The drugs that kill adult heartworms can't always be used on dogs, either. The treatment is, in essence, poison the worms and hope they die before the dog does. It's an arsenic compound. Then deal with all the dead worms. If the dog has already been weakened, then they can't bear the treatment.

http://www.veterinarypartner.com/Content.plx?A=610 (dogs)

http://www.veterinarypartner.com/Content.plx?P=A&C=&A=593&SourceID= (cats)

We use Sentinel year-round on our dogs - it ROCKS.