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

Glycogen Metabolism and Its Role in Growth and Encystation in Entamoeba histolytica

Entamoeba histolytica is a parasitic protozoan that causes diarrheal disease in approximately 100 million people worldwide every year. E. histolytica has two forms, the growing trophozoite and the infectious cyst. Trophozoites colonizing the large intestine form cysts that are released into the environment. The ingestion of the cysts in contaminated food and water continues the disease cycle. Here, we investigated the role of glycogen in trophozoite growth and encystation. Glycogen is thought to provide precursors for the synthesis of chitin, a major component of the protective cyst wall. We propose that glycogen also serves as an energy source during metabolic adaptation to different nutrient environments. We examined the role of glycogen in E. histolytica by analyzing the growth and encystation of RNAi strains with reduced expression of the single gene-encoding glycogen synthase (GYS) or two of three genes encoding glycogen phosphorylase (PYG). The GYS RNAi strain had a greatly reduced glycogen accumulation, and both the GYS and PYG RNAi strains exhibited reduced growth in the glucose-poor medium. Both RNAi strains also showed reduced cyst production. Our results suggest glycogen synthesis and degradation are vital to the growth and adaptation of E. histolytica to a low-glucose environment such as that encountered in the large intestine.

Diversity and Plasticity of Virulent Characteristics of Entamoeba histolytica

The complexity of clinical syndromes of amebiasis, caused by the parasite Entamoeba histolytica, stems from the intricate interplay between the host immune system, the virulence of the invading parasite, and the surrounding environment. Although there is still a relative paucity of information about the precise relationship between virulence factors and the pathogenesis of Entamoeba histolytica, by accumulating data from clinical and basic research, researchers have identified essential pathogenic factors that play a critical role in the pathogenesis of amebiasis, providing important insights into disease development through animal models. Moreover, the parasite's genetic variability has been associated with differences in virulence and disease outcomes, making it important to fully understand the epidemiology and pathogenesis of amebiasis. Deciphering the true mechanism of disease progression in humans caused by this parasite is made more difficult through its ability to demonstrate both genomic and pathological plasticity. The objective of this article is to underscore the heterogeneous nature of disease states and the malleable virulence characteristics in experimental models, while also identifying persistent scientific issues that need to be addressed.

Epidemiology of irritable bowel syndrome in Asia: something old, something new, something borrowed

In this review we have unearthed epidemiological data that; support the 'old' concept of irritable bowel syndrome (IBS) as a disorder of civilization, build a 'new' symptom profile of IBS for Asia, and persuade us against the use of 'borrowed' Western diagnostic criteria and illness models by Asian societies. In the 1960s, IBS was described as a disorder of civilization. Early studies from Asia suggested a prevalence of IBS below 5%. Recent studies from Asia suggest a trend for the more affluent city states like Singapore and Tokyo, to have higher prevalence of 8.6% and 9.8%, respectively, while India had the lowest prevalence of 4.2%. Furthermore, there was a trend among the better educated and more affluent strata of society in several urban Chinese populations for a higher prevalence of IBS, as well as a trend for a higher consultation rate. Across Chinese and Indian predominant populations, a majority of patients with IBS criteria report upper abdominal symptoms such as epigastric pain relieved by defecation, bloating and dyspepsia. Bloating and incomplete evacuation appear to be more important determinants of consultation behavior, than psychological factors. The failure of the Rome criteria to recognize the relationship to meals, may have led to a substantial misclassification of IBS as dyspepsia. The relevance of the Western model of psychological disturbance as a determinant of consultation behavior is questionable because of the accessibility and acceptability of medical consultation for gastrointestinal complaints in many Asian communities.

Irritable bowel syndrome and inflammatory bowel disease: infectious gastroenteritis-related disorders?

Infectious gastroenteritis may be one of the important factors in the development of irritable bowel syndrome (IBS), with affected individuals often categorized as having post-infectious IBS (PI-IBS), and is linked to the onset of symptoms in approximately 10-20% of patients diagnosed with IBS. Intestinal mucosal infiltration of T cells and mast cells, and enterochromaffin cell hyperplasia are significant immunological and pathological findings that reveal the pathogenesis of PI-IBS, and results of laboratory studies using animal models of PI-IBS clearly support clinical evidence. Recently, infectious gastroenteritis has also been suggested to be associated with the development of inflammatory bowel disease (IBD), and various studies have suggested that individuals with IBS or IBS-like symptoms may be susceptible to initiation of IBD. However, it is still unclear whether infectious gastroenteritis is directly or indirectly (through PI-IBS) linked to the initiation of IBD. Additional studies are necessary to understand the clinical overlap among infectious gastroenteritis, IBS, and IBD.

Prospective evaluation of lactose malabsorption by lactose hydrogen breath test in individuals infected with and passing cystsEntamoeba histolytica

The aim of the present prospective study was to detect lactose malabsorption in subjects in northern India infected with Entamoeba histolytica and passing cysts. The study group included forty-one patients with E. histolytica cysts in at least one of three consecutive faecal samples. Lactose malabsorption was detected by a lactose H2 breath test. The results were compared with those of forty controls subjects. Thirty-two of forty-one (78·0%) subjects passing E. histolytica cysts had lactose malabsorbtion compared with seventeen of forty (42·5%) control subjects (P>0·01). In conclusion, the present study shows that lactose malabsorption is significantly more common in individuals infected with E. histolytica and passing cysts compared with control subjects.

Almost all irritable bowel syndromes are post-infectious and respond to probiotics: controversial issues

Acute infectious gastroenteritis is the strongest known risk factor for the development of irritable bowel syndrome (IBS), one of the most common functional gastrointestinal disorders. However, knowledge about the incidence and prevalence of post-infectious IBS (PI-IBS) in the general population is still limited. Some of the published epidemiological studies on PI-IBS lack an appropriate control population, and were limited by a short follow-up symptom assessment post-infection. A number of risk factors have been associated with the development of PI-IBS, including the virulence of the pathogen, younger age, female sex, the long duration of the initial illness and the presence of psychological disturbances. However, much work has to be done to establish whether multifactorial mechanisms actually concur to the pathophysiology of PI-IBS. The discovery that an infective episode may trigger the development of IBS has not substantially changed the clinical management of this subset of patients compared to the classical (non-infective) form of IBS. Probiotics have been claimed to be of some benefit in IBS, but the majority of studies have been performed in non-specific IBS rather than in PI-IBS and a number of issues still remain to be elucidated.

Irritable bowel syndrome: a review on the role of intestinal protozoa and the importance of their detection and diagnosis

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder in which abdominal pain is associated with a defect or a change in bowel habits. Gut inflammation is one of the proposed mechanisms of pathogenesis. Recent studies have described a possible role for protozoan parasites, such as Blastocystis hominis and Dientamoeba fragilis, in the etiology of IBS. Dientamoeba fragilis is known to cause IBS-like symptoms and has a propensity to cause chronic infections but its diagnosis relies on microscopy of stained smears, which many laboratories do not perform, thereby leading to the misdiagnosis of dientamoebiasis as IBS. The role of B. hominis as an etiological agent of IBS is inconclusive, due to contradictory reports and the controversial nature of B. hominis as a human pathogen. Although Entamoeba histolytica infections occur predominately in developing regions of the world, clinical diagnosis of amebiasis is often difficult because symptoms of patients with IBS may closely mimic those patients with non-dysenteric amoebic colitis. Clinical manifestations of Giardia intestinalis infection also vary from asymptomatic carriage to acute and chronic diarrhoea with abdominal pain. These IBS-like symptoms can be continuous, intermittent, sporadic or recurrent, sometimes lasting years without correct diagnosis. It is essential that all patients with IBS undergo routine parasitological investigations in order to rule out the presence of protozoan parasites as the causative agents of the clinical signs.

Mucosal immunity to asymptomatic Entamoeba histolytica and Entamoeba dispar infection is associated with a peak intestinal anti-lectin immunoglobulin A antibody response

We monitored 93 subjects cured of amebic liver abscess (ALA) and 963 close associate controls in Durban, South Africa, and determined by enzyme-linked immunosorbent assay that the intestinal immunoglobulin A (IgA) antibody response to the Entamoeba histolytica galactose-inhibitable adherence lectin is most accurately represented by a complex pattern of transitory peaks. One or more intestinal anti-lectin IgA antibody peaks occurred in 85.9% of ALA subjects over 36 months compared to 41.6% of controls (P < 0.0001). ALA subjects exhibited a greater number of anti-lectin IgA antibody peaks (P < 0.0001) than controls. In addition, their peak optical density values were higher (peak numbers 1 to 3, P < 0.003), peaks were of longer duration (for peaks 1 and 2, P </= 0.0054), and there was a shorter time interval between peaks (between 1 and 2 or 2 and 3, P </= 0.0106) than observed for control subjects. A prior E. histolytica infection was associated with the occurrence of an anti-lectin IgA antibody peak (79.1%, P < 0.0001) more so than for Entamoeba dispar infection (57.2%, P < 0.001). The annual number of anti-lectin IgA antibody peaks in ALA subjects was 0.71 per year, compared to just 0.22 in controls (P<0.0001), indicating a higher rate of exposure to the parasite than previously appreciated. Anti-lectin IgA antibody peaks were of higher amplitude following a E. histolytica infection compared to E. dispar (P = 0.01) and, for either, were of greater height in ALA subjects than controls (P < 0.01). ALA subjects demonstrated greater clearance of amebic infection after an anti-lectin IgA antibody peak compared to controls, and only 14.3% remained with a positive culture after the peak, compared to 38.9% in controls (P = 0.035). In summary, this prospective controlled longitudinal study elucidated the dynamic nature of the human intestinal IgA antibody response to E. histolytica and E. dispar infection and revealed that ALA subjects exhibit heightened intestinal anti-lectin IgA antibody peaks that are associated with clearance of E. histolytica and E. dispar infection.

Enhanced pro-inflammatory chemokine/cytokine response triggered by pathogenic Entamoeba histolytica : basis of invasive disease

The virulence of Entamoeba histolytica is governed by adhesion/colonization in the gut which is mediated by a galactose specific lectin. Two morphologically identical but distinct species i.e. pathogenic E. histolytica and non-pathogenic E. dispar, can be differentiated by distinct epitopes in the lectin. Both species bind to colonic epithelial cells, but only E. histolytica infection induces an inflammatory response and subsequent pathogenesis. Thus, comparing the responses of the intestinal cells to pathogenic and non-pathogenic lectins is a point of interest. The pathogenic lectin causes cytolysis of epithelial and immune-competent cells. Our data (both qualitative and mRNA quantitation) indicate that the epithelial cells responded to E. histolytica lectin with an increased expression of pro-inflammatory IL-2, IL-6, IL-8, MIP-1alpha, MCP-1, RANTES, GROalpha and GMCSF as compared to E. dispar lectin. The pathogenic LCM induced a significant increase in intracellular calcium concentration, proliferative response and chemotaxis of lymphocytes from ALA patients as compared to non-pathogenic LCM. High RANTES and IL-6 were induced in patients' lymphocytes by pathogenic LCM, along with their receptors CCR5 and CD126 as compared to NP-LCM. The local release of such a complex network of cytokines/chemokines could explain the histopathology of E. histolytica infection. The comparative low levels of these chemokines/pro-inflammatory cytokines and high levels of anti-inflammatory IL-10 in response to non-pathogenic E. dispar could explain the absence of an acute inflammatory response and the disease process. The cytokines and chemokines may provide a mechanism for initiation, amplification or containment of inflammation during disease state.

Irritable bowel syndrome in developing countries--a disorder of civilization or colonization?

While irritable bowel syndrome (IBS) is common in the West, early studies suggest that the prevalence is low in developing countries. However, recent studies point to increasing prevalence in newly developed Asian economies. The presentation appears to differ from the West, with a lack of female predominance, a greater frequency of upper abdominal pain and defecatory symptoms perceived as being less bothersome. This difference, together with the preoccupation with organic disease, could explain why we may be missing IBS in Asia and also why excess surgery has been observed in some Asian countries. While a recent study from China, consistent with western studies, support an important role for infection and inflammation, early studies from India reporting no association between amoebic infection and IBS appear to dispute this observation. To reconcile these seemingly contradictory observations, an hygiene hypothesis model is proposed. Exposure to a variety of microorganisms early in life could result in the colonization of the intestine with microflora that can respond more efficiently to an episode of gastroenteritis. Together with the changes with evolution of Asian economies such as westernization of the diet and increased psychosocial stress, it is proposed that loss of this internal protective effect, could give rise to a more uniform worldwide prevalence of IBS.

Efficacy of saccharomyces boulardii with antibiotics in acute amoebiasis

AIM

To compare the efficacy of antibiotics therapy alone with antibiotics and saccharomyces boulardii in treatment of acute amebiasis.

METHODS

In a double blind, random clinical trial on patients with acute intestinal amoebiasis, 57 adult patients with acute amoebiasis, diagnosed with clinical manifestations (acute mucous bloody diarrhea) and amebic trophozoites engulfing RBCs found in stool were enrolled in the study. Regimen 1 included metronidazole (750 mg Tid) and iodoquinol (630 mg Tid) for 10 days. Regimen 2 contained capsules of lyophilized saccharomyces boulardii (250 mg Tid) orally in addition to regimen 1. Patients were re-examined at two and four weeks after the treatment, and stool examination was performed at the end of week 4. Student's t-test, chi(2) and McNemar's tests were used for statistical analysis.

RESULTS

Three patients refused to participate. The other 54 patients were randomized to receive either regimen 1 or regimen 2 (Groups 1 and 2 respectively, each with 27 patients). The two groups were similar regarding their age, sex and clinical manifestations. In Group 1, diarrhea lasted 48.0+/-18.5 hours and in Group 2, 12.0+/-3.7 hours (P<0.0001). In Group 1, the durations of fever and abdominal pain were 24.0+/-8.8 and 24.0+/-7.3 hours and in Group 2 they were 12.0+/-5.3 and 12.0+/-3.2 hours, respectively (P<0.001). Duration of headache was similar in both groups. At week 4, amebic cysts were detected in 5 cases (18.5 %) of Group 1 but in none of the Group 2 (P<0.02).

CONCLUSION

Adding saccharomyces boulardii to antibiotics in the treatment of acute amebiasis seems to decrease the duration of clinical symptoms and cyst passage.

Postinfectious Irritable Bowel Syndrome

Postinfectious irritable bowel syndrome (PI-IBS) should be considered in patients who present with a change in bowel habits or an onset of new abdominal pain or discomfort following a recent confirmed or presumed exposure to infectious organisms, or in those who have recently returned from a tropical or developing country. In patients who are greatly distressed by their symptoms, an extended workup early in the course of their illness may give physician and patient confidence in focusing on IBS. The author favors a proactive, multicomponent approach to management, as it gives the physician and patient a sense of control. Treatment should include stress management, dietary advice to minimize exposure to trigger foods, and pharmacotherapy to alleviate anxiety and target disturbed physiology.

Host-pathogen interaction in amebiasis and progress in vaccine development

Entamoeba histolytica, the causative organism of invasive intestinal and extraintestinal amebiasis, infects approximately 50 million people each year, causing an estimated 40 to 100 thousand deaths annually. Because amebae only infect humans and some higher non-human primates, an anti-amebic vaccine could theoretically eradicate the organism. Uncontrolled epidemiologic studies indicate that acquired immunity to amebic infection probably occurs and that such a vaccine might be feasible. Application of molecular biologic techniques has led to rapid progress towards understanding how Entamoeba histolytica causes disease, and to the identification of several amebic proteins associated with virulence. These proteins are now being evaluated as potential vaccine components. Parenteral and oral vaccine preparations containing recombinant amebic proteins have been effective in preventing disease in a gerbil model of amebic liver abscess. Although systemic and mucosal cellular and humoral immunity both appear to play a role in protection against Entamoeba histolytica, the relative importance of each in the human immune response remains unknown. No animal model of intestinal amebiasis currently exists, moreover, so it has been impossible to evaluate protection against colonization and colitis. Further investigation of the fundamental mechanisms by which Entamoeba histolytica causes disease and of the human immune response to amebic infection is necessary to assess the true feasibility of an anti-amebic vaccine.

Screening returning travelers

Posttravel screening is the clinical and laboratory assessment of an individual aimed at uncovering occult infections, pathology, or health risks, the treatment of which will yield a significant health benefit to the individual. Screening must be tailored to the different risk patterns associated with different travel categories (e.g., missionary, tourist). Screening, predominantly a secondary prevention strategy, is most cost-effective when integrated with primary prevention strategies aimed at preventing future travel related illness (Table 6). The screening process begins with a medical history that allows a definition of risks and a tailored approach to laboratory tests. The screening tests currently available for STDs, tuberculosis, and parasitic infections have been reviewed, and although cost-effectiveness data are not available for most post-travel screening tests, recommended approaches are proposed. Traditionally, screening has been directed at uncovering occult infectious disease (STDs, tuberculosis, and parasitic infections). Important benefits can be gained, however, by including screening questions and tests for those diseases that are the major causes of mortality, both in nontraveling and in traveling North Americans, that is, the atherosclerotic and neoplastic diseases and trauma, especially vehicular.

Does non-dysenteric intestinal amoebiasis exist?

BACKGROUND

Chronic abdominal pain and frequent bowel disturbance are common symptoms experienced by more than 15% of apparently healthy people. In areas endemic for Entamoeba histolytica infection, these symptoms are often diagnosed as non-dysenteric intestinal amoebiasis even though no causal relation between such symptoms and E histolytica has been established and clinical presentation of non-dysenteric intestinal amoebiasis and irritable bowel syndrome (IBS) is not distinct. This study was done to assess the clinical significance of E histolytica infection in causation of such symptoms.

METHODS

Patients with symptoms suggestive of non-dysenteric intestinal amoebiasis were recruited from a survey to assess the prevalence of abdominal symptoms in the general population (group A; n = 78) and from medical outpatient clinics (group B; n = 66). Participants who had symptoms as well as symptom-free controls (group C; n = 100) were clinically examined and underwent stool examination, amoebic serology, colonoscopic examination, histopathological examination of colonoscopic biopsy samples, and a trial of antiamoebic therapy (only for participants with symptoms) with metronidazole and mebendazole.

FINDINGS

There were no significant differences between the 144 patients with symptoms and the 100 symptom-free controls in the proportion with E histolytica in stools (26 [18%] vs 18 [18%]), serological evidence of E histolytica infection (61 [42%] vs 41 [41%]), colonoscopic abnormalities (five of 66 vs one of 33), or histopathological abnormalities (36 [49%] of 73 vs ten [30%] of 33). Cyst-positive and cyst-negative individuals showed no significant difference in serological evidence of E histolytic infection, histological abnormalities, or response to therapeutic trial with metronidazole. A diagnosis of IBS was suggested on the basis of consensus criteria and Kruis diagnostic index in 127 of 144 patients with symptoms. The diagnosis of non-dysenteric intestinal amoebiasis could be made in only one patient, who had relapse of symptoms within 6 weeks of antiamoebic therapy and therefore the relapse did not meet criteria for the diagnosis of non-dysenteric intestinal amoebiasis. More than 60% of cyst-positive as well as cyst-negative patients with symptoms showed either complete or partial response to treatment strategy for IBS.

INTERPRETATION

Chronic bowel symptoms, such as pain in abdomen and frequent bowel disturbance, have no association with either past or present infection with E histolytica. Most patients with such symptoms are likely to have IBS. The clinical entity of non-dysenteric intestinal amoebiasis, if it exists, must be extremely rare.

Microscopic colitis is a cause of large bowel diarrhea in Northern India

Chronic diarrhea is a common clinical problem. To determine the possible causes in North India, we studied prospectively 71 patients with chronic diarrhea of the large bowel type. A definite diagnosis could be established in 70 patients. Ulcerative colitis was found in 18 patients, colorectal malignancies in three, colonic polyps in three, and irritable bowel syndrome in 32. In addition, seven patients with seronegative polyarthritis and chronic diarrhea were found to have chronic inflammation of the colon on histology. Two patients had pseudodiarrhea, and no diagnosis could be established in one patient. The remaining five patients with chronic diarrhea showed histologic evidence of chronic colonic inflammation with predominantly mononuclear cell infiltration of the lamina propria and increased intraepithelial lymphocytes, but results of their radiologic and endoscopic studies were normal. These five patients were classified as having microscopic (lymphocytic) colitis. We conclude that the causes of chronic diarrhea in North India patients are similar to a large extent to those seen in Western populations. Microscopic (lymphocytic) colitis is a definite clinicopathologic entity that should be considered in the differential diagnosis of chronic diarrhea.

Entamoeba histolytica trophozoites induce an inflammatory cytokine response by cultured human cells through the paracrine action of cytolytically released interleukin-1 alpha

Infection with the protozoan parasite Entamoeba histolytica results in a high mortality worldwide. To initiate infection, E. histolytica trophozoites in the bowel lumen penetrate the epithelium, and cause extensive lysis of host cells. The acute amebic lesions in animal models are characterized by infiltration with inflammatory cells, particularly neutrophils. The acute host response is likely important for determining whether the infection will spread systemically, but little is known regarding the signals which initiate an acute inflammatory response to E. histolytica. In the studies reported herein, we used an in vitro model system to define the proinflammatory signals produced by epithelial and other host cells in response to infection with E. histolytica trophozoites. Coculture of human epithelial and stromal cells and cell lines with trophozoites is shown to increase expression and secretion of an array of chemoattractant and proinflammatory cytokines, including IL-8, GRO alpha, GM-CSF, IL-1 alpha, and IL-6. Moreover, high-level secretion of those cytokines is regulated by the paracrine action of cytolytically released IL-1 alpha. A second mechanism for trophozoite-induced IL-8 production involves trophozoite-target cell contact via a galactose-inhibitable amebic adherence protein, and appears to be mediated through increased intracellular calcium levels. These studies define novel mechanisms through which acute inflammation can be initiated in the host in response to a cytolytic pathogen, such as E. histolytica.

Entamoeba histolytica cyst passers. Clinical profile and spontaneous eradication of infection

The present study was carried out to examine whether Entamoeba histolytica cyst passers suffer from any parasite-related bowel symptoms and to assess the frequency of spontaneous eradication of this infection. The study was carried out in two parts. In part I, stool samples were collected at random from 3536 individuals living in rural communities around Delhi. E. histolytica was isolated by the culture technique in 345 (9.7%) subjects. There was no increase in the prevalence rate of bowel symptoms in the culture-positive compared to the culture-negative subjects. One hundred twenty-four (36%) of the culture-positive subjects agreed to take part in a longitudinal study; the subjects were left untreated and clinical assessment and stool examinations were carried out at three-month intervals. One hundred eighteen (95.2%) subjects had eradicated their parasite spontaneously at the end of one year; none developed any features of invasive amebiasis. Part II of the study was carried out on 625 patients attending our Gastroenterology Clinic. Positive cultures of E. histolytica were obtained from 99 (15.2%) patients. Again, there was no increase in the prevalence rate of bowel complaints in the culture-positive compared to the culture-negative subjects. Moreover, histological appearances of the rectal biopsy specimens were not significantly different between the two groups. Twenty-eight (28.2%) patients agreed to the longitudinal study and all eradicated the parasite spontaneously within five months; none developed any evidence of invasive amebiasis.(ABSTRACT TRUNCATED AT 250 WORDS)

Antigenicity, immunogenicity and vaccine efficacy of the galactose-specific adherence protein of Entamoeba histolytica

Entamoeba histolytica is an enteric protozoan that causes amoebic colitis and liver abscess. Human immunity to E. histolytica is apparently mediated by a serum antibody response and amoebicidal cellular mechanisms. The galactose-specific adherence protein of E. histolytica is a 260 kDa glycoprotein which mediates amoebic in vitro adherence to human colonic mucins, epithelium, and inflammatory cells. Amoebic lysis of cells is dependent upon binding by this adherence protein. Serum IgG and salivary IgA antibodies from greater than 90% of subjects with invasive amoebiasis recognize the adherence protein's 170 kDa heavy subunit. Incubation of peripheral blood mononuclear cells from antibody-positive subjects with the purified galactose-specific adherence protein induces in vitro T lymphocyte proliferation, IL-2 and gamma interferon production, and direct lymphocyte amoebicidal activity. The rationale for an adherence protein vaccine includes sIgA blockage of amoebic binding to colonic mucins, humoral IgG prevention of parasitic adherence in tissues, and development of amoebicidal cell-mediated immunity. Immunization of gerbils with purified adherence protein in Freund's adjuvant provides protection against intrahepatic challenge with the trophozoites. In summary, the galactose specific adherence protein of E. histolytica contains highly conserved B- and T-cell epitopes, and has a high degree of vaccine efficacy in the gerbil model of amoebic liver abscess.

The amoeba enigma

After more than 70 years of intermittent debate over the true relationship between the 'pathogenic' and 'non-pathogenic' forms of Entamoeba histolytica, the application of molecular biology has finally yielded an unambiguous answer: these are not interconvertible phenotypes of the same parasite, a kind of unicellular Jekyll and Hyde, but two quite distinct genetic entities that just happen to look the same. But given the overwhelming evidence now available from gene sequences, pointing to an evolutionary divergence some tens of millions of years ago, why is it that certain eminent workers in the field are still claiming that, at least in vitro, conversion between the two phenotypes can take place? In this article Bill Spice and John Ackers review recent developments in the molecular biology of E. histolytica and assess the continuing controversy over the status of this enigmatic parasite.

Non-pathogenic entamoeba histolytica in Italian HIV-infected homosexuals

A cohort of 51 homosexuals who were either HIV-positive or had AIDS was followed prospectively with parasitologic stool examination and in vitro culture in order to determine the incidence of E. histolytica infection. Amoebic isolates were further characterized by electrophoretic isoenzyme study. Five subjects (9.8%) were found to be infected with E. histolytica. None of the amoebic isolates were found to be pathogenic by isoenzyme analysis.

Amebiasis

Entamoeba histolytica, the causative agent of amebiasis, was first described in 1875. Although a large number of people throughout the world are infected with this organism, only a small percentage will develop clinical symptoms. Morbidity and mortality due to E. histolytica vary from area to area and person to person. Recent findings have suggested that there are pathogenic and nonpathogenic strains of E. histolytica that can be differentiated by isoenzyme (zymodeme) analysis, monoclonal antibodies, and DNA probes. Whether pathogenicity is a genotypic trait or can be changed by environmental influences has not been resolved. Exchange of genetic material between strains of amebae can influence zymodeme patterns. Currently, detection of E. histolytica infections depends on examinations for ova and parasites and on serologic tests; however, the development of monoclonal antibodies and DNA probes specific for pathogenic zymodemes may be beneficial for clinical laboratory testing and therapeutic decisions when approved tests become available. A better understanding of the mechanisms of pathogenicity at the molecular level is evolving and should promote the development of vaccines and better target selection for therapeutic agents.

Protective immunity to entamoeba histolytica infection in subjects with antiamoebic antibodies residing in a hyperendemic zone

To investigate the relationship between amoebic gut infection and the presence of serum antiamoebic antibodies in a community where amoebiasis is highly endemic, a cross-sectional population survey was conducted covering 982 subjects in 2 adjacent villages in rural India. E. histolytica was detected in fecal smears in 18.1% (88/487) and antiamoebic antibodies by micro-ELISA in 28.7% (251/874) subjects. In 450 subjects in whom there was no clinical evidence of invasive amoebic disease or history of ingestion of antiamoebic drugs in the previous year and from whom both stool and matched serum samples were obtained, correlation of gut infection with seropositivity was attempted. Seropositivity in 80 cyst passers (23.8%) was significantly lower than in 370 non-cyst passers (35.1%; p less than 0.05). Conversely the rate of gut infection with E. histolytica was lower in seropositive subjects (19/149; 12.8%) as compared to seronegative subjects (61/301; 23%, p less than 0.05). In the population where E. histolytica infection was highly endemic, the rate of gut infection with the parasite was inversely related to seropositivity. Our data suggests that invasive E. histolytica infection (mostly asymptomatic) evokes good gut immunity in the host with clearing of the parasite from the colon and/or resistance to reinfection. High prevalence rate of amoebic antibodies indicates good "herd" immunity.

Amebiasis: clinical and laboratory perspectives

Entamoeba histolytica, the premier intestinal protozoan, has traversed time in its relentless quest for survival in its dichotomous role of parasite and pathogen. Enigmatic in its transition from human intestinal commensal to invader of human tissue, diverse in its pathogenicity for the human host, and intricate in its bacterial interrelationship in the bowel, E. histolytica has become the focal point of intensive investigation in its basic biology underscoring human pathogenicity. This review will focus on facets of cell biology, pathophysiology, clinical, therapeutic, and epidemiologic, correlates, along with diagnostic modalities and future research trends.

Zymodemes of Entamoeba histolytica in Dhaka, Bangladesh

Stool samples containing Entamoeba histolytica were obtained from two sources: an urban hospital and an urban slum. Using the enzyme patterns of three enzymes (E.C.2.7.1.1 hexokinase, E.C.5.3.1.9 glucose phosphate isomerase and E.C.2.7.5.1 phosphoglucomutase) stained after cellulose acetate electrophoresis, four zymodemes (I, II, XIV and XVI) were identified in 71 isolates. Zymodemes considered to be pathogenic (II and XIV) were identified from 31 of 34 isolates from the urban hospital, and these zymodemes were strongly associated with dysentery and trophozoites containing ingested red blood corpuscles. Blood visible to the naked eye was more commonly seen in stools from which zymodeme XIV was isolated than in those containing zymodeme II (P less than 0.001). Zymodemes considered to be non-pathogenic (I and XVI) were identified in 34 of 37 isolates from slum dwellers and were not associated with blood in the stools.

Zymodeme conversion of isolates of Entamoeba histolytica

A non-cloned and a cloned isolate of non-pathogenic (zymodeme I) Entamoeba histolytica derived from homosexual males were incubated in Diamonds' TP-S-1 medium in the presence of Crithidia for 52 d. A single addition of Escherichia coli irradiated at 1000 Gy rendered the culture axenic. Analysis of zymodeme patterns established that both isolates had transformed to zymodeme II. The re-establishment of cultures in the presence of microflora from the original cultures resulted in a return to zymodeme I, whereas incubation with microflora from an amoebic isolate exhibiting a zymodeme II pattern maintained the pathogenic pattern.

Nondysenteric intestinal amebiasis. Colonic morphology and search for Entamoeba histolytica adherence and invasion

There is controversy regarding the presence of colonic mucosal abnormalities or mucosal invasion by Entamoeba histolytica in patients with "nondysenteric intestinal amebiasis." To determine the role of E. histolytica in causing symptoms and mucosal changes and to detect if mucosal invasion by E. histolytica is present in nondynsenteric intestinal amebiasis, we evaluated 24 E. histolytica-infected patients (stool microscopy positive for E. histolytica) and 12 noninfected controls who presented with chronic gastrointestinal symptoms, but without dysentery, to a clinic in Calcutta. The colonic mucosa was evaluated at colonoscopy, and mucosal biopsies obtained from the cecum, sigmoid colon, and rectum were evaluated by light microscopy, indirect immunofluorescence microscopy, and scanning electron microscopy. At colonoscopy mucosal ulcerations were absent in all the controls and all except one of the E. histolytica-infected patients. E. histolytica trophozoites or cysts were not seen in the lamina propria or on the luminal surface in any infected patient by light and immunofluorescence microscopy. On scanning electron microscopy, structures that resembled rounded E. histolytica trophozoites were seen on the luminal surface in two of 19 cecal specimens from the infected patients. Moderate or severe mucosal inflammation was frequent on light microscopy in both the E. histolytica-infected patients and the noninfected controls with the cecum involved in two thirds of both groups. Antibodies to E. histolytica were detected in serum of 25% of study patients and 58% of controls. Mucosal inflammation did not correlate with stool positivity for E. histolytica or seropositivity for ameba antibody.(ABSTRACT TRUNCATED AT 250 WORDS)

Therapy of entamebiasis

Therapy of entamebiasis is critical in that, if untreated, the disease can be fatal. Recently, a new method for differentiating pathogenic and non-pathogenic amebae has been standardized. This method relies upon the electrophoretic analysis of 4 isoenzymes which allow the identification of 20 different zymodemes. It is now widely accepted that non-pathogenic strains of Entamoeba histolytica are not a hazard for humans and therefore don't need therapy. As a consequence, treatment must be addressed only toward infections caused by pathogenic strains. As there are different drugs available for treating amebiasis, from a therapeutical point of view the disease must be divided into two forms: intestinal and extraintestinal. For the former, drugs which reach therapeutical levels in the gut are required. The mainstay for the treatment of asymptomatic carriage of pathogenic strains is DILOXANIDE FUROATE, a very well tolerated luminal amebicide. METRONIDAZOLE and other 5-nitroimidazole compounds such as ORNIDAZOLE are indicated for the treatment of symptomatic intestinal infections as they reach good concentrations in tissues, including the bowel where ulcerations develop. In order to ensure the clearance of amebae from the gut, a subsequent cycle with diloxanide furoate is advisable. Extraintestinal forms include amebic abscesses which can develop in many sites, but most commonly in the liver. Metronidazole and related compounds are the drugs of choice; in case of liver abscess, the addition of CHLOROQUINE is indicated because of its good concentration in tissues. A subsequent cycle with diloxanide furoate is also indicated.(ABSTRACT TRUNCATED AT 250 WORDS)

Outcome of untreated infection with Entamoeba histolytica in homosexual men with and without HIV antibody

Among homosexual men the prevalence of infection with Entamoeba histolytica is high. To determine the clinical importance of this infection 55 homosexual men carrying the parasite were investigated in detail. No clinical, serological, or histological evidence of invasive amoebiasis was found in any of them. The patients were not treated and were followed up for 12 to 29 months (mean 21.6 months), during which period none developed symptoms that could be attributed to E histolytica. Spontaneous loss of the parasite occurred in 17 patients, some of whom later became reinfected. Sixteen patients had antibody to human immunodeficiency virus, and infection with E histolytica showed the same benign course in them as in the patients who did not have antibody. Throughout the study classification of the isolates of E histolytica consistently showed that they belonged only to non-pathogenic zymodemes. The findings provide further evidence that E histolytica in homosexual men is a commensal organism.

Entamoeba histolytica as a commensal intestinal parasite in homosexual men

Entamoeba histolytica is considered to be an uncommon, imported organism in the United Kingdom and in many parts of North America, but recent attention has been drawn to the possibility of sexual transmission of this parasite among homosexual men. To determine the prevalence and clinical importance of enteric parasitic infections in men attending a clinic in London for the treatment of sexually transmitted diseases, we studied 354 randomly selected patients who provided a single stool sample that was examined for E. histolytica and other intestinal parasites. Forty-five of the 225 homosexual patients (20 percent) were infected with E. histolytica, but no such infections were found among the 129 heterosexual subjects (P less than 0.0001). With the use of isoenzyme electrophoresis, 34 of the 45 E. histolytica isolates were classified according to zymodeme. All were Zymodeme I or III, which are considered to be nonpathogenic. There was no correlation between the presence of E. histolytica and gastrointestinal symptoms. These findings suggest that E. histolytica is a common commensal in the homosexual population and that, in the absence of evidence of invasive disease, treatment of persons passing cysts of the organism may have little practical benefit.

Is Entamoeba histolytica in homosexual men a pathogen?

Entamoeba histolytica (EH) in homosexual men is generally considered to be pathogenic. To test this hypothesis, the generally accepted features of invasion (haematophagous trophozoites in faeces; high-titre serum antibody; moderate to severe acute inflammatory change; and presence of EH in the mucosa on rectal biopsy) and the zymodeme pattern of cultured trophozoites were assessed in twenty-three EH excretors and eleven control homosexual men. No trophozoites or antibody to EH were found in either group. When other pathogens were excluded, no patient in either group had severe, acute histopathological proctitis. Moderately severe change was seen in 38% of EH excretors and 18% of controls (not significant). All the zymodemes were non-pathogenic. Successful eradication of EH did not result in even a trend towards normalisation of the moderate inflammatory histopathology. There are, therefore, no data here to suggest that EH is a pathogen in homosexual men.

Rectal histology in acute bacillary dysentery

A recent epidemic of acute Shigella dysentery in West Bengal (India) provided us with an opportunity to examine the rectal mucosal abnormalities seen in this condition. One hundred two patients were investigated using sigmoidoscopy, rectal biopsy, and rectal swab for culture. Pure culture of Shigella was obtained in 37 cases, and the rectal biopsy specimens from these patients were assessed in detail. The mean (+/- SD) duration of illness was 47.8 +/- 27.4 h (range 8-120 h), and most patients (31 of 37, 84%) had diarrhea with blood and mucus in the stools. Significant findings at histology were as follows. (a) Cellular infiltrate was predominantly round cell or mixed round cell and neutrophilic in the majority of patients (27, 73%). (b) Disorganization of crypts was seen in as many as 31 patients (84%); in most subjects the distorted architecture was mild, but in a few the defect was severe with crypt branching and dilatation. (c) In the majority of patients the inflammatory process extended to the muscularis mucosae and submucosa; edema with or without increased cellular infiltrate was seen in the muscularis mucosae in 92% and in the submucosa in 80%. (d) There was no difference in the rectal histology of patients with a short history of disease (less than 48 h) compared with those with a longer history, except for goblet cell depletion which was more in those with diarrhea for more than 48 h. (e) The mucosal abnormalities in patients with watery diarrhea were, in general, milder than in those with dysentery, although the difference was statistically not significant; 2 of 6 patients with watery diarrhea had severe colitis. (f) The mucosal abnormalities were more severe in patients with Shigella dysenteriae infection compared with Shigella flexneri.