Trematodes Overview Quiz

Study Notes

Trematodes are flatworms belonging to the phylum Platyhelminthes, which also includes cestodes (tapeworms).
Adult trematodes, often called flukes, can be found in various human organs including the intestinal tract, bile ducts, lungs, and blood.
Trematodes are unable to reproduce within their human host.
They are typically flat and leaf-like, with some species having a globular shape.
Their tegument (outer surface) may contain embedded spines.
Trematodes exhibit bilateral symmetry, meaning they have two distinct sides that are mirror images of each other.
They lack a coelom (body cavity).
Adults possess two suckers: an oral sucker for feeding and a ventral sucker for attachment to the host.
The excretory system functions via flame cells (protonephridium).
Most trematodes are hermaphroditic, meaning they possess both male and female reproductive organs (except schistosomes, which are dioecious).
Reproduction occurs through oviparity (laying eggs).
All trematodes have intricate aquatic life cycles involving a definitive host (often a human or other animal) and one or more intermediate hosts, which can include snails, fish, and crabs.
Transmission to humans can happen through ingestion of encysted metacercariae (free or embedded in the tissue of a second intermediate host) or through skin penetration by the cercarial form.
The nervous system of trematodes is modified for their parasitic lifestyle.

Paragonimiasis is a chronic disease caused by the trematode Paragonimus westermani.
The adult P. westermani resides in the lungs of its final host, causing symptoms like cough, chest pain, rusty sputum, and hemoptysis.
Infection occurs through the ingestion of raw crayfish and freshwater crabs, containing the metacercariae stage of the parasite.

Adult Worm: Monoecious (with both male and female reproductive organs); the reproductive organs are parallel, and the worm has two suckers.
Eggs: Golden brown, oval-shaped, with an operculum (lid); measuring 80-120 x 48-60 micrometers.
Metacercariae: Round or oval; diameter of 300-400 micrometers, with 1-3 layers of cyst wall.

The life cycle begins with adult worms residing in the lungs of a human host.
These worms release eggs into sputum, which are then expelled through coughing or swallowing.
Eggs are released into the environment via feces or sputum (if swallowed).
Eggs mature in water within 21 days, releasing miracidia, the first larval stage.
Miracidia infect a specific type of snail (the first intermediate host), where they develop into sporocysts.
Sporocysts produce cercariae, the second larval stage.
Cercariae leave the snail and encyst in the tissues of a second intermediate host, such as crabs or crayfish, forming metacercariae.
Infection of humans occurs when they eat raw or undercooked crabs or crayfish containing metacercariae.
Metacercariae excyst in the human intestine and penetrate the intestinal wall.
The worm travels via the bloodstream to the lungs, where they mature into adult stages within 3 months.
Adult worms lay eggs, and the cycle continues.