Paragonimus westermani: Third Time's a Charm

Form and Function

Paragonimus westermani has four major forms that should be considered. The first is the miricidia (free-living larva), the second is the cercaria (or larval worm), the third is the metacercaria (encysted worm), and the final is the adult fluke.  Each form has unique characteristics and fascinating adaptations that allow it to thrive in each of the species’ several hosts.

   P. westermani miricidia

The miricidia is the smallest form. It hatches from the egg in an auquatic environment, and uses its cilia to swim to its first host, the freshwater snail (Liu et al. 2008). There is not much information about the specific physical features of P. westermani miricadia, but the larger group digenea miricidia have an anterior opening leading to penetration ducts within, and a posterior region that contains structures that will aid in asexual reproduction later in life (Animal Diversity Web 2012). 


    P. westermani cercaria



Once inside their first intermediate host (Paludomus snails), the miricidia grow into cercaria and are 174.5 um in length and 73.2 um in width on average. The cercaria is oval shaped, has a sucker on one end, and a flagellate tail for locomotion on the other (Iwagami et al. 2007).


P. westermani metacercaria



The metacercaria can live in several second intermediate hosts (freshwater crabs, crayfish), are oval shaped, can be thick or thin walled, and are larger than the cercariae (between 724 and 916 um in length, and between 598 and 771 um in width). The metacercariae can infect the muscles, hepatopancreas, and gills of the crabs (Rekah Devi et al. 2012). The metacercariae do not move, and once in the crab, they encyst (form a tough layer) around themselves. They each contain a single worm that will become the adult fluke in the definitive host (mammals). The metacercariae find their way to human (and other mammal) hosts when the mammal ingests the raw meat of infected crabs, boars, or sometimes (rarely) crayfish (Kuk-Na, et al. 2005).

Once inside the definitive mammal host, the metacercaria excysts (comes out of its cyst). The newly emerged metacercaria then does something very interesting. It begins secreting an enzyme that allows it to: evade the host immune system, intake nutrients from the host body, and dissolve the host tissue to aid in the movement of the metacercaria (Kuk-Na, et al. 2005). The metacercaria then begins traveling directly through the host intestines until it reaches the lungs. Once in the lungs, it forms a thick fibrous layer around itself, and begins developing into an adult fluke. (Rekha Devi et al. 2012).


The adult fluke is mostly flat and ovoid in shape, between ~6 -7mm in length, and between ~4-5 mm in width. The fluke has two suckers, one oral and P. wester adultone ventral (belly). The skin is covered in spines, and eggs laid by the fluke were dark brown in color (Rekha Devi et al. 2009). Perhaps the most interesting adaptation displayed by the adult fluke is its ability to thrive in both low oxygen and high oxygen environments. This adaptaion is necessary because (as previously stated) the fluke can end up not only in the lungs, but in many low-oxygen environments in the host body. The fluke can do this because it actually has three distinct populations of mitochondria! Each population serves to produce ATP most efficiently in different environments. (Takamiya et al, 2010). For another organism that employs INCREDIBLE adaptations to survive hostile environments, see this page.                                 P. westermani tegument             


OS: oral sucker CE: cecum OV: ovary TE: testes EB: excretory bladder
UT: uterus AC: acetabulum (ventral sucker)


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