Picture courtesy of sciencedirect.com
The parasite’s complicated life cycle begins with egg development in the host’s intestinal tract. The eggs are excreted into the water. In the water, fully developed miracidiae hatch from the eggs and penetrate aquatic snails, which are the intermediate hosts. The miracidiae gather in the digestive gland of the snail where they grow to sporocysts within two months. Under favorable light (top 5 cm of water) and water conditions (>17 degrees Celsius), the snails release thousands of cercariae, which are capable of infecting water birds. Snails are typically found in shallow water with sandy or vegetated bottoms. Cercariae are light sensitive and are released from snails especially in the early morning. Tiny and light, they move readily with air and water currents. These features have implications for human chances of encountering cercariae. Water exposures in shallow water, in early or mid-morning, and at places with onshore winds are likely to increase contact with cercariae.
In addition to thermotactile stimulation, cercariae follow non-selective chemoattractants such ceramide and cholesterol, which facilitate the sticking of the cercariae on the host's skin. Humans are final hosts because the cercariae are not able to enter the blood vessels. They penetrate the stratum corneum (this is facilitated by the release of proteolytic enzymes), and die within a few hours in the epidermis. The enzymes of the cercariae are immunogenic and lead to sensitization 2 weeks after the first contact; skin eruptions develop only a few hours following subsequent contact.