As of this afternoon ProHeart 6 is back on the market per Jim James, a Fort Dodge representative.  This is a tremendously safe product that is effective for six months against heartworm.  Since dog owners can't forget to administer it for six months, the risk of heartworm is greatly reduced. 

There is also a long-acting antiboitic on the market that I will be talking up soon.  It's approved for skin infections, and one shot is as effective as giving pills twice a day for two weeks.  (More, actually.)