Nonpathogenic enteric protozoa

INTRODUCTION — Several nonpathogenic protozoa inhabit the intestinal tract and may be identified in stool specimens sent to the clinical laboratory for ova and parasite examination [1]. Since these nonpathogenic parasites can be reported by the diagnostic laboratory, it is important to be able to distinguish between organisms that require treatment and organisms that do not.

The classification of nonpathogenic protozoa will be reviewed here. Several other enteric organisms, including Blastocystis hominis and Dientamoeba fragilis, are discussed separately. (See "Blastocystis species" and "Dientamoeba fragilis".)

NONPATHOGENIC PROTOZOA — The nonpathogenic protozoa can be divided into two groups: amebae and flagellates. The nonpathogenic amebae include:

●Endolimax nana

●Entamoeba bangladeshi

●Entamoeba coli

●Entamoeba dispar

●Entamoeba gingivalis

●Entamoeba hartmanni

●Entamoeba polecki

●Entamoeba moshkovskii

●Iodamoeba buetschlii

The nonpathogenic flagellates include:

●Chilomastix mesnili

●Trichomonas hominis

The nonpathogenic amebae and flagellates listed above are enteric organisms, with the exception of E. gingivalis, which is encountered around teeth (pyorrhea alveolaris), in tonsillar crypts, and in vaginal and cervical smears of women with intrauterine devices.

Identification — The nonpathogenic protozoa listed above can be identified based on morphologic features, with the exception of E. dispar, E. bangladeshi, and E. moshkovskii, which cannot be morphologically distinguished from one another or from E. histolytica (picture 1) [2]. Molecular genetic criteria can be used to differentiate the species, however [3-6]. In addition, in the setting of invasive disease due to E. histolytica, trophozoites of this species that contain ingested erythrocytes may be observed (picture 2) [7]. (See "Intestinal Entamoeba histolytica amebiasis".)

Some studies report that E. dispar and E. moshkovskii are observed more frequently than E. histolytica. In one study of 109 stool samples from children in Bangladesh, polymerase chain reaction (PCR) analyses were positive for E. histolytica, E. dispar, and E. moshkovskii in 16, 36, and 21 percent of cases, respectively; among those positive for E. moshkovskii, 74 percent were also positive for E. histolytica or E. dispar [7]. In another study of 1260 stool samples in northeast India, rates of nonpathogenic E. dispar and E. moshkovskii were 12 percent and 8 percent, respectively [8]. In contrast, a study from Iraq among asymptomatic individuals found that the overall Entamoeba prevalence rate was 7.4 percent, with over one-third of these having a missed infection, and 6 percent positivity for E. histolytica, 4.3 percent for E. dispar, and 0.3 percent for E. moshkovskii [9].

The first description of E. bangladeshi outside Bangladesh was in South Africa in 2017 [5]. Further epidemiologic study is needed.

E. dispar, E. moshkovskii, and E. bangladeshi have been identified from patients with gastrointestinal symptoms including diarrhea, but the causal link between the presence of these species and clinical symptoms remains uncertain [10]. Although E. moshkovskii is generally considered nonpathogenic and is listed above as such, reported infections are becoming more frequent and there is some emerging evidence of its potential pathogenicity [11-13]. Additionally, the pathogenic potential of E. bangladeshi is not completely clear [12,14]. It has been suggested that there may be genetic diversity accounting for occasional pathogenic potential of these species, but some reports of possible pathogenicity due to species other than E. histolytica may be due to difficulties in discriminating between Entamoeba species and/or due to coinfection with E. histolytica plus one of these other species [11,12,15,16]. As species-specific detection and study of genetic profiles of isolates improve, understanding of the epidemiology and pathogenicity of these amebae may evolve [13,17]. E. dispar has been identified from patients with nondysenteric but symptomatic colitis and patients with amoebic liver abscess, but it is debated whether some strains can be pathogenic and invasive or whether E. histolytica trophozoites which have invaded the intestinal mucosa also facilitates invasion by E. dispar trophozoites [18].

Among gay males in Europe and the United States, almost all enteric Entamoeba isolates are E. dispar, and invasive disease is uncommon [19]. In Japan, in contrast, E. histolytica is more common than E. dispar; up to 20 percent of gay males are seropositive for E. histolytica, and invasive amebiasis has been reported in gay and HIV-infected males [20].

Given the difficulty associated with distinguishing morphologically between E. histolytica, E. dispar, E. moshkovskii, and E. bangladeshi, ideally, diagnosis of E. histolytica infection should be based on detection of specific antigen or DNA [3,4,21,22]. (See "Intestinal Entamoeba histolytica amebiasis", section on 'Diagnosis'.)

Entamoeba coli is a species complex, with at least two subtypes recognized, and substantial intra-subtype diversity for ST2 [23].

Clinical significance — For circumstances in which the clinical laboratory reports nonpathogenic species exclusively, no therapy is required. The presence of nonpathogenic protozoa suggests the possibility of ingested contaminated water or food or suboptimal fecal-oral hygiene.

In the setting of ongoing intestinal symptoms with identification of only nonpathogenic organisms, symptoms should not be attributed to the nonpathogenic organisms. Misidentification and speciation of enteric protozoa should be considered [24]; in addition, further diagnostic testing should be pursued. Nonpathogenic protozoa may also alter the gut microbiota [25,26]. Excretion rates of pathogenic protozoans may fluctuate from day to day; therefore, serial stool examinations may be required to detect pathogens such as Giardia lamblia or E. histolytica. Ideally, a minimum of three samples should be obtained on at least three different days. In addition, acid-fast or fluorescent staining techniques for detection of Cryptosporidia, Cystoisospora, or Cyclospora infections should be performed. (See "Epidemiology, clinical manifestations, and diagnosis of Cystoisospora (Isospora) infections" and "Cyclospora infection".)

SUMMARY

●Nonpathogenic protozoa – Several nonpathogenic protozoa inhabit the intestinal tract and can be divided into two groups: amebae and flagellates. (See 'Nonpathogenic protozoa' above.)

●Identification

•The nonpathogenic protozoa can be identified based on morphologic features, with the exception of E. dispar, E. bangladeshi, and E. moshkovskii, which generally cannot be morphologically distinguished from one another or from E. histolytica (picture 1). (See 'Identification' above.)

•E. dispar and E. moshkovskii have both been identified from patients with gastrointestinal symptoms, but a definitive evidence of a causal link between the presence of these two species and clinical symptoms remains uncertain. (See 'Identification' above.)

●Clinical significance – In the setting of ongoing intestinal symptoms with identification of only nonpathogenic organisms, symptoms should not be attributed to the nonpathogenic organisms. Misidentification or speciation of enteric protozoa should be considered, and further diagnostic testing should be pursued. (See 'Clinical significance' above.)