Vicky and anyone else who might be interested.
 
Ferrets ARE succeptible to heartworm.  I don't know about whether or not
they are an optimum host, but I have been told by a very good ferret vet
and I have read in Fox's "Biology and Diseases of the Ferret" that as few
as i to 2 worms can kill a ferret where a dog can host many more and still
have a chance to recover.
 
Our ferrets don't get heartworm treatment as they very seldom go outside
and we are not in a very risky area anyway.  If you are in a risky area
i'd suggest getting another opinion from a different vet.
 
Best Wishes,
Leigh and Fargo, Fonzi, Kami, and Moon Boy
[Posted in FML issue 3088]