I have treated 3 that i can remember Diabetic ferrets in the last 2 years.
 
In my experience when the DM (diabetes Mellitus) happens after Insulinoma -
especially long-standing insulinoma it is a result of pancreatic
exhaustion - it has run out of cells to produce insulin.  (i'm not sure
this totally makes sense or not physiologically but i like the idea so i;m
sticking with it for now) ALSO there is very likely risk that the tumor
has metastasized (moved) to the liver and is making the liver ineffective
in glucose regulation.  OR there may be a primary cancer in the liver or
other organs (lymphomasarcoma)
 
BUT Lymphoma cannot be diagnosed by physical exam !  There must be a
biopsy of some type to have a diagnosis - high rate of suspicion may occur
based on physical exam findings, historical information and signalment.
Most oncologists will not even treat cancer unless they have 2 separate
biopsy from separate organs.  (ie a liver and a lymphnode, ect)
 
As for treatment of DM:
here are a couple of things:
 
THE PUBLISHED DOSE FOR INSULIN IN MOST REFERENCES IS A TYPO THAT CARRIED
OVER FROM THE FIRST ED OF QUESENBERRY / PINK BOOK !!!!!
 
i KNOW THAT FROM EXPERIENCE AND MORE IMPORTANTLY I heard it from the
horses mouth !!  ( not that Dr. Q is a horse !!)
 
IT SHOULD BE:  1 unit / kg  NOT 0.1 U / KG
 
also you might have your vet get a hold of some PZI insulin.  It works
better in cats because it mimics the shape of their insulin better than
human does.  I;m not sure about the shape of ferret insulin - someone
ought to call dr.  fox and ask him?
 
Since ferrets are more obligate carnivores i think their insulin might
work more like cats.
 
I have had GLIPAZIDE work in a couple of ferrets.
 
I have had 3 ferrets spontaneously recover from their DM - just as quickly
as it occurred they went back to being insulinomic.
 
I had one case that occurred because a ferret had been given treat ECE - a
human oral replacement electrolyte solution for children.  (i just cant
remember name I'm not trying to obtuse i do that enough as it is.  )
 
Hope this helps.
 
--ben
 
Benjamin A. Otten, DVM
Practice Limited to Avian and Exotic Pets
[Posted in FML issue 3608]