I don't want to scare you but I was looking up duck diseases today - only one it comes up as an obvious symptom:
Riemerella anatipestifer Infection
This bacterial disease of ducks is also known as Pasteurella anatipestifer infection, infectious serositis and New Duck disease. Anatipestifer infection causes high mortality, weight loss and condemnation. In the acute form, listlessness, eye discharge and diarrhea are commonly seen. Ducks show incoordination, shaking of the head and twisted neck. Birds are commonly found on their backs, paddling their legs. Typical lesions found in dead birds are infected air sacs, membranes covering the heart and liver, and meningitis. Preventive management and vaccination are effective means of control. Penicillin, enrofloxacin and sulfadimethoxine-ormetoprim (0.04-0.08% in feed) are effective in reducing mortality.
...good luck with yours!
