Interview with a Heartworm Specialist: Part 2

By Jennifer Coates, DVM on Apr. 24, 2012

Yesterday, I talked with Dr. Cristiano von Simson, Director of Veterinary Technical Services of Bayer HealthCare LLC, Animal Health Division, about how heartworm preventives work and a bit about the unique aspects of the heartworm disease in cats. Today, we’ll tackle the subject of drug resistance as it applies to heartworm disease. Full disclosure: Bayer makes Advantage Multi, a product to which Dr. von Simson refers.

Dr. Coates: What do you have to say about the reports primarily coming from the southeastern United States about heartworm infection becoming more prevalent in dogs that by all reports have been on monthly, year round preventives?

Dr. von Simson: Years ago, the FDA announced that they were getting more reports about a lack of efficacy in heartworm prevention products. We also heard directly from veterinary practitioners in central and southern states that in addition to those cases where they didn’t think dogs had received all their preventive doses at the right time, which is unfortunately very common, they were also seeing cases where they were very confident that owners had given all the doses correctly and they believed that the product had really failed to prevent heartworm disease.

There have been some researchers working on this as well. Dr. Byron Blagburn is the one who really took the lead. He went there, collected some samples, took them to his lab, and did some in vitro assays and clinical studies on heartworms and the lack of efficacy of preventives. Dr. Dan Snyder did some work on that too, and so did researchers from the University of Georgia. So, there is a lot of information out there. I think the best summary that is available is in the American Heartworm Society’s new canine guidelines that have just been revised.

What they are saying, and I’ll paraphrase here, is that in some areas, certain isolates of heartworm show some resistance to preventive products. In clinical studies where dogs are infected with one strain of heartworm called MP3 that was isolated in Georgia, several different preventive products were challenged by Dr. Blagburn and only one was able to prevent 100 percent of heartworm cases, and that was Advantage Multi. The other monthly products that were tested had seven out of eight dogs in their groups develop heartworms in that trial. Some other studies by Dr. Snyder corroborated that evidence of failure.

So this is sort of a new area. We thought we knew everything about heartworms, but there’s a lot of new evidence and we’re still trying to understand the mechanisms of resistance and why we’re seeing heartworms now and where those resistant isolates are.

Dr. Coates: How do you recommend that owners use this information?

Dr. von Simson: The biggest problem we have today is still the number of dogs and cats that are not on prevention or are not on 12 doses all year round. Talk to your veterinarian about what the best product is for your pet. That will depend on a series of factors, including the spectrum of the product. The Companion Animal Parasite Council (CAPC) recommends broad spectrum products that will kill heartworms and intestinal parasites (including whipworms). So the veterinarian and owner should discuss the best parasite prevention protocol, including the topic of resistant strains of heartworms, and choose a product that will give peace of mind.

Do you have any questions for Dr. von Simson? He has graciously offered his time to provide answers; so ask away and I’ll pass on the ones that get the most "likes."