Parasite Mimics and Pitfalls

Banana material (parasite mimic)

The images below show banana material; a parasite mimic commonly found in stool specimens. These structures are NOT actually banana seeds as is commonly taught in parasitology texts. Instead, they are polymerized tannins associated with xylem strands. The Cavendish bananas that are most often sold in grocery stores do not actually develop seeds. They are naturally sterile (triploid) and can only be propagated vegetatively. However, each aborted ovum has a vascular network consisting of xylem strands and associated cells containing astringent tannins. Upon ripening, the tannins polymerize into a semi-solid mass called 'tannin bodies' which fill the cells. The tannin bodies sometimes incorporate red-brown pigments from polyphenol oxidase activity (like the browning reaction in cut apples) as the cells age, and can therefore be seen as the red-brown bodies in this case. They are associated with the xylem strand which give them a chain-like appearance.

Onion skin (parasite mimic)

Sections of onion skin as shown here superficially resemble a length of tapeworm proglottids. They can be differentiated from proglottids by their lack of the expected internal structures.

Mushroom spores (parasite mimic)

A variety of mushroom spores may be found in stool and superficially resemble parasite eggs such as those of Enterobius vermicularis. The spores shown here are from morel mushrooms. They are smaller than E. vermicularis eggs and are not flattened on one side.

Ciliated host cells (detached ciliary tufts; ciliocytophthoria)

Ciliated host cells and detached ciliary tufts (a.k.a. ciliocytophthoria) are commonly shed in certain types of clinical specimens (e.g. respiratory specimens) and remain motile for some time after being shed. (See a video HERE). As a result, they present convincing parasitic mimics and have been mistaken for ciliated or flagellated parasites such as Balantidium coli and Trichomonas tenax. Fortunately, the morphologic features allow for clear differentiation of these objects. Detached ciliary tufts (DCTs) are relatively small (5-15 micrometers in diameter), the cilia are present in a dense mat attached to a terminal plate, and the cilia beat in a rhythmic motion without propelling the object forward. In comparison, the true ciliated parasite, Balantidium coli, is very large in its ciliated trophozoite form (40-200 micrometers) and is covered in short cilia rather than a localized patch of long cilia" (as seen in this case). It also has a distinct peristome and kidney-shaped macronucleus which is not present in DCTs. Similarly, Trichomonas tenax is easily differentiated from DCTs by morphology: it has only 4 apical flagellae (rather than a tuft of dense cilia) and demonstrates a 'jerky' motility pattern. Of note, the free-living amebae that infect humans (e.g. Naegleria fowleri) do not have a flagellate form in the human body and therefore should not enter the differential diagnosis when DCTs are seen. Also, Lophomonas blattarum is a parasite of cockroaches and has not been convincingly described from humans; instead, it has been misdiagnosed in cases where DCTs were observed. Here is a link to a recent paper I was apart of discussing the misdiagnosis of L. blattarum: https://journals.asm.org/doi/10.1128/jcm.00845-23

For future reference, here is a list of sites in the human body where ciliated epithelial cells may be found: upper respiratory tract, fallopian tubes, parts of the endometrium, ependymal cells that line the ventricles in the brain, and caput epididymis/efferent ducts. Ciliated epithelial cells may also be found in any number of locations where the epithelium has undergone a change called ciliary metaplasia (the epithelium converts from non-ciliated cells to ciliated cells). This can occur just about anywhere in the body.

Pollen grains (parasite mimic)

Pollen grains are microscopic structures produced by seed plants that can be found in abundance in certain foods such as honey. When seen in human stool specimens, they may be mistaken as helminth eggs, but can usually be identified by their morphologic features. Pollen grains may be round, oval, elongate, triangular, square, or even star-shaped, and commonly have features not seen on helminth eggs such as pores, spines or furrows.

Rat-tailed maggot (parasite mimic)

Rat-tailed maggots, otherwise known as hoverfly larvae, may be found in stagnant water, and occasionally, in toilet bowels. In the latter scenario, they may be mistaken for human parasites and submitted to the microbiology laboratory for identification. They can be differentiated from myiasis-causing fly larvae by their long respiratory siphon located at their posterior end.

Charcot-Leyden crystals

Charcot-Leyden crystals are break-down products of eosinophils. They may be found in stool, nasal secretions and other specimens when there is an abundance of eosinophils. When seen in stool specimens, they may be indicative of a helminth infection.

Howell-Jolly bodies, thick blood film

Howell-Jolly bodies are erythrocyte nuclear remnants seen mostly commonly in asplenic patients. Their primary significance in the clinical parasitology laboratory is that they may be mistaken for chromatin dots of Plasmodium trophozoites when seen on thin or thick blood films. However, unlike Plasmodium trophozoites, Howell-Jolly bodies don't have associated cytoplasm.

Howell-Jolly bodies, thin blood film

Hemozoin (malaria) pigment, thin blood film

Hemazoin pigment is formed when hemoglobin is digested by Plasmodium parasites. The pigment ranges from yellow to brown-black pigment and is most commonly seen in association within the intraerythrocytic Plasmodium parasites. Less commonly, it may be seen within phagocytes such as macrophages, particularly in heavy infections. These images were taken by Emily F.

Crystals, urate (urine)

Urate or uric acid crystals are commonly found in urine and may be mistaken for Schistosoma haematobium eggs. They can be differentiated by their smaller and more pleomorphic sizes, irregular shapes (often with points at both ends), refractile nature, and occasional cracking artifact.