A field guide to emerging enteric protozoa

February 1, 2001

Although relatively common in children, certain pathogens remain unknown to most office clinicians. This review discusses fo?5{oa implicated as causes of chronic diarrhea and other gastrointestinal symptoms. Awareness of these organisms will improve the management of challenging GI illnesses, the identification of outbreaks, and the collection of epidemiologic data.


A field guide to emerging enteric protozoa

Jump to:Choose article section...LEARNING OBJECTIVES Dientamoeba fragilis Blastocystis hominis Entamoeba coli Cyclospora cayetanensis ACCREDITATION CONTINUING MEDICAL EDUCATION CREDIT HOW TO APPLY FOR CME CREDIT FACULTY DISCLOSURES

By John D. Cowden and Peter J. Hotez, MD, PhD

Although relatively common in children, certain pathogens remain unknown to most office clinicians. This review discusses four emerging protozoa implicated as causes of chronic diarrhea and other gastrointestinal symptoms. Awareness of these organisms will improve the management of challenging GI illnesses, the identification of outbreaks, and the collection of epidemiologic data.

Parasites have long been associated with tropical and exotic environments, but their importance as pathogens in developed countries has become increasingly clear in recent years. A highly publicized example is the addition of Cryptosporidium parvum to the list of clinically relevant parasitic pathogens in North America. This protozoan, once considered unimportant, has become a popular subject of research and is commonly listed among the most important parasitic causes of diarrheal disease in children.



After reviewing this article the physician should be able to:

Identify clinical situations suggestive of protozoan infection.

Discuss appropriate treatment for patients infected with Dientamoeba fragilis, Blastocystis hominis, Entamoeba coli, or Cyclospora cayetanensis.

Understand the strength of evidence supporting the pathogenic status of each protozoan.

Recognize that labs must specifically seek these protozoans for them to be diagnosed.


Despite the growing appreciation of C parvum, a 1997 study of 511 physicians in Connecticut revealed a lack of awareness of risk factors and symptoms associated with cryptosporidiosis.1 Notably, one quarter of pediatricians in the study were unaware that watery diarrhea is a symptom of this treatable disease.

A major hurdle to raising awareness of parasitic pathogens in North America is the general lack of knowledge regarding these organisms and their relationship to the human host. For many parasites, incidence and prevalence are unknown, life cycle and modes of transmission are described by probable theories and assumptions, and descriptions of pathogenicity are often anecdotal and actively debated in the current literature. Furthermore, the practice of managing intestinal diseases such as diarrhea without identifying the etiology has precluded the development of clinical experience with parasitic pathogens and hindered the accumulation of epidemiologic data.

Although incomplete, the available information about common and clinically important intestinal parasites is nevertheless valuable in the assessment of gastrointestinal and abdominal disease. We present the following field guide as an introduction to four lesser known protozoa recently implicated as intestinal pathogens in children: Dientamoeba fragilis, Blastocystis hominis, Entamoeba coli, and Cyclospora cayetanensis. The first three are widely distributed parasites that historically have been described as nonpathogens. Of the three, D fragilis has gained the most support as a potential pathogen. B hominis has a controversial pathogenic status and Ent coli is thought to cause disease rarely, if at all. Broad epidemiologic data are not available, but isolated studies in North America indicate that these three are among the most important parasites in terms of prevalence. For example, in a survey of stool samples from 1,532 Canadian children in the Alberta Children's Hospital, the most frequently identified parasites after Giardia lamblia (31% of parasite-positive samples) were D fragilis (23%), Ent coli (16%), and B hominis (13%).2

C cayetanensis, the fourth protozoan we will describe, was not definitively identified until the 1990s. In North America it causes foodborne and waterborne outbreaks as well as traveler's diarrhea. Exploration of the full extent of its distribution and pathogenicity has just begun.

Our hope is that an introduction to these four organisms will broaden the pediatric clinician's understanding of emerging parasites (that is, organisms previously unrecognized as pathogens) and provide a basis for assimilating future discoveries into practice.

Dientamoeba fragilis

D fragilis (Figure 1) is an unusual protozoan first described in 1918. Because of its amoeboid morphology, it was once grouped taxonomically with Entamoeba, but recent molecular analyses suggest a closer relation to Trichomonas vaginalis, a flagellate associated with sexually transmitted disease. Consequently, D fragilis is now officially classified as an amoebo-flagellate even though it has no flagella.



Many protozoa take on at least two forms during their life cycles: a trophozoite that lives and reproduces in the host, and a hardy infectious cyst that is shed from the body as a vehicle for transmission between hosts. Interestingly, D fragilis has no known cyst form, and the trophozoite is fragile outside of the host environment. This raises the question of how D fragilis passes from person to person. One likely mode is fecal-oral, but an unusual and intriguing second mode has been proposed. For many years a strong association between Enterobius vermicularis (pinworm) and D fragilis infections has been noted. Studies have reported the association to be nine to 20 times what would normally be expected.3,4 Furthermore, investigators have shown that ingestion of pinworm eggs can result in D fragilis infection, implying that D fragilis may be transported on or in the eggs of E vermicularis.5 A similar mode of transmission has been documented for Histomonas meleagridis, a protozoan bird parasite that travels between hosts embedded in the egg of the helminth, Heterakis gallinae. Clinically, the association between D fragilis and E vermicularis suggests that a child found to harbor the former should be examined for the latter as well.

The prevalence of D fragilis in North America is unknown, but studies of specific communities have shown a range of 1.4% to 19%.6 Figures of 2% to 4% have been quoted for the general population, but these estimates are believed to be very low, for two reasons: laboratories do not routinely look for the protozoan, and, when they do, detection can be difficult. The former reason may be the more important of the two, as diagnostic methods have been refined with increasing use. In a 1999 letter in the British Medical Journal, Windsor and Johnson urged laboratories to test for D fragilis, having found it to be the most common enteropathogen (5.1% of fecal samples) in the Sultanate of Oman once a suitable fecal stain technique was adopted.7 They noted that most of the 450 diagnostic laboratories in the United Kingdom do not look for D fragilis and that a recent increase in D fragilis reports in the UK was correlated with an increase in the number of laboratories actively seeking the protozoan. They also encouraged clinicians to add D fragilis to the differential diagnosis for diarrhea and abdominal pain, predicting that the practice of requesting D fragilis searches in appropriate situations would reveal that the parasite is more important than previously thought.

The pathogenicity of D fragilis has been the subject of debate for decades, but an accumulation of evidence in recent studies has led to a wider recognition of D fragilis as a cause of GI disease. The most commonly reported symptoms associated with infection are diarrhea and abdominal pain, though flatulence, nausea, vomiting, fatigue, malaise, and weight loss also occur (Table 1). D fragilis resides in mucosal crypts of the large intestine and is thought to cause symptoms secondary to irritation of the colonic mucosa. Left untreated, diarrhea may persist, eventually giving way to chronic abdominal pain later in the course of the disease. Eosinophilia, generally uncommon in protozoan infections, is reported in approximately 50% of children infected with D fragilis.8 Concomitant infection by E vermicularis may cause this unusual finding, though unlike other intestinal worms, pinworm generally is not associated with eosinophilia.


Emerging protozoal pathogens at a glance

PathogenMode of transmissionStrength of evidence of pathogenicitySigns and symptoms
Probably fecal-oral. Pathogen may also be transported on or in the eggs of Enterobius vermicularis (pinworm)StrongPersistent watery or foul-smelling diarrhea lasting more than a week. Abdominal pain, flatulence, nausea vomiting, fatigue, malaise, weight loss, eosinophilia
Fecal-oral. Foodborne, waterborne, and person to person. Zoonotic transmission has been theorizedControversialDiarrhea (may be watery and profuse) abdominal pain, cramps, nausea flatulence, fatigue, anorexia
Fecal-oralWeakDiarrhea and other GI symptoms
Foodborne, waterborneStrongWatery diarrhea, commonly intermittent with episodes separated by periods of constipation. Nausea, abdominal cramps, anorexia, weight loss


Cuffari and colleagues, in a report of D fragilis infection presenting as allergic colitis, alerted pediatric clinicians to the possibility that milk-protein allergy refractory to diet modification could be complicated or even caused by chronic D fragilis infection.9 They described a child younger than 5 years old, diagnosed with milk protein allergy, who presented with persistent diarrhea and abdominal pain despite adherence to a strict bovine protein-free diet. Following the identification and subsequent elimination of D fragilis, the symptoms resolved, and, interestingly, the patient's protein allergy disappeared.

Suspect D fragilis infection in any child with persistent watery or foul-smelling diarrhea lasting more than one week with or without abdominal pain. Maintain a higher index of suspicion for children with eosinophilia, those living in institutions or areas of poor hygiene, and children attending day care. Day-care centers are infamous for fecal-oral transmission of parasites among children and employees. Keystone and colleagues studied children attending 22 Toronto day-care centers and found that 19% of children were infected with intestinal parasites; 8.6% of children were infected with D fragilis.10

Iodoquinol, paromomycin, and tetracycline (for children over 8 years old) are the drugs of choice for treatment of D fragilis infection.11 (Current dosage recommendations for these and other drugs are listed in Table 2.) Iodoquinol is administered in three doses totaling 40 mg/kg/d (maximum 2 g/d) for 20 days, and should be taken with meals. Possible adverse effects include abdominal discomfort, diarrhea, headache, dysesthesias of the extremities, and optic neuritis. The dosage for paromomycin, a nonabsorbable aminoglycoside, is three doses totaling 25 to 30 mg/kg/d for seven days. Ironically, aminoglycosides may exacerbate diarrhea in children with gastroenteritis. Tetracycline may be given in four doses totaling 40 mg/kg/d (maximum 2 g/d) for 10 days, but is contraindicated in younger children due to toxicity. Doxycycline may be used instead of tetracycline. A follow-up fecal examination is recommended at three to four weeks post-therapy.


Pharmacologic treatment strategies

PathogenDrug(s) of choiceDosage (oral)
Dientamoeba fragilisIodoquinol Paromomycin Tetracycline (for children over 8 years old)*40 mg/kg/d (maximum 2 g/d) in 3 divided doses for 20 days. To be taken with meals. 25–30 mg/kg/d in 3 divided doses for 7 days 40 mg/kg/d (maximum 2 g/d) in 4 divided doses for 10 days
Blastocystis hominisMetronidazole** Iodoquinol**20–35 mg/kg/d in 3 divided doses for 10 days 40 mg/kg/d (maximum 2 g/d) in 3 divided doses for 20 days. To be taken with meals.
Entamoeba coliDiloxanide furoate20 mg/kg/d in 3 divided doses for 10 days
Cyclospora cayetanensisTrimethoprim-sulfamethoxazole5 mg/kg TMP and 25 mg/kg SMX bid for 7 days


Blastocystis hominis

B hominis (Figure 2) was first described in 1911 but attracted only sporadic attention until reports of possible pathogenicity appeared in the 1970s. Three decades of inquiry and debate have yet to resolve most of the fundamental questions regarding the role of B hominis as a human parasite. The taxonomy, biology, epidemiology, pathogenicity, treatment, and even identity of B hominis remain enigmatic for parasitologist and clinician alike.



Taxonomically, B hominis is an orphan of sorts. The organism has been placed with the protozoa because of morphologic characteristics and a susceptibility to antiprotozoal drugs. B hominis is a misfit among the protozoa, however, and its classification remains in question. Evidence that the species is composed of several "demes," or subspecies, has emerged from molecular analysis. The unfolding of this discovery may eventually clarify contradictory findings regarding this organism's pathogenicity.

B hominis is strictly anaerobic, and its life cycle has three stages: a vacuolar form most commonly found in fecal examinations, an infectious cystic form, and a smaller, avacuolar form found to predominate in the human intestine. The development of these stages and the process of infection remain poorly understood. The vacuolar form is thought to develop into the cystic form, which is transmitted by the fecal-oral route. Transmission may be foodborne, waterborne, or person to person. Because Blastocystis species infect a variety of mammalian, avian, and reptilian species, transmission by a zoonotic route has been theorized.

Though its true prevalence in North America is unknown, B hominis has been the most common parasite identified in several studies of both symptomatic and healthy individuals.12 Underestimation of prevalence can be assumed for most parasites, as they are difficult to identify and often are not sought by laboratories. Summarizing a number of studies, Stenzel and Boreham estimate the prevalence of B hominis in developed countries to be 1.5% to 10%.12 Some studies have reported a higher prevalence in children than in adults, but other studies contradict this finding.12

Multiple reports have demonstrated an association between B hominis infection and GI disease, but pathogenicity has never been proven. Much of the evidence in favor of B hominis pathogenicity comes from case reports, some of which cite the disappearance of symptoms following eradication of B hominis with antiprotozoal therapy.12 Though these observations are compelling, they do not account for the possibility that treatment eradicated undiscovered pathogens along with B hominis, giving a false impression of causality. In their comprehensive review of the organism, Stenzel and Boreham list a large number of studies supporting each side of the question and conclude that essential steps in the analysis are missing.12 Case control studies have not been conducted, and a reliable way to exclude all other potential causes of disease has not been found. Perhaps most important is the inability to fulfill Koch's postulates due to a lack of experimental animal models. Nonetheless, many investigators and clinicians, while acknowledging that most B hominis carriers probably remain asymptomatic, believe a preponderance of evidence has been presented and call for the acceptance of B hominis as a bona fide pathogen. Some of those who are unsure, such as Stenzel and Boreham, have stated that the prudent approach would be to treat B hominis as a potential cause of disease.12 As evidence of various demes within the species is further explored, it is possible that pathogenic and nonpathogenic strains will be differentiated, vindicating both viewpoints. Alternatively, B hominis may cause disease only in special circumstances, such as hyperinfection, poor nutrition, or immunosuppression.

Symptoms commonly associated with B hominis infection are nonspecific, including diarrhea, abdominal pain, cramps, nausea, flatulence, fatigue, and anorexia. The diarrhea may be watery and profuse. B hominis has been proposed as a potential cause of both irritable bowel syndrome and inflammatory bowel disease, and associations with diabetes and leukemia have been reported. All of these associations remain unsubstantiated. A diagnosis of pathologic B hominis infection usually requires that B hominis be the only organism identified on fecal examination from a symptomatic patient. Accordingly, treatment for B hominis infection is recommended only after attempts to exclude all other causes of the symptoms have been made.

Since few controlled studies of treatment alternatives for B hominis exist, recommendations are based primarily on anecdotes, and therapy tends to be empiric. Furthermore, questions surrounding the pathogenicity of B hominis cloud the setting and assessment of therapeutic goals. It is not known, for example, whether complete eradication of the parasite from the enteric tract is necessary to alleviate symptoms, if alleviation of symptoms alone might determine therapeutic success, or if treatment is needed at all. Complicating matters are reports of spontaneously resolving infections and the recent demonstration of irregular shedding of B hominis in human feces.13 In either situation, coincidental disappearance of organisms from fecal samples after therapy might falsely imply efficacy.

Despite these doubts and difficulties, successful treatment of B hominis has been described and may provide guidance when therapy is indicated. Metronidazole, known for its efficacy against anaerobes, has been used with some success in adults.12 The recommended dosage in children is three doses totaling 20 to 35 mg/kg/d for 10 days. Iodoquinol also has been effective and is commonly listed with metronidazole as a drug of choice for B hominis infection. Its lack of activity against the organism in vitro and potential toxicities make it somewhat less desirable. The recommended dosage for iodoquinol is three doses totaling 40 mg/kg/d (maximum 2 g/d) for 20 days.

In contrast, furazolidone has been effective against B hominis in vitro and may be a reasonable alternative for treatment of infected children. Adverse effects are rare and mild, including abdominal pain, diarrhea, headache, nausea, and vomiting. In patients with G6PD deficiency, furazolidone may cause hemolysis.

Other potential treatments for B hominis include quinicrine, ornidazole, tinidazole, and trimethoprim-sulfamethoxazole (TMP-SMX), though reports of their efficacy are based on a small number of treated patients.

Entamoeba coli

Ent coli (Figure 3) is a widely distributed relative of Entamoeba histolytica that has always been described as a nonpathogenic protozoan. In 1991, case reports from Northern Europe suggested that Ent coli caused diarrhea and other GI symptoms in 12 patients, including at least two children.14,15 Repeated fecal examinations were performed in an exhaustive search for known causes of diarrhea, to no avail. Ent coli was the only parasite discovered, sometimes in large numbers. Following antiamebic therapy, symptoms disappeared in all patients and subsequent fecal examinations detected no Ent coli. Though this limited evidence clearly does not prove that Ent coli is a pathogen, future inquiry may show the association to be more than incidental. We know of no other reports suggesting Ent coli pathogenicity.



The presence of Ent coli may be a useful marker for fecal-oral exposure to other pathogens, but it probably is not an indication for therapy unless symptoms are present and more likely pathogens have been ruled out. The presumptive treatment of choice is diloxanide furoate, which is available in the US from the Centers for Disease Control and Prevention Drug Service. The recommended dosage is 20 mg/kg/d in three divided doses for 10 days.

Cyclospora cayetanensis

In 1993, an oft-described "cyanobacterium-like body" associated with diarrhea and other gastrointestinal symptoms was revealed to be a previously undescribed species of the protozoan genus Cyclospora. Initially, C cayetanensis (Figure 4) was thought to be unusual in North America, detected only in the occasional traveler returning from overseas. The emerging pathogen soon gained widespread attention after significant outbreaks in 1996 and 1997 that were linked to consumption of raspberries from Guatemala. Now there is evidence of possible endemicity in North America,16 and research has begun to uncover details about the pathogen's genetics, life cycle, transmission, epidemiology, and clinical importance.



The life cycle of C cayetanensis is thought to resemble that of its relative, Isospora. Noninfectious oocysts are shed in feces and sporulate outside the host. Following ingestion, the sporulated oocyst invades enterocytes of the small intestine and reproduces, first asexually, then sexually, to form a noninfectious oocyst. The oocyst is shed and the cycle begins anew.

Person-to-person transmission of C cayetanensis is unlikely given the requirement that the excreted oocyst sporulate outside the host. Foodborne and waterborne transmission of C cayetanensis are probably the major routes of infection, and both have been linked to disease outbreaks in Nepal, Peru, and the US.16 In North America, fresh produce imported from countries where C cayetanensis is endemic has been implicated in foodborne infections. In addition to raspberries, basil and lettuce have been identified as carriers. C cayetanensis oocysts can adhere stubbornly to produce despite washing.

Waterborne transmission of C cayetanensis has not been proven, but investigation of one outbreak and several isolated cases in the US, as well as two outbreaks in Nepal, has raised strong suspicions.17 The risk appears to be highest with exposure to sewage or untreated water. In addition to evidence of waterborne infection in the US, a preliminary link has been made to working with soil.16

Though the suggestion has been made that fresh produce becomes contaminated by contact with waterborne C cayetanensis, contamination by birds or other animals is also possible. Coccidian parasites are known to cause diarrhea in birds, and investigators have suggested that perhaps the bird parasite Eimeria and C cayetanensis are one and the same.18

The epidemiology of C cayetanensis is virtually undescribed. Distribution is thought to be worldwide. An accumulation of reports suggests that individuals of all ages, both immunocompetent and immunocompromised, are susceptible to infection. Cyclosporiasis has been made a reportable disease by the CDC, and increased surveillance by laboratories and clinicians should begin to reveal the full extent of infection in the US.

Symptoms of cyclosporiasis include watery diarrhea, nausea, abdominal cramps, anorexia, and weight loss. The presentation is comparable to that of cryptosporidiosis, and because of the similar microscopic appearance, the two protozoa can easily be confused. In laboratories where careful measurement of cysts is not routine, many cases of cyclosporiasis may have been called cryptosporidiosis in past years. Though both organisms cause watery diarrhea, the specific pattern may be used to help differentiate C cayetanensis from C parvum. The watery diarrhea seen in cyclosporiasis is commonly intermittent, with episodes typically separated by periods of constipation. This is in contrast to the cholera-like diarrhea of cryptosporidiosis.

The differential diagnosis for a child presenting with chronic diarrhea with or without other nonspecific GI symptoms should include cyclosporiasis, as well as infection by any of the protozoa described in this article. A special request to the laboratory and multiple stool samples may be necessary for accurate protozoan assays. Until more is known about the mechanism by which C cayetanensis causes disease, any symptomatic child found to be infected should be treated.

Trimethoprim-sulfamethoxazole is the apparent drug of choice.17 Reports of successful eradication of C cayetanensis from immunocompetent patients have come from Nepal and Peru.19,20 In a child diagnosed with C cayetanensis infection, the recommended oral dosage of TMP-SMX is 5 mg/kg TMP and 25 mg/kg SMX taken twice daily for seven days.21

Enteric protozoan infections have gained increasing recognition in North America with the discovery that protozoa previously described as nonpathogens may cause disease. Familiarity with the four organisms discussed in this article will aid the pediatric clinician in diagnostic and therapeutic decisions when other causes of disease have been excluded from the differential diagnosis.


1. Morin CA, Roberts CL, Mshar P, et al: What do physicians know about cryptosporidiosis? A survey of Connecticut physicians. Arch Intern Med 1997; 157(9):1017

2. Kabani A, Cadrain G, Trevenen C, et al: Practice guidelines for ordering stool ova and parasitic testing in a pediatric population. Am J Clin Pathol 1995;104(3):272

3. Yang J, Scholten T: Dientamoeba fragilis: A review with notes on its epidemiology, pathogenicity, mode of transmission, and diagnosis. Am J Trop Med Hyg 1997;26(l):16

4. Chang SL: Parasitization of the parasite. JAMA 1973;223:1510

5. Frenkel LM: Dientamoeba fragilis infection, in Feigin RD, Cherry JD (eds): Textbook of Pediatric Infectious Disease, vol 2, ed 4. Philadelphia, Saunders, 1998, pp 2403-2406

6. Butler WP: Dientamoeba fragilis: An unusual pathogen. Dig Dis Sci 1996;41(9):1811

7. Windsor JJ, Johnson EH: More laboratories should test for Dientamoeba fragilis infection. Br Med J 1999;318(7185):735

8. Aucott JN, Ravdin JI: Amebiasis and "nonpathogenic" intestinal protozoa. Infect Dis Clin North Am 1993; 7(3):467

9. Cuffari C, Oligny L, Seidman EG: Dientamoeba fragilis masquerading as allergic colitis. J Pediatr Gastroenterol Nutr 1998;26(l):16

10. Keystone JS, Yang J, Grisdale D, et al: Intestinal parasites in metropolitan Toronto day-care centres. Can Med Assoc J 1984;131(7):733

11. The Medical Letter. Drugs for parasitic infections. Med Lett Drugs Ther 1998;40(1017):1

12. Stenzel DJ, Boreham PFL: Blastocystis hominis revisited. Clin Microbiol Rev 1996;9:563

13. Vennila GD, Kumar GS, Anuar AK, et al: Irregular shedding of Blastocystis hominis. Parasitol Res 1999; 85:162

14. Wahlgren M: Entamoeba coli as a cause of diarrhea? Lancet 1991;337:675

15. Corcoran GD, O'Connell B, Gilleece A, et al: Entamoeba coli as a possible cause of diarrhea. Lancet 1991;338:254

16. Sterling CR, Ortega YR: Cyclospora: An enigma worth unraveling. Emerg Infect Dis 1999;5(1):48

17. Marshall MM, Naumovitz D, Ortega Y, et al: Waterborne protozoan pathogens. Clin Microbiol Rev 1997;10(1):67

18. Osterholm MT: Cyclosporiasis and raspberries—lessons for the future. N Engl J Med 1997;336(22):1597

19. Hoge CW, Shlim DR, Ghimire M, et al: Placebo-controlled trial of cotrimoxazole for Cyclospora infections among travelers and foreign residents in Nepal. Lancet 1995;345:691

20. Madica G, Gilman RH, Miranda E, et al: Treatment of Cylospora infections with cotrimoxazole. Lancet 1993; 342:122

21. Merck Manual of Diagnosis and Therapy, ed. 17. Whitehouse Station, NJ, Merck Research Laboratories, 1999

MR. COWDEN is a student at Yale University School of Medicine, New Haven, CT.
DR. HOTEZ is Professor and Chairman, Department of Microbiology and Tropical Medicine, George Washington University Medical Center, Washington, DC.



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Jefferson Medical College, in accordance with accreditation requirements, asks the authors of CME articles to disclose any affiliations or financial interests they may have in any organization that may have an interest in any part of their article. The following information was received from the author of "A field guide to emerging enteric protozoa."

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Peter J. Hotez, MD, PhD, has nothing to disclose.



Peter Hotez, John Cowden. A field guide to emerging enteric protozoa. Contemporary Pediatrics 2001;2:40.