I agree with separating her from others just in the case of Mareks. Vitamin B is great for any neurological problem. Keep us updated please, this is a quite strange case.
Change the water bowl of your ducks ..maybe consider one with small holes so the mice can not get inside... another thing, consider getting a cat. I have 3 they have done miracles on my farm.
I have 3 cats. Two of them eat together and are very comfortable with each other. The third, however, don't eat in the same room as others. Maybe she is not comfortable eating with other cats around.