Last spring, a client named Maria brought her three-year-old Labrador mix, Biscuit, into the clinic. Biscuit had been slowing down on walks, coughing occasionally, and just seemed “off.” Maria thought it might be allergies. When I ran the bloodwork and the heartworm test came back positive, she sat in silence for a full minute before asking, “But he’s barely outside — how did this happen?” It’s a conversation I’ve had more times than I can count, and every single time, I wish the outcome had been different. Because heartworm disease is one of those conditions where the best treatment is never getting it in the first place.
Meet the Silent Invader
Heartworm disease is caused by a parasitic worm called Dirofilaria immitis, and it does exactly what the name suggests — these worms take up residence in the heart, lungs, and associated blood vessels of infected animals. Transmitted exclusively through mosquito bites, the lifecycle of this parasite is quietly devastating. A mosquito bites an infected animal, picks up microscopic baby worms called microfilariae, and then passes them along to the next animal it bites. Those larvae migrate through tissue, mature over several months, and eventually land in the pulmonary arteries and heart, where they grow into foot-long adults.
What makes heartworm so treacherous is the silence of the early stages. Dogs can be infected for months without showing a single symptom. By the time a cough, exercise intolerance, or weight loss appears, the infection is often well-established and the worms have done significant damage to the cardiovascular system. Cats can show even more unpredictable signs — sometimes the first symptom is sudden collapse or even sudden death.
“But My Pet Never Goes Outside”
If I had a dollar for every time an owner told me their indoor cat or mostly-indoor dog didn’t need heartworm prevention, I could fund a free clinic for a year. I understand the logic — if your pet isn’t out in the woods or rolling in the dirt, how could mosquitoes find them? But here’s the reality: mosquitoes don’t respect property lines, window screens, or the cozy indoor life you’ve built for your pet.
Mosquitoes get indoors through open doors, torn screens, gaps around air conditioning units, and even tiny cracks you’d never notice. I’ve treated heartworm-positive cats who literally never set a paw outside their apartment. According to the,[1] heartworm infections have been diagnosed in all 50 states, and the risk is real regardless of housing situation. One of my patients, a strictly indoor Siamese named Cleo, tested positive two years ago. Her owner was stunned. “She sits on the windowsill and watches birds. That’s as wild as she gets.” It took one mosquito. That’s all it ever takes.
The Myth of the Easy Fix
Treatment Is Not the Safety Net You Think It Is
Many pet owners operate under the assumption that even if their dog gets heartworms, it’s treatable — so what’s the rush on prevention? I genuinely understand that thinking. Prevention feels like an extra monthly expense for something that might never happen. But the reality of heartworm treatment is far from simple, and I’ve watched enough clients go through it to know it’s not something anyone would choose if they understood what was involved.
The treatment protocol for dogs involves a series of injections with a medication containing arsenic — yes, arsenic — to slowly kill the adult worms. It’s typically spread over several months with strict exercise restriction throughout. I mean strict. No running, no playing, no long walks. For a young, energetic dog, this is emotionally and physically taxing. As Maria said during Biscuit’s treatment, “He just stares at me with those eyes, wanting to chase his ball, and I have to tell him no. It breaks my heart every single day.”
The financial cost is substantial as well — heartworm treatment often runs into hundreds or even thousands of dollars, depending on the severity and the region. Compare that to the cost of a monthly preventative, and the math speaks for itself. Prevention isn’t just the safer medical choice; it’s the more responsible financial one too.
For Cats, There Is No Treatment at All
Here’s where the conversation takes a turn that I never enjoy delivering. There is currently no approved drug therapy to treat heartworm infection in cats. None. The medication used to treat dogs is explicitly unsafe for felines — it can cause severe, even fatal reactions. When a cat is diagnosed with heartworm disease, veterinary care shifts entirely to managing symptoms and hoping the cat’s immune system can clear the infection on its own. Sometimes they do. Sometimes they don’t.
This is why I am absolutely resolute about heartworm prevention for cats, even though many cat owners have never even heard of feline heartworm risk. The.[2] is clear on this point, and every veterinary professional I know echoes the same message. With dogs, we have a difficult but effective treatment path. With cats, we have nothing. Prevention is the only medicine that exists.
Why You Can’t Just Buy Preventatives Online
Heartworm preventatives require a veterinary prescription. I know this frustrates some owners — it feels like an unnecessary hurdle, especially when you’re just trying to reorder the same medication you’ve always used. But there’s a critically important medical reason behind this protocol that many people don’t understand.
Heartworm preventatives do not kill adult heartworms. They work by eliminating the larval stages that a mosquito may have transmitted in the previous 30 days. If you give a preventative to a dog that’s already carrying a mature heartworm infection, you can trigger a dangerous reaction. A sudden die-off of microfilariae — those microscopic baby worms circulating in the bloodstream — can cause anaphylactic shock, organ damage, or even death. This is why veterinarians require a heartworm test before prescribing preventatives and why we retest annually. We’re not being bureaucratic. We’re protecting your pet from a potentially fatal medication interaction.
I’ve changed my own thinking on this over the years. Early in my career, I’d sometimes feel awkward insisting on the test before refilling a prescription for an owner I knew well. But after seeing one close call with a dog whose owner had skipped prevention for several months and then resumed without testing — the dog spent three days in emergency care — I stopped hesitating. The test is quick. It’s affordable. And it can save a life.
Choosing the Right Preventative
When selecting a heartworm preventative, there are several options to discuss with your veterinarian, and the right choice depends on your pet’s specific needs. Some preventatives come as monthly chewable tablets. Others are topical applications. There are also injectable options that provide extended coverage. Beyond heartworm protection, many of these products also guard against intestinal parasites like roundworms and hookworms, and some include protection against fleas and ticks.
What matters most is consistency. The most effective preventative in the world won’t help if it’s sitting in a cabinet while mosquitoes are active. I tell every client the same thing: pick a day of the month that’s easy to remember — the first, the day you pay rent, a birthday — and stick with it. Year-round. Even in winter. Mosquito seasons are lengthening and becoming less predictable, and the [3]
The Bottom Line
Heartworm prevention isn’t a marketing ploy or an upsell. It’s one of the most reliable, straightforward ways to protect your pet from a disease that is painful to treat in dogs and impossible to treat in cats. I think about Biscuit often — he completed his treatment successfully, and Maria now sets a monthly phone reminder for his preventative. She told me last visit, “I spent more on his treatment than I would have spent on prevention for his entire life. I just didn’t know.” That’s the thing. Most people don’t know until they’re sitting across from me hearing the diagnosis. My hope is always to change that — one conversation, one pet, one preventative dose at a time.
Sources
- American Heartworm Society — heartwormsociety.org
- FDA — fda.gov
- concept of a "mosquito-free season" is increasingly unreliable — magnolia.vet