Dog & cat health · Vet-reviewed guide

Dog Heartworm Symptoms: Treatable, Unlike in Cats, but Prevention Wins

Heartworm treatment for dogs exists, but it's hard, slow, and expensive enough that every vet will tell you the same thing: it's a disease you want to prevent, not treat.

Quick answer: Unlike in cats, dog heartworm is treatable, over 95% of dogs survive treatment, but the treatment is a months-long, expensive, and physically demanding ordeal. That is exactly why prevention is so strongly recommended. Heartworm is spread by mosquito bites and, left untreated, the worms grow up to a foot long inside the heart and lungs, causing serious and eventually fatal disease.

The good news is there is an FDA-approved cure for dogs; the hard news is what it involves: a series of deep injections of an arsenic-based drug, antibiotics beforehand, and about 60 days of strict crate rest, because as the dead worms break down they can cause a fatal lung clot if the dog exercises. Treatment commonly runs many hundreds to a few thousand dollars, about ten times the cost of a year of prevention. So while a heartworm-positive dog can usually be cured, the far easier and cheaper path is a monthly preventive, because waiting to treat is not always the answer when prevention avoids the whole ordeal. This guide explains the disease, the treatment, and why prevention wins.

What heartworm does to a dog

Mosquitoes transmit heartworm larvae, which mature over about six months into adult worms that live in the heart, lungs, and associated blood vessels.¹ A heavy infection can involve dozens of worms up to 10–12 inches long, damaging the heart and lungs. Early on there are often no signs; later ones include a mild persistent cough, tiredness after activity, reduced appetite, and weight loss, progressing to heart failure.

Yes, it's treatable, here's what that means

This is the key contrast with cats, where no approved cure exists. In dogs, an adulticide drug called melarsomine, an arsenic-based injection, kills the adult worms, and more than 95% of dogs survive the full protocol.¹ But the process is demanding:

• Pre-treatment with antibiotics (doxycycline), steroids, and a monthly preventive to stabilize the dog and weaken the worms.
• A series of deep melarsomine injections into the back muscles over roughly two months.
• About 60 days of strict crate rest, which is non-negotiable. As the dead worms fragment and are absorbed, exertion can send worm material to the lungs and cause a fatal clot.¹ Some dogs need sedatives to stay calm.

The cost, and why prevention wins

Treatment commonly costs several hundred to a few thousand dollars, and more in severe cases, which is about ten times the cost of a year of prevention.¹ Prevention is a monthly chewable or spot-on that costs a few dollars a month and also protects against intestinal worms and often fleas. Given the expense, the months of restricted activity, and the real risks of treatment, keeping a dog on year-round prevention is far easier on both the dog and the owner.

Is heartworm contagious?

No, not directly at all. Heartworm spreads only through mosquito bites, not from dog to dog.¹ A dog cannot catch it from an infected dog by contact, though both are at risk wherever mosquitoes are present, which is everywhere.

Prevention and testing

Vets recommend year-round heartworm prevention for all dogs, with an annual blood test, because preventives are given to dogs not already infected.¹ Preventives are prescription products. If your dog needs one, our licensed veterinarians can review your dog and issue a prescription when appropriate. See how our vet prescription service works. For combined mosquito, flea, and tick protection, see our prevention guide.

When to see a vet

See a vet if your dog has a persistent cough, tires easily, or is losing weight, and get an annual heartworm test and year-round prevention regardless.¹