Cinda - This is what Dr. Klein gave me for heartworm preventive:
0.3cc injectable ivermectin in one ounce propylene glycol (100mcg/cc)
Dosage:  0.1cc/lb body weight once monthly; i.e. 0.3cc for a 3-lb ferret.
Your vet can call to confirm with Dr. Klein at 915-855-4100.
 
Since 2 - 3 adult worms can kill a ferret, I consider this a very worthwhile
investment.  Hope this helps.
 
Blessings,
Carla Smith
Rio Grande Domestic Ferret Club
[Posted in FML issue 1554]