Guide: heartworm prevention for puppies
Heartworm Prevention for Puppies: Why to Start by 8 Weeks
Heartworm prevention for puppies should begin as early as the product label allows, and no later than eight weeks of age, according to the American Heartworm Society. Heartworms are spread by mosquitoes, and they have been found in all 50 states. A monthly preventive is simple and inexpensive, while treating an infected dog takes months.
What heartworm disease is
Heartworms are worms that can grow up to a foot long. They live in the heart, lungs and nearby blood vessels, where they can cause lung disease, heart failure and damage to other organs (American Heartworm Society).
Early on, a dog may show no signs at all. Later come a mild cough, tiredness, reluctance to exercise and weight loss.
How it spreads
Only mosquitoes spread heartworm. A mosquito bites an infected animal and picks up microscopic larvae. Over 10 to 14 days, the larvae develop inside the mosquito. Then the mosquito bites a dog, and the larvae enter through the bite. About six months later, they are adult worms.
So a dog cannot catch heartworm directly from another dog. However, one infected dog in a neighborhood supplies the local mosquitoes.
When to start heartworm prevention for puppies
- Start by eight weeks, or as early as the label allows.
- No test is needed first. Puppies under seven months can begin without one.
- Test later. The American Heartworm Society advises a test six months after starting, another six months after that, and then once a year.
Most preventives are a monthly chew or a spot-on liquid, and many are dosed by weight. Therefore, a growing puppy needs to be weighed often.
Year-round heartworm prevention for puppies and adult dogs
The society’s advice is “Think 12”: test every 12 months, and give prevention 12 months a year. Mosquitoes survive indoors and in mild spells, and people forget to restart in spring.
Although heartworm is most common near the Atlantic and Gulf coasts and along big rivers, it has been diagnosed in every state. In addition, dogs travel. So living in a dry state such as Utah is not a reason to skip it.
If you miss a dose
Call your vet, restart the monthly preventive straight away and plan a test six months later. It takes about seven months before an infection shows up on a test, which is why a test the next day tells you nothing.
If a dog tests positive
Treatment is possible, but it is hard on the dog and the owner. It involves a series of injections given by a vet, strict rest for weeks and several months of follow-up. In short, prevention is far easier.
Heartworm prevention for puppies often covers worms too
Many monthly heartworm preventives also control roundworms and hookworms, which is why parasite experts recommend moving puppies onto one after their early deworming. See worms in puppies, flea and tick prevention for puppies and our puppy vaccine schedule.
Related guides
- Puppy Vaccine Schedule and Deworming: What Is Done Before Go-Home and What Comes Next
- Flea and Tick Prevention for Puppies: When to Start and What Is Safe
- Dog Ear Infections: Signs, Causes and Prevention
- Dog Dental Care: Brushing, Chews and Professional Cleanings
- Luxating Patella in Dogs: Signs, Grades and Treatment
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Sources
- American Heartworm Society. Heartworm Basics. heartwormsociety.org
- Companion Animal Parasite Council. General guidelines for dogs and cats. capcvet.org
Sources were checked on October 7, 2026. This article is general information and does not replace advice from your veterinarian.
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