Trematodes – Finals Review

80 vocabulary flashcards summarizing key trematode species, life-cycle stages, morphology, clinical features, diagnosis, and treatment to aid exam preparation.

Trematodes

Unsegmented, flat, leaf-like helminths with two muscular suckers and a well-developed hermaphroditic reproductive system (except in schistosomes).

Anterior muscular organ of flukes used for attachment and ingestion.

Oral sucker

Ventral sucker (acetabulum)

Ventral muscular attachment organ of flukes situated midway on the body.

Gonotyl

Additional genital sucker found in schistosomes and Heterophyes heterophyes.

Miracidium

Ciliated, free-swimming larva that hatches from the egg and infects the snail.

Sporocyst

Asexual larval stage inside the snail that produces rediae or daughter sporocysts.

Redia

Larval stage in snail tissue that produces cercariae by asexual reproduction.

Cercaria

Free-swimming larva with a tail; infective for humans in schistosome life cycles.

Metacercaria

Encysted larval stage on plants, fish, crabs, etc.; infective for most non-schistosome flukes.

Oviparous

Term describing organisms that lay eggs; all trematodes are oviparous.

Operculated egg

Egg with a lid-like operculum; typical for trematodes except schistosomes.

Definitive host

Host in which the adult, sexually mature fluke lives (usually humans or mammals).

Intermediate host

Host in which larval or asexual stages occur; always a freshwater snail for trematodes.

First intermediate host

Freshwater snail species that harbors miracidia, sporocyst, redia and cercaria stages.

Second intermediate host

Plant or animal that carries metacercariae (crabs, fish, aquatic plants, etc.).

Blood flukes

Schistosomes that reside in mesenteric or pelvic veins.

Lung fluke

Paragonimus westermani, which encysts in lung parenchyma.

Intestinal flukes

Group including Fasciolopsis buski, Echinostoma ilocanum, Heterophyes heterophyes, etc.

Liver flukes

Fasciola spp., Clonorchis sinensis, and Opisthorchis felineus that inhabit biliary ducts.

Schistosome

Dioecious blood fluke causing schistosomiasis; sexes are separate.

Gynecophoric canal

Long groove on male schistosome that holds the slender female worm.

Schistosoma haematobium

Blood fluke of bladder plexus; terminal-spined eggs passed in urine; causes urinary schistosomiasis.

S. haematobium egg

Ovoid, non-operculated egg ~150×50 µm with large terminal spine.

S. haematobium snail host

Bulinus species freshwater snails.

Schistosoma mansoni

Blood fluke of inferior mesenteric veins; eggs bear prominent lateral spine.

S. mansoni egg

Yellow-brown, non-operculated egg 112-182 µm long with large lateral spine.

S. mansoni snail host

Biomphalaria species freshwater snails.

Schistosoma japonicum

Oriental blood fluke of superior mesenteric veins; smallest eggs with minute lateral spine.

S. japonicum egg

Round/oval 50-85 µm egg with tiny lateral spine; most fertile of schistosomes.

S. japonicum snail host

Oncomelania species amphibious snails.

Cercarial dermatitis (swimmer’s itch)

Pruritic papular rash from penetration of cercariae into the skin.

Katayama fever

Acute systemic reaction during early egg laying of schistosomes (especially S. japonicum).

Terminal hematuria

Painless blood at end of micturition, classic sign of S. haematobium infection.

Praziquantel

Broad-spectrum antitrematode drug; first-line therapy for most fluke infections.

Paragonimus westermani

Oriental lung fluke acquired from undercooked freshwater crabs/crayfish; causes paragonimiasis.

Paragonimiasis

Pulmonary disease with chronic cough, hemoptysis; may be cerebral if ectopic.

Paragonimus egg

80-118 × 48-60 µm operculated egg with shoulders and terminal shell thickening.

Paragonimus metacercaria

Encysted larva in crab or crayfish musculature; infective to humans.

Triclabendazole (Paragonimus)

Effective alternative drug for human paragonimiasis.

Fasciolopsis buski

Giant intestinal fluke of humans/pigs; infection from aquatic plants carrying metacercariae.

Fasciolopsiasis

Intestinal disease; heavy loads may cause obstruction, edema, and intoxication.

Fasciolopsis egg

Large 130-140 × 80-85 µm operculated egg resembling Fasciola eggs.

F. buski intermediate hosts

Segmentina/Hippeutis snails (1st IH) and aquatic plants (vehicle for metacercariae).

Echinostoma ilocanum

Garrison’s fluke with crown of circumoral spines; acquired from raw snails/fish.

Echinostomiasis

Intestinal fluke infection causing abdominal pain, diarrhea, ulceration.

Heterophyes heterophyes

Tiny intestinal fluke with gonotyl; infection via raw freshwater fish.

Heterophyiasis

Mild to severe intestinal infection; ectopic eggs may cause cardiac or neurologic disease.

Clonorchis sinensis

Chinese liver fluke living in bile ducts; transmitted by undercooked freshwater fish.

Clonorchiasis

Chronic biliary infection leading to cholangitis, gallstones, and cholangiocarcinoma.

Clonorchis egg

Small 30 × 15 µm operculated egg with pronounced shoulders and terminal knob.

Fasciola hepatica

Sheep/temperate liver fluke; adults measure 18-51 mm and have pronounced shoulders.

Fasciola gigantica

Tropical liver fluke; larger (25-75 mm) with less developed shoulders.

Fasciola metacercaria

Encysted larva on aquatic vegetation or in water; infective when ingested.

Fascioliasis acute stage

Invasive hepatic phase causing fever, hepatomegaly, right upper quadrant pain, eosinophilia.

Fascioliasis chronic stage

Biliary obstruction, fibrosis, cirrhosis, and possible pancreatitis due to adult flukes.

Triclabendazole (Fasciola)

Drug of choice for fascioliasis.

Opercular shoulders

Thickened rim around operculum seen in eggs of Clonorchis and Paragonimus.

Katayama disease

Oriental schistosomiasis caused by S. japonicum; severe systemic manifestations.

Snail hosts of schistosomes

Bulinus (S. haematobium), Biomphalaria (S. mansoni), Oncomelania (S. japonicum).

Schistosome diagnostic stage

Characteristic eggs in urine (S. haematobium) or feces (S. mansoni, S. japonicum).

CAA / CCA tests

Detection of circulating anodic and cathodic antigens for schistosome diagnosis.

Kato-Katz thick smear

Quantitative stool technique for detecting S. mansoni and other fluke eggs.

LAMP test

Loop-mediated isothermal amplification assay for detecting Paragonimus DNA.

FAST-ELISA

Falcon assay screening test for anti-S. haematobium antibodies (using HAMA antigen).

Oxamniquine

Alternative drug effective mainly against S. mansoni infections.

Metrifonate

Organophosphate alternative therapy for urinary schistosomiasis (S. haematobium).

Fairley’s intradermal test

Obsolete skin test for delayed hypersensitivity to S. haematobium antigen.

Flame cells

Excretory structures of trematodes connected to collecting tubules and bladder.

Asexual snail development

Miracidium → sporocyst (1° & 2°) → redia → cercaria sequence inside snail.

Miracidial penetration

Active invasion of snail tissue by ciliated larvae shortly after egg hatching.

Cercarial longevity

Free-swimming cercariae survive 1–3 days in fresh water while seeking a host.

Schistosomulum

Tail-less cercarial form that enters circulation after skin penetration.

Pulmonary distomiasis

Older term for pulmonary paragonimiasis caused by P. westermani.

Sandy patch

Granulomatous bladder lesion seen in chronic S. haematobium infection.

Clay pipe-stem fibrosis

Periportal fibrosis of the liver in S. japonicum infection.

Operculated egg absence

Schistosome eggs lack an operculum, unlike most other trematodes.

Metacercarial excystation

Release of juvenile fluke in duodenum after ingestion of metacercariae.