, USA, 30333, (800) CDCINFO (232-4636), email@example.com, http://www.cdc.gov .
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Margaret Alic, PhD