I just gave my dog Apollo his first summer dose of heartworm prevention. I have to fess up and say that when it comes to heartworm prevention, I recommend that most of you do as I say, not as I do … but I have scientific evidence to support this apparent contradiction.


Take a look at the most recent heartworm incidence map put together by the American Heartworm Society (AHS). The map is based on 2013 testing data from 3.5 million patients from more than 4,300 veterinary clinics and shelters across the United States. I live in that little finger of white (<1 case/clinic) that dips down into Northern Colorado. Note, however, that I don’t use that information to justify not giving Apollo heartworm prevention at all. Every year I give his first dose well before heartworm disease could be transmitted through the bite of an infected mosquito and give a final dose after mosquito season is over. Apollo gets to take a few months off during the winter; that’s all.


Unless you happen to live in one of the “white zones” on the AHS map and never travel with your dog outside of a “white zone,” I strongly recommend that you give your dogs heartworm prevention every month, all year round. And I still recommend that dogs in the “white zones” receive prevention in all but the coldest months of the year. The reason is simple. Even if the chance of your dog contracting heartworm disease is relatively low, the condition is so devastating and expensive to treat that any analysis of risks and benefits almost always comes down in favor of prevention.


I’ve heard all the excuses as to why people don’t give heartworm prevention to their dogs. Two of the most common are:


  1. It’s too expensive – While the all-in-one products that protect dogs against heartworms, fleas, ticks, mange mites, intestinal parasites, and more can get pricey, generic heartworm prevention alone is a bargain, coming in at less than $5 per month in many cases.
  2. It’s too dangerous – Dogs have gotten sick after being given some types of parasiticides, but if you stick with the old standby, ivermectin, you have nothing to worry about. The dose of ivermectin in heartworm preventatives is so low it can be safely used even in ivermectin-sensitive breeds of dogs. Where resistance to heartworm preventatives is becoming a problem (mostly in the South and Southeast), other products (e.g., those containing moxidectin) might be a better choice.


The AHS heartworm incidence map should also put to rest the notion that dogs in certain states are at no risk of contracting heartworm disease. I cringe whenever I hear an owner say something along the lines of “We live in Arizona. There isn’t heartworm disease here.” Heartworm disease can now be transmitted to dogs in every state except Alaska.


Veterinary medicine should never be a one-size-fits-all endeavor. Talk to your veterinarian about what type of heartworm (and other parasite) prevention is appropriate based on your dog’s health and risk of exposure.


Dr. Jennifer Coates


Image: Photographee.eu / Shutterstock


Last reviewed on July 31, 2015