Vicky
Ferrets can get heartworm.  I worked as a Licensed Veterinary Technician
until I was disables recently, and we found a number of "unexplained'
deaths to be heartworm related in both cats and ferrets when we did the
necropsies.  You may want to suggest to your vet that she read Biology and
Diseases of the Ferret by Dr. Fox-it will tell her a lot about it.
 
I live in MI and one of our vets gave me a liquid preventative made with
ivermectin in a suspension that the ferrets take readily.
 
Diane and the raucus 7
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[Posted in FML issue 3088]