You just never know how long it will take to rectify issues like this. If it were me, I'd also see that it's getting some extra vitamin e as well.
Here's a good source for foods that contain vitamin e and although it's for chickens, ducks can have the same...
That's 156% of the recommended daily dose for an adult. I believe its 25mg if that's the brand. You would give 10mg for each duck. So, if you're just feeding the one, it would just need a 1/2 tablet crushed. I would make sure it's on or in something it eats. If your feeding the whole flock, it...
I would opt for a stronger version of niacin. Nutritional yeast has a higher level of niacin than brewers. But, for the one with issues, I recommend the liquid super b complex. It says injectable, but you can add it to their feed or a treat. I'd make sure the 1 was getting a full dose.
How long and what type of niacin did you supplement with? I'd also add vitamin e to the diet. Even though the rest of the ducks are ok, some will need supplementation. Ducks wouldn't have down syndrome because that's due to an extra chromosome and it's a human issue. They could have neurological...