16.2 Intestinal/Urogenital Protozoa and Helminths

16.2 Intestinal/Urogenital Protozoa and Helminths

Quick Answer: Cryptosporidium oocysts (about 4–6 µm) stain on modified acid-fast smears. Separate E. histolytica (compact central karyosome, even peripheral chromatin, up to 4 cyst nuclei, ingested RBCs) from E. coli (blotchy eccentric karyosome, coarse chromatin, up to 8 nuclei). Giardia trophozoites show falling-leaf motility and a face-like pair of nuclei; the cyst has four nuclei. Trichomonas is a motile trophozoite with no cyst stage. Helminth keys: Enterobius cellulose-tape eggs, hookworm thin-shelled eggs, Ascaris mammillated eggs, Trichuris barrel-shaped eggs with bipolar plugs, Schistosoma spine location (lateral S. mansoni, terminal S. haematobium), Strongyloides larvae with autoinfection, Taenia striated eggs, Diphyllobothrium operculated eggs plus B12 deficiency, and Trichinella muscle nurse cells. Filaria and flukes are survey-level. Parasite culture is SM-only.

Official outline III.C.2 names intestinal and urogenital protozoa and intestinal and tissue helminths, plus additional parasites (filaria, flukes) at example level. Methods that prove the morphology (trichrome, modified AF, tape, Knott) are taught as identification here and as procedure in 16.3. Culture of Trichomonas or Strongyloides is SM-only and is not an M-tested identification method.

Intestinal and urogenital protozoa

Stool is the default source. Add a genital wet mount for Trichomonas and a modified acid-fast smear or antigen/NAAT when coccidia are in the differential. Respiratory specimens appear when Strongyloides hyperinfection or Cryptosporidium in AIDS produces pulmonary findings, but those are not the first-line stools of a routine O&P.

Cryptosporidium is a coccidian, not an ameba. Oocysts are small (about 4–6 µm), round, and stain red on modified acid-fast (or appear unstained ghosts on some trichrome smears). They are immediately infectious, survive chlorinated water, and cause outbreak diarrhea from pools and municipal water. Disease is self-limited in immunocompetent hosts and chronic and voluminous in AIDS. Do not look for a karyosome; this is not Entamoeba. Size-split from Cyclospora (about 8–10 µm, also modified AF, often autofluorescent) so you do not call every red ball crypto. Antigen EIA and NAAT (16.3) are more sensitive than smear alone.

Entamoeba histolytica trophozoites have a small compact central karyosome and even, fine peripheral chromatin on Wheatley trichrome. Cytoplasm may contain ingested RBCs — the finding that supports true E. histolytica rather than morphologically identical E. dispar. Cysts have up to four nuclei and chromatoid bodies with rounded, cigar-shaped ends. Disease is invasive intestinal amebiasis (flask-shaped ulcers, dysentery) and extraintestinal liver abscess. E. coli is the commensal trap: trophozoites have a large eccentric, blotchy karyosome, coarse uneven peripheral chromatin, and sluggish motility; cysts have up to eight nuclei and splinter-shaped chromatoids. Calling E. coli E. histolytica over-treats a nonpathogen; missing ingested RBCs under-calls invasive amebiasis. Poorly fixed stool yields pale nuclei you cannot speciate — re-collect rather than guess.

Giardia duodenalis (also called G. lamblia or G. intestinalis) colonizes the duodenum and proximal jejunum. The trophozoite is pear-shaped with a ventral sucking disk, two nuclei, axonemes, and median bodies — the old-man face. On a wet mount it shows falling-leaf (leaf-like, tumbling) motility. The cyst is oval with four nuclei and is the form that survives in water and on hands. Stools are greasy and foul; trophozoites may be intermittent, which is why antigen/NAAT outperform a single O&P. Do not confuse falling-leaf motility with the jerky corkscrew motility of Trichomonas.

Trichomonas vaginalis is a urogenital flagellate. The trophozoite has an undulating membrane, four anterior flagella, and jerky, nondirectional motility on a warm saline wet mount of vaginal or urethral discharge. There is no cyst stage, so the organism does not persist in the environment the way Giardia cysts do. A delayed, cooled wet mount loses motility and becomes a low-sensitivity test; NAAT is more sensitive (16.3). Do not order a stool O&P to diagnose trichomoniasis, and do not use culture as the M-tested method — Trichomonas culture is SM-only.

Intestinal and tissue helminths

Eggs, larvae, proglottids, and (for Trichinella) muscle larvae are the diagnostic forms. Adults are occasionally submitted as macroscopic worms.

Ascaris is the largest intestinal nematode. Fertile eggs are oval with a thick mammillated coat. Larvae migrate through the lungs (eosinophilic Loeffler pneumonia) before returning to the gut; a bolus of adults can obstruct bowel or biliary tree. Eggs are abundant on a concentrated O&P.

Enterobius (pinworm) females migrate out the anus at night and glue eggs to perianal skin. The collection is a cellulose-tape preparation first thing in the morning, not a random stool. Eggs are oval with one flattened side (D-shaped). A negative stool O&P does not exclude pinworm.

Hookworm eggs are thin-shelled and already cleaved when passed. Filariform larvae in soil penetrate skin (ground itch). Chronic infection drains blood and causes iron-deficiency anemia. Do not confuse hookworm rhabditiform larvae (if eggs hatch in an old stool) with Strongyloides: hookworm has a long buccal capsule and an inconspicuous genital primordium.

Strongyloides is the autoinfection nematode. Diagnostic stool forms are larvae, not eggs. Rhabditiform larvae have a short buccal capsule and a prominent genital primordium. Infective filariform larvae have a notched tail and can penetrate perianal skin or the gut wall, completing the cycle inside one host. Autoinfection explains decades-long infection and hyperinfection when steroids or HTLV-1 collapse immunity — then larvae appear in sputum and Gram-negative bacteremia follows gut translocation. Agar-plate or charcoal culture is used in some reference labs but is SM-only as tested content; for M, identify the larva and name autoinfection.

Trichuris (whipworm) eggs are football- or barrel-shaped with bipolar plugs. Heavy burdens cause colitis and rectal prolapse in children. They are stool eggs, not tape eggs.

Schistosoma eggs are the speciation. Cercariae from freshwater snails penetrate skin. S. mansoni: large lateral spine, recovered in stool (inferior mesenteric / colon). S. haematobium: terminal spine, recovered in urine (especially terminal urine) and linked to bladder disease and squamous-cell bladder cancer. S. japonicum: small rudimentary spine, stool, more eggs per female. Spine location plus specimen (stool versus urine) is the exam item.

Taenia eggs are spherical with a radially striated embryophore and are not speciated by the egg. T. saginata (beef) has an unarmed scolex and many uterine branches (typically more than 15) in gravid proglottids. T. solium (pork) has an armed scolex and fewer branches (typically fewer than 13). Ingesting T. solium eggs (not cysts in meat) causes cysticercosis, including neurocysticercosis — a tissue larval disease, not a stool-egg problem.

Diphyllobothrium latum (broad fish tapeworm) eggs are operculated, unembryonated, and lack a spine — do not call them Schistosoma. The worm is the longest human tapeworm and competes for vitamin B12, producing megaloblastic anemia. Source is undercooked freshwater fish.

Trichinella is not diagnosed on stool. Encysted larvae in skeletal muscle nurse cells follow ingestion of undercooked pork, bear, or other game. Clinical clues are fever, myalgia, periorbital edema, and marked eosinophilia. Serology and muscle biopsy, not O&P, are the tests.

Additional parasites at survey level

Filariae: Wuchereria bancrofti and Brugia microfilariae circulate in blood with nocturnal periodicity; use Knott concentration or thick films collected at night (16.3). Loa loa is diurnal with Calabar swellings. Onchocerca volvulus is diagnosed on skin snips, not blood, and causes river blindness. Vectors are mosquitoes, Chrysops, and blackflies respectively — arthropod details in 16.3.

Flukes (trematodes): Clonorchis/Opisthorchis small operculated eggs from raw fish, bile-duct disease; Fasciola large operculated eggs, watercress, liver; Paragonimus operculated eggs in sputum, crayfish, mimics TB; Fasciolopsis intestinal. Operculated eggs without a spine are flukes or Diphyllobothrium, not schistosomes.

Do not spend M study time on parasite culture protocols for Trichomonas or Strongyloides. Those bullets are SM-only on the 2025-09-25 guideline.