How To Prevent Heartworm Disease in Puppies

Published Nov. 28, 2023
A Golden Retriever puppy runs through a field.

Heartworm disease in dogs is a very dangerous infection that occurs when a parasitic worm enters a dog’s bloodstream. Once in the bloodstream, the worm grows in the major blood vessels of the heart and lungs. Adult heartworms cause deep damage to these organs and can be difficult, dangerous, and expensive to treat.

While heartworm disease is on the rise in the United States, the good news for dog parents is that it’s preventable with the right medications. Heartworm medicine, usually given monthly, is important for every puppy and should be continued for your dog’s entire life.

What Is Heartworm Disease in Puppies?

Heartworm disease refers to infection by a very specific parasite, Dirofilaria immitis. Clinical disease is mostly seen in domestic and wild dogs, though cats, ferrets, and sea lions have also been known to be affected.

Mosquitoes pick up larval worms (called microfilaria) from infected animals and spread them with their bite to new hosts.

These worms grow to maturity, becoming nearly a foot long, in the major blood vessels of the heart and lungs. After about 5–7 months, adult female worms will create new microfilaria, which can then be spread to new hosts, again by mosquito bite.

Because heartworms take 5–7 months to grow to maturity, visible illness in young puppies is rare.

That said, dogs can be infected with microfilaria at any age by a single mosquito bite.

Mosquitoes are often found inside the home, so even puppies kept only inside are at risk. Young puppies who are not started on preventive medicine can have life-threatening infections by the time they are 7–8 months old. 

As soon as a heartworm is mature, it can affect a puppy’s heart and lung function. Treatment of heartworm disease is very expensive, and the side effects of both the medication and worm/microfilaria death can be life-threatening for a dog.

Puppies undergoing treatment can develop severe allergic reactions (anaphylaxis), and blood clots can form around microfilaria and cause organ and brain damage.

Though your veterinarian will try to avoid these complications with other medications like steroids, preventing infection in the first place is much safer for your puppy.

Symptoms of Heartworm Disease in Puppies

Because heartworms affect the heart and lungs, most symptoms of heartworm disease are related to the respiratory and cardiovascular systems.

Cough and fatigue are most common.

Decreased appetite and weight loss can also be noted. Severe heart damage can lead to the buildup of fluid in the chest or abdomen. These symptoms are usually seen only with severe infections, but they are occasionally noted in young puppies.

The most frightening aspect of heartworm disease is how healthy a puppy can look while having this deadly infection. In fact, most dogs who test positive for heartworm disease show no clinical signs whatsoever.

These dogs are still at risk for complications with treatment and mosquitoes can spread the infection to other dogs.

The American Heartworm Society recommends testing all puppies and dogs over 7 months of age for heartworm disease at least once a year to catch infections that might otherwise go unnoticed.

How You Can Prevent Heartworm Disease in Puppies

Heartworm preventatives are very effective and easily accessible through your veterinarian. Puppies should be started on prevention by no later than 8 weeks of age. Most products are given every 30 days. Marking the date on a calendar or setting a reminder on your phone can be helpful in ensuring there is no break in coverage.

Most preventatives have doses based on weight, so your veterinarian may dispense doses one at a time until your puppy reaches a certain size or weight.

This is especially important for larger puppies who may grow into the next weight range within a month. Giving the wrong size preventative will not keep your puppy protected.

Another product that can help prevent heartworm disease in your puppy is mosquito repellent. Products like Advantix® II don’t directly address heartworms but can help reduce exposure by decreasing mosquito bites.

This should not be the only protection you use for your puppy. Talk to your vet about what products might fit your puppy’s lifestyle.

Best Heartworm Prevention for Puppies

There are many great products on the market—and choosing the best one for your pup can be overwhelming. The best product for heartworm prevention is the one that you are comfortable with, you can afford, and is easy to give to your puppy. Here are some once-a-month options:

Other safe options are on the market as well. Ask your veterinarian to help you pick one that works for you and your puppy.

When Should You Start Your Puppy on Heartworm Prevention?

Most puppies can be started on heartworm prevention as soon as they come home, depending on their size.

All puppies should be on heartworm prevention by 8 weeks of age.

If you have adopted an older puppy (older than 5 months) who hasn’t been on prevention or if there has been more than a month lapse in protection, your veterinarian will recommend testing them for active infection before starting medications.

Heartworm preventatives should not be given to dogs with an active heartworm infection; this can cause severe reactions and even death.

How to Prevent Heartworm Disease in Puppies FAQs

What are the dangers of heartworm medication?

Heartworm medications are very safe, and most side effects are limited to upset stomach, vomiting, or diarrhea.

What is a dog’s life expectancy after heartworm treatment?

If heartworm disease has been caught before organ damage can occur, treated dogs are expected to live a full and happy life.

If your dog has heartworm disease, your veterinarian will test for common forms of organ damage and discuss risks and potential complications for your pup.

Why do dogs need a heartworm test before medication?

Heartworm preventatives target the larval stage of the heartworm life cycle. If these larvae are allowed to mature because of missed doses, they can grow to adulthood and cause heartworm infection.

 If there are adult worms creating new larvae, the death of large numbers of larvae can also lead to allergic reactions and blood clots that can cause organ damage in your dog.

Featured Image: MatthewCrissall/iStock / Getty Images Plus via Getty Images

Jamie Lovejoy, DVM


Jamie Lovejoy, DVM


Dr. Jamie Lovejoy graduated from Tufts Cummings School of Veterinary Medicine in 2012 after an undergraduate degree in Marine Biology. ...

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