General Characteristics

Endolimax nana, one of the smaller nonpathogenic amebae, has a worldwide distribution and is seen as frequently as E. coli.

E. nana has the same life cycle stages as E. dispar and the other nonpathogenic amebae. The trophozoite usually measures 6 to 12 µm in diameter (normal range, 8 to 10 µm) (see Figures 1 to 3). Although rarely seen, motility is sluggish and nonprogressive with blunt, hyaline pseudopods. In the permanent stained smear, normally no peripheral chromatin is seen on the nuclear membrane, and the karyosome is large, with either a central or an eccentric location in the nucleus (see Figures 2 and 3). E. nana shows more nuclear variation than any of the other amebae, and occasionally E. nana can mimic D. fragilis or E. hartmanni. The cytoplasm may have small vacuoles containing ingested debris or bacteria, but it also may appear relatively clean.

Fig1. 1 to 5, Trophozoites of Endolimax nana. 6 to 10, Cysts of E. nana. 11 to 13, Trophozoites of Iodamoeba bütschlii. 14 to 16,  Cysts of I. bütschlii. (From Garcia LS: Diagnostic medical parasitology, ed 4, Washington, DC, 2001, ASM Press.)

Fig2. A to C, Trophozoites of Endolimax nana. D and E, Cysts of E. nana. (A-C courtesy Lynne Garcia, Santa Monica, CA.)

Fig3. A, Endolimax nana trophozoite. B, E. nana cyst,  iodine stain. C, E. nana cyst. D, E. nana cyst. (B courtesy Dr. Henry  Travers, Sioux Falls, S.D.)

Cysts usually measure 5 to 10 µm in diameter (normal range, 6 to 8 µm). Cysts as large as 14 µm have been seen. The cyst is usually oval to round, with the mature cyst containing four nuclei. The nuclei typically have no peripheral chromatin and are some what evenly distributed in the cyst. Occasionally, very small, slightly curved chromatoidal bars are present. The two-nucleated stage is not commonly seen, and frequently both trophozoites and cysts are present in clinical specimens.

Epidemiology

 Transmission occurs through the ingestion of mature cysts from contaminated food or water. The cysts of E. nana are less resistant to desiccation than those of E. coli. E. nana is also found in warm, moist climates and in other areas with a low standard of personal hygiene and poor sanitary conditions.

Pathogenesis and Spectrum of Disease

 E. nana is considered nonpathogenic and does not cause disease.

Laboratory Diagnosis

Although cysts sometimes can be seen in a wet preparation, definitive identification of E. nana relies on examination of permanent stained smears.

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