Covers the two medically important Opisthorchis liver flukes, their distribution in Southeast Asia and Europe with Central Asia, their morphology, life cycle through snail and fish hosts, and clinical effects. Then turns to Schistosoma blood flukes, their three human-infecting species and geographic ranges, dioecious biology, skin-penetrating cercariae, and organ damage from trapped eggs. Includes recap questions, four mnemonics contrasting the two genera, and several pictures.
Topic 89 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 4 sections, about 1,047 words (7 min of reading), with 10 flashcards.
In this note
Genus Opisthorchis. Genus Schistosoma.
Genus Opisthorchis
Genus Schistosoma
Mnemonics
Genus Opisthorchis
General Characteristics
The genus Opisthorchis includes parasitic flatworms that belong to the Class Trematoda and the Phylum Platyhelminthes.
Two major species of medical importance are:
Opisthorchis viverrini, which is found mainly in Southeast Asian countries such as Thailand, Laos, Cambodia, and Vietnam.
Opisthorchis felineus, which is found in parts of Europe and Central Asia, including Italy, Germany, Belarus, Russia, Kazakhstan, and Ukraine.
These two species are very similar in both structure and behavior.
Morphology
Adult Opisthorchis flukes are small and flattened, measuring between seven millimeters and twelve millimeters in length.
They inhabit the bile ducts of their definitive hosts.
Life Cycle
Adult worms lay eggs that are passed out of the host through feces.
These eggs are ingested by a snail, which serves as the first intermediate host.
Inside the snail, the parasite undergoes several stages of development:
The egg hatches into a miracidium.
The miracidium becomes a sporocyst, which develops into rediae, and eventually into cercariae.
The cercariae exit the snail and penetrate freshwater fish, which serve as the second intermediate host.
Inside the fish, the cercariae form metacercariae, which encyst in the muscles or beneath the scales.
These metacercariae represent the infective stage for mammals.
Mammals, including humans, cats, and dogs, become infected by eating raw or undercooked fish containing these metacercariae.
Once ingested, the parasites excyst in the duodenum and migrate into the bile ducts, where they mature into adults.
Clinical Aspects and Distribution
Most infections in humans are asymptomatic.
In some cases, pathology occurs due to inflammation and obstruction of the bile ducts, which may result in abdominal pain, liver dysfunction, or secondary bacterial infections.
The geographical range of opisthorchiasis overlaps with that of fascioliasis in some regions, particularly in parts of Europe and Asia.
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