Surveys for amoebiasis; interpretation of data and their implications

Cyst production and transmission of Entamoeba and Endolimax

We studied the cyst production of 340 asymptomatic carriers with single or mixed infections of Entamoeba histolytica/Entamoeba dispar, Endolimax nana and Entamoeba coli. It was found that the main source of transmission was a small group of 37 (10.9%) carriers who produced 70.2% of all cysts. Gender was an important factor for cyst production of Ent. histolytica/Ent. dispar, since males produced 6-fold more cysts than females. Females produced 'large' (13-15 microns) Ent. histolytica/Ent. dispar cysts and males small (10-12 microns) cysts when concentration was > 2000 cysts/g. When concentration was < 2000 cysts/g, cysts were the same size for both sexes (11.6 +/- 0.3 microns). In Ent. coli infections, an inverse relationship between the number and size of cyst was found for both sexes. Cysts of End. nana were not affected by gender or cyst concentration. Following cyst production by a carrier infected with Ent. coli and Ent. histolytica for 28 days, we identified synchronized cycles of cyst production for both species. Altogether, these data suggest that cyst-mediated transmission is a highly regulated process in which synchronization among different species and gender of the host have an important role in transmission.

Gender distribution in asymptomatic and invasive amebiasis

OBJECTIVE

The majority of individuals infected by the protozoan parasite Entamoeba histolytica experience subclinical infections. However, a small proportion of parasitized individuals develop severe invasive disease such as amebic dysentery or amebic liver abscess. Invasive amebiasis affects predominantly men; the usual explanation for this has been that men have a higher rate of asymptomatic infections and therefore experience a higher rate of invasive disease. To date, there is no convincing evidence of an increased rate of asymptomatic infection of men as compared with women. The purpose of this study was to evaluate the evidence supporting the hypothesis that men have higher rates of asymptomatic infection and thus an increased frequency of invasive amebiasis.

METHODS

We reviewed published reports of invasive amebiasis and population-based parasitological studies from 1929-1997 to compare the gender ratio of asymptomatic and symptomatic E. histolytica infection. Infections with E. histolytica were differentiated from the nonpathogenic E. dispar whenever possible.

RESULTS

The reports of invasive amebiasis (dysentery, liver abscess, colonic perforation, peritonitis, appendicitis, and ameboma) showed a higher proportion of men than women (ratio, male:female = 3.2:1, p < 0.05). This contrasts with the epidemiological surveys, where the rate of asymptomatic infection with E. histolytica was the same (1:1) for both genders (p > 0.05).

CONCLUSIONS

Asymptomatic E. histolytica infection is equally distributed between the genders. The high proportion of men with invasive amebiasis may be due to a male-related susceptibility to invasive disease.

Lymphocyte subpopulations transformation studies in an experimental model of intestinal and hepatic amoebiasis

The lymphocyte subpopulations in peripheral blood and blast transformation were studied on days 0, 3, 7, 11, 15, 20 and 35 after infection in progesterone-treated guinea-pigs which had been experimentally infected with Entamoeba histolytica by intracaecal inoculation. In animals with intestinal infection, lowered T cell numbers and blast transformation were seen during the acute phase with a subsequent tendency to recover. In guinea-pigs with hepatic amoebiasis, both these parameters were significantly depressed throughout the period of study with no tendency to recovery. In progesterone-treated and sham-operated animals, a less marked depression of T cells, was observed. The B cell population did not vary greatly in either group. This study has thus shown that in hepatic amoebiasis there is highly significant depression in both number and function of T cells in comparison to intestinal amoebiasis.

Entamoeba histolytica cyst passers: clinical features and outcome in untreated subjects

To examine whether Entamoeba histolytica is the cause of a variety of intestinal disorders, as often implicated, a two-part study was conducted. In part 1, 184 patients attending a gastroenterology clinic and needing a sigmoidoscopy had a rectal biopsy specimen and a stool sample cultured for E histolytica. Part of the biopsy specimen was examined histologically. In addition, serum was tested for antibody to E histolytica. 34 (18.7%) patients were culture positive. There was no correlation between the culture results and patients' symptoms except for diarrhoea, which was more common in the culture-negative group, or with histological or serological findings. In the second part of the study, 15 culture-positive patients were followed up untreated for a mean (+/- SD) of 8.6 (+/- 4.9) months, range 1-18. In all 15 patients spontaneous eradication of the parasite occurred, and in none did symptoms of invasive amoebiasis develop. Thus E histolytica seems not to be responsible for many of the bowel complaints commonly attributed to it. In these cyst passers the cysts are probably incidental to an irritable bowel syndrome or some other organic disease. There seems to be little justification for treating cyst passers since eradication of the parasite occurs spontaneously and the danger of invasive amoebiasis is negligible. Such a practice would result in an enormous saving on the cost of drugs.

The epidemiology of infection with Entamoeba histolytica in the Gambia, West Africa

A country-wide stool survey of The Gambia showed the prevalence of Entamoeba histolytica cysts to range from 13.7% up-country in the dry season to 52.3% near the coast. A longitudinal survey showed a near 100% infection rate over one year and a sharp rise in prevalence as the rains commence with an equally sharp fall as the rains progress. Specific antibody levels are elevated and reasonably constant through the year. Carriers generally show no specific lymphocyte reactivity to amoebic antigen but consistently parasite-negative individuals tended to show elevated lymphocyte reactivity. Attempts to discover the presence of cysts in the environment of villages by cultivation of specimens of water, soil, food, flies and washing from clothes and hands generally failed though E. histolytica was recovered once from a well.