I'd never heard of it until I started googling my ducks symptoms. My first hen dropped dead without me even realizing there was anything wrong so I can't say there were any symptoms, which is why I suspected poisoning. My Pekin Quacky duck who is very noisy is not quacking. I haven't noticed her "gaping" as much the last couple days but she seems to go through bouts of laboured breathing and head shaking. When she's sitting I can see her tail bobbing up and down and her chest really sucks in. Other than that she is really going about her normal routine. I have a missing hen that seemed perfectly fine as well. She has 7 ducklings and when I got home from work they were alone. I've looked for her but can't find anything. I read that they are common through out Canada but I don't really know and it seems that it could many other things but I've never had illnesses before. I'm not sure I mentioned already I also had a duckling come down with this mysterious aliment. He was fine in the am, after work he was wondering aimlessly alone with his beak in the air like he was trying to catch imaginary flies. He seemed lost in space. I put him in a tote box and by the time I got my kid to bed he was dead. I checked his mouth as best I could and he seemed fine although I didn't get a good look down the throat or think to use a flash light or anything.
There's a video on you tube of a hen with gapes you can watch it might help you figure it out. Seemed pretty much like what my 2 are/were doing.
I'm also wondering if it is in fact gapeworm can the ducks catch from each other?