5/26/2023 0 Comments Human liver fluke![]() Animals such as pigs, dogs, and a variety of feline species can also harbor P westermani. Infections may persist for 20 years in humans. Time from infection to oviposition is 65-90 days. However, when this occurs, completion of the life cycle is not achieved because the eggs laid cannot exit these sites. The worms can also reach other organs and tissues, such as the brain and striated muscles, respectively. The metacercariae excyst in the duodenum (7), penetrate through the intestinal wall into the peritoneal cavity, and then through the abdominal wall and diaphragm into the lungs, where they become encapsulated and develop into adults (8) (7.5-12 mm X 4-6 mm). Human infection with Paragonimus westermani occurs by eating inadequately cooked or pickled crab or crayfish that harbor metacercariae of the parasite (6). This is the infective stage for the mammalian host (5). The cercariae invade the second intermediate host, a crustacean such as a crab or crayfish, in which they encyst and become metacercariae. Miracidia go through several developmental stages inside the snail (4): sporocysts (4a), rediae (4b), with the latter giving rise to many cercariae (4c), which emerge from the snail. In the external environment, the eggs become embryonated (2), and miracidia hatch and seek the first intermediate host, a snail, and penetrate its soft tissues (3). Image courtesy of the US Centers for Disease Control and Prevention.Įggs are excreted unembryonated in the sputum, or, alternately, they are swallowed and passed with stool (1). Various animals serve as reservoirs for S japonicum and Schistosoma mekongi. Human contact with water is thus necessary for infection by schistosomes. Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host, and migrate through several tissues and stages to their residence in the veins. The stages in the snail include 2 generations of sporocysts and the production of cercariae. Under optimal conditions, the eggs hatch and release miracidia, which swim and penetrate specific snail intermediate hosts. The eggs are moved progressively toward the lumen of the intestine (S mansoni and S japonicum) and of the bladder and ureters (S haematobium), and they are eliminated with feces or urine, respectively. The females (size 7-20 mm males slightly smaller) deposit eggs in the small venules of the portal and perivesical systems. Adult worms in humans reside in the veins in various locations: Schistosoma mansoni in the inferior mesenteric veins, Schistosoma japonicum in the superior mesenteric veins, and Schistosoma haematobium in the vesical veins (these locations are not absolute).
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