Heartworm in Dogs: The Questions Worth Actually Asking

Heartworm gets mentioned at nearly every vet visit, which means it becomes background noise for a lot of dog owners. It shouldn't — the disease is preventable, and the prevention costs a fraction of what treatment costs. Here's the information that actually matters.

What heartworm is and how dogs get it

Heartworm (Dirofilaria immitis) is a parasitic worm that lives in the heart, lungs, and associated blood vessels of affected dogs. Worms can reach over 30 centimeters in length in adult females. The transmission route is mosquitoes — an infected mosquito bites a dog and deposits larvae that migrate through tissue over several months, eventually reaching the cardiovascular system and maturing into adult worms.

This means the risk correlates with mosquito exposure. Dogs in areas with year-round mosquito activity face constant risk. In colder climates, risk is seasonal — but mosquitoes survive longer into fall and emerge earlier in spring than most owners assume, which is why veterinarians recommend year-round prevention even in northern climates.

What happens when a dog has heartworm

Early heartworm infection often has no visible symptoms. Dogs may appear completely healthy while adult worms are establishing. As the worm burden increases, symptoms develop: persistent coughing, exercise intolerance, fatigue, and eventually difficulty breathing. Severe infestations can cause heart failure. By the time symptoms are obvious, the disease has been active for an extended period.

Treatment exists but is involved, expensive (often $1,000-$3,000), and requires weeks of strict rest while the dying worms are cleared from the body. Dead worms can block blood vessels, which is why the dog's activity must be severely restricted during treatment. The process is genuinely hard on the animal.

Prevention: straightforward and inexpensive by comparison

Monthly preventive medication — either oral chewable tablets like heartworm prevention for dogs or topical treatments — stops heartworm larvae from developing after a mosquito bite. Most preventive medications also cover other internal parasites including roundworms, hookworms, and whipworms. Given that the preventive cost runs around $6-15 per month depending on the product and dog size, and treatment runs into the thousands, the math is not complicated.

Dogs need to be tested before starting prevention if they've been without coverage — giving prevention to a dog that already has adult worms isn't dangerous the way it once was with older drugs, but it also isn't effective against existing adult worms. Annual heartworm testing is standard, even for dogs on prevention.

Missed doses: what to do

If a monthly preventive dose is missed by a few days, give it when remembered and continue the schedule. If it's been significantly longer — more than a couple of months — the dog should be tested before resuming prevention. A vet can advise based on the specific duration and the drug being used. Don't quietly restart prevention after a long gap without testing; the test gives you accurate information about current status before you commit to long-term prevention again.

What I'd skip

I'd skip treating heartworm prevention as optional because your dog mostly stays indoors, or because you live somewhere with cold winters. Mosquitoes are opportunistic, they get inside, and the seasons in which they're active have been expanding. Year-round prevention is inexpensive protection against a disease that is genuinely difficult and costly to treat once established. The comparison isn't even close.