Parasitology: Intestinal Protozoa and Helminths

Only E. histolytica is an established invasive member of its morphologic complex. E. dispar, E. moshkovskii, and E. bangladeshi can be indistinguishable from it by light microscopy. A validated E. histolytica-specific antigen assay or NAAT is needed for a species-level result. Molecular methods may also resolve the other members. Serology is most useful for extraintestinal amebiasis, especially amebic liver abscess, but antibody may persist after infection and can reflect prior exposure. Stool microscopy has low sensitivity in isolated liver abscess.^{88 Centers for Disease Control and Prevention. DPDx: Amebiasis.}

The complete life cycle and pathogenic significance of D. fragilis remain unsettled. Human stool usually contains trophozoites, asymptomatic detection is common, and transmission within Enterobius eggs remains a hypothesis.

Blastocystis sp. is a stramenopile (a eukaryote in a lineage separate from the protozoa and fungi) retained in diagnostic parasitology. The usual fecal form is a spherical or oval vacuolar form, 5 to 40 µm and commonly 8 to 10 µm, with a large central body and a thin rim of cytoplasm containing peripheral nuclei. On trichrome the central body stains variably from red to blue and the nuclei stain purple. The proposed cyst and transmission cycle are not fully established. Because the organism occurs in symptomatic and asymptomatic people, detection alone does not prove that it caused the patient's diarrhea.^{99 Centers for Disease Control and Prevention. DPDx: Blastocystis.}

Stool antigen, direct fluorescent antibody, or NAAT usually detects Giardia more sensitively than a single O&P examination. Duodenal aspirate, string-test material, or small-bowel biopsy may show trophozoites when stool tests remain negative.^{1010 Centers for Disease Control and Prevention. DPDx: Giardiasis.}

Trichomonas vaginalis is a urogenital trichomonad with no recognized cyst. The 7 to 30 µm trophozoite has 4 anterior flagella, a recurrent flagellum associated with an undulating membrane extending about half the body, one anterior nucleus, and a posteriorly projecting axostyle. NAAT is preferred because wet-mount sensitivity is only 44% to 68% and falls rapidly when examination is delayed. Specimen types cleared by the Food and Drug Administration (FDA) differ by platform, and some assays accept male urine as well as female genital or urine specimens. A Papanicolaou test suggesting T. vaginalis requires confirmation with a sensitive diagnostic test.^{1111 Centers for Disease Control and Prevention. Trichomoniasis. 2021.}

Cryptosporidium occupies an intracellular but extracytoplasmic position at the intestinal brush border. Because the oocyst is immediately infectious and resists routine chlorination, person-to-person and waterborne transmission occur. Yeast can mimic the small oocysts by size, so confirm the identification with the acid-fast reaction, DFA, antigen testing, or NAAT. In severe immunodeficiency, infection may extend to the biliary, pancreatic, or respiratory tract.^{1212 Centers for Disease Control and Prevention. DPDx: Cryptosporidiosis.}

Cyclospora oocysts are about twice the diameter of Cryptosporidium oocysts and autofluoresce. Cryptosporidium oocysts do not autofluoresce. Shedding is low-level and intermittent, so concentrate and examine specimens collected on different days. No animal reservoir for C. cayetanensis has been confirmed. Cystoisospora shedding may also be scant despite profuse diarrhea, and peripheral eosinophilia may occur. Most routine trichrome stains do not identify either organism reliably.^{13,1413 Centers for Disease Control and Prevention. DPDx: Cyclosporiasis.14 National Institutes of Health. Cystoisosporiasis. 2025.}

Microsporidia are obligate intracellular, spore-forming eukaryotes closely related to fungi. The resistant spore is infective and contains a coiled polar tubule that everts to inject the sporoplasm into a host cell. Enterocytozoon bieneusi spores measure about 0.8 to 1.4 µm. Encephalitozoon spp. and several other human microsporidia measure about 1.5 to 4 µm. Modified trichrome or Chromotrope 2R stains spores pink-red, often with an equatorial band. Quick-hot Gram-chromotrope stains them dark violet. Examination requires oil immersion and an experienced reader. Light microscopy does not reliably determine species. Polymerase chain reaction (PCR), species-specific antibodies, or ultrastructural examination provides species identification. Stool is used for enteric disease, and urine may contain spores during disseminated Encephalitozoon infection.^{1515 Centers for Disease Control and Prevention. DPDx: Microsporidiosis.}

Hookworm rhabditiform larvae have a long buccal canal and inconspicuous genital primordium, the reverse of Strongyloides. Hookworm larvae in stool usually indicate delayed processing or culture. Strongyloides larvae are passed from an active infection. Decorticated Ascaris eggs retain a thick smooth shell. Do not mistake them for hookworm eggs, which have a thin shell and segmented contents. The morphology and life cycles in this table follow current DPDx organism descriptions.^{6,16-206 Centers for Disease Control and Prevention. DPDx: Intestinal parasites, comparative morphology tables.16 Centers for Disease Control and Prevention. DPDx: Enterobiasis.20 Centers for Disease Control and Prevention. DPDx: Intestinal capillariasis.}

Routine O&P microscopy has low sensitivity for uncomplicated strongyloidiasis. Serial fresh stools examined by Baermann sedimentation or Koga agar-plate culture improve larval recovery. Charcoal and Harada-Mori cultures are alternatives. Formalin-ethyl acetate processing can remove larvae. Agar culture plates can produce infective filariform larvae and must remain sealed and be handled with exposure precautions. Serology helps with screening and low-burden infection, but cross-reactions occur, a positive result does not distinguish active from past infection, and immunosuppression can produce false-negative results. PCR requires a validated non-formalin-fixed specimen.^{2121 Centers for Disease Control and Prevention. DPDx: Strongyloidiasis.}

Autoinfection allows S. stercoralis to persist for decades. Hyperinfection is accelerated autoinfection with increased larvae along the usual gastrointestinal and pulmonary route. In dissemination, larvae spread to organs outside that route. Corticosteroids are the most important precipitant. Human T-cell lymphotropic virus type 1 (HTLV-1) infection, hematologic malignancy, transplantation, and other immunosuppression also increase risk. Larvae may appear in stool, sputum, or bronchoalveolar lavage. In severe disease, enteric bacteria can enter the blood or cerebrospinal fluid (CSF). Eosinophilia may be absent.^{2121 Centers for Disease Control and Prevention. DPDx: Strongyloidiasis.}

Handle unfixed Taenia specimens as though they may contain immediately infectious T. solium eggs until the species is established. Undercooked pork containing cysticerci causes intestinal taeniasis. Cysticercosis follows ingestion of eggs shed by a human carrier. Egg microscopy cannot separate the human Taenia species or distinguish them from other taeniid eggs such as Echinococcus.^{2222 Centers for Disease Control and Prevention. DPDx: Taeniasis.}

H. nana can persist through internal autoinfection. Diphyllobothriid taxonomy has changed substantially, and many historical cases assigned to Dibothriocephalus latus lacked species confirmation. A minority of broad fish tapeworm infections cause vitamin B12 deficiency. Dipylidium caninum is usually recognized when motile proglottids or egg packets are recovered, and flotation may miss the heavy packets.^{23-2523 Centers for Disease Control and Prevention. DPDx: Hymenolepiasis.25 Centers for Disease Control and Prevention. DPDx: Dipylidium caninum.}

Recent consumption of animal liver can explain Fasciola-compatible eggs in stool without an established infection. This dietary passage is called pseudofascioliasis. Record the observed eggs, review the collection history, and recommend repeat stool examination after several days without liver before interpreting the finding as infection. Egg morphology alone cannot establish whether the eggs originated in the patient.^{2626 Centers for Disease Control and Prevention. DPDx: Fascioliasis.}

For suspected intestinal schistosomiasis, examine repeated stool specimens by a sedimentation or validated quantitative method. For S. haematobium, urine collected around noon to 3 PM and examined as centrifuged sediment or by standardized membrane filtration improves recovery. When African exposure could involve both intestinal and urinary species, examine both stool and urine. Serology supports diagnosis in previously untreated travelers or immigrants when eggs are not recovered, but antibody does not distinguish active from treated infection. Cercarial dermatitis, acute Katayama syndrome (the systemic illness that follows heavy cercarial exposure), and chronic egg-induced fibrosis are possible manifestations, and a patient may have any one of them without the others.^{26-2926 Centers for Disease Control and Prevention. DPDx: Fascioliasis.29 Centers for Disease Control and Prevention. DPDx: Schistosomiasis infection.}

Migrating larvae and tissue cestodes

Humans are accidental or dead-end hosts for many tissue helminths, so stool O&P is negative. Toxocara larvae do not mature into egg-producing adults in humans. Diagnosis is based on exposure, compatible visceral or ocular disease, and serology. Demonstration of larvae in tissue is definitive but uncommon. The same principle applies to Baylisascaris procyonis. Anisakis and Pseudoterranova larvae are usually recovered by endoscopy, surgery, or histology after raw marine fish or squid exposure. Cutaneous larva migrans is usually diagnosed clinically. Trichinella antibody may remain negative for 3 to 5 weeks after infection, so early negative serum may need a later specimen. Muscle biopsy confirms infection when larvae are seen, but it has limited sensitivity.^{32-3532 Centers for Disease Control and Prevention. DPDx: Toxocariasis.35 Centers for Disease Control and Prevention. DPDx: Trichinellosis.}

In cysticercosis, a human becomes an intermediate host after ingesting T. solium eggs. Neurocysticercosis diagnosis combines exposure, clinical findings, both noncontrast computed tomography (CT) and magnetic resonance imaging (MRI), and a glycoprotein enzyme-linked immunotransfer blot when confirmation is needed. Antibody sensitivity is high with multiple viable cysts but poor with a single lesion or only calcifications. Antibody does not distinguish active from inactive infection.^{3636 Centers for Disease Control and Prevention. DPDx: Cysticercosis.}

In echinococcosis, humans are accidental intermediate hosts. Canids are the definitive hosts of E. granulosus sensu lato, E. multilocularis, and E. vogeli. Wild felids are the definitive hosts of E. oligarthrus. Echinococcus granulosus sensu lato causes cystic echinococcosis, E. multilocularis causes infiltrative alveolar disease, E. vogeli causes polycystic neotropical disease, and E. oligarthrus causes a very rare unicystic form. Imaging is the main diagnostic basis and serology is supportive. Negative serology does not exclude disease, and invasive sampling requires a planned expert procedure because cyst leakage can cause an allergic reaction and secondary implantation.^{3737 Centers for Disease Control and Prevention. DPDx: Echinococcosis.}

Humans acquire Dracunculus medinensis by drinking water containing infected copepods. Diagnosis is usually clinical when a gravid female emerges from a skin ulcer, and lesion fluid may contain larvae. Humans are accidental hosts for Angiostrongylus cantonensis, which causes eosinophilic meningitis after ingestion of larvae from mollusks, paratenic hosts, or contaminated produce. Human stool is not diagnostic. Specialized real-time PCR of cerebrospinal fluid is available through reference laboratories. A. costaricensis causes abdominal disease, and eggs, larvae, or adults may be found in intestinal surgical or biopsy tissue. They are not found in stool.^{38,3938 Centers for Disease Control and Prevention. DPDx: Dracunculiasis.39 Centers for Disease Control and Prevention. DPDx: Angiostrongyliasis.}

The laboratory needs the immune defect and exposure history, because a routine O&P examination may omit acid-fast staining and microsporidia stains, and a multiplex panel may omit one or more opportunistic parasites. Persistent diarrhea with negative stool studies may require additional specimens, a different method, or examination of duodenal or biopsy material.^{4141 Shane AL, et al. 2017 Infectious Diseases Society of America clinical practice… 2017.} At the bench, check which organisms the ordered test can detect before you report a negative result.