"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

March 20, 2010

March 20 - Trichobilharzia ocellata

The next time you emerge from a lake and soon find yourself covered in itchy, red papules, you probably have a schistosome like Trichobilharzia ocellata to blame. Cercarial dermatitis, or Swimmers’ Itch, is a skin rash caused by the larval stage of a schistosome flatworm like T. ocellata mistaking a human for their primary host and burrowing into their skin. In humans, small blisters form around the larvae, which soon die because they cannot continue developing. The infected person’s immune response leads to tingling, burning, and itching of the skin.

Here’s what the T. ocellata was supposed to do: The adult schistosomes live in the blood of infected waterfowl, such as ducks and geese. Eggs produced by the adults are passed through the host’s feces and, if the eggs land in water, they hatch and release small, free-swimming larvae called miracidia. These larvae then find, and infect their intermediate hosts, freshwater snails. Once inside a snail the larvae multiply and continue to develop, eventually become cercariae. Cercariae are released through the snail’s feces, and this larval stage is the one that will infect the parasite’s primary host and become an adult. If the cercariae infect you instead, they won’t become adults but you will have an itchy few days.

Contributed by Kate Bowell.


  1. Swimmer's Itch was the first parasite I was exposed to as an undergrad. Up at the University of Michigan Biological Station, there is a wonderful course offered: "Biology of Animal Parasites". This class is taught by Harvey Blankespoor, an expert in schistosomes and Swimmer's Itch. When we got to the schistosome section of the class, many of us even infected ourselves with Swimmer's Itch! How we love our parasites!

  2. Does this parasite have a redia stage? I am curious because i am in a parasitology class in which we covered similar parasites.

  3. It's a kind of schistosome, so it has sporocyst stages - which are a bit like rediae, except that while rediae often have a mouth and a rudimentary blind digestive tract, a sporocyst is basically a sac filled with developing cercariae.

    In terms of the parasite's life-cycle, sporocysts are functionally equivalent to rediae, in that their role is the asexual production cercariae free-living stages that are released into the environment.

  4. I did some work on T. ocellata at Western University in London ON in the late '60s and early 70's. I do not believe, as the article suggests, that cercariae are released through the feces of the snail. Cercariae develop in sporocsyts in the snail's liver. They do not need to enter the gut to achieve access to the outside environment.