 The current monthly oral and topical heartworm preventives, as well as ProHeart6, the injectable six-month formulation for dogs, are all members of the same pharmaceutical class: the macrocyclic lactones. These are compounds or chemical derivatives of compounds produced by various soil-dwelling species of actinomycete bacteria within the genus Streptomyces.
Ever since the introduction to small animal veterinary medicine of the first member of the class, ivermectin (as Heartgard), these compounds have become the mainstay of heartworm prevention in the United States and around the world. These are amazing compounds with efficacy at remarkably low doses against internal and external parasites. With the exception of the well-known blood-brain barrier problem in certain collie breeds, this drug class poses almost no significant safety concerns. These compounds have moved heartworm prevention from the world of the daily into the monthly (or semi-annual with ProHeart6) and, either on their own or in formulations with other products, also provide concurrent protection against internal parasite infections. All in all, from the point of view of the dog and cat, health care now is similar to that of people after World War II when penicillin and related antibiotics jumped onto the scene and provided a remarkable method of combating bacteria. Life is good! The safety of the molecules and their great efficacy are unfortunately lulling people into a sense of complacency. All practicing veterinarians know of a colleague who treats heartworm-infected dogs with the simple administration of a monthly preventive. All veterinarians have argued with clients or themselves about the need to annually test animals that are on year-round (or even soundly prescribed seasonal) prevention. Veterinarians who treat heartworm cases with Immiticide are aware of the recommendation to treat dogs first with a monthly preventive (usually ivermectin), and thus are aware that the medications can be safely administered to microfilaremic dogs. And if you read the labels for the canine products Heartgard Plus, Interceptor, Sentinel, Revolution, Iverhart Max and Advantage Multi, you will see that they all say they are safe for dogs with circulating microfilariae. Thus, there are those who argue that there is no reason to check a dog before beginning a preventive program, because it is not dangerous for the dog, it will “slowly kill” the adult heartworms over a number of months, and any microfilaremia will eventually resolve after the adult worms are removed. If the above arguments are true, if you can treat existing infections with a monthly product, if you do not have to check heartworm status before you start preventive and if annual checks are pointless because the worms will ultimately be killed by the product anyway--then, why not just sell heartworm preventives over the counter? Make the products available in Target and Wal-Mart. There are people out ther... | 
