Paragonimus westermani

Paragonimus westermani egg/CDC

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

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

Prevention

• Stress the importance of fully cooking crustaceans
• Proper disposal of sputum and feces

Paragonimus westermani adult/CDC