Echinostomatoidea

Life cycle of the trematode parasites Fasciola hepatica and F. giganticaImmature eggs are discharged in the biliary ducts and in the stool (1). Eggs become embryonated in water (2) and release miracidia (3), which invade a suitable snail intermediate host (4), including snails in the genera Galba, Fossaria, and Pseudosuccinea. In the snail, the parasites pass through several developmental stages (sporocyst (4a), redia (4b), and cercaria (4c)). The cercariae are released from the snail (5) and encyst as metacercariae on aquatic vegetation or other surfaces. Mammals acquire the infection by eating vegetation containing metacercariae. Humans can become infected by ingesting metacercariae-containing freshwater plants, especially watercress (6). After ingestion, the metacercariae excyst in the duodenum (7) and migrate through the intestinal wall, the peritoneal cavity, and the liver parenchyma into the biliary ducts, where they develop into adults (8). In humans, maturation from metacercariae into adult flukes takes approximately 3 to 4 months. The adult flukes (Fasciola hepatica: up to 30 mm by 13 mm; F. gigantica: up to 75 mm) reside in the large biliary ducts of the mammalian host. Fasciola hepatica infect a variety of mammals, but mostly herbivores.From Centers for Disease Control Parasites and Health website.

Life cycle of Echinostoma trematode flatworms, the agents causing echinostomiasis in humans Many animals may serve as definitive hosts for various Echinostoma species, including aquatic birds, carnivores, rodents, and humans (a definitive host is the host in which the adult parasites occur). Unembryonated eggs are passed in feces (1) and develop in the water (2). On average, the ciliated miracidium larva takes around 10 days to mature before hatching (3) and penetrating the first intermediate host, a snail (4). Several genera of snails may serve as the first intermediate host. The intramolluscan stages include a sporocyst (4a), one or two generations of rediae (4b), and cercariae (4c). The cercariae may encyst as metacercariae within the same first intermediate host or leave the host and penetrate a new second intermediate host (5). Depending on the species, a range of animals may serve as the second intermediate host, including other snails, bivalve mollusks, fish, and tadpoles. The definitive host becomes infected after eating a second intermediate host (6). Metacercariae excyst in the duodenum (7) and adults reside in the small intestine (8) of the definitive host.From Centers for Disease Control Parasites and Health website.