You might assume that because heartworm disease makes its way to dogs via mosquitoes, those dogs who live where temperatures fall too low to support mosquito life don’t need year-round protection from this illness, which can cause lasting damage not only to the heart but also the lungs and arteries. But mosquito species keep adapting to better withstand cold climates. And some mosquitoes do perfectly fine huddling up in homes over the winter months. That’s no doubt at least part of the reason that heartworm disease has found new footholds in cooler, dryer regions.
For 2022, the latest year for which data are available, “states with historically low heartworm rates…saw unexpected increases and the development of new ‘hot spot’ areas,” reports the American Heartworm Society (heartwormsociety.org). These states include Washington, Oregon, Kansas, North Dakota, Massachusetts, and Connecticut. And the illness continues to plague dogs in Southern states, which have a climate particularly conducive to mosquito proliferation. In fact, Arkansas, Mississippi, Texas, Georgia, Florida, Tennessee, and the Carolinas all saw expansions of high-density areas last year.
The illness is simply awful. Dozens, or even hundreds, of worms that can grow to a foot long take up residence in a dog’s heart and lungs and associated blood vessels. They can live for 5 to 7 years — half a dog’s life. Early signs of the disease include mild persistent cough, reluctance to exercise, fatigue even after moderate activity, decreased appetite, and weight loss. As the disease progresses, a dog may develop heart failure and a swollen belly as a result of excess fluid accumulating in the abdomen. A dog with a large number of heartworms can even develop a sudden blockage of blood flow within the heart itself, leading to life-threatening cardiovascular collapse.
The biggest culprit
The unfortunate irony is that the rise in cases of this devastating illness is not all about climate and mosquito adaptability. Poor compliance with heartworm prevention is a big factor, either because people with dogs are forgoing it altogether or are haphazard about when they administer the protective medicine. Even if you give the monthly preventative just a few days late, you leave your dog vulnerable — and other dogs, too. When a mosquito bites an infected dog to feed off its blood, it picks up baby worms, which quickly mature into what is known as the infective larvae stage. When the mosquito then goes to bite its next dog, it deposits the larvae onto that dog’s skin. From there they make their way into the bloodstream through the wound left by the bite, thriving and growing to adult size.
How significant is non-compliance in the spread of heartworm disease? Veterinary practitioners responding to a survey for the American Heartworm Society blamed more than 50 percent of the rise in heartworm rates on a combination of infected dogs coming to their area and poor compliance with heartworm disease prevention. Only about 20 percent said the uptick was a result of weather conditions. Additionally, more than 60 percent of those veterinarians who saw a drop in heartworm disease rates in their practices cited an increase in pet owners administering heartworm preventatives and better compliance with the timing of doses. In other words, you, not nature, hold the cards here.
Think 12
The Heartworm Society has a “Think 12” prevention strategy with two main components.
1. Give your dog a heartworm preventative 12 months a year.
2. Have your dog tested for heartworm disease every 12 months.
You might reasonably ask why annual testing is necessary if you administer a heartworm preventative to your dog every 12 months (in the form of a pill or topical obtained by prescription — or a once- or twice-yearly injection at the vet’s office). The reason is that you might inadvertently skip a dose. Or your dog could spit out or vomit up a pill or rub off a topical application. Then, too, even if you’re just a little late in giving the medication, a window opens for heartworms to infiltrate. Testing requires just a small blood sample taken during your dog’s annual exam. Best not to skip it.
