Paragonimus westermani
Parasitology 101 is an educational blog that that can be used as a study guide for microbiology, infectious disease and medical technology students. The “bullet-point” format keeps the information concise and to the point.
General Information
There are numerous species in the genus Paragonimus that are found worldwide; primarily in Asia, but also Africa (P.africanus and P. uterobilteralis) and South America (P. peruvianus)
P. kellicotti is found in North America
Paragonimus westermani (most common)is also known as the “Oriental Lung Fluke”.
Geography
Found throughout Asia
China is the major endemic area with 20 million people infected
Also seen with low prevalence in Japan, Korea, the Philippines, Thailand
Morphology (adults)
Large, plump flukes
7-16 mm length x 4-8 mm width x up to 5 mm thick
Found in pairs in the lungs; looks like coffee beans
Morphology (eggs)
Large, thick-shelled
large, flattened operculum
80-120 um x 50-65 um
Life Cycle
Unembryonated eggs passed into lung tissue and excreted in sputum; or swallowed and excreted in the feces
Eggs embryonate in the water and miracidia emerge from egg and search for specific species of snail (first intermediate host)
In the snail, miracidia goes transforms-sporocyst> rediae> cercariae
Cercariae leaves snail and invades second intermediate host, a crustacean, typically a crab or crayfish
The cercariae encysts into metacercariae (infective stage for human or animal host)
Human infected after eating inadequately cooked crustacean
Metacercariae excysts in small intestine and penetrates the gut wall
Peritoneal cavity> abdominal wall> diaphragm> lungs
Develop into adults in the lungs
Prepatent period is 2-3 months
Infection can last for years to decades
Pathology
Migration of young worms (acute)may produce diarrhea and abdominal pain
When established in the lungs: Fever, cough, rusty-brown sputum, hemoptysis (often thought to be tuberculosis at first)
Migration of worms to other organs(erratic paragonimiasis) can be very severe, particularly when brain is involved
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Diagnosis
The presence of eggs in sputum or stool.
Serodiagnosis
Treatment
Praziquantel is the drug of choice
Bithionol is alternate treatment
Epidemiology
Humans or other mammals are definitive host
Humans get infected eating undercooked crabs or crayfish
China- “Drunken crabs” soaked in rice wine
Philippines- roast crabs
Thailand- raw crayfish salad
Korea- soak crab in soy sauce
Use of crab juice for seasoning and medicinal purposes
Africa- undercooked crabs for fertility
Pickled crabs
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Prevention
Stress the importance of fully cooking crustaceans
Proper disposal of sputum and feces