I talked to Pfizer today.  They recommend the 5-15 lb cat size at this
time.  When they did some small studies with ferrets, 1 ferret did get
heartworms with the 0-5 lb kitten size.  A small safety study was done with
the 5-15 lb size, and it was safe at this high dose.  They thought there
will be more info on ferrets at the Heartworm Society Meeting in April.
 
Jerry Murray, DVM
[Posted in FML issue 3371]