Here’s a wild one for you—”My dog never leaves the yard,” or “Our property is fenced,” or even “We don’t have any neighbors.” These arguments are based on the supposition that mosquitoes conduct themselves according to an honor code that prevents them trespassing on another person’s property (or airspace). The mosquitoes that carry heartworm disease actually wander for miles and miles as they search for a meal. It should be noted that foxes and coyotes are also a source of heartworm infection, and that these animals are the usual neighbors of people who think they don’t have neighbors.
“I have a herding breed dog.” The MDR1 genetic mutation, present in some herding dogs, affects patient response to specific pharmaceuticals, and that list includes multiple anti-heartworm compounds. However, concentrations found in parasite preventatives are far below the threshold and do not pose a threat to dogs of any breed.
“We don’t have that in my part of the country.” Not so long ago, there were a few geographic zones where heartworm was virtually unheard-of. Those zones have shrunken rapidly for two major reasons. One is the ease and readiness of transporting animals over very large distances, thus introducing disease into new areas. This was clearly illustrated by the rescue and relocation of homeless pets after Hurricane Katrina. A large number of dogs were trucked to all corners of the U.S. in search of safer homes, and some of them were carrying invisible, parasitic cargo. The second influence here is climate change, which has permitted mosquitoes to set up shop in parts of the world which were previously off limits. One of those previously protected areas was Iceland, and island on which mosquitoes were recently observed for the very first time.
“Not needed in the winter.” Mosquitoes do require certain weather conditions to survive, breed, and transmit disease. Here in the Midwest, the weather was never anything short of wildly unpredictable, and our changing climate is breaking many preexisting norms. Geography-based weather patterns are rendered even less reliable by the tendency of urban areas to retain heat. It’s the denser population, lack of vegetation, and all those flat, dark-colored surfaces…plus the presence of features that have outdoor access while still retaining cold weather protection (e.g., parking structures). This disproportionate warmth allows urban mosquitoes a much longer active season than their country cousins, even at the same latitude. To further complicate the issue of timing, one must understand that monthly heartworm medicines give protection for the PREVIOUS thirty days—not the upcoming thirty days. This somewhat counterintuitive aspect of the medication, paired with continually evolving weather patterns, make it impossible to safely pause your preventions during any season of the year.
Foregoing heartworm prevention has precisely one benefit, and that is keeping its purchase price in your pocket. Discuss the options with your veterinarian, because saving a couple of bucks by skipping a couple of months is just not worth the gamble.
Dr. M.S. Regan