Parasitic Worms - Trematode Infections

Trematodes, or flukes are often refered to as flatworms. Most trematode infections of humans are acquired via the digestive tract.

Trematodes can range in size from 0.16 mm to 5.7 cm in length. They are usually oval in shape, bilaterally symetrical and flattened dorsal-ventrally. They typically possess an oral sucker leading to a digestive tract and sometimes a ventral sucker which is used for attachment to the host. These suckers are sometimes decorated with spines and small crater like depressions called papillae. Trematodes have a rudimentary gut, but usually no anus and a simple nervous system. Most trematodes are hermaphroditic and are often capable of self-fertilization.



Trematode eggs typically possess a lid or operculum that can easily be seen in this Paragonimus egg (Lung worm). The egg measures about 80-110 x 50-60 microns.
The life cycle of typical trematode worms is illustrated by that of Paragonimus westermani commonly known as the oriental lung worm. This infection is found primarily in isolated areas of the far East, occasionally in West Africa and Central and South America.


Adult worms are found in pairs located in a cystic cavity in the lungs. Eggs are coughed up and usually swallowed, passing out in the feces. The eggs require water to hatch. A form of the worm called a miracidium hatches from the egg and proceeds to penetrate the body of a freshwater snail. In the snail the miracidium develops into a sporocyst, then rediae which finally give rise to many cercaria. The short-lived cercaria must infect a freshwater crab by penetrating and encysting in the crabs gills or muscles. This stage of the parasite is called a metacercaria and the crab is called an intermediate host.


If the crab is eaten raw, the metacercaria hatch in the intestine and the worm burrows from the intestines through the diaphram and into the lungs. Eggs are laid 5-6 weeks later and the cycle continues. Although these worms are hermaphroditic, two worms are necessary for fertilization to take place.

The clinical features of this infection resemble chronic bronchitis with a morning cough producing redish-brown mucous. The infection can be diagnosed by finding the trematode eggs in the feces. The infection can be prevented by simply cooking crabs prior to eating them (no sushi please). The infection can be treated with a drug called praziquantel (Biltricide).

Next time we will discuss Schistosomiasis, the most important trematode infection of humans.