Paragonimus westermani: Third Time's a Charm

Habitat and Geography

Around the wGlobe focused on the Asain continent; Image by and retrieved from GEOATLAS.comorld:
Although almost exclusively prominent in eastern and southern Asia, Paragonimus westermani still exhibits much variation (Devi et al. 2010). This is seen as the fluke is often referred to as a complex species that was originally divided into two groups. The first group inhabited countries in northeastern Asia, namely Japan, Korea, China, and Taiwan, while the second group inhabited the southeastern Asian countries of Malaysia, the Philippines, and Thailand. Since these groups were formed, however, P. westermani has also been discovered in Vietnam, India, and Sri Lanka (Doanh et al. 2013). The parasite has also been identified in tropical and/or temperate countries outside of the Asian continent, such as in North America, but usually just in individuals infected with the pulmonary disease paragonimiasis (Lane et al. 2009).


No place like a host:
Aside from geographical location, the immediate habitat of Paragonimus westermani depends on what point of its lifecycle it is in. Going through about six different life stages, this parasite generally occupies three different hosts, as well as a freshwater environment.  The habitat first occupied is that of the aquatic (freshwater) environment, as this is where the eggs are dispersed. After hatching, the parasite will infect and embed itself in the tissues of its first intermediate host, a freshwater snail (Iwagami et al. 2009). Upon transforming and leaving the snail in the next stage of its life (as a cercaria) the next mission of the Paragonimus westermani is to infect its second intermediate host - a crab, crayfish, or other freshwater crustacean. It does this in one of two ways: it leaves the snail, becomes free-living again, and then invades the crustacean host on its own accord, or, while still in the snail, the snail is ingested by the crustacean, and it transfers hosts then. Either way, the encysted cercaria (now a metacercaria) makes its new home in the gills, muscles, or other tissues of its new crustacean host (Liu et al. 2008). The final host and home of the fluke is that of various body parts, such as the lungs, brain, etc., of humans and other mammals that happen to ingest the raw or undercooked, infected crustaceans (Chen et al. 2010).

Freshwater snail; Retrieved from:                    Japanese freshwater crab; Retrieved from:                     Third host: the human lungs; Credited to Shutterstock

Living conditions:
Because of the variety of environments/hosts that this parasite inhabits, it must endure a variety of conditions, namely anaerobic versus aerobic. During its free-living stage, P. westermani prefers and is more commonly found in moving water where it is exposed to a sufficient supply of oxygen (Fuller 2012). SiOxygen (as seen as an element on the periodic table) Retrieved from:, the preferable habitat of the adult worm is the well-oxygenated pulmonary tissues of its mammalian host. However, when imbedded in the tissues of an intermediate host’s small intestine, or some equally anaerobic area, the parasite undergoes much oxygen stress and must adjust accordingly. As a result, the fluke is exceedingly efficient at adapting its metabolic needs to match the amount of oxygen present (Takamiya et al. 2010).

As Paragonimus westermani is a parasite, and one of three different hosts in a single lifespan, it is clear that it shares the habitat of many other creatures. As aforementioned, the first of these organisms are its variety of hosts, from its initial snail residence, to its intermediate crustacean home, to its final mammalian host. Aside from these obvious co-inhabitants, though, this fluke also has relatives as ‘neighbors’ - the other species of the Paragonimus genus. Similar in nature, these species utilize the same or similar hosts as P. westermani. In fact, all six other species found in Vietnam: P. heterotremus, P. proliferus, P. viatnemensis, P. skrjabini, P. bangkokensis, and P. harinsutai, were found to coexist in the same intermediate crab host. That being said, however, there was not found to be any competition between the cohabiting Paragonimus spp. metacercariae (Doanh et al. 2013).  


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