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Center for Food Safety & Applied Nutrition Foodborne Pathogenic Microorganisms and Natural Toxins Handbook |
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1. Name of the Organism: Entamoeba histolytica |
This is a single celled parasitic animal, i.e., a protozoa, that 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. |
| 2. Name of Acute Disease: | Amebiasis (or amoebiasis) is the name of the infection caused by E. histolytica. |
| 3. Nature of the Acute Disease: |
Infections that sometimes last for years may be accompanied
by 1) no symptoms, 2) vague gastrointestinal distress, 3)
dysentery (with blood and mucus). Most infections occur in
the digestive tract but other tissues may be invaded.
Complications include 4) ulcerative and abscess pain and,
rarely, 5) intestinal blockage. Onset time is highly
variable. It is theorized that the absence of symptoms or
their intensity varies with such factors as 1) strain of
amoeba, 2) immune health of the host, and 3) associated
bacteria and, perhaps, viruses. The amoeba's enzymes help
it to penetrate and digest human tissues; it secretes toxic
substances.
Infectious Dose--Theoretically, the ingestion of one viable cyst can cause an infection. |
| 4. 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 by its appearance. |
| 5. Transmission: | 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. |
| 6. Frequency of Infections: | The infection is "not uncommon" in the tropics and arctics, but also in crowded situations of poor hygiene in temperate-zone urban environments. It is also frequently diagnosed among homosexual men. |
| 7. Usual Course of the Disease and Some Complications: | 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. Fatalities are infrequent. |
| 8. Target 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. |
| 9. Analysis of Foods: | 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. 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.) |
| 10. Selected Outbreaks: | 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. |
| For more information on recent outbreaks see the Morbidity and Mortality Weekly Reports from CDC. |
mow@cfsan.fda.gov
January 1992 with periodic updates
