The image on the right shows the life cycles of three of the Schistosomes. In addition to their markedly different morphology, the aspect of their life cycle that differs the most from the other flukes is that when their cercariae emerge from the snail, they do not encyst on something and become metacercariae. Instead, the cercariae directly penetrate the human skin. Thus, there is a requirement for contact with water, rather than just eating something from the water. Despite that seemingly unlikely way of getting schistosomiasis, it is still a threat in many parts of Africa, South America, the Caribbean, Southeast Asia, and the Middle East. In the US, we see cases from people who have lived in or traveled to these areas.
The eggs of the schistosomes all have spines and special enzymes which enable them to penetrate the tissue in order to make their way into the intestines and thus be excreted with the feces. Note that whereas S. mansoni, S. japonicum, S. intercalatum, and S. mekongi all live in vessels associated with the gastrointestinal tract and are thus seen in the feces, S. haematobium lives in vessels by the urinary bladder and thus are more frequently seen in the urine.
