Heartworm in Dogs
Dog Health Guides · Diseases
Heartworm Symptoms in Dogs: Early Signs, Testing and Prevention
If you are reading about heartworm symptoms, your veterinarian is the best first step. This guide explains what owners commonly see and what to do next. It does not diagnose or prescribe.
Heartworm symptoms in dogs often start so quietly that owners miss them: a mild, lasting cough, tiring easily on walks, and a dog that is less keen to play. As the worms grow in the heart and lungs, signs become serious, including weight loss, a swollen belly, fainting and heart failure.
Heartworm disease is spread by mosquitoes and is found in all 50 US states. It is much easier, safer and cheaper to prevent than to treat. This guide covers heartworm in dogs symptoms, how dogs get heartworm, why a yearly vet heartworm test matters, and monthly prevention.
Important: Sudden collapse, labored breathing, pale gums, or dark, coffee-colored urine in a dog that may have heartworm is an emergency. Never start heartworm prevention in a dog that has not had a vet heartworm test: giving prevention to an infected dog can cause a serious reaction. Some herding breeds, such as collies, shelties and Australian shepherds and their mixes, can be sensitive to ivermectin and some other parasite medicines, so ask your vet before giving your dog any parasite product.
What Heartworm Disease Is
Worms in the Heart and Lungs
Heartworms (Dirofilaria immitis) are long, slender worms that live in the heart and the large blood vessels of the lungs, where they damage the vessels and strain the heart.
A Slow Disease
After a mosquito bite, larvae take about six months to grow into adult worms. Adults can live for years, so damage builds over time.
Not Spread Between Dogs
A dog cannot catch heartworm from another dog. Only an infected mosquito can pass it on.
Heartworm in Dogs Symptoms by Stage
First Signs
Often none. Some dogs have a mild, lasting cough or tire a little sooner on walks.
Moderate Signs
Coughing, tiring after mild exercise, reduced appetite, and weight loss.
Late-Stage Signs
Fainting, trouble breathing, a swollen belly from fluid, and heart failure.
Caval Syndrome
A sudden blockage of blood flow by many worms: collapse, pale gums and dark urine. This needs emergency surgery.
How Dogs Get Heartworm
Mosquito Bites
A mosquito bites an infected dog, coyote or fox, picks up young larvae, and passes them to the next dog it bites.
Indoor Dogs Are at Risk Too
Mosquitoes get indoors, so heartworm can affect dogs that rarely go outside.
Year-Round Risk
Mosquito seasons vary, and many vets recommend prevention all year so there are no gaps.
Heartworm Testing and Prevention
A Vet Heartworm Test First
A simple blood test finds adult heartworm. Dogs over about seven months old should be tested before starting prevention, then every year, even when on prevention.
Monthly Prevention
Monthly chews or tablets, topical products, or a vet-given injection kill the young larvae before they become adults. Your vet chooses the product that suits your dog.
Breeds That Need Extra Care
Some herding breeds, including collies, shelties, Australian shepherds and their mixes, can be sensitive to ivermectin and related medicines, so your vet may suggest a simple cheek-swab DNA test before choosing a product.
Puppies
Puppies usually start prevention at about eight weeks old, as your vet advises, and are tested later as they grow.
If Your Dog Tests Positive
Treatment Is Done by Your Vet
Heartworm treatment for dogs follows a careful vet protocol over several months, with checks along the way. It is never done at home.
Strict Rest
Dogs must stay calm and rested during treatment, because dying worms can block blood vessels if the dog is active.
Outlook
Most dogs treated before severe heart damage recover well and go on to live normal lives. Your vet will discuss side effects and what to expect.
What To Do Tonight
Routine
No signs and on prevention: keep up monthly prevention and a yearly vet heartworm test.
Urgent
Collapse, breathing trouble, pale gums or dark urine: emergency vet now. A cough with tiring: vet visit soon.
Related products
Heartworm Tablets for Dogs - Heartworm Prevention for Dogs
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Monthly heartworm prevention for dogs. Ask your vet for a heartworm test first and to confirm this product suits your dog. It does not treat an existing heartworm infection. Some herding breeds, such as collies, can be sensitive to ivermectin, the active ingredient, so check with your vet before use.
View product →Free shipping on orders $75+. Prices and stock update automatically. Products are listed for education; medicines are used only when a veterinarian directs care. This page does not diagnose or prescribe.
Related Dog Health Guides
Back to Dog Health Guides →Heartworm in Dogs FAQ
What are the first signs of heartworms in dogs?
Often there are none. Early signs can be a mild, lasting cough and tiring sooner on walks. A yearly vet heartworm test finds infection before signs appear.
What are the symptoms of late-stage heartworms in dogs?
Fainting, breathing trouble, weight loss, a swollen belly and heart failure. Sudden collapse with dark urine is an emergency.
How common is heartworm in dogs?
Heartworm is found in all 50 US states and is most common in the Southeast, along the Mississippi River, and in warm, humid areas, but any dog can be at risk.
Do I need a heartworm test before starting prevention?
Yes, for dogs older than about seven months. Prevention does not cure adult heartworm, and giving it to an infected dog can cause a reaction.
What is the life expectancy of a dog after heartworm treatment?
Most dogs treated before severe heart damage live normal lives. Dogs with advanced disease may have lasting heart or lung changes. Your vet can give you an outlook for your dog.
Are herding breeds sensitive to heartworm medicines?
Some are. Collies, shelties, Australian shepherds and their mixes can carry a gene that makes them sensitive to ivermectin and some other medicines, so ask your vet before giving any parasite product.
Is this heartworm in dogs guide a substitute for a vet visit?
No. It is educational only. Your veterinarian should examine your dog and guide any diagnosis and treatment.
Can I work out a medicine dose from this page?
No. Never work out doses from product strengths, other pets’ medicines or internet pages. The right medicine and dose depend on your dog’s weight, age, breed, health and other medicines, so they must come from your veterinarian.