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Nematodes, cestodes and trematodes

16 min

Skills you'll build3 skills
  • Differentiate intestinal nematodes using the observed egg or larval stage
  • Identify Taenia species from segments, scolex, or molecular testing
  • Match each trematode egg to the specimen it appears in, such as urine or stool

Try first

Try first

Stool is sent to look for Strongyloides stercoralis before a patient starts high-dose corticosteroids. Which stage does the laboratory look for?

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Strongyloides larvae in stool

Most intestinal nematodes are recognized by their eggs in a concentrated sediment. Strongyloides stercoralis eggs hatch in the intestinal mucosa, so stool contains rhabditiform larvae and rarely eggs. Routine O&P microscopy misses most uncomplicated infections. Serial fresh stools examined by Baermann sedimentation or agar plate culture recover more larvae, and formalin-ethyl acetate processing can remove them. Agar plates can produce infective filariform larvae, so they are kept sealed and handled with exposure precautions.1

The rhabditiform larvae of Strongyloides and hookworm are distinguished by the buccal canal and the genital primordium. Strongyloides has a short buccal canal and a prominent genital primordium. Hookworm has a long buccal canal and an inconspicuous genital primordium. Hookworm larvae in stool usually indicate that the eggs hatched during delayed processing or culture.2,3

Taenia eggs

Eggs of Taenia saginata, T. solium and T. asiatica look the same. Each is about 30 to 40 µm, with a thick, radially striated shell around a 6-hooked oncosphere, so the report says Taenia species. A gravid proglottid separates the two common species by its primary uterine branches per side, 12 to 30 in T. saginata and 7 to 13 in T. solium. Molecular testing also identifies the species. Until then, unfixed specimens are handled as if they contain T. solium eggs, which are immediately infectious.4

Trematode eggs and specimens

EggDistinguishing featuresSpecimen
Schistosoma mansoni114 to 180 µm, prominent lateral spineStool
S. japonicum70 to 100 µm, small inconspicuous lateral spineStool
S. haematobium110 to 170 µm, terminal spineUrine collected from noon to 3 PM
Paragonimus68 to 118 µm, thick-shelled and operculatedSputum, or stool when swallowed
Fasciola hepaticaAbout 130 to 150 µm, operculatedStool, duodenal fluid or bile

Most trematode eggs are operculated. Schistosome eggs have no operculum and have a spine. When exposure could involve both intestinal and urinary schistosomes, the laboratory examines stool and urine.3,5

Follow the bench demonstration

Sources for this step
References
  1. Centers for Disease Control and Prevention. DPDx: Strongyloidiasis. Accessed September 27, 2026. https://www.cdc.gov/dpdx/strongyloidiasis/index.html
  2. Centers for Disease Control and Prevention. DPDx: Intestinal hookworm. Accessed September 27, 2026. https://www.cdc.gov/dpdx/hookworm/index.html
  3. Centers for Disease Control and Prevention. DPDx: Intestinal parasites, comparative morphology tables. Accessed September 27, 2026. https://www.cdc.gov/dpdx/diagnosticprocedures/stool/morphcomp.html
  4. Centers for Disease Control and Prevention. DPDx: Taeniasis. Accessed September 27, 2026. https://www.cdc.gov/dpdx/taeniasis/index.html
  5. Centers for Disease Control and Prevention. DPDx: Schistosomiasis infection. Accessed September 27, 2026. https://www.cdc.gov/dpdx/schistosomiasis/index.html

Watch one

Read each decision and its explanation.

Fresh stool from a man about to start corticosteroids was set up on an agar plate culture for Strongyloides. At 48 hours the plate shows larval tracks. The larvae recovered from it have a long buccal canal and an inconspicuous genital primordium. The O&P concentrate from the same stool showed thin-shelled eggs about 65 by 38 µm with segmented contents. Which nematode is present?

  1. Keep the plate sealed and handle the larvae with the exposure precautions in the procedure.

    Culture plates can produce infective filariform larvae, and the larvae can penetrate skin.

  2. Read the buccal canal: long.

    A long buccal canal is the hookworm pattern. Strongyloides rhabditiform larvae have a short one.

  3. Read the genital primordium: inconspicuous.

    Strongyloides larvae have a prominent genital primordium. Hookworm larvae have an inconspicuous one.

  4. Compare the larvae with the concentrate: hookworm eggs.

    Thin-shelled, segmented eggs of 60 to 75 by 35 to 40 µm are hookworm eggs, and those eggs hatch on a culture plate.

  5. Note the negative finding: no Strongyloides larvae on this single culture.

    Routine methods have low sensitivity for Strongyloides, and serial fresh stools improve recovery.

Hookworm is present. The larvae hatched from hookworm eggs on the plate. No Strongyloides larvae were seen, and one negative culture does not exclude it.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A fresh stool wet mount shows rhabditiform larvae with a short buccal canal and a prominent genital primordium. What does this finding indicate?

Correct. A short buccal canal and a prominent genital primordium mark a Strongyloides rhabditiform larva. Its eggs hatch in the intestinal mucosa, so fresh stool contains larvae and rarely eggs.

Incorrect. Hookworm larvae have a long buccal canal and an inconspicuous genital primordium, the reverse of this pattern.

Incorrect. A short buccal canal and a prominent genital primordium are the features of a Strongyloides rhabditiform larva, so the morphology indicates infection. Contamination with free-living nematodes is considered only when the collection history suggests contact with soil or water.

Hint
  1. Compare the two features the question names with the hookworm larva.
  2. Think about where this nematode's eggs hatch.

Review Intestinal nematodes

Problem 2 of 3 · Hints available

An O&P requisition asks to rule out Schistosoma haematobium in a traveler. Which specimen does the laboratory need?

S. haematobium passes its terminal-spined eggs in urine, so a stool examination misses it.

Searched stool for Schistosoma haematobium eggs

S. haematobium passes its terminal-spined eggs in urine, so a stool search misses it. Urine collected between noon and 3 PM gives the best recovery.

The eggs are in urine, and a collection from noon to 3 PM recovers more of them.

Schistosome eggs are not found in blood films.

S. haematobium eggs pass in urine. A midday collection, examined as centrifuged sediment or by membrane filtration, gives the best recovery.

Hint
  1. Consider how each schistosome's eggs are passed.
  2. Timing of the collection affects recovery for this species.

Review Trematodes

Problem 3 of 3 · On your own

The formalin concentrate from a stool shows round eggs about 35 µm with a thick, radially striated shell and 6 hooklets inside. What does the laboratory report?

Eggs of the human Taenia species look the same. A species named from the egg can miss T. solium, whose eggs are immediately infectious.

Assigned a Taenia species from an indistinguishable egg

Eggs of the human Taenia species and other taeniids look the same, so an egg cannot identify T. solium or T. saginata. Naming a species from the egg can miss T. solium, whose eggs are immediately infectious. The specimen is handled as T. solium until segments, scolex, or molecular testing identify the species.

H. nana eggs are thin and colorless, with polar thickenings and 4 to 8 polar filaments.

The striated shell and 6-hooked oncosphere fit Taenia. The egg identifies only the genus, and a gravid proglottid's uterine branches or molecular testing identifies the species.

Diphyllobothriid eggs are 55 to 75 µm, unembryonated and operculated.

Review Cestodes

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Kofi Lott, 29, MRN 3308461, is a hydrologist home from 8 months of lake fieldwork in southern Africa. He describes it as mostly swimming with notebooks.
  • The order is a stool O&P only.
  • The formalin concentrate shows eggs about 140 by 60 µm with a prominent lateral spine.
  • The same sediment shows a few round eggs about 35 µm with a thick, radially striated shell.
Decision 1 of 3

What are the large eggs?

S. haematobium eggs have a terminal spine and pass mainly in urine.

Fasciola eggs are operculated and have no spine.

An egg of 114 to 180 µm with a prominent lateral spine, found in stool, is S. mansoni.

S. japonicum eggs are smaller, 70 to 100 µm, with a small inconspicuous spine.

Review Trematodes

Decision 2 of 3

The requisition names only stool. What else does his exposure require?

S. haematobium passes its eggs in urine. A stool examination misses it.

Searched stool for Schistosoma haematobium eggs

S. haematobium passes its terminal-spined eggs in urine, so a stool search misses it. Urine collected between noon and 3 PM gives the best recovery.

African freshwater exposure can involve intestinal and urinary species, so stool and urine are both examined.

Repeat stools improve detection of intestinal schistosomiasis. They do not recover eggs passed in urine.

Review Trematodes

Decision 3 of 3

How are the round eggs reported?

The egg identifies the genus only. Any unfixed portion of the stool is handled as if it contains infectious T. solium eggs until the species is known.

The egg cannot separate T. saginata from T. solium.

Assigned a Taenia species from an indistinguishable egg

Eggs of the human Taenia species and other taeniids look the same, so an egg cannot identify T. solium or T. saginata. Naming a species from the egg can miss T. solium, whose eggs are immediately infectious. The specimen is handled as T. solium until segments, scolex, or molecular testing identify the species.

The egg cannot identify T. solium either. A proglottid or molecular testing is needed.

Review Cestodes

The workup depends on the spine position and his exposure. A lateral spine in stool is S. mansoni. Southern African lake water can also transmit S. haematobium, whose terminal-spined eggs are passed in urine, so a midday urine is added to the workup. The Taenia eggs are reported to the genus.

For reference

Sources checked