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Darkfield Digital Image Gallery

Trematode Flukes (Echinostoma revolutum)

The digenetic trematode Echinostoma revolutum is an endoparasitic helminth having a wide variety of hosts including ducks, geese, pigeons, chickens, and humans. With part of its life history spent in water bodies, these members of the phylum Platyhelminthes (flatworms) use freshwater snails as their primary hosts and frogs or snails as their secondary intermediate hosts.

View a second image of a trematode fluke.

Humans are usually infected by ingesting infected raw snails or frogs serving as hosts of the parasites, which are found throughout North America. Prevention of the parasitism by this trematode in humans is easily accomplished, but infestations are much harder to control in avian hosts. Although it is difficult to prevent free-roaming birds from eating raw snails and frogs, it is possible to treat the freshwater habitats with molluscides since the first parasitic stage is dependent on snails. After two asexual generations in the snail, the cercariae emerge from the snail and encyst in a second snail or a tadpole, developing into metacercariae. The cercariae find the snail hosts via chemotaxis by following the increasing gradients of small peptides associated with snail slime. The first larval stage, the miracidium, seems to be attracted to macromolecular glycoconjugates associated with potential snail hosts. In addition, they use environmental stimuli associated with gravity and light in a complex searching pattern. The chemotactic responses of free-living larval trematodes seeking their hosts may lead to clues for the development of larval traps to reduce outbreaks.

In humans, clinical signs of this type of fluke parasitism are emaciation and weakness, and with heavy infestations, possible hemorrhagic enteritis is exhibited. With human parasite loads, benzimidazoles, particularly albendazole and praziquantel are effective when prescribed against E. revolutum. Diagnosis is usually confirmed by finding parasite eggs in the feces of avian or human hosts and continuation of the life cycle is halted if fecal matter is kept from water bodies.

The adult trematode uses an oral and a ventral sucker to hold onto the lining of the bird cecum or human rectum. As hermaphrodites, each mature parasitic flatworm contains an ovary, with a uterus, eggs, and a pair of testes. The generic name is derived from the Greek roots for spiny (echino) and mouth (stoma) as they are characterized by a collar of spines (papillae) around their anterior ends. As with other members of the flatworm phylum, these parasitic flukes cannot biosynthesize their own fatty acids and sterols, and thus depend on parasitism to complete their nutritional needs.

Contributing Authors

Cynthia D. Kelly, Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.


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