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Epidemiology

Humans usually acquire amebiasis by ingesting cysts of the protozoan Entamoeba histolytica in contaminated food or drinking water (Fig. 1.1). In most Western countries the infection is transmitted mainly by carriers who pass cysts and who work in restaurants or other public places. These human sources of infective parasites are either healthy hosts or convalescent patients with amebic disease; during the initial stage of the disease, they may discharge trophozoites in small numbers or up to millions a day. The quiescent stage of the disease may last from a few months to two years.

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Fig. 1.1 Modes of transmission of amebiasis.

Throughout the tropics, water from polluted wells or broken plumbing, and food contaminated by human "night soil" used for fertilization are significant sources of infection. Flies, cockroaches, and other insects may be vectors of the disease. Man is the principal host and reservoir of E. histolytica, even though amebic cysts can be found in the stools of various animals, including dogs, cats, pigs, rats, and some primates; however, these are not a frequent source of human infection. The parasite has also been experimentally inoculated in some mammals.

Amebiasis affects both sexes equally. Infections tend to be more common in adults than in children, but this certainly does not apply everywhere in the world. For instance, a study in Lagos (Nigeria) showed that 62% of all patients were under the age of 10 years and 75% of these were under 5 years. Infected infants under the age of 4 and 6 months have been reported in other series.

The amebic trophozoite is sensitive to minor temperature changes, cannot tolerate the low PH environment of the stomach, and can only survive out of its host for a brief period of time. Conversely, the infectious form of the parasite is the tetranucleated mature cyst, which is resistant to gastric secretions and other gastrointestinal enzymes and is known for its ability to survive adverse environmental conditions within or outside the host. Depending on temperature and degree of dehydration, the cysts retain their infective capacity in soil and water for 8 days when the temperature ranges between 28 and 34C and for up to a month at 10C. The parasite is resistant to the conventional chlorine concentrations used to purify drinking water. The cysts are destroyed when exposed to 200 p.p.m. of iodine, to acetic acid, or to temperatures greater that 68C. They can also be removed by sand filtration.

Transmission of the disease occurs by ingestion of tetranucleated mature cysts which can be found in food and water contaminated by improper food processing, improper use of contaminated fertilizers, or faulty irrigation practices, and in unprocessed drinking water. Self-contamination can also occur due to poor habits of hygiene. However, it should be emphasized that only a small percentage of infested individuals become symptomatic from tissue penetration of the parasite. Clinical manifestations from intestinal or extraintestinal pathologic lesions are designated as invasive amebiasis.

The natural history of amebiasis shows wide regional differences, with the percentage of infected patients and those with active disease varying from one area to another. In Mexico, it is estimated that one in every five infected patients will develop invasive amebiasis. However, in one sampling, 95% of 3,000 infected Mexicans had ulcerative proctocolitis, a much higher percentage than in most endemic areas; there is a similar variation in the frequency of ameboma. These geographic variations are not fully understood. One possibility is that there is a synergistic action of the amebae with other elements in the flora of the bowel. Variations in host response and immunity, virulence and stage of the parasite, and possibly spontaneous mutations of avirulent into virulent strains of E. histolytica are other factors that might influence the progress of infection. The clinical picture may also depend on the degree of exposure, and whether the infection is heavy, prolonged or repetitive. Diet also influences the course of infection: malnutrition predisposes to more severe disease.

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Copyright: Palmer and Reeder
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