I'm in Virginia and personally I do use heartworm preventative for my baby.
The vets I've spoken with around here don't seem to think it's necessary
for ferrets because they are generally indoor pets.  The point I always
make to them though is that we do take ours out occasionally and mosquitoes
can, and do, get inside.  One bite from the wrong bugger is all it takes.
So they made me up a homemade liquid mixture of ivermectin which was pretty
cheap and will last practically forever with just one ferret.
 
I already had one dog get heartworms and go through treatment (I had lapsed
on their prescription).  It wasn't cheap (don't tell the old man) and it
was not easy emotionally.  We had to separate our dogs for six weeks, to
keep the stricken one calm.  They were not happy.
 
My advice would be - if nobody in the area can give advise on ferrets - ask
your vet if they recommend the preventative for dogs in your area.  If so,
I would use it for my fert(s).
 
Good luck,
Keron (and Chloe the hobbler)
[Posted in FML issue 3029]