Our vet says if they have a predisposition to the heart failure, or
have had fluids accumulating there in unnoticeable amounts (meaning,
you haven't seen side effects), pred can tip the scale as it does
cause water retention. In January we lost a ferret that had insulinoma
because she had to be on prednisilone, and this caused her heart
failure to accelerate. You can put them on diuretics, but with the
doses of pred she had to have, it was a losing battle and the pred was
canceling out the diuretics. She did pee a lot more, but it did not
help get rid of or stop the accumulation of the fluid in her lungs.

[Posted in FML 7030]