If the necropsy again shows nothing, I'd start thinking incubation problems. It's like the sudden death syndrome of ducks. Irregular temperatures during incubation can cause heart defects that cause sudden death when the birds body outgrows the hearts ability to pump blood.
Sudden death in Ducks? More Dead! - Page 2
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We had one of our two pekin ducks die three months ago, no signs or symptoms, in the coop, plenty of food and water, and no signs of an attack. Our second duck was very upset and lonely, so we got two more, and the three of them have been healthy and we have not had another problem since then. It was very sudden, but at least the rest of our ducks have not had a problem.
I just had a Pekin and Brown Khaki die suddenly last , both their necks were stretched back along their bodies. They were acting a little lethargic the previous days. We have a small plot of corn, hay in the small duck house next to pond and had just put Hydrillia from the Potomac River in there last weekend..... My neighbors idea. They are/were 6 months old. Read about mold in Corn and Hay. protozoans maybe from river? 2 Khakis are well this morning. A Fox may have got one Khaki 3 days ago, but we were outside in 70 degree temps last night when they died. Any help appreciated :(
thanks very helpful, im sure one of these caused it, i have narrowed it down to Aflotoxin , Asperilligus or virus Enteritus
I think you can knock aspergilosis off the list. Usually that's only a problem in poorly ventilated, damp indoor conditions. The ducks either breath the mold or pick at it and eat it. In outdoor conditions they are very unlikely to do either.
If you suspect that it's aspergillus you can check their bedding. It would Smell moldy and possibly have visible evidence of mold.
If they live on an open waterway with contact with wild waterfowl, you're probably right about the enteritis.
thanks we brought Hydrillia from the Potomac River into pond from a stagnant cove, im sure it was contaminated by wild waterfowl. two of the brown khakis survived, one looked like it was going to die , but recovered. Got 9 non fly away Mallards on Craigslist for free :)