I've been meaning to post this for years actually. I've been reading posts on here for years, but just now got around to joining.
Back in October 2009 we got a Muscovy and her ducklings. All I-forget-how many were fine and healthy except one. Pippin, while s/he was supposed to be these same age was half the size of the others. I quickly noticed s/he also couldn't walk straight but instead went around and around in circles. With closer inspection I could see she was blind. Cloudy eyeball blind, one eye was much worse than the other. We don't have a vet for ducks where I live. I put Pippin inside with me in a plastic container as it was all I had at the time. I didn't have a heat lamp so I put her by a space heater. I had to guide Pippin to her food and water at first, but as long as I kept them in the same area she learned where they were. I made sort of squeaking noises at her and she'd squeak back. I let her swim around in my deep bathroom sink and when it was plastic container cleaning time, I took her outside and let her splash around in the bird bath (it was still warm out). I have a collie mix and my neighbor had a great dane mix. The times I did let her play on the ground I had to cover her up with grass clippings. Sounds silly but it worked.
Unfortunately though, she never got better. If anything she got worse. Her neck and head would twist and stiffen. The eye got worse and at times looked as it it would melt out of her hea. I wondered if maybe there was some sort of parasite, but there was nothing I could have done at the time. If I was nearby I would gently hold her head in place during the spell. Pippin still grew and when she started getting feathers, she had to leave the house. By then the others were on the pond and seemingly healthy. We put her in the duck pen to see how she'd do. She did okay for a while, she got all her feathers and was a full grown duck, but still passed away one night.
We have raised several ducklings, and she's the only one I've ever had to have that problem. (FYI I say she, but I have no idea what gender Pip was.)
Some photos and video of Pippin. Okay so it's more like pic-spam. I don't know what the limit for photos per post is yet, some will be links.
http://www.youtube.com/watch?v=8oHTFxblA40 - first day I had her in my house. You can see the circling here
https://www.youtube.com/watch?v=7AI_zcWndw0 - falling asleep.
http://elistariel.deviantart.com/art/Muscovies-139134422 - Mama with other babies
http://elistariel.deviantart.com/art/Blind-Ducky-139317765 - Pippin, here you can see the misshapen eye.
http://elistariel.deviantart.com/art/Cataracts-139318393 - both eyes were affected.
http://elistariel.deviantart.com/art/Dead-to-the-World-139377457 - playing in the birdbath.
http://elistariel.deviantart.com/art/A-Duck-and-a-Truck-140938579 - she fell out, also meet Ranger. Ranger doesn't bother ducks.
http://elistariel.deviantart.com/art/Duckoflauge-140938932 - My attempt at camoflauge.
Photos I managed to take during her spells, seizures, whatever they were:
Pippin, fully grown or close to it.