Yes, unfortunately there lots of other infections so it would be interesting to know what symptoms @WaterfowlsPoultry 's flock displayed.
The D.Agr only kills birds when one has tested positive.
Avian 'flu is a notifiable disease. Big fines for people who don't report and are found out. If a vet is involved, they will report. If a commercial flock is involved, Agriculture inspectors come round to any known neighboring flocks and test. Two or three years ago, several BYC members...
The problems with wild ducks include Avian 'flu which they carry but are often symptomless -- last year was a bad year. I think that the best care for your ducks would be to build an enclosed area for them to forage that keeps out the wild ducks; and to ensure that your duck food isn't feeding...