Usually refers to the Capillaria plica (or Pearsonema plica ), a thread-like nematode that lives in the bladder of dogs, cats, and foxes.

The term "bladder worm" is actually used to describe a few different things depending on who—or what—is infected.

Parasites often have complex life cycles that involve "middlemen" or intermediate hosts. The Pet Perspective (Dogs and Cats)

Humans aren't typically infected by the animal version. Instead, "bladder worm" often refers to Urinary Schistosomiasis , caused by Schistosoma haematobium . These worms live in the blood vessels surrounding the bladder.