"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.