Entamoeba histolytica is an anaerobic parasitic protozoan, belonging among the entamoebae. It infects predominantly humans and other primates. Diverse mammals such as dogs and cats can become infected but usually do not shed cysts (the environmental survival form of the organism) with their feces, thus do not contribute significantly to transmission. The active (trophozoite) stage exists only in the host and in fresh feces; cysts survive outside the host in water and soils and on foods, especially under moist conditions on the latter. When swallowed they cause infections by excysting (to the trophozoite stage) in the digestive tract.
Nature of Disease
Amebiasis (or amoebiasis) is the name of the infection caused by E. histolytica.
Infections that sometimes last for years may be accompanied by
- no symptoms,
- vague gastrointestinal distress,
dysentery (with blood and mucus).
Most infections occur in the digestive tract but other tissues may be invaded. Complications include
- ulcerative and abscess pain and, rarely,
- intestinal blockage. Onset time is highly variable. It is theorized that the absence of symptoms or their intensity varies with such factors as
- strain of amoeba,
- immune health of the host, and
- associated bacteria and, perhaps, viruses.
The amoeba secretes toxic substances, including enzymes that destroy cell membranes and help it penetrate and digest human tissues, resulting in flask-shaped ulcers. E. histolytica also feeds on cells by phagocytosis and is often seen with red blood cells inside. Especially in Latin America, a granulomatous mass may form in the wall of the colon due to long-lasting cellular response, and is sometimes confused with cancer.
Theoretically, the ingestion of one viable cyst can cause an infection.
Diagnosis of Human Illness
Human cases are diagnosed by finding cysts shed with the stool; various flotation or sedimentation procedures have been developed to recover the cysts from fecal matter; stains (including fluorescent antibody) help to visualize the isolated cysts for microscopic examination. Since cysts are not shed constantly, a minimum of 3 stools should be examined. In heavy infections, the motile form (the trophozoite) can be seen in fresh feces. Serological tests exist for long-term infections.
It is important to distinguish the E. histolytica cyst from the cysts of nonpathogenic intestinal protozoa such as Entamoeba coli by its appearance. E. histolytica cysts have a maximum of four nuclei and may contain ingested cells, while the commensal E. coli has up to 8 nuclei and no ingested cellular material. Additionally, in E. histolytica, the endosome is centrally located in the nucleus, while it is off-center in E. coli. Finally, chromatoidal bodies in E. histolytica are rounded, while they are jagged in E. coli.
Amebiasis is transmitted by fecal contamination of drinking water and foods, but also by direct contact with dirty hands or objects as well as by sexual contact. Additionally, geophagy is a common route of infection in certain cultures.
Relative Frequency of Disease
Approximately 500 million people are infected with E. histolytica worldwide, and approximately one hundred thousand die each year. It is most common in warmer areas, both because of poorer hygiene and because the parasites live longer. It is also frequently diagnosed among homosexual men, where it has spread by anilingus in the United States.
Iodoquinol and metronidazole are both used to treat infections, though both have side effects.
In the majority of cases, amoebas remain in the gastrointestinal tract of the hosts. Severe ulceration of the gastrointestinal mucosal surfaces occurs in less than 16% of cases. In fewer cases, the parasite invades the soft tissues, most commonly the liver. Only rarely are masses formed (amoebomas) that lead to intestinal obstruction.
At Risk Populations
All people are believed to be susceptible to infection, but individuals with a damaged or undeveloped immunity may suffer more severe forms of the disease. AIDS / ARC patients are very vulnerable.
E. histolytica cysts may be recovered from contaminated food by methods similar to those used for recovering Giardia lamblia cysts from feces. Filtration is probably the most practical method for recovery from drinking water and liquid foods. E. histolytica cysts must be distinguished from cysts of other parasitic (but nonpathogenic) protozoa and from cysts of free-living protozoa as discussed above. Recovery procedures are not very accurate; cysts are easily lost or damaged beyond recognition, which leads to many falsely negative results in recovery tests. (See the FDA Bacteriological Analytical Manual.)
The most dramatic incident in the USA was the Chicago World's Fair outbreak in 1933 caused by contaminated drinking water; defective plumbing permitted sewage to contaminate the drinking water. There were 1,000 cases (with 58 deaths). In recent times, food handlers are suspected of causing many scattered infections, but there has been no single large outbreak.
Last updated: 08-30-2005 12:57:52