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The Heartworm Conundrum


The Heartworm Conundrum

by Dr. Michael Trapani

Throughout all of my 35-year career in veterinary medicine, I have been fighting a battle against heartworms. While this hasn’t exactly been a losing battle, I certainly cannot claim anything like “victory.” Dirofilaria immitis, the mosquito-borne blood vessel parasite of dogs has been a constant threat to my pets and patients. 

When I first practiced in the rural Sierra foothills, heartworms were everywhere. At 2,000 feet of elevation the mixed conifer/hardwood forests provides a perfect habitat for mosquitos capable of transmitting the worms from one dog to another. Coyotes examined by the county trapper were consistently found to have 10 to 12 inch worms inside their hearts and pulmonary arteries. Those worms would breed, producing circulating larvae, which would be picked up by mosquitos and injected into the blood of the next dog who was bitten. It was a perfect storm of parasitism and dogs not protected with medication became quickly infected. It was not uncommon to see 8- to 12-month-old puppies with heart failure caused by massive numbers of worms clogging their circulation. It was a no-brainer: every dog had to be on preventive medication OR ELSE.

But life in Bodega Bay is different. Our sea level location doesn’t support the right kind of trees to breed mosquitos necessary to transmit heartworms. We don’t have a reservoir population of infected canines needed to infect those mosquitos. We don’t have a long enough period of consistently warm weather needed for the worms to mature inside the mosquitos to become infective. It’s a dream come true. Stay-at-home dogs were very safe and required only yearly monitoring. Traveling dogs were medicated as necessary to keep them from picking up the worms and bringing them home.

The relative safety of our little ocean-front town is not mirrored in the rest of Sonoma County. Inland areas, with their warmer temperatures, differing tree and mosquito populations, and higher elevations suffer a much greater risk from heartworms.

But Bodega Bay has changed so much in just the last five years. Ocean waters are several degrees warmer than historic normals. We have more hot days and longer periods of sustained warm weather. Mosquitos, which once seemed uncommon, are now a much more regular annoyance, and new, different kinds of mosquitos are seen with regularity. Clearly, there is trouble in Paradise.

All of this came to a head recently when we had our first-ever positive heart worm test. The worms were found in an older dog with a history of travel to the Sierra foothills, and she probably picked up the worms there. The dog’s owner was aware that travel created risk for his dog, but failed to either use regular monthly heartworm preventive medication OR periodic post-travel deworming that would have prevented the problem before it started. Fortunately, the dog’s disease had not progressed to the heart failure stage. BUT this dog had infective larvae in her bloodstream: Heartworms have been introduced into Bodega Bay.

One needs only follow the Zika stories in the news to see just how quickly a disease agent can spread when conditions are right. A single infected dog can infect scores of mosquitos, each of whom is capable of infecting scores of dogs or foxes or coyotes. In a trice, heartworms could become a threat to every local dog, even here in heavenly, safe Bodega Bay.

And now we know the worms are here.

In the past, I made an effort to inform dog owners of the risk of heartworms, including details about the relatively safe environment of Bodega Bay. It seemed fair to tell them that I keep my own dogs on regular heartwood preventive medication, but also to tell them that we had never had a positive heartworm test in Bodega Bay. I like to inform my clients and allow them to make their own decisions. Some chose to give monthly heartworm preventive medication to their dogs, others decided to bear the risk and go without medication.

But I’m not so comfortable about heartworms anymore. Look at Zika. 

So, while the heartworm risk is still pretty low, I’ve had a change of heart (pun intended). I don’t want anyone to be caught short by this: Heartworm cause a terrible, debilitating, life threatening disease. I think it is wise for every dog to be protected against heartworms and other parasites using any of the safe, effective, once-monthly heartworm preventive medications available..

Copyright © 2012 by Dr. Michael Trapani