Sunday, February 20, 2011

Paragonimus westermani - These suckers will crab on to your lungs!

Introduction: The common human lung fluke, Paragonimus westermani, was first discovered from the lungs of a tiger which died in Amsterdam Zoo. It infects millions in Southeast Asia and Japan. Millions are affected in these areas because of the consumption of raw seafood is very popular. Human infections (paragonimiasis) with the lung fluke are acquired by eating fresh crustaceans containing live metacercariae or consuming raw seafood that is carrying the parasite.

Symbiont Description: Over 30 species of flukes of the genus Paragonimus have infected animals and humans. The most common is Paragonimus westermani, the oriental lung fluke [1]. The fluke is bean shaped andyellowish when it is an egg and is a reddish-brown color and is 7.5 to 12mm by 4 to 6mm as an adult. The tegument if of the fluke is covered with scalelike spines. There are 2 suckers on this fluke, the oral sucker is located in the front and the ventral sucker is located slightly anterior of the center of the body. These suckers allow the parasite to hold on to tissue [2]. These parasites have lobed ovaries and testes. They are located parallelly in the posterior end of the body and the vitellaria is located laterally.

Host Description: Paragonimus westermani has the ability to affect carnivores such as felids, canids, rodents, weasels, pigs and humans. The definitive host can be any animal that consumes crustaceans, commonly humans. These parasites have 2 intermediate hosts, the first intermediate host is a snail and the second intermediate host is a crustacean (example: crab). These parasites have no vectors. The transmission of Paragonimus westermani to humans is commonly through the consumption of raw or undercooked seafood. Sometimes lung fluke larvae accidentally travel to the brain or other organs and reproduce there. But because the secretion of the eggs from the brain is blocked the life cycle will not happen. If the worm goes to the spinal cord instead of the lungs, the host might become paralyzed. If it infects the heart, the host could die [2]. Below we can take a look at the life cycle of Paragonimus westermani.

Life Cycle: [3]
  1. Eggs are released unembryonated in the sputum, or passed as stool
  2. The eggs become embryonated in the external environment
  3. Once the miracidia hatch, the miracidia look for the first intermediate host (a snail) and enters the snails soft tissues
  4. The miracidia inside the snail go through many stages of development: sporocysts, rediae, cercariae.
  5. The cercariae attack the second intermediate host (crab or crayfish). Here the cercariae encyst and become metabercariae
  6. Once a human ingests raw or undercooked seafood, the infection starts to occur
  7. The metacercariae excyst in the duodenum
  8. The metacercariae enters the lungs where they develop into adults
Ecology: P. westermani is disturbed in Southeast Asia and Japan. Consuming raw or undercooked seafood will lead to the disease, Paragonimiasis. Symptoms are coughing, chest pain, dyspenae, and dyspnea, hemoptysis, allergic reactions, and central nervous system abnormalities [4]. Infections can last for 20 years in humans.

Example of Paragonimus westermani: Of the four categories of parasites, Paragonimus westermani is a parasitee Type D parasite. Type D parasites are passive in that they are ingested by the host, often with the host’s food. They become mesoparaistes. Some of them might puncture the wall of the digestive tract to become endoparasites [5].


[5] Combes, Claude. "The Profession of Parasite." The Art of Being A Parasite. Chicago: University of Chicago, 2005. 51


  1. Why are this particular type of flukes fond of growing in lungs?

  2. You said the eggs are embryonated while in the external environment, does that mean that while here the male parasite fertilizes the egg or am I totally wrong?

  3. They either come as a pair of adults (live together for >10years) or they are hermaphrodite (posses both sex organs).

    They thrive in the lungs (respiratory system) as one of their transmission routes is via spit/sputum or being coughed out basically, and this is only possible if they are within lungs/trachea/oesophagus somewhere.

    Hope this helps, I'm doing a university phD in parasitology and came across this as i was revising :)