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.

Paragonimus westermani egg/CDC
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”.
  • 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
  • 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
  • The presence of eggs in sputum or stool.
  • Serodiagnosis
  • Praziquantel is the drug of choice
  • Bithionol is alternate treatment
  • 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
  • Stress the importance of fully cooking crustaceans
  • Proper disposal of sputum and feces
Paragonimus westermani adult/CDC