The epidemiology of amoebiasis

Amoebiasis

Amoebiasis is the second leading cause of death from parasitic disease worldwide. The causative protozoan parasite, Entamoeba histolytica, is a potent pathogen. Secreting proteinases that dissolve host tissues, killing host cells on contact, and engulfing red blood cells, E histolytica trophozoites invade the intestinal mucosa, causing amoebic colitis. In some cases amoebas breach the mucosal barrier and travel through the portal circulation to the liver, where they cause abscesses consisting of a few E histolytica trophozoites surrounding dead and dying hepatocytes and liquefied cellular debris. Amoebic liver abscesses grow inexorably and, at one time, were almost always fatal, but now even large abscesses can be cured by one dose of antibiotic. Evidence that what we thought was a single species based on morphology is, in fact, two genetically distinct species--now termed Entamoeba histolytica (the pathogen) and Entamoeba dispar (a commensal)--has turned conventional wisdom about the epidemiology and diagnosis of amoebiasis upside down. New models of disease have linked E histolytica induction of intestinal inflammation and hepatocyte programmed cell death to the pathogenesis of amoebic colitis and amoebic liver abscess.

Amebic hepatic abscess in children

The authors retrospectively reviewed all case histories of children with amebic hepatic abscess treated from 1975 to 1993 at their hospital. Twenty boys and 12 girls were diagnosed. Their ages ranged from 10 months to 12 years, with a mode of 1 and 2 years. In 17 (53%) of the patients, the abscess remained confined to the liver and was treated medically with dehydroemetine and metronidazole. Imminence of complication was present in 9 patients (52%), and required percutaneous needle aspiration. Imminence of complication was evidenced by: (1) clinical worsening of the patient despite adequate medical treatment, (2) presence of an abscess of 6 cm or more in a septic patient, or (3) clinical or ultrasonographic findings of an abscess on the verge of rupture. All 9 patients did satisfactorily. Fifteen cases (47%) were complicated by rupture and required surgical treatment. One of these patients died of sepsis. Medical treatment alone was excellent for small abscesses. Percutaneous needle aspiration was a successful approach in patients with imminence of complication. Surgery was reserved for ruptured abscesses.

Prevalence of Entamoeba histolytica and other intestinal parasites in a community from Maracaibo, Venezuela

The prevalences of Entamoeba histolytica and other intestinal parasites were assessed in a suburban community of Maracaibo, Venezuela, by examination of a stool specimen from each of 342 individuals, using iron-haematoxylin stained faecal smears and formalin-ether concentration. The overall parasitic infection rate was 80.4%, and 65.8% of the population had multiple infections. The overall amoebic infection rate, which was highest in female adults, averaged 39.7%. The E. histolytica infection rate was 8.7% and most of those infected were passing cysts. Entamoeba polecki was observed in two samples. Amongst the protozoa, Entamoeba coli was observed most frequently (24.8%) and Giardia lamblia was the predominant pathogen (13.0%). Trichuris trichiura (71.9%) and Ascaris lumbricoides (54%) were the most common parasites, particularly in school-children. The high rates of parasitic and multiple infections reflect the low socio-economic status of the community studied.

[Seroepidemiologic study of amebiasis in a community of the State of Zulia, Venezuela]

In the present evaluation, a community of low socioeconomical conditions from Zulia State, Venezuela, was analyzed for the prevalence of antibodies to E. histolytica. Two hundred and eighty three serum samples were collected and examined by the indirect hemagglutination test according to a microtiter modification of the KESSEL and LEWIS method, as used by MILGRAM et al. Antigen prepared from axenically-grown. E. histolytica strain HK9 in Diamond's medium was used. The seropositivity rate obtained was 46.6% and the frequency of positive cases was dependent on age. The antibody profiles obtained suggest a high endemicity for this parasitic infection in the area studied, with a much higher level of transmission than invasive amebiasis.

Non-pathogenic Entamoeba histolytica: functional and biochemical characterization of a monoxenic strain

We have cultured under monoxenic conditions and characterized an Entamoeba histolytica clone, MAV-I CINVESTAV (MAV-I), obtained from feces from an asymptomatic carrier. The clone shows the non-pathogenic E. histolytica zymodeme type I, which did not change through the process of monoxenization. Clone MAV-I was non-pathogenic in both in vivo and in vitro tests, and it did not have a functional 112-kDa adhesin. As far as we know, this is the first non-pathogenic monoxenic strain reported. Clone A (strain HM1:IMSS), a highly virulent clone with pathogenic zymodeme type II, and which has the 112-kDa adhesin, was used as a control. Protein patterns from both clones were almost identical in one-dimensional gels. In two-dimensional gels, differences in high-molecular-weight proteins were detected. Clone MAV-I adhered and phagocytosed only 12% of the red blood cells adhered and phagocytosed by clone A. MAV-I trophozoites did not destroy cell culture monolayers and did not produce hepatic abscesses in hamsters. They also showed deficiency in protease activity. The absence of virulence in clone MAV-I correlated directly with the absence of a functional 112-kDa adhesion, supporting the role that this protein plays in virulence.

HLA antigens associated to amoebic abscess of the liver in Mexican mestizos

Worldwide prevalence of amoebiasis is estimated at 4 x 10(8) cases/year, yet only one of about 300 individuals harbouring Entamoeba histolytica suffers tissue invasion and these cases are mostly concentrated in certain areas of Asia, Africa and Latin America. Patients with amoebic abscess of the liver (AAL) represent only a small fraction of that. These contrasting figures have been tentatively explained on the one hand through variations in sex, immunocompetence, nutritional and other socioeconomic features of the host, and on the other hand through differences in parasite virulence. In order to explore a possible association between the major histocompatibility complex (MHC) and AAL susceptibility, we studied the HLA profile in 31 Mexican mestizos with AAL and compared it to race and socioeconomically matched controls. Mexican mestizo patients with AAL revealed a significant increase in HLA-Bw16 and HLA-DR3 which could suggest an HLA-related susceptibility to liver invasion by E. histolytica.

Immunocytochemical detection of Entamoeba histolytica

Human anti-Entamoeba histolytica immunoglobulin was used to detect Entamoeba histolytica in 74 positive samples from several different sources, using an indirect immunoperoxidase method. In 73 samples, the protozoan was easily identified. Trophozoites and cysts of all cultured Entamoeba strains examined were strongly stained, and as few as 3 trophozoites per microscope slide could be detected. In addition, 51 negative control samples were also tested and non-specific reactions were not observed. These preliminary results show that this method is both sensitive and specific, and can easily detect trophozoites and cysts of different E. histolytica strains.

Ameba-bacterium relationship in amebiasis

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Amebic infections in asymptomatic homosexual men, lack of evidence of invasive disease

A survey for enteric infections in 140 asymptomatic homosexual men who attended a community clinic revealed a high prevalence of infection with Entamoeba histolytica (27.1 per cent) and Giardia lamblia (15.7 per cent). In contrast, the prevalence of elevated indirect hemagglutination (IHA) titers (greater than or equal to 1:128), which indicate invasive amebiasis, was low (5.7 per cent). Our findings suggest that only a limited amount of invasive amebic disease is occurring in this group of homosexual men.

Application of the leucocyte adherence inhibition test to the assessment of cell mediated immunity in invasive amoebiasis

The leucocyte adherence inhibition (LAI) test was assessed as a means for measuring specific cell mediated immunity (CMI) in invasive amoebiasis. The LAI test and the amoebic gel diffusion (AGD) test were performed on 20 patients admitted to hospital with amoebic liver abscess (ALA) and 24 control subjects with no demonstrable clinical amoebiasis. The AGD results were acceptable for such a study in an endemic area as all the proven ALA subjects gave positive responses while all except one of the controls gave a negative reaction. A percentage adherence in the LAI test of more than 80% was considered to be negative and the mean percentage adherence of leucocytes for the control subjects was found to be 86.0 +/- 4.9 while that for the experimental group was found to be 76.1 +/- 15.4; the difference between these groups is statistically significant. It was noticed that the LAI test response was possibly linked with severity of disease in the ALA subjects. The LAI test has been found to be a relatively rapid test of specific CMI and although fastidious, it does not require the use of radio-isotopes.

Electrophoretic isoenzyme patterns of the pathogenic and non-pathogenic intestinal amoebae of man

Cultured stocks of Entamoeba hartmanni, Endolimax nana, Iodamoeba buetschlli and Dientamoeba fragilis were compared with the four Entamoeba histolytical groups already described (SARGEAUNT et al., 1978), by the electrophoretic patterns of three enzymes: glucose phosphate isomerase (GPI), phosphoglucomutase (PGM) and L-malate: NADP+ oxidoreductase (oxalacetate-decarboxylating) (ME). All the species were easily distinguished by their characteristic patterns.

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.

Pathogenicity of entamoeba histolytica

The pathogenicity of Entamoeba histolytica is discussed from an immunologic point of view. The evidence that there is some "trigger" mechanism which converts a commensal dwelling organism into a tissue invasive pathogen is rejected as inadequate. The number of liver abscess cases in comparison with the number of intestinal amebic infections in a population is so low that this in itself suggests that tissue invasion is a rare event in the life history of the ameba. A review is made of the experimental evidence that some type of sensitization is necessary before ameba can invade tissue. In postulating an immunologic basis for the pathogenicity of ameba, a parallel between the behavior of malignant cells in the body and an amebic infection in the gut is made. An appealing hypothesis which deserves further research effort is that an altered immune response is the basis for the pathogenic mechanism in the host.

Surveys for amoebiasis; interpretation of data and their implications

In survey work, stool microscopy for cysts and serology are the only really practicable measures of amoebic infection. The diagnostic sensitivity of stool microscopy should be estimated. Using a simple deterministic mathematical model, the rates of gain and loss of infection and seropositivity may be estimated from survey data. The low value of the constants creates difficulties but they can be estimated from: (1) cumulative data from frequently sampled cohorts; (2) analysis of the curves of age prevalence; (3) analysis of curves of long-term follow-up studies. The second method is the simplest and normally gives valid results. Appropriate epidemiological studies can give useful information about protective immunity, incubation period, superinfection, and interactions between amoebic infection and various host factors, including other disease states. Knowledge of the rate constants of infection allows predictions to be made about the likely effect of control measures.