Lifecycle and Reproduction

From the egg...
Although Paragonimus westermani is a parasite pretty much its whole life, that is not to say that it is a parasite to one host. Rather it makes its home in three different organisms, and which one it inhabits simply depends on which stage of its life it is in. Starting out as an egg, after at least two weeks this fluke hatches as a free-living and ciliated miracidium, or larva (Liu et al. 2008). The larva will immediately start looking for its first intermediate host, a freshwater snail. Upon the discovery of one, the miracidium will embed itself in the tissues of the snail and shed its cilia while maturing into a sporocyst. At this next stage in its life, the fluke will asexually produce two generations of cylindrical larva called redia, the second of these which will asexually reproduce cercariae. The cercariae are disk-shaped larva with tail-like appendages that, from egg until now, have taken about four months to develop (Fuller 2012).

...to the 1st..2nd...3rd..hosts
From this first intermediate (snail) host, the P. westermani will inhabit its second intermediate host - a crab, crayfish, or other freshwater crustacean. It does this either by being ingested by the crustacean while still in the snail and then transferring to the body of the new host, or by first swimming out of the snail and penetrating the skin of the crustacean. At this point, the cercariae transform into metacercariae, as they become enclosed in a thick-membraned cyst (Liu et al. 2008). Humans or other mammals, the final hosts, then get infected by this parasite when they ingest the raw or undercooked crustaceans populated with metacercariae. Once inside this definitive host, the fluke will maneuver its way to the animal’s lungs or other tissues, ultimately causing paragonimiasis in this organism (Chen et al. 2010). However, not all mammals (such as mice) or tissues (such as the heart) are favorable to the fluke and therefore disallow Paragonimus westermani to develop into an adult worm. If this is the case, and it remains in the metacercariae stage, eggs will not be produced, the lifecycle will not be complete, and the mammalian host will not be infected (Shibahara 1993 and Liu et al. 2008). Just the opposite, if and/or when the fluke does fully mature, it carries out its final duty of sexually reproducing eggs. These eggs will then leave the mammal’s body and be distributed out into the environment by way of saliva or feces, and the cycle will repeat (Fuller 2012).

Forms of reproduction
When it comes to reproduction, then, this fluke will reproduce either sexually or asexually depending on which stage of its life it is in. As aforementioned, when maturing from the sporocyst to the redia and the redia to the metacercariae, the parasite reproduces asexually. In contrast, when producing and fertilizing eggs as adult worms, sexual reproduction takes place. Hermaphroditic in nature, sexual reproduction typically requires the worms to find a mate, form a cyst, and exchange sperm to produce the eggs. Discoveries of cysts containing a single worm, however, suggest that self-fertilization is also an option for this parasite. Because of the number of asexually produced offspring (redia and metacercariae) within this lifecycle, Paragonimus westermani does not invest in its young.  Sheer numbers are expected to ensure the survival of the species (Fuller 2012).

Sexual reproduction will also only take place if the fluke is diploid. Discoveries have shown that this is not always the case, though, but rather that triploid and even polyploidy P. westermani exist. Because chromosome-crossing would get complicated in sexual reproduction between flukes with excess chromosomes, those that do exceed the diploid number usually do not reproduce in this way. Instead, they engage in asexual reproduction, self-fertilization, or, as is usually the case with triploid individuals, they are parthenogenic (Park et al. 2003).

 

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References