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Heartworm disease is one of the most dangerous threats to your dog's health, and it only takes a single mosquito bite to transmit it. Many owners don't realize their dog is infected until the damage is already severe. The good news is that heartworm disease is almost entirely preventable with the right approach.
I've worked with hundreds of dog owners over the years, and I've seen what happens when prevention takes a back seat. A dog who could have stayed healthy ends up needing months of painful treatment and strict crate rest. You can avoid that outcome with a simple routine.
This guide walks you through what heartworms are, how dogs get them, the signs to watch for, and the steps you can take to keep your dog safe year-round.
What Heartworm Disease Is
Heartworm disease is caused by a parasitic worm called Dirofilaria immitis. These worms live in the heart, lungs, and associated blood vessels of infected dogs. Adult heartworms can grow up to a foot long and live for five to seven years inside your dog.
A single infected dog can carry dozens of worms at once. Over time, the worms cause inflammation, block blood flow, and damage the heart and lungs. Left untreated, heartworm disease is fatal. The worms don't go away on their own, and the damage compounds the longer they stay.
The tricky part is that early infections show no symptoms at all. Your dog might look and act completely fine while the worms are quietly maturing and multiplying. That's why testing and prevention go hand in hand — you can't rely on watching for signs alone.
Key fact: Heartworms are not the same as intestinal worms. They live in the heart and lungs, not the digestive tract, and deworming medications for roundworms or hookworms do not protect against them.
How Dogs Get Heartworms
Dogs get heartworms through mosquito bites. When a mosquito bites an infected dog, it picks up microscopic baby worms called microfilariae. Those larvae develop inside the mosquito for about two weeks, then get transmitted to the next dog the mosquito bites.
Once inside your dog, the larvae migrate through tissue and eventually reach the heart and lungs, where they mature into adult worms over about six months. During this time, you won't see any symptoms, and standard heartworm tests won't detect the infection yet.
This is why year-round prevention matters so much. You never know when your dog will encounter an infected mosquito, and the gap between infection and detection gives the worms a head start. Prevention kills the larvae before they can mature and cause damage.
Signs Your Dog Might Have Heartworms
In the early stages, heartworm disease has no visible symptoms. By the time you notice something is wrong, the infection is already advanced. That's why annual testing is non-negotiable, even if your dog is on preventatives.
As the worms multiply and the disease progresses, you might see:
- A persistent, dry cough that lingers after exercise
- Reluctance to go on walks or play fetch
- Fatigue after mild activity that used to be easy
- Weight loss or a dull, thinning coat
- A swollen or distended belly from fluid buildup
- Pale gums or labored breathing in severe cases
If you notice any of these signs, call your vet right away. Do not wait to see if things improve on their own. Heartworm symptoms only get worse, and delaying treatment makes recovery harder.
Heartworm Testing: What to Expect
Heartworm testing is simple and fast. Your vet draws a small blood sample and runs a test that detects adult female heartworm proteins. Most clinics can do this in about ten minutes during your regular visit.
Most vets recommend annual testing even if your dog is on year-round preventatives. No preventative is 100% effective, and a missed or late dose can leave a window for infection. A yearly test catches anything that slips through.
Puppies under seven months can start preventatives without a test, because it takes six months for larvae to mature into detectable adults. But if you adopt an older dog with an unknown history, test them right away and again six months later to be sure.
Vet tip: If you miss more than one monthly dose or your dog spits out a pill without you noticing, tell your vet. They may recommend testing six months after the gap instead of waiting for the annual checkup.
Prevention: Medications and Strategies
Prevention is the single most important thing you can do for your dog's heart health. Monthly heartworm preventatives kill any larvae your dog picks up before they mature into adults. You give them year-round, not just during mosquito season.
Common preventative options include:
- Oral chewables: Monthly pills like Heartgard, Iverhart, or Tri-Heart that taste like treats
- Topical liquids: Monthly applications like Revolution that absorb through the skin
- Injectable options: A six-month shot called Pro Heart 6 that your vet administers
Talk to your vet about which option fits your dog's lifestyle and your schedule. The best preventative is the one you'll actually give consistently. If you travel a lot or tend to forget monthly doses, the six-month injection might be worth considering.
Beyond medication, you can reduce your dog's risk by limiting mosquito exposure. Keep your dog indoors at dawn and dusk when mosquitoes are most active. Dump standing water from planters, buckets, and birdbaths in your yard. If you live near a lake or marsh, ask your vet about extra precautions.
Treatment Options If Your Dog Tests Positive
If your dog tests positive, don't panic. Heartworm disease is treatable, and most dogs recover fully when you follow the treatment plan. But treatment is long, expensive, and requires strict exercise restriction, so prevention is always the better path.
The standard treatment protocol involves three injections of a medication called melarsomine over two months. Your dog stays at the clinic for each injection and comes home with strict crate-rest orders. Exercise must be limited to short leash walks for bathroom breaks only — no running, no play, no stairs.
Here's what to expect during treatment:
- Your vet confirms the diagnosis with a second test and possibly chest X-rays
- Your dog starts antibiotics and steroids to weaken the worms and reduce inflammation
- The first melarsomine injection is given, followed by one month of rest
- Two more injections are given 24 hours apart, about one month later
- Your dog stays on strict exercise restriction for the full treatment period
- A follow-up test six months after treatment confirms the infection is gone
The hardest part for most owners is the exercise restriction. Dogs don't understand why they can't run and play, so you may need to use a crate or baby gates to keep them calm. Leash walks for bathroom breaks only — that's the rule for the entire treatment window.
