On the rise

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Heartworm disease is an extremely common and often life-threatening parasitic infection that affects about 1 million dogs every year in the US. Despite available preventives, infection rates are increasing within certain parts of the country. Adam Christman, DVM, MBA, of dvm360® Live, spoke with Michael Dryden, DVM, MS, PhD, DACVM,about changing how veterinarians and pet owners view the infection as well as ways to better protect our pets.

Adam Christman, DVM, MBA: What do we know about heart disease and how it has compared to the previous years?

Michael Dryden, DVM, MS, PhD, DACVM: It is hard to get an accurate number from year to year, but heartworm rates have trended upward. We have data from the Companion Animal Parasite Council, but that is a small fraction of the dogs actually getting tested or data being uploaded. The rough estimate is a little over 1.34% of the dog population. 

Christman: What are some factors causing this increase?

Dryden: If you look at the American Heartworm Society incidence maps, you see dark red in the lower Mississippi Delta and valley area. Dog relocation across the US and Canada has played a major role in this, especially when thinking about the major hurricane season of 2005. A lot of these dogs come from no-kill shelters where prior ownership was not the best, therefore, they aren’t on preventives. Also, dogs with heartworm won’t always test positive upon arrival if they are within the first 6 months of infection.

Another factor is climate change. Heartworm transmission seasons are longer than they used to be. Warmer and wetter weather, even in northern regions, create an explosion for mosquitoes. There are 25 different species that cause heartworm.

Lastly, preventive adherence has gone down. From the American Heartworm Society’s data, 64% of dogs leave a private practice without prevention. Tie that in with not always getting medicine regularly from pet owners or only being given medication seasonally, and it leaves a gap in care.

Christman: What are some recommendations to better educate pet owners?

Dryden: When I was in practice, we had a visual with the heart. Now it’s mostly posters on the wall of a veterinary office. While it may be graphic, we need to show pet owners what heartworm disease is doing to dogs. There are a lot of pictures that encapsulate the damage done in the lungs and pulmonary arterial system. A dog will never be what it could have been after heartworm because the damage is permanent and lifelong. If there is a 12-inch worm that is killed with medication, where does it go? It degrades within the dog. Pet owners need to see the consequences up front and realize an ounce of prevention is worth the cure.

Christman: We’ve heard a lot of reports about macrocyclic lactones (ML)–resistant isolates of heartworm. What do we currently know about this topic?

Dryden: The explosion of ML resistance in the lower Mississippi Delta changed everything. It rejuvenated research in heartworm and made us more aware of the L3 larvae that infect a dog. They have resistance to heartworm preventives, and it varies how much resistance within each product. It appears to be manageable if we do things correctly with preventive utilization.

If you suspect resistance, there are a few things to check: if enough products were bought to treat the dog and if the dog has microfilaria. If the latter is true, you can do a microfilaria suppression test and then treat with a transdermal moxidectin product. To see if microfilaria is reducing, you run a notch test, which involves taking blood, splicing the red blood cells, spinning them down, and seeing the number of microfilaria. You then run this same test again 4 weeks after treatment. If you’ve reduced them by 90%, they are probably not resistant. If not, it is an indicator they might be.