This could be caused by many things, you'd be best off having a vet do
an autopsy to find out exactly what the problem was.  I'd lean towards a
heart problem which caused a fluid build-up, or something like ADV which
caused abdominal bleedout.  It also sounds possibly indicative of
lymphoma.
 
If it was his heart, regretfully it's possible it could have been treated
to give him a better life quality and even possibly extend his life.  If
it was ADV, you need to have the other tested.
 
Any time a ferret slows down noticably, a vet visit with a full workup is
in order to determine the cause.  Of course as a ferret ages, it will slow
down.  But it should not be that drastic over a six month period.
 
I'd also find a new petsitter.  Fleas are never acceptable with all the
safe remedies on the market these days.
 
[DA]
[Posted in FML issue 3649]