When Should Puppies Start Heartworm Prevention? A Vet's Complete Guide

He’s stolen your favorite shoes, the best spot on the couch, and your entire heart. It’s only a fair trade that you protect his. Heartworm disease is a serious, potentially fatal condition caused by parasitic worms that live in a dog's heart and lungs. The good news? It's entirely preventable with consistent monthly medication.

Puppies should start heartworm prevention no later than 8 weeks of age, according to the American Heartworm Society. Heartworm disease is a serious, potentially fatal condition caused by parasitic worms that live in a dog's heart and lungs.

All it takes is one bite from an infected mosquito to put your puppy at risk. This guide covers everything you need to know about protecting your new puppy, including when to start prevention, the different types of heartworm medicine available, costs, and testing schedules.

What Are Heartworms?

Heartworms (Dirofilaria immitis) are parasitic worms that live in a dog's heart, lungs, and surrounding blood vessels. Adult heartworms can grow up to 12 inches long. It's common for dozens of worms to infest an infected dog at once.

These worms cause significant damage to the heart and lungs, preventing the heart from pumping blood correctly throughout the body. Unlike intestinal worms in puppies that affect the digestive system, heartworms target the cardiovascular system and require specific and different prevention methods.

Once adult heartworms establish themselves in a dog's body, they can live for 5 to 7 years. During that time, they continue to reproduce and cause progressive damage to vital organs.

How Do Puppies Get Heartworms?

Mosquitoes are the only way heartworms spread from one dog to another. Dogs cannot transmit heartworms to each other through direct contact, shared water bowls, or any other means. The entire transmission cycle depends on mosquitoes.

Here's how the cycle works: When a mosquito bites an infected dog, it picks up microscopic baby worms called microfilariae. These larvae mature inside the mosquito over the course of about two weeks. When that mosquito bites another dog or puppy, it deposits the larvae into the skin. From there, the larvae migrate through the body's tissues and eventually reach the heart and lungs, where they grow into adult worms.

Because mosquitoes are found everywhere, including indoors, keeping your puppy inside isn't enough to prevent infection. Heartworm disease has been diagnosed in dogs in all 50 states. This is why consistent prevention is so important.

Can Puppies Get Heartworms?

Yes. Puppies can be infected with heartworms from their very first day of life if they're bitten by an infected mosquito. One bite is all it takes.

However, there's an important distinction between infection and disease. Heartworm disease, where symptoms become apparent, is rare in puppies under 6 months old. This is because it takes approximately 6 months for the larvae to mature into adult heartworms that cause noticeable damage.

This doesn't mean young puppies are safe. A puppy infected at 8 weeks old won't show symptoms right away, but those larvae are quietly developing. By the time the puppy is 7 or 8 months old, adult worms could already be living in their heart.

When Should Puppies Start Heartworm Prevention?

The American Heartworm Society recommends starting puppies on heartworm prevention no later than 8 weeks of age. Some products, like Heartgard Plus, are FDA-approved for puppies as young as 6 weeks old.

Each heartworm prevention product has specific age and weight requirements. Your veterinarian will help you choose the safest and most appropriate option for your puppy based on their size, age, and overall health. Don't wait to have this conversation, most vets will bring it up at your puppy’s first vaccination appointment. The earlier you start prevention, the better protected your puppy will be.

All heartworm preventives require a prescription from a veterinarian. This ensures your puppy receives the correct dosage and that the medication is safe for them to take. Your vet will also establish a testing and follow-up schedule to make sure your puppy stays protected.

Does My Puppy Need a Heartworm Test First?

Whether your puppy needs a heartworm test before starting prevention depends on their age and history.

Puppies under 7 months old can start heartworm prevention without a test. Because it takes at least 6 months for heartworms to mature enough to be detected, testing a very young puppy wouldn't show an infection even if one existed. However, your vet will schedule follow-up testing at 6 months after starting prevention, again 6 months later, and then annually.

Puppies 7 months or older who have never been on prevention should be tested before starting medication. At this age, there's been enough time for a potential infection to become detectable. Giving prevention to a dog with an active heartworm infection can cause dangerous reactions.

Puppies with lapsed prevention (missed doses for more than a month) should see their vet immediately. Your vet will restart prevention and schedule testing 6 months later to check for any infection that may have developed during the gap.

Puppy Heartworm Testing Schedule

Types of Heartworm Prevention for Puppies

Several types of heartworm preventives are available, and your veterinarian will recommend the best option for your puppy. All require a prescription.

Oral chewables are the most popular option. These flavored tablets or soft chews are given monthly and are often designed to taste like treats. Many oral preventives also protect against intestinal parasites like roundworms and hookworms. Most are safe for puppies starting at 6 to 8 weeks of age, depending on the specific product.

Topical preventives are liquid medications applied directly to your puppy's skin, typically between the shoulder blades. These are a good option for puppies who won't take pills or are picky eaters. Some topical products also protect against fleas and ticks. Avoid bathing your puppy or letting them swim immediately after application.

Injectable preventives (such as ProHeart) are administered by your veterinarian and provide protection for 6 or 12 months, eliminating the need to remember monthly doses. However, these are only approved for dogs 6 months of age and older, so they're not an option for young puppies.

Important: Flea and tick medications do not prevent heartworms unless they specifically include a heartworm preventive ingredient. Always check the label and confirm with your vet.

Heartworm Prevention Types Compared

*Varies by specific product. Always check the label and consult your veterinarian.

What Months Do Dogs Need Heartworm Pills?

All 12 of them. The American Heartworm Society, the FDA, and most veterinarians recommend year-round heartworm prevention regardless of where you live.

Some pet owners wonder if they can skip prevention during colder months when mosquitoes seem less active. This approach is risky for several reasons:

Mosquitoes don't follow calendars. They can survive indoors year-round in basements, garages, and heated spaces. Unseasonably warm days in winter can also activate dormant mosquitoes outdoors.

Prevention works retroactively. Each monthly dose kills larvae your dog may have been exposed to during the previous 30 days. Skipping even one month creates a window where larvae can mature beyond the point where prevention can eliminate them.

Consistency is easier. Maintaining a year-round schedule eliminates confusion about when to start and stop. It's much easier to remember a monthly routine than to track seasonal timing.

How Much Does Heartworm Prevention Cost?

Heartworm prevention typically costs between $6 and $18 per month, depending on your dog's size and the specific product. That works out to roughly $72 to $216 per year.

Compare that to the cost of treating heartworm disease: $600 to $6,000 or more, depending on the severity of the infection. Treatment requires multiple veterinary visits, blood tests, X-rays, injections of an arsenic-based medication, and months of strict exercise restriction. In severe cases requiring surgery, costs can exceed $6,000.

Beyond the financial cost, treatment is hard on your dog. The medications used to kill adult heartworms can cause serious side effects, and the worms themselves can cause complications as they die. Even after successful treatment, some dogs suffer permanent heart and lung damage.

Prevention vs. Treatment: Cost Comparison

Managing the Costs of Heartworm Prevention for Puppies

The first year of owning a puppy comes with a lot of moving parts. Between the initial rounds of shots, the follow-up exams, and constantly restocking heartworm and flea prevention, the vet bills tend to show up one after another.

If you’d rather not pay for everything in large chunks, you can add an optional Wellness Rewards program to your Embrace Pet Insurance plan to help space things out. It’s a way to take those expected annual costs, like your puppy's meds and heartworm tests, and break them down into more manageable monthly payments.

Here is how it actually works in practice:

• No waiting periods: You can use these funds right away. If you sign up before your puppy’s 8-week appointment, you can use the rewards to pay for that first dose of heartworm prevention immediately.

• It’s flexible: Many plans put strict limits on how much they’ll pay for specific items. With Embrace, you get a set amount of rewards to use however you see fit for preventive care. If you want to use the majority of it to buy a 6-month or 12-month supply of heartworm meds at once, you can, or you can use it on flea and tick treatment, training classes, or even grooming.

While the wellness plan handles the routine costs, the pet insurance side is there for the unexpected. If your dog does end up with heartworm disease, the treatment is a major medical undertaking. Having a policy in place means the costs of specialized medications and vet stays are covered, so you can focus on your puppy's recovery.

It’s a straightforward way to keep your puppy protected while making the costs of that first year a lot more predictable.

What Happens If I Don't Use Heartworm Prevention?

Without prevention, your puppy is vulnerable to heartworm infection from a single mosquito bite. Once infected, the larvae develop inside your dog's body, and prevention medication can no longer eliminate them after about 51 days.

Heartworm disease often doesn't cause obvious symptoms until significant damage has already occurred. When symptoms do appear, they may include:

Persistent cough: Caused by worms in the lungs and surrounding blood vessels.

Reluctance to exercise: Dogs may tire easily or avoid physical activity they previously enjoyed.

Decreased appetite and weight loss: As the disease progresses, dogs often eat less and lose weight.

Difficulty breathing: Worms interfere with blood flow and oxygen delivery.

Swollen belly: In advanced cases, fluid accumulates in the abdomen due to heart failure.

Without treatment, heartworm disease is fatal. Even with treatment, some dogs experience permanent heart and lung damage. Prevention is far safer, easier, and less expensive than treatment.

Is There a Natural Heartworm Prevention for Puppies?

No. There is no FDA-approved natural heartworm prevention available. While various natural remedies are marketed online, none have been scientifically proven to prevent heartworm infection.

Heartworm disease is too serious to risk on unproven methods. Prescription preventives have decades of research supporting their safety and effectiveness. They remain the only reliable way to protect your puppy from this potentially fatal disease.

Talk to your veterinarian about the best FDA-approved prevention option for your puppy's specific needs.