Paragonimus westermani: Third Time's a Charm


As Paragonimus westermani is a parasite, it interacts with several different species across several different phyla, usually to that species’ detriment. The dynamic of the interaction is invariably that of parasite/host. P. westermani has three major forms, and each form is associated with a different species or group of species as the host. In order of P. westermani’s lifecycle, I will list the host species and the parasite’s effect on them.

First, the snail....
The first form of P. westermani is called the miracidia, or ciliated free-living larval form. Once hatched from its egg, the miracidia swims to its first host, the freshwater snail. The miracidia infects the snail, and begins growing into a cercaria (Liu et al. 2008).

Freshwater snailThe cercaria grow inside a group of snail species in the genus Paludomus. These snails are P. westermani’s first intermediate host. The cercaria infest the snails’ reproductive organs, and they do so at a rate of approximately .2 percent. (Iwagami et al. 2007). After some time, the cercariae emerge from the snail, or the snail is eaten by a predator. Cercariae emergence from an infected snail significantly increases the snail’s mortality (Koprivnikar et al. 2011). There are several different second intermediate hosts of P. westermani, which we will cover next.



Then, the crab.....
After emerging from or being eaten while inside of their snail host, the cercaria can “set up shop” in several different organisms. The most common is likely the freshwater crab. In these crabs, the cercaria will encyst (make a tough layer around) itself while it begins to develop into an adult fluke. Once encysted, the cercaria is called a metacercaria. The metacercaria can be found in the crab’s muscles, hepatopancreas, and gills (Rekha Devi et al. 2012). Another candidate for the cercaria to infect are crayfish, which led to human cases of paragonimiasis in the United States after the crayfish were eaten raw (Lane et al. 2009).

P. westermani adult

Lastly, us!!!!
While the first two stages of P. westermani are certainly troublesome to the organisms infested with them, we are typically most concerned with the damage done by the adult version of the fluke. To infest their definitive host (mammals), the infested second intermediate host must be eaten raw. The metacercaria may then migrate from the mammal’s digestive tract to their final destination (mammal lung tissue). They are able to bore through tissue in this way due to an enzyme that they secrete. This enzyme dissolves mammalian tissue, allowing the metacercaria to: migrate, ingest nutrients, and evade the host’s immune system. Once in place, the metacercaria develop into adult flukes, which then build a fibrous granuloma (image right) around themselves (Na et al. 2005).


Not The Pinky!!
The metacercaria may also infest other organs in the mammal’s body, such as the peritoneum (lining of the intestines) or the brain. There was even one P. westermani in little fingerdocumented case of the fluke that had migrated to the tip of a patient’s little finger! If in human lung tissue, the fluke will almost certainly cause symptoms that resemble tuberculosis (TB) or lung cancer (violent, blood-tinged coughing). Many people with a P. westermani infestation exhibiting these symptoms are treated for TB initially. Infestation of nearly any part of the mammal’s body is considered paragonimiasis. (Sim et al. 2010). Treatment for paragonimiasis involves the drug praziquantel, which paralyzes the fluke, allowing it to be expelled. Another drug used to treat a parasitic infection (malaria) is called quinine, which is obtained from this amazing tree. Finger photo credit: (Sim et al. 2010)


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