The other intestinal protozoa: Enteric infections caused by Blastocystis hominis, Entamoeba coli, and Dientamoeba fragilis

The Other Intestinal Protozoa: Enteric Infections Caused by Blastocystis hominis, Entamoeba coli, and Dientamoeba fragilis Peter Hotez, MD, PhD Pediatricians frequently are confronted with clinical microbiology laboratory reports about a fecal examination that was determined to be positive with Dientamoeba fragilis, Blastocystis hominis, or Entamoeba coli. Increasingly, these organisms are becoming leading causes of parasitic infection in North America. Evidence is provided that these unique organisms may be pathogenic under some circumstances. Some guidelines for management and treatment are also discussed. Copyright 娀 2000 by W.B. Saunders Company

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ediatricians and pediatric nurse practioners frequently obtain fecal specimens from children with diarrhea or other gastrointestinal symptoms. Sending the specimen to a clinical parasitology laboratory allows the technologist to examine the stool for the presence of parasites. In North America, a good possibility is that the specimen will be identified as being positive for the presence of Blastocystis hominis, Entamoeba coli, or Dientamoeba fragilis. Next to Giardia lamblia, these 3 eukaryotic protists are the enteric parasites reported most frequently. For example, in a study conducted at Alberta Children’s Hospital, 146 of 2,652 stool specimens from 1,532 children were positive for the presence of intestinal parasites. Of these positive fecal examinations, G lamblia was identified from 31 percent, D fragilis from 23 percent, E coli from 16 percent, and B hominis from 13 percent.1 This information has since been used to design and implement practice guidelines for ordering fecal examinations.1 Because decisions about the clinical management of pediatric infections caused by D fragilis, E coli, and B hominis have been hampered by controversies about their true taxonomies and their actual roles in the pathogenesis of childhood gastroenteritis, the salient features of each of the 3 intestinal parasites are outlined here. The goal is to provide the clinician with relevant information for making a decision about possible treatment.

Blastocystis hominis Infection In many respects, B hominis remains an enigma for biologists, as well as pediatric health care providers. Not only do its taxonomy, life cycle, and mode of transmission remain controversial,2 but

From the Departments of Epidemiology and Public Health and Pediatrics, Yale University School of Medicine, New Haven, CT. Address correspondence to Peter Hotez, MD, PhD, Yale University School of Medicine, Laboratory of Epidemiology and Public Health, 60 College St, New Haven, CT 06520; e-mail: [email protected]. Copyright 娀 2000 by W.B. Saunders Company 1045-1870/00/1103-0006$10.00/0 doi:10.1053/pi.2000.6228

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the role that this organism plays in eliciting gastrointestinal pathology and symptoms is equally uncertain. By electron microscopy, B hominis has ultrastructural features, including the presence of mitochondria-like organelles and at least one nucleus, indicating that it is a eukaryotic parasite that resembles a protozoan.2,3 Because B hominis is anaerobic, the mitochondria presumably do not function in oxidative phosphorylation. In vitro, the organism is sensitive to antiprotozoal agents but not to antibacterial or antifungal drugs. However, the technique of small-subunit rRNA sequencing (used to determine taxonomic status of microorganisms) has failed to establish the taxonomic relationship of B hominis with other protozoa or even other simple eukaryotes. Possibly, the organism will be assigned its own subphylum.2-4 The genus Blastocystis appears to have multiple species, each of which may have its own animal host. Although B hominis is considered the major human parasite, new molecular evidence points to separate ‘‘demes’’ of this organism.5 This finding ultimately may result in new assignments or species. If some demes are found to be more pathogenic than are others, the controversy about whether B hominis is a true human pathogen might be resolved. The life cycle of B hominis, like almost every other aspect of this organism, remains controversial. In at least one part of its life history, the parasite manifests a typical-appearing vacuolar form, which under light microscopy appears as a thin peripheral band of cytoplasm surrounding a large membrane-enclosed central vacuole.2-4 The vacuolar form is the one most commonly identified during fecal examination. The central vacuole may contain material used by the parasite for energy storage. The vacuolar stage of the organism typically is between 4 and 15 µm in diameter, although vacuolar forms with diameters greater than 200 µm have been described.2 A second smaller avacuolar form has been recovered from infected patients by colonoscopy or from the stools of patients who produce large volumes of diarrheal fluid.2 Likely, the avacuolar form is the form most frequently present in the human intestine. As B hominis passes down the intestinal tract, small vesicles within its cytoplasm coalesce into a vacuole. Eventually, the vacuolar form of the

Seminars in Pediatric Infectious Diseases, Vol 11, No 3 ( July), 2000: pp 178-181

Pediatric Blastocystis and Dientamoeba Infections parasite forms a cyst. The cyst probably is the infectious form,2 which then is transmitted by a fecal-oral route. Parasite cyst transmission may be water-borne,6,7 food-borne,8 or person-toperson, especially in child care centers6,7,9 or other institutional settings. Transmission also may occur by means of a zoonotic route. Because fecal examinations usually are sought only in patients with gastrointestinal symptoms, the true prevalence of B hominis parasitism in North America or elsewhere is not known. B hominis has not been definitively determined to be a true enteric pathogen. Most of the literature implicating B hominis as pathogenic is based on case reports. However, many of these reports do not exclude the possibility that individuals are asymptomatically colonized with the organism. Indeed, most likely many or most cases of B hominis infection are asymptomatic. Some case reports conclude that B hominis is an etiologic agent on the basis of symptomatic improvement in the patient after antiprotozoal therapy, a conclusion that fails to consider that many of the agents used could have eradicated other copathogens, including bacteria.2 The organism seldom has been seen to invade colonic mucosa on endoscopic biopsy, even on occasions when the mucosa was noted to be edematous and inflamed.2 On the other hand, some case reports have shown evidence of histopathologic invasion, including one investigation in a child in whom a substantial effort was made to rule out other infectious causes.10 B hominis has been linked clinically to a wide variety of gastrointestinal symptoms. Heading the list of abnormal gastrointestinal findings are diarrhea, abdominal cramping and pain, and anorexia. This constellation has led some investigators to postulate that B hominis may be an etiologic agent of irritable bowel syndrome (IBS).2,11 Indeed, anti–B hominis IgG2 subclass antibodies have been noted to increase significantly in IBS patients.11 For similar reasons, efforts have been made to investigate B hominis as an etiologic or exacerbating agent for inflammatory bowel disease, including terminal ileitis and ulcerative colitis.2,12,13 Another suggestion is that B hominis becomes an agent of clinical illness when it occurs in large numbers or when it parasitizes the intestines of an immunocompromised patient, such as one with AIDS.2 The observation that B hominis is the most common parasite among homosexual men, regardless if they were co-infected with the human immunodeficiency virus, renders designation of this organism as an opportunistic pathogen in AIDS difficult.14 Stenzel and Boreham2 offer several reasons why definitively implicating B hominis as a cause of enteric disease has been difficult: (1) the absence of a laboratory animal model precludes the possibility of experimentally fulfilling Koch’s postulates using defined isolates of the organism; (2) exhaustively excluding other co-pathogens in individuals who harbor B hominis is difficult; (3) B hominis may be pathogenic only under certain conditions such as immunosuppression or poor nutrition; and (4) only one or a few of the multiple B hominis demes may be pathogenic. For these reasons, some clinicians recommend anti-Blastocystis therapy for children only when an extensive effort has been made to rule out other infectious and noninfectious causes in a patient with relevant gastrointestinal symptoms. Typically, a diagnosis of B hominis infection is established by identifying B hominis as the exclusive organism in a trichrome

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stain of a permanent fecal smear. More invasive techniques, including endoscopic aspiration and biopsy, are not recommended routinely. Serologic testing for detection of B hominis– specific antibodies currently is available only through research laboratories.

Antimicrobial Treatment Given the problems outlined above in implicating B hominis as a cause of specific gastrointestinal signs and symptoms, the lack of a consensus for a treatment of choice is not surprising. The antimicrobial agents and their doses used for B hominis infections are based largely on anecdotes. Metronidazole has been used to target the anaerobic metabolic machinery of the organism, with some reported success. Stenzel and Boreham2 have summarized briefly the uses of this agent in adults, with dosages ranging from 250 to 750 mg three times per day for 5 to 10 days; 200 mg four times per day for up to 1 week; and 2 g/d for 5 days. Furazolidone may represent an effective alternative agent for treating infected children. This agent, like metronidazole, was superior at inhibiting the growth of B hominis in vitro compared with a panel of other antimicrobial agents.15 Furazolidone also was found to be effective in vitro at inhibiting the incorporation of a radiolabeled nucleotide into parasite nucleic acid.16 The drug is available in suspension. Iodoquinol also has been tried with some success in patients with B hominis infection. However, this agent has low in vitro activity against B hominis and, therefore, probably is not a first-line treatment.17 Antibacterial agents, such as the ␤-lactams, cephalosporins, and aminoglycosides, and the antifungal agent amphotericin B do not inhibit the growth of B hominis in vitro.2,16

Entamoeba coli Infection Unlike Entamoeba histolytica, E coli was always considered an entirely nonpathogenic enteric protozoan until 1991, when case reports appeared in northern Europe suggesting otherwise.18-20 These patients experienced persistent diarrhea and had repeated fecal examinations to exclude other organisms. The search was exhaustive and included electron microscopy and anaerobic stool culture. One of the patients was a parasitology laboratory technician,18 and 2 were children,19 all of whom responded to specific antiamebic therapy. The identification of E coli in a child with diarrhea is not, generally speaking, an indication for initiating antiamebic therapy unless efforts to exclude other pathogens are attempted. Diloxanide furoate is the probable treatment of choice. In the United States, the drug is available from the Centers for Disease Control and Prevention Drug Service.

Dientamoeba fragilis Infection D fragilis is an intestinal protozoan that, because of its amoeboid morphology, was thought to taxonomically resemble Entamoeba. At one time, a common clinical pearl to state was that D fragilis was the only parasitic amoeba of humans that did not form a cyst. However, a more recent analysis of the organism’s 16S-like rRNA provides another explanation. D fragilis is now thought to share a recent common evolutionary history with the tricho-

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monads21 and more closely resembles the human endoparasitic protozoan Trichomonas vaginalis. The organism is, therefore, an amoebo-flagellate, which typically is binucleated. D fragilis inhabits the mucosal crypts of the human large intestine, where it ingests bacteria and red blood cells.22,23 The trophozoites are sensitive to aerobic environments. D fragilis has been postulated to have at least two modes of transmission, one of which is fecal-oral. The second mode of transmission is considered novel because it requires the presence of a second human parasite. Several investigators have noted that patients who harbor D fragilis in their colon also are co-infected with the human pinworm Enterobius vermicularis.24,25 Subsequently, ingestion of E vermicularis eggs was shown experimentally to also result in acquisition of D fragilis infection.22,26 Of interest is that similar coparasitisms have been described in related amoeboflagellates of some birds. Therefore, a diagnosis of D fragilis infection in a child might raise the suspicion that E vermicularis infection also is present. The true prevalence of D fragilis infection in the United States is not known. In addition to the occurrence of watery or foul-smelling diarrhea, an important clinical manifestation of D fragilis infection in a child is abdominal pain that sometimes is described as crampy or colicky and usually located in the lower abdominal quadrants.22 The abdominal pain that accompanies dientamoebiasis sometimes is accompanied by abdominal tenderness.22 Left untreated, both the pain and tenderness can become chronic.27-29 The presence of D fragilis should be suspected in a child with either diarrhea or abdominal pain that lasts for more than a week, with the index of suspicion being even higher for children who live in institutional settings or in environments where the level of hygiene is inadequate.22 The organism can be demonstrated reliably by fecal examinations of stool placed in a preservative such as polyvinyl alcohol or by a permanent stained smear of a fresh sample. A single stool examination can identify up to 71 percent of known cases of D fragilis infection. The sensitivity increases to 93 percent when at least three stool samples are obtained.30 The major diagnostic criterion is the presence of a high percentage of binucleate trophozoites with characteristic nuclei that do not contain peripheral chromatin, but instead have 4 to 8 chromatin granules in a central mass.22

Antimicrobial Therapy The major drugs of choice for the treatment of children with dientamoebiasis are iodoquinol, paromomycin, or tetracycline (for children older than 8 years).31 Metronidazole also may be effective. Iodoquinol (40 mg/kg/d [maximum, 2 g] in 3 doses, administered for 20 days) comes in a tablet that should be taken with meals. Side effects may include abdominal discomfort, diarrhea, headache, dysesthesias of the distal extremities,22 and optic neuritis. Paromomycin (25-30 mg/kg/d in 3 doses, administered for 7 days) is a nonabsorbable aminoglycoside. In some cases, the drug actually may exacerbate diarrhea in children with gastroenteritis. Tetracycline (40 mg/kg/d [maximum, 2 g] in 4 doses, administered for 10 days) is appropriate for older children. Some investigators suggest that patients on antiamebic therapy should have a fecal examination repeated approximately 3 to 4 weeks after therapy.

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epidemiology, pathogenicity, mode of transmission and diagnosis. Am J Trop Med Hyg 26:16-22, 1977 30. Hiatt RA, Markell EK, Ng E: How many stool examinations are necessary to detect pathogenic intestinal protozoa? Am J Trop Med Hyg 53:36-39, 1995 31. The Medical Letter: Drugs for parasitic infections. Med Lett Drugs Ther 40(1017):1-12, 1998