mysterious death of muscovy duckl

We have had 2 healthy muscovy ducks die of unknown causes and have a
third come down ill tonight. One day they appear healthy the next they
sit, and are listless. That night or the next day they are dead. No
signs of trauma, body weight good. We do not have any poisons laying
about and are very concerned that others in our flock are in danger.
Has anyone had a similar experience with muscovies or other ducks or
any knowledge of what might cause these deaths? Thanks for your time.
Jim & Faye

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