Life Cycle of Paragonimus Kellicotti
Paragonium kellicotti are an American species of lung fluke exhibiting extreme similarity with the more familiar Asian lung fluke, paragonium westermani. The two species share a basic life-cycle pattern that involves three hosts. Both are parasites that can infect human hosts, though westermani is more commonly implicated than kellicotti in human infections at this time.
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Lung Flukes
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Lung flukes are parasites in the class of flat worms. Lung flukes resemble small, flattened slugs. Kellicotti lung flukes are usually a drab brown. The flukes are seven to 12 mm long and four to seven mm wide, with suckers on both tips allowing them to fasten tightly to a host. Adult lung flukes occupy the lungs of mammalian hosts, feeding off of the blood of the victim.
Hosts
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Paragonium kellicotti pass through a total of three hosts in completing their life cycle. The definitive hosts (those that house adult, reproducing flukes) are always mammalian. Larval forms of the fluke pass first through an aquatic snail of the Pomotiopsis spp, and then pass to a crustacean host before being eaten by a mammal and taking up residence and beginning the life cycle over. Thus the cycle moves from mammal to snail to crustacean, over and over. Due to the aquatic habitat of the two intermediate host forms Paragonium kellicotti's life cycle is permanently tied to lakes, rivers and other wetlands.
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Eggs
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Adult flukes lay their eggs near the bronchial tubes of their mammalian host. The eggs are ejected through two methods: sputum is coughed up and either ejected through the mouth or swallowed and eventually passed with the feces. When the eggs enter a wetland environment they begin to form an embryo, which eventually hatches as a mobile, free living larva called miracidea.
Miracidea
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The miracidea is a transportation larval form only. It does not eat, and if it fails to find a snail host within a day it will die. It is a single celled animacule which moves by the use of cilia.
Snail Phases
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When the miracidea finds a snail host it will penetrate the shell and enter the snail's body. In the body it will feed and grow, passing through three larval stages in the snail: sporocysts, rediae and cercariae. The larvae multiply, using asexual reproduction, so that one successfully hosted miracidea can give rise to hundreds of emerging cercariae. Cercariae proceed to search for a crustacean host, most often a crayfish of some local species or a freshwater crab.
Crustacean to Mammal
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Cercariae form cysts in the muscles of their crustacean hosts, and settle down to wait. They will remain in the host for the duration of its life. The lung fluke larvae in the form of the encysted cercariae will only find a new, mammalian host when the crustacean is eaten by a mammalian predator. When the mammal eats the raw (or undercooked) crustacean the encysted cercariae is released into the mammalian gut. It penetrates the wall of the gut, passes into the peritoneal cavity, and migrates from there up into the lungs, where it proceeds to mature and take up permanent residence as an adult lung fluke.
Effects and Treatment
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Paragonium kellicotti can infect humans, but is currently far more prevalent as a parasite of wild mammals and domestic animals that might come in contact with either raw crustaceans or, occasionally, may prey on animals that have recently eaten crustaceans. Thus cats, dogs and pigs are the most common victims. Both humans and animals experience pneumonia-like symptoms. The infection, when diagnosed, can be treated with antibiotics, but severe infections must often be surgically treated and the complete removal of all larvae is problematic.
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References
- Photo Credit Public Domain - Wikimedia Commons
