For many people, the word "parasites" makes them wrinkle their nose and say "Ewwww." Parasites were one of the first things I learned about when I started working with animals, and heartworms still give me that feeling. The disease is serious, it can stay hidden for a long time, and it is preventable.
The short version: test your dog every year, keep them on a vet-prescribed preventive all year (indoor dogs too), and call your veterinarian if a dose is missed.
What is heartworm in dogs?
Heartworm disease is caused by a parasitic worm called Dirofilaria immitis. Despite the name, the adult worms live mainly in the arteries of the lungs; in heavy infections they move into the right side of the heart and the large veins leading to it. They cause severe lung disease, heart failure and damage to other organs, and the disease can be fatal, especially when found late.
Heartworm has been diagnosed in all 50 U.S. states and in many other parts of the world, including European countries such as Spain.
What is the life cycle of the heartworm?
You might be thinking, "I don't want to go back to biology class." Fair enough, but the life cycle explains how dogs get infected and why the disease stays silent for so long. Three terms help:
- Vector: the carrier that transmits the disease. For heartworm, that is the mosquito.
- Intermediate host: where the larvae develop. Again the mosquito: the offspring of adult heartworms (microfilariae) grow into infective larvae inside it.
- Definitive host: where the adult worms live and reproduce: the dog, and wild canids such as coyotes, wolves and foxes.
Here is the cycle:
- Adult female heartworms release microfilariae (microscopic baby worms) into the dog's bloodstream.
- A mosquito bites the infected dog and takes up microfilariae with the blood.
- Inside the mosquito, they become infective larvae in about two weeks in warm weather. Cool weather slows this; cold weather stops it.
- When that mosquito bites another dog, the larvae are left on the skin and enter through the bite wound.
- The larvae mature for about two months in the tissues under the skin and in the muscles. Young adults reach the lung arteries roughly 2-4 months after the bite.
- The worms mate, and their offspring can appear in the blood from about 6-7 months after infection.
Adults can live 5 to 7 years, so every mosquito season can add worms to an untreated dog. For those who love visuals, here is a useful life-cycle graphic from the U.S. Food and Drug Administration (FDA).
How is heartworm disease transmitted from one pet to another?
Only through the bite of an infected mosquito. Heartworm is not contagious: a dog cannot catch it by living with an infected dog. (For the vet techs: microfilariae can rarely pass through the placenta or a blood transfusion, but without a mosquito they never become adult worms.)
What are the signs of heartworm disease in dogs?
Early on, many dogs show few signs or none. The longer the infection goes on, the more likely signs become, especially in active or heavily infected dogs. The American Heartworm Society grades the disease into four classes by signs and test results:
- Class 1 (mild): no signs, or an occasional cough.
- Class 2 (moderate): cough, tiring easily with activity, and abnormal lung sounds.
- Class 3 (severe): the above plus trouble breathing, abnormal heart sounds, an enlarged liver, fainting, and a swollen belly from fluid buildup (heart failure). Weight loss and poor appetite are common. Class 3 disease can be fatal.
- Class 4 (caval syndrome): an emergency. Masses of worms block blood flow through the right heart. It starts suddenly with labored breathing, pale gums and dark, bloody or coffee-colored urine. Without prompt surgery to remove the worms, few dogs survive.
Chest X-rays do not show the worms themselves. They show the damage: enlarged, abnormal lung arteries and, in advanced cases, an enlarged right side of the heart.
How significant is my dog's risk for heartworm infection?
Real, even for indoor dogs and in places where heartworm seems rare. Outdoor dogs have more exposure, but mosquitoes get inside homes. Infected dogs get relocated after disasters or through rescue transport, healthy dogs travel to high-risk areas, and coyotes, wolves and foxes carry the worm near many cities. Rescue and relocated dogs should be tested.
The American Heartworm Society sums it up as "think 12": test your dog every 12 months, and give prevention 12 months a year.
How do vets test for heartworm?
The American Heartworm Society recommends testing every year, even for dogs on prevention, because prevention can fail quietly. A late or missed dose, a pill spat out or vomited, or a topical that rubbed off leaves a gap. No preventive is 100% effective, and resistant heartworms have been documented. The earlier heartworm is found, the better the chances of recovery.
Vets use two blood tests:
- An antigen test looks for proteins made by adult female heartworms. This is the main screening test.
- A microfilaria test looks for baby worms in a blood sample, often under the microscope.
A positive result should be confirmed with a second, different test. Very recent infections do not show up yet, because the tests only detect older worms. After a positive, chest X-rays or a heart ultrasound (echocardiogram) show how much damage there is; changes like those on X-rays of an enlarged heart in dogs help the vet stage the disease.
What happens if your dog tests positive?
Most infected dogs can be treated successfully. Your veterinarian builds the plan around your dog's class of disease and the American Heartworm Society guidelines:
- Confirm the diagnosis with a second, different test.
- Restrict exercise right away. Activity speeds up the damage the worms cause.
- Stabilize the dog first if they have heart failure or other complications.
- Weaken the worms. Heartworms carry bacteria called Wolbachia that help them survive, so vets often give an antibiotic aimed at them, plus a preventive to clear young larvae and baby worms.
- Kill the adult worms with melarsomine, the only FDA-approved drug for adult heartworms, given as injections at the clinic on a schedule your vet sets.
- Recover quietly. Dying worms break into fragments that lodge in small lung vessels. Most complications come from this, so strict rest matters most; your vet sets how strict and how long.
- Retest to make sure the worms are gone, then keep the dog on prevention for life.
Dogs with caval syndrome need emergency surgery first, usually through a large vein in the neck. You may also read about "slow-kill" with a preventive alone; the American Heartworm Society does not recommend it.
Call your veterinarian during recovery if breathing becomes labored, a cough worsens or brings up blood, or your dog runs a fever, refuses food or seems unusually flat.
How can I prevent my dog from getting heartworms?
Prevention is much safer and cheaper than treatment. In the U.S., heartworm preventives are prescription only, and your vet may test your dog first to make sure they are heartworm-free.
They come as chewables or tablets, topical liquids, and an injection only a veterinarian can give. All of them belong to one drug class, the macrocyclic lactones, and your veterinarian chooses the product and schedule. A few rules hold whichever one you use:
- Give it year-round, with no seasonal breaks. Your vet starts puppies early, at the first puppy visits.
- If a dose is missed or late, call your vet about restarting and retesting.
- Skip natural remedies: none is proven, and there is no vaccine. Use only FDA-approved preventives.
- Mosquito repellents made for dogs can add a layer but never replace the preventive; ask your vet.
Can cats get heartworms?
Yes, but it is a different story. The cat is an atypical host: most worms die before adulthood, and many infected cats have no adult worms at all. Immature worms still cause real damage, called heartworm associated respiratory disease (HARD). There is no approved treatment for cats, and the dog drug cannot be used, so prevention is their only protection.
Questions and answers for pet owners
How much is heartworm treatment?
It varies with your dog's size and disease stage, so ask your clinic. With repeated tests, X-rays, medications, injections and follow-ups, it costs far more than a year of prevention.
How much does a heartworm test cost?
It depends on the clinic; the test is usually part of the yearly visit.
How long after heartworm treatment can a dog be active?
Your veterinarian decides from follow-up checks. Returning to activity too early can cause serious complications.
What is the best flea and heartworm preventive for dogs?
There is no single best product. Many heartworm preventives also cover some intestinal worms, fleas or ticks; your veterinarian picks one for where you live and your dog's lifestyle.
Can I get heartworm from my pets?
No. In rare cases a person bitten by an infected mosquito is infected, but the larvae die before they become adult worms.
What does heartworm look like?
Like spaghetti (apologies if you are reading this over dinner). Females grow up to about 12 inches long and males up to about 6 inches.
How common is heartworm?
The American Heartworm Society surveys clinics every few years and publishes incidence maps of average positive cases per clinic; the latest is the 2025 map. Besides dogs, the worm infects cats, ferrets, foxes, wolves, coyotes, sea lions and, rarely, people.
Summary
Heartworm can kill dogs, but it is preventable. Think 12: test every 12 months and give a vet-prescribed preventive 12 months a year, indoor dogs included. Missed a dose? Call your vet. Positive test? Rest starts right away, and your veterinarian builds the plan from there.
Originally written by Arais. Updated by the I Love Veterinary editorial team.
Sources
- American Heartworm Society: Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs (revised 2024)
- American Heartworm Society: Heartworm in Dogs
- American Heartworm Society: Incidence Maps
- Companion Animal Parasite Council (CAPC): Heartworm guidelines
- U.S. FDA Center for Veterinary Medicine: Keep the Worms Out of Your Pet's Heart! The Facts about Heartworm Disease
- Parasites & Vectors: Prevalence of heartworm in dogs and cats of Madrid, Spain
- VCA Animal Hospitals: How to Treat Heartworm Disease in Dogs
This article is for general information and is not a substitute for advice from your veterinarian.
