We GET swimmer’s itch…so you don’t HAVE to!


Swimmer’s itch is not caused by snails. Nor is it caused by ducks. Swimmer’s itch is caused by tiny larval parasites that penetrate our skin accidentally.

There are well over 20 different species of parasites that cause swimmer’s itch. All members of this group, collectively called avian schistosomes, have the same general life cycle, involving the metamorphosis of different lifestyle stages (similar to a butterfly that goes through an egg, caterpillar, and pupil stage before turning into an adult).

Basic Life Cycle

Let’s start with an adult worm (1), which can live for several years. During its lifespan an adult worm produces hundreds of thousands of eggs (2), each which hatch into a tiny larva called a miracidium (3). A miracidium lives only 1-2 days before it starts growing into a sporocyst (4), A sporocysts takes 4 weeks to fully develop before it starts to release (via asexual reproduction) hundreds of thousands of cercariae (5). A cercaria lives only 1-2 days before developing into an adult worm, which completes the life cycle.That’s the basic life cycle of an avian schistosome. But remember, these tiny animals are parasites, so now we really need to incorporate their host species into their basic life cycle if we are to better understand swimmer’s itch. (See “Basic Life Cycle + Hosts” tab above)

Basic Life Cycle + Hosts

Eggs (2), miracidia (3), and cercariae (5) are free-living stages within the life cycle. There are 2 parasitic stages in avian schistosomes life cycles: Adult worms (1), which live in the definitive host (A), most commonly a waterfowl species, and sporocysts (4), which live in an intermediate host (B), always a certain species of snail.So if humans aren’t part of the natural life cycle of these parasites, how do they end up causing swimmer’s itch? (See “Basic Life Cycle + Host + Humans” tab above)

Basic Life Cycle + Hosts + Humans

Humans (C) become unwilling participants in the life cycle when a cercaria (5) leaves an infected snail and accidentally penetrates our skin (instead of the skin of its suitable definitive host species). Each raised, red papule in a case of swimmer’s itch represents your body’s immune response to one invading cercaria. So in places where swimmer’s itch is a huge problem, are all the ducks and snails in the lake carry the parasite? (See “Life Cycle of Trichobilharzia stagnicola” tab above)

T. stagnicola

All avian schistosome species exhibit a high degree of host specificity, meaning each species utilizes its own unique combination of definitive and intermediate hosts. The swimmer’s itch species that causes the greatest problem in the northern United States is Trichobilharzia stagnicola, which infects common mergansers and Stagnicola snails.So does that mean that only common mergansers carry the swimmer’s itch parasite? (See “Life Cycle of Trichobilharzia physellae” tab above)

T. physellae

Mallards, Canada geese, and many other waterfowl species can serve as the definitive host for swimmer’s itch parasites. But these avian schistosome species usually aren’t as common in the northern US, and therefore aren’t usually responsible for massive swimmer’s itch outbreaks. One such species is Trichobilharzia physellae, which utilizes mallards and Physa snails as its definitive and intermediate hosts, respectively.