Paragonimus Westermani

• Paragonimus westermani

  A lung fluke that causes Paragonimiasis/Pulmonary distomiasis/Oriental lung fluke infection
• Infection of P. westermani occurs in areas where local people eat improperly cooked crustaceans
• P. westermani is widely distributed in Africa and Asia and is the most important lung fluke among several in the genus Paragonimus capable of infecting man
• This disease is normally a zoonosis in some areas and man is infected accidentally
• Paragonimus westermani

  • Diagenetic Trematode
  • Infects the lungs of definitive host (humans)
  • Life span usually 6 years but can live up to 20 years
  • Most common oriental lung fluke to infect humans
  • Genus includes 45 species of lung fluke
  • Only 12 species infect man
• Species of Paragonimus

  • Paragonimus africanus
  • Paragonimus mexicanus
  • Paragonimus uterobilateralis
  • P. peruvianus
• Transmission of Paragonimus westermani

  1. Eating raw, undercooked or pickled crustaceans

  2. Infection occurs

  3. Larval fluke penetrates gut wall

  4. Enters peritoneal cavity

  5. Matures in lungs
• Culinary habits that involve raw crustaceans

  • Drunken crab in China and Philippines
  • Sushi crab in Japan
  • Gye muchim in Korea
• Marginal utility

  The additional utility (satisfaction) gained from the consumption of an additional product
• Adult Paragonimus

  • 7-12 mm long
  • Thick and fleshy
  • Reddish-brown
  • Hermaphroditic with oral and ventral suckers
• Paragonimus ovum


  Yellowish-brown in colour, ovoid, thick shelled and bears flattened operculum
• The egg of Paragonimus is asymmetrical in shape and slightly flattened on one side
• The shell of Paragonimus ovum is thicker at the end opposite the operculum
• Life cycle of Paragonimus westermani

  1. Eggs coughed up or passed in faeces

  2. Egg reaches water

  3. Hatches after 3 days

  4. Releases ciliated miracidium

  5. Miracidium enters freshwater snails
• Life cycle continuation

  1. Miracidium develops into sporocyst

  2. Sporocyst becomes rediae

  3. Mature rediae replicate into cercariae

  4. Cercariae encyst as metacercariae in crustaceans

  5. Man is infected by swallowing metacercariae
• Pathogenesis of Paragonimus

  • Pulmonary or Extra-pulmonary
  • Causes inflammatory reactions
  • Cysts filled with thick brownish fluid
  • Can lead to broncho-pneumonia
• Adult worms can lay their eggs in the brain
• Initial invasion has little pathological effect on the host
• Clinical manifestations of Paragonimiasis

  • Acute phase
  • Chronic phase
  • Pulmonary type
  • Abdominal type
  • Brain type
• Diagnosis is done by detecting eggs in sputum or stool
• Paragonimiasis should be suspected in patients diagnosed as having pulmonary tuberculosis who have no acid-fast bacilli in sputum smear
• Treatment for Paragonimus

  Praziquantel is the drug of choice
• Prevention includes avoiding eating raw, undercooked or pickled crabs or crayfish
• Detect and treat infected persons in endemic areas
• Identify and destroy the snail hosts of local Paragonimus species where this is possible
• Treat the sewage system
• Life cycle

  1. Large operculate eggs are either coughed up or passed in faeces of definitive host

  2. Egg reaches water, hatches after 3 days and releases ciliated miracidium some weeks later

  3. Miracidium enters body of some freshwater snails

  4. Miracidium develops into sporocyst in the snail and then becomes a rediae

  5. Mature rediae replicate into cercariae

  6. Cercariae encyst as metacercariae in various freshwater crustaceans, mainly crayfish and crabs

  7. Metacercariae may be found in gills, muscles and liver of the crustaceans where they encyst

  8. Man is infected by swallowing metacercariae in raw or slightly prickled crustaceans

  9. Metacercariae excyst in duodenum, and larval fluke rapidly penetrate gut wall and enters the peritoneal cavity
• paragonimiasis can be acquired through ingestion of raw or undercooked crabs or snails that contain metacercariae
• paragonimiasis is caused by paragonimus westermani (lung fluke)
• the larvae are found in the lungs, liver, pleura, pericardium, and abdominal cavity
• the life cycle of paragonimus westermani involves an intermediate host such as crab or crayfish
• symptoms of paragonimiasis include coughing up blood, chest pain, fever, weight loss, and abdominal pain.