Swimmer's Itch - Life Cycles of Non-human Schistosomes

Life Cycles of Non-human Schistosomes

The non-human schistosomes use two hosts in their life cycles. One is an aquatic snail, the other is a bird or mammal.

• Schistosomes are gonochoristic and sexual reproduction takes place in the vertebrate host. In genera that infect birds, adult worms occur in tissues and veins of the host’s gastrointestinal tract, where they produce eggs that are shed into water with host feces.

Note: One European species, Trichobilharzia regenti, instead infects the bird host’s nasal tissues and larvae hatch from the eggs directly in the tissue during drinking/feeding of the infected birds.

• Once a schistosome egg is immersed in water, a short-lived, non-feeding, free-living stage known as the miracidium emerges. The miracidium uses cilia to follow chemical and physical cues thought to increase its chances of finding the first intermediate host in its life cycle, an aquatic snail.

• After infecting a snail, it develops into a mother sporocyst, which in turn undergoes asexual reproduction, yielding large numbers of daughter sporocysts, which asexually produce another short-lived, free-living stage, the cercaria.

• Cercariae use a tail-like appendage (often forked in genera causing swimmer’s itch) to swim to the surface of the water; and use various physical and chemical cues; in order to locate the next and final (definitive) host in the life cycle, a bird.

• After infecting a bird, the parasite develops into a schistosomulum and migrates through the host’s circulatory system (or nervous system in case of T. regenti) to the final location within the host body where it matures and, if it encounters a mate, sheds eggs to begin the cycle anew.