Blastocystis hominis: a neglected cause of diarrhoea?

No correlation between clinical symptoms and Blastocystis hominis in immunocompetent individuals

Abstract Background and Aim: Previous reports regarding the clinical significance and pathogenicity of Blastocystis hominis have been contradictory. The aim of this study was to examine the association between Blastocystis and gastrointestinal symptoms in immunocompetent individuals.

METHODS

We monitored over 2800 healthy people for a period of 15 months, and took stool specimens during both asymptomatic periods and during periods of gastrointestinal symptoms.

RESULTS

After exclusion of individuals who had simultaneous identification of other fecal pathogens, we compared the proportions of asymptomatic versus symptomatic individuals positive for Blastocystis and found no significant difference (P = 0.5). Symptom status did not correlate with parasite abundance. We found that some individuals were likely to have Blastocystis detected during both asymptomatic and symptomatic periods, possibly suggesting carriage of the organism.

CONCLUSION

In conclusion, we found no correlation between clinical symptoms and the presence or absence of Blastocystis among this healthy cohort.

Blastocystis hominis, a long-misunderstood intestinal parasite

For over 50 years, Blastocystis hominis has been held to be a harmless intestinal yeast-probably frequent in stool samples from man and other primates, but usually ignored except as a possible source of confusion with Entamoeba histolytica. More recently, its status as a protozoan parasite has been accepted, and it is now increasingly recognized as an agent of intestinal disease - usually self-limiting but occasionally fatal in monkeys. Here, Charles Zierdt reviews the status o f this intriguing protozoan, drawing attention to its unusual biochemistry.

The effects of extracts from anti-diarrheic Thai medicinal plants on the in vitro growth of the intestinal protozoa parasite: Blastocystis hominis

The activities of n-hexane, dichloromethane and methanol extracts from five anti-diarrheic Thai medicinal plants, Acacia catechu (Fabaceae) resin, Amaranthus spinosus (Amaranthaceae) whole plant, Brucea javanica (Simaroubaceae) seed, Piper longum (Piperaceae) fruit and Quercus infectoria (Fagaceae) nut gall were tested against the in vitro growth of fresh isolates of the intestinal protozoan parasite, Blastocystis hominis. The extracts at concentrations that ranged from 62.5 to 2000 microg/mL, were incubated with several isolates of Blastocystis hominis for 48 h. The activities were classified as killed, inhibited, moderately inhibited and not-inhibited. Dichloromethane and methanol extracts from the Brucea javanica seed and a methanol extract from Quercus infectoria nut gall showed the highest activity. At a concentration of 2000 microg/mL, the three extracts killed 82, 75 and 67% of the Blastocystis hominis samples tested and inhibited 94, 100 and 76% of them, respectively. Metronidazole, used as a reference antiprotozoan drug, at a concentration of 40 microg/mL, killed 97% of the Blastocystis hominis isolates and inhibited all samples tested at concentrations that ranged from 1.25 to 20 microg/mL.

Effect of trimethoprim-sulfamethaxazole in Blastocystis hominis infection

OBJECTIVE

Blastocystis hominis (B. hominis) is a common intestinal parasite that has long been considered nonpathogenic. Recently there have been many reports supporting a role for the organism as a potential pathogen. We performed a study to examine the pathogenicity of B. hominis and the effect of trimethoprim-sulfamethaxazole (TMP-SMX) on this organism.

METHODS

Stool samples of patients, who came to the Department of Parasitology, Faculty of Medicine, Celal Bayar University, were examined by direct wet-mount, trichrome staining, formalin-ethyl acetate concentration, and Kinyoun acid fast techniques for intestinal parasites, and bacteriological stool cultures were performed. Fifty-three symptomatic patients (38 children and 15 adults) with two consequent stool samples positive for abundant B. hominis (five or more organisms per x400 field) and negative for other parasitic and bacterial pathogens were treated with TMP-SMX for 7 days, children 6 mg/kg TMP, 30 mg/kg SMX, and adults 320 mg TMP, 1600 mg SMX, daily. On the seventh day, at the end of treatment, stool samples of all patients were examined by same methods, and clinical symptoms were again evaluated.

RESULTS

B. hominis was eradicated in 36 of 38 (94.7%) children, and 14 of 15 (93.3%) adults. Clinical symptoms disappeared in 39 (73.6%), decreased in 10 (18.9%), and no change was observed in one (1.9%) patient, whereas symptoms persisted in all three (5.7%) patients in whom B. hominis could not be eradicated. Mean number of stools per day was significantly decreased from 4.3 to 1.2 in the 33 children (p < 0.001), and decreased from 3.5 to 1.0 in the four adults (p = 0.06) with diarrhea.

CONCLUSIONS

These results suggested that B. hominis may be pathogenic, especially when it is present in large numbers, and TMP-SMX is highly effective against this organism. Although there are some anecdotal reports, to our knowledge this is the first study examining the effect of TMP-SMX on B. hominis in humans.

Blastocystis hominis revisited

Blastocystis hominis is a unicellular organism found commonly in the intestinal tract of humans and many other animals. Very little is known of the basic biology of the organism, and controversy surrounds its taxonomy and pathogenicity. There morphological forms (vacuolar, granular, and ameboid) have been recognized, but recent studies have revealed several additional forms (cyst, avacuolar, and multivacuolar). The biochemistry of the organism has not been studied to any extent, and organelles and structures of unknown function and composition are present in the cells. Several life cycles have been proposed but not experimentally validated. The form used for transmission has not been defined. Infections with the organism are worldwide and appear in both immunocompetent and immunodeficient individuals. Symptoms generally attributed to B. hominis infection are nonspecific, and the need for treatment is debated. If treatment appears warranted, metronidazole is suggested as the drug of choice, although failures of this drug in eradicating the organism have been reported. Infection is diagnosed by light microscopic examination of stained smears or wet mounts of fecal material. Most laboratories identify B. hominis by observing the vacuolar form, although morphological studies indicate that other forms, such as the cyst form and multivacuolar form, also should be sought for diagnosis.

Blastocystis hominis infection and intestinal injury

Blastocystis hominis is an enteric protozoan associated with clinical illness. To determine the prevalence of intestinal injury in patients with B. hominis infection, the authors prospectively evaluated 18 patients with B. hominis infection by endoscopy and a test of intestinal permeability. Seventeen patients had gastrointestinal symptoms. Colonic mucosa appeared normal by lower endoscopy in 12 of 13 patients, and was friable slightly in 1. Duodenal mucosa was normal by upper endoscopy in nine patients. Pathologic examination of mucosal biopsy specimens did not demonstrate evidence of mucosal invasion. 51Cr-edetic acid (51Cr-EDTA) was given to the 18 patients with stools positive for B. hominis and to 32 healthy control subjects. Approximately 100 uCi of 51Cr-EDTA was given orally after an overnight fast, and urine was collected for the following 24 hours. Mean 24-hour urinary excretion of 51Cr-EDTA, calculated as a percent of the administered dose, was 1.31% (0.34-2.76%) in patients with B. hominis infection and 1.99% (0.59-3.48%) in the control subjects. The intestinal permeability to 51Cr-EDTA in blastocystis-infected individuals was not increased, but was decreased significantly compared with healthy subjects (p < 0.005). Therefore, in a group of symptomatic patients with B. hominis infection, endoscopy typically did not show evidence of significant intestinal inflammation, and results of intestinal permeability testing with 51Cr-EDTA did not suggest impaired barrier function of the intestinal mucosa. The clinical literature on B. hominis infection and intestinal injury is reviewed.

Evidence of an epidemic of Blastocystis hominis infections in preschool children in northern Jordan

Blastocystis hominis is now gaining acceptance as an agent of human intestinal disease. A case-control study of the cause of gastroenteritis in children less than 6 years old was conducted. A total of 500 stool specimens were examined by wet mount preparation, formalin-ether concentration, Sheather's sugar flotation technique, and permanent stains when necessary. B. hominis was found in 63 (25%) of 250 stool specimens of the cases examined; 38 (15%) of these specimens contained this parasite alone. The appearance of severe symptoms was associated with increased numbers of the parasite in the diarrheic specimens (more than five parasites per field at a magnification of x 400). The most common symptoms were abdominal pain, recurrent diarrhea, cramps, anorexia, and fatigue. Contaminated water was suspected to be the major source of infection, since several cases were associated with Giardia infection. These findings support the concept of B. hominis pathogenicity in children with gastroenteritis.

Epidemiology and clinical significance of Blastocystis hominis in different population groups in Salamanca (Spain)

A prospective study was carried out to investigate the epidemiology and clinical significance of Blastocystis hominis in the following groups of the population of the city of Salamanca (Spain): in children attending 11 day care centres and 7 primary schools, two fecal samples were obtained from each child, and in 1231 patients attending the Clinical Hospital. A B. hominis incidence of 5.3-10.3% was found in the day care centres and an incidence rate of 13.4-19.4% was found in the primary schools. All the cases were observed in asymptomatic children. The incidence of B. hominis was greater in children older than 3 years in the day care centres and in the 10-14 year-old group in the primary schools. A heavier parasitization was observed in the boys than in the girls and in the students of schools in areas of low socio-economic level. B. hominis was identified in 40 patients attending the Clinical Hospital (3.25% of all those studied). The maximum peak of incidence was found in subjects with ages between 10 and 14 years. A follow up study was performed on 18 patients parasitized exclusively by B. hominis; 7 of these were considered to have acute gastroenteritis and one chronic gastroenteritis associated with the protozoan. No statistically significant association was observed between the number of B. hominis cells and the presence of diarrhoea. Our results show that despite the high number of asymptomatic carriers of B. hominis in the juvenile population, this protozoan may be, on other occasions, responsible for gastrointestinal symptoms.

Blastocystis hominis--past and future

The history of B. hominis is unique. Few infectious agents have provoked the many misconceptions that plague this enigmatic parasitic ameba. Conflicting descriptions of its nature and pathogenesis have continued throughout the 20th century. As seen by the greatly expanded number of reports in recent years, B. hominis is now a major subject of study, particularly for evidence of disease causation. Physicians are treating patients with intestinal disease caused by B. hominis. Many mild cases resolve in about 3 days without treatment, but others are acute and chronic disease is common. As with E. histolytica, the carrier state is often seen without symptoms. Treatment is usually with metronidazole, but emetine (for refractory infections), trimethoprim-sulfamethoxazole, and pentamidine are also effective. In fecal samples, this complex protozoan appears in a variety of cell forms which makes microscopic diagnosis difficult. As yet, no specific fluorescent-antibody test is available for diagnosis. A culture method to demonstrate the more easily recognized CB form is available, but probably not feasible for most diagnostic laboratories. The common cell forms are the CB form, the granular (mitochondria) form, and the ameba form. The unexpected size range of these forms in clinical material, from yeast size (ca. 7 microns) to giant cells of 20 to 40 microns, makes diagnosis difficult Pseudopodia may be demonstrated by the ameba form in heated microscope stage culture chambers. The anaerobic B. hominis has no cyst form. Its mitochondria are uniquely anaerobic and have no cytochrome protein or oxidative mitochondrial enzymes. Because of its many cell forms and anaerobic mitochondria, B. hominis is an organism of great interest for morphologic and biochemical study. Reproduction is asexual, usually by binary fission. Shizogony occurs in cultured cells. The CB appears to be an organelle whose specific purpose is for reproduction by shizogony. From 2 to 30 progeny are derived from schizogony. The ameba form reproduces by plasmotomy; it has no CB. The pathology of B. hominis infections has been studied in gnotobiotic guinea pigs in which inflammation of the intestinal mucosa and invasion of the superficial layers were seen. Only limited studies of human pathology are available. Those who have studied mucosal histopathology report inflammation and cellular changes that resolve after treatment. More study in this area is strongly indicated (32, 44, 57, 62, 67, 75). Ultrastructural details of B. hominis major forms, except for the schizont, are complete. The organism has no cell wall. The concentric CB takes up as much as 95% of the cell. The major organelles, which include multiple nuclei, Golgi apparatus, mitochondria, endoplasmic reticulum, fat, and other inclusions, are confined in two or four opposed pods in a thin band of peripheral cytoplasm between the spherical entire plasma membrane and the CB membrane. The pods buldge the CB membrane inward. There is evidence of a bacteroid endosymbiont. Education about B. hominis is needed. Entry of recent findings into new textbooks is imperative for its understanding among medical practitioners. Laboratory workers need to be aware of it for many reasons. The College of American Pathologists includes B. hominis in its proficiency testing samples and requires that it be reported from clinical samples.

Blastocystosis: a new disease in the acquired immunodeficiency syndrome?

The protozoon Blastocystis hominis may cause episodes of diarrhoea with abdominal pain, tenesmus, fever and eosinophilia. We have observed 5 cases of blastocystosis in male subjects with symptomatic HIV infection. All patients had a complete response to metronidazole. This report confirms that Blastocystis hominis may be responsible for HIV-related diarrhoea.

Clinical significance of Blastocystis hominis

A total of 19,252 stool specimens from 12,136 patients were examined by direct microscopy and the ethyl acetate-Formalin concentration method during the last 2 years. All liquid specimens and those in which parasite identification was difficult or equivocal were also examined in trichrome-stained preparations. A total of 3,070 intestinal parasites were seen in 2,889 patients. Blastocystis hominis was found in fecal material from 647 patients (17.5%). A total of 132 cases (25.6%) were observed to be in association with other enteric pathogens. B. hominis in large numbers was present as the only parasite or with other commensals in 515 specimens from patients (79.6%). Of these patients, 239 (46.4%) had symptoms, the most common being abdominal pain (87.9%), constipation (32.2%), diarrhea (23.4%), alternating diarrhea and constipation (14.5%), vomiting (12.5%), and fatigue (10.5%). Forty-three (18%) of the patients were treated with metronidazole (0.5 to 1.0 g/day) because of recurrent symptoms and the presence of large numbers of B. hominis cells in repeated stool specimens. After 7 to 10 days of treatment, all patients became asymptomatic with negative stools on follow-up examinations for B. hominis.

Epidemiology and clinical features associated with Blastocystis hominis infection

A retrospective chart review was performed on 100 patients infected with Blastocystis hominis (Bh) and 50 randomly selected age and sex matched controls to examine the clinical and epidemiologic features associated with this organism. The finding of greater than 5 Bh per oil immersion field (1,000 X) was significantly associated with acute presentation of symptoms but was not predictive of the presence of gastrointestinal symptoms. Of patients infected with Bh, 57.5% had recently travelled to the tropics or had consumed untreated water as compared to 12.2% of controls (p less than 0.001). Forty Bh-positive patients were assessed on more than one occasion. No significant differences appeared to exist in the clinical responses of those treated with Metronidazole (14/18; 77.8%) or with dietary management (6/6; 100%) as compared with those not receiving treatment (13/16:81.2%). Patients tended to become less symptomatic with time and in the absence of specific treatment, and therefore treatment with Metronidazole may not be warranted in light of the natural history of Bh infection.

Light-microscopical appearance and ultrastructure of Blastocystis hominis, an intestinal parasite of man

A study on Blastocystis hominis was undertaken for the purpose of clarifying the morphology of the organism using the following techniques; Giemsa, Heidenhain Iron Hematoxylin, and DAPI stains, phase-contrast microscopy, and transmission- and scanning-electron microscopy. Microscopic examinations of the lumen fluids aspirated at the endoscopical examination revealed the habitation of B. hominis in the lower ileum and cecum of a patient. When examined light-microscopically, organisms from stool materials, cultures, and aspirated intestinal lumen contents of a patient showed morphological resemblances to each other except for variations in size. Vacuolated cells, which were spherical in shape and characteristically had a large central vacuole and a narrow rim of cytoplasm containing nuclei and some inclusions, were the only form of the organism observed in this study, although the contents of the vacuole notably varied. DAPI stains clearly revealed the nucleus and the possible mitochondrion in the narrow rim of cytoplasm. Phase-contrast microscopy of fresh material prepared with physiological saline was recommended for diagnosis. When examined electron-microscopically, the organisms were coated with a capsule that was composed of fine filamentous materials. All the organisms contained a central vacuole although the contents of it varied considerably. The cytoplasm gave the organism a signet ring appearance and contained cristate mitochondria, a great number of ribosomes, Golgi's apparatuses, cytoplasmic microtubules, and nuclei with a nucleolus. Very few of the ultrastructures are those that would be expected of a yeast. Recent occurrences of B. hominis infection in Kyoto City, Japan, during a two-year period (June 1983 to August 1985) were also reported.

Blastocystis hominis: evidence for human pathogenicity and effectiveness of metronidazole therapy

1. Clinical symptoms and oral treatment with metronidazole were studied in 103 patients with pure infections by Blastocystis hominis. 2. The results showed that excessive flatulence is the chief gastrointestinal symptom associated occasionally with diarrhoea and abdominal cramps. All the patients showed good responses with treatment of metronidazole and 74 patients whose stools were reexamined 1-2 months after the treatment demonstrated no signs of infections. 3. It is concluded that B. hominis is a pathogenic intestinal parasite and the infection could be eradicated successfully by oral metronidazole.

Association of Blastocystis hominis with signs and symptoms of human disease

Purged stools from 389 patients were evaluated microscopically for the presence of Blastocystis hominis. A total of five or more B. hominis cells per 40X field were observed in 43 patients (11%), and B. hominis was the only intestinal parasite present in 23 (6%) of these patients. Of the 23 patients, 19 had symptoms which included abdominal discomfort (15 patients), anorexia (10 patients), diarrhea (9 patients), and flatus (9 patients). The remaining four patients were asymptomatic. The proportion of eosinophils in the peripheral blood ranged from 4 to 12% in 11 (58%) of the symptomatic patients. Absolute eosinophil counts were greater than 250/microliter in 8 patients and greater than 400/microliter in 5 patients. Eosinophilia was not observed in the remaining symptomatic or asymptomatic patients. This study supports the emerging concept of the role of B. hominis as an intestinal parasite causative of human disease.

Etiology and epidemiology of diarrheal diseases in the United States

Accurate data on the frequency of acute diarrheal illness and the distribution of pathogens are not available for several reasons, including the facts that only a small fraction of cases come to the attention of physicians and that available diagnostic tests establish an etiology in only about half of these. In a survey of three groups of patients in a community (upper-middle class and lower class outpatients and hospitalized infants), a possible cause was found in fewer than 20 percent of outpatients, and rotavirus accounted for the majority. However, asymptomatic colonization with rotavirus is very common in early infancy, and demonstration of the antigen correlates with a causative role in diarrhea in only about one half of cases. Data on cases of shigellosis over a 15-year period in Dallas compared with the Centers for Disease Control national data demonstrate that epidemiologic patterns in any given community can differ substantially from the national data. Day-care centers are important sources of Giardia and Shigella infections in the community. The epidemiologic roles of newly reported causes of diarrhea (Campylobacter laridis, Blastocystis hominis, Cryptosporidium species, and Aeromonas hydrophila) are being explored. Generalizations about etiology and epidemiology of pathogens in diarrheal disease are invalid unless the population group is defined socioeconomically and geographically.

Polymicrobial aetiology of travellers' diarrhoea

Of 35 US Peace Corps volunteers in Thailand, 20 (57%) had a total of 30 episodes of diarrhoea during their first 6 weeks in the country. Enteric pathogens were associated with 90% of the episodes. A single pathogen was identified in 17 (57%) episodes, 2-4 pathogens were identified in 10 (33%) episodes, and there were 15 symptomless infections. Enterotoxigenic Escherichia coli (ETEC) was identified in 37% of these episodes, and various salmonella serotypes were isolated in 33%. Infections with 9 other enteric pathogens were also identified: Campylobacter jejuni (17%), Plesiomonas shigelloides (13%), Aeromonas hydrophila (10%), Blastocystis hominis (7%), Norwalk virus (7%), Vibrio parahaemolyticus (3%), non-O1 Vibrio cholerae (3%), Vibrio fluvialis (3%), and rotavirus (3%). In total, 56 enteric infections were documented in 35 volunteers.