Interview with a Heartworm Specialist: Part 1

By Jennifer Coates, DVM on Apr. 23, 2012

Did spring arrive early where you live? It sure did here in Colorado (my last ski trip of the year involved almost as much mud as it did snow). An unusually warm spring could certainly mean that we are in for one heck of a mosquito season, spelling trouble on the heartworm front.

I wrote a post a few months back about the mounting evidence that some mosquito populations are developing resistance to widely used heartworm prevention medications. A combination of larger than normal numbers of mosquitoes potentially carrying drug-resistant heartworm larvae would be very worrisome, to say the least. That’s why I thought I’d share with you parts of a conversation I recently had with Cristiano von Simson, DVM, MBA. Dr. Simson is the Director of Veterinary Technical Services of Bayer HealthCare LLC, Animal Health Division.

Full disclosure: Bayer makes Advantage Multi, a product you will see referenced in the second half of this post that will be available tomorrow. Today, we’ll focus on background information about heartworm disease in dogs and cats.

Dr. Coates: Could you talk a little about how heartworm preventives work and how the term "prevention" is a bit of a misnomer?

Dr. von Simson: That’s a very good point. We call them heartworm prevention products, and that is correct if you are thinking of heartworm disease. They do prevent adult worms from invading the heart and blood vessels in the lungs, but they don’t prevent mosquitoes from infesting the dog with immature heartworms on a regular basis. When these products are given once a month, they kill the baby heartworms before they can grow into the adults that cause all the damage in the heart and blood vessels. That is why we need to give these products on a monthly basis, on the same day of the month, all year long, because you never know when a pet might get exposed to mosquitoes and be infected again.

Dr. Coates: What are some of the unique aspects of heartworm disease in cats?

Dr. von Simson: In cats, the disease is slightly different than it is in dogs. Cats can get the adult worms in their heart and blood vessels in the lungs. The fact that cats are generally smaller than dogs, and their hearts and vessels are too, means that a smaller burden of worms will cause significant clinical disease in cats. But cats also have a very unique inflammatory reaction to many things, including worms in general. This inflammation causes Heartworm Associated Respiratory Disease (HARD), with symptoms like coughing, difficulty breathing, and exercise intolerance, which can be hard to appreciate in cats.

So even if an owner does not recognize the symptoms, the cat is experiencing significant problems. That is why it is important to get your cat checked at least once a year for heartworms. Owners also need to understand that we cannot use the medication that kills adult heartworms in dogs on cats, meaning it’s even more important to prevent the disease in cats.

Tomorrow: Dr. von Simson talks about resistance to heartworm preventives.