Heartworms in Dogs: The Silent Threat You Don’t Want to Miss

Early Signs of Heartworm in Dogs Are Easy to Overlook—But the Damage Is Anything but Small

Heartworms in dogs are one of those threats that can start quietly…and become devastating if ignored.

Heartworm disease is spread through infected mosquito bites and caused by a parasitic worm called Dirofilaria immitis. Over time, adult worms live primarily in the pulmonary arteries and can affect the heart and lungs.

Early on, dogs often look perfectly fine.

That’s what makes this disease so sneaky.

By the time obvious symptoms of heartworm in dogs appear, the infection may have been developing for months and damage may already be occurring.

The good news?

👉 Heartworm disease is preventable.

And that’s the part I really want you to lean into.

 

Table of Contents

1. What Heartworm Disease Really Is

2. How Do Dogs Get Heartworm?

3. What Are the First Signs of Heartworms in Dogs?

4. Symptoms of Heartworm in Dogs as Disease Progresses

5. Why Heartworm Disease Is So Dangerous

6. Heartworm Prevention for Dogs

7. Annual Heartworm Testing Matters

8. A Special Note for Herding Breeds

9. Aly’s Bottom Line

10. FAQs About Heartworms in Dogs

 

What Heartworm Disease Really Is

Heartworm disease begins when a mosquito carrying infective heartworm larvae bites a dog.

Those larvae enter the dog’s body and develop over several months into mature heartworms.

Adult heartworms can:

• Grow to approximately a foot long
• Live for years inside a dog's body
• Damage the pulmonary arteries, lungs, and heart
• Interfere with normal blood flow as disease progresses
• Cause serious illness before an owner realizes something is wrong

This isn't a skin parasite.

It isn't simply an annoying worm.

Heartworms in dogs can become a serious cardiopulmonary disease.

And here's the frustrating part:

A dog may appear healthy during the early stages.

That's why prevention and testing matter so much.

 

How Do Dogs Get Heartworm?

If you've ever wondered how do dogs get heartworm, the answer is surprisingly simple:

👉 Mosquitoes.

Heartworm disease is not spread directly from one dog to another.

Here's what happens:

• A mosquito bites an infected dog, coyote, fox, or other susceptible animal
• The mosquito picks up microscopic heartworm larvae called microfilariae
• Those larvae develop inside the mosquito
• The infected mosquito later bites another dog
• Infective larvae enter through the bite wound
• Over the following months, the larvae mature into adult heartworms

So when people tell me:

"But my dog barely goes outside."

I understand the thinking.

But mosquitoes come inside.

They enter houses.

They enter garages.

They find dogs resting on patios and porches.

And heartworm disease has been diagnosed in all 50 states.

Geographic risk of heartworms in dogs varies significantly, but living in a lower-risk region doesn't mean the risk is zero.

Indoor dogs are not automatically protected.

 

What Are the First Signs of Heartworms in Dogs?

This is probably one of the most important questions owners ask:

What are the first signs of heartworms in dogs?

And unfortunately, the answer isn't always satisfying.

There may be few or no obvious early signs.

That's exactly why annual testing matters.

When early signs of heartworm in dogs do appear, they may be subtle:

• A mild persistent cough
• Getting tired more easily
• Less enthusiasm for exercise
• Reduced stamina
• Taking longer to recover after activity
• Seeming unusually tired or “flat”

Maybe your normally energetic dog suddenly doesn't want to finish the walk.

Maybe playtime ends sooner.

Maybe your hiking buddy starts lagging behind.

Those changes are easy to blame on:

• Age
• Hot weather
• Being out of shape
• “Just having a lazy day”

But changes from your dog's normal always deserve attention.

Know your dog. Know their normal. Notice when normal changes.

 

Symptoms of Heartworm in Dogs as Disease Progresses

As the worm burden and damage increase, symptoms of heartworm in dogs can become much more noticeable.

Watch for:

• Persistent cough — especially following activity
• Exercise intolerance — normal activity suddenly becomes difficult
• Fatigue — less stamina, less enthusiasm, less spark
• Reduced appetite
• Weight loss
• Difficulty breathing
• Swollen abdomen — potentially from fluid accumulation associated with advanced heart disease
• Weakness or collapse in severe cases

This is why waiting for obvious symptoms isn't a heartworm strategy.

By the time the body is shouting, the disease may have been whispering for a long time.

 

What About Caval Syndrome?

Severe heartworm disease can sometimes progress to caval syndrome.

This is a life-threatening emergency in which a large burden of heartworms interferes with blood flow through the heart.

Signs may include:

• Sudden weakness
• Collapse
• Pale gums
• Difficulty breathing
• Signs of severe cardiovascular distress

A dog suspected of having caval syndrome needs immediate veterinary treatment.

This isn't something to monitor overnight.

This isn't a “let's see how he feels tomorrow” situation.

This is an emergency.

 

Why Heartworm Disease Is So Dangerous

Heartworm disease isn't simply difficult to treat.

Treatment itself requires careful management because killing adult heartworms creates its own risks.

Treatment may involve:

• Medication to kill adult heartworms
• Additional medications as part of the treatment protocol
• Strict exercise restriction
• Multiple veterinary visits and testing
• Careful monitoring for complications

One of the biggest challenges?

Exercise restriction matters enormously.

As adult worms die, fragments can travel into smaller blood vessels in the lungs, creating potentially serious pulmonary complications.

That means the dog who wants to run, play, jump, and live normally may need significant activity restriction during treatment and recovery.

This is one of those moments where leadership isn't optional.

The dog doesn't understand why we're saying:

"Nope. You cannot chase that squirrel."

We do.

And our job is to make the responsible decision for them.

 

Heartworm Prevention for Dogs

This is where we get to turn the story around.

Heartworm prevention for dogs is far easier than treating established heartworm disease.

The American Heartworm Society recommends year-round prevention and annual testing.

FDA-approved heartworm preventatives come in several forms, including:

• Monthly oral medications
• Monthly topical medications
• Veterinarian-administered injectable preventatives lasting 6 or 12 months

Depending on the product, active ingredients may include:

• Ivermectin
• Milbemycin oxime
• Moxidectin
• Selamectin

Some products also protect against additional parasites, but coverage varies.

That's why this isn't about grabbing whatever box happens to be convenient.

Talk with your veterinarian about:

• Your dog's age
• Weight
• Health history
• Travel
• Lifestyle
• Local parasite exposure
• Other parasite protection your dog may need

Then choose the prevention plan that fits the dog standing in front of you.

 

Annual Heartworm Testing Matters

Here's something owners sometimes ask:

"If I'm giving prevention, why does my dog still need a heartworm test?"

Because prevention is highly effective—but no preventive program is absolutely foolproof.

Dogs can:

• Spit out medication
• Vomit a dose without an owner realizing it
• Miss or receive a monthly dose late
• Lose some topical medication after application

And life happens.

That's why annual testing provides another layer of protection.

Current American Heartworm Society guidance recommends annual screening using both an antigen test and a microfilaria test.

Adult dogs starting prevention after a period without protection may require additional testing based on timing and exposure.

If you miss a dose?

Call your veterinarian rather than guessing.

The timing of heartworm infection and testing matters because a newly acquired infection isn't immediately detectable.

 

Understanding the Risk of Heartworms in Dogs

The risk of heartworms in dogs isn't identical everywhere.

Some areas have much higher infection rates because of:

• Climate
• Mosquito populations
• Humidity
• Standing water
• Wildlife reservoirs
• Movement of infected animals

But here's what matters:

Heartworms have been diagnosed in all 50 states.

And infection patterns can change.

That's why “we don't really have heartworm here” isn't something I'd use as my entire prevention plan.

Mosquitoes move.

Dogs travel.

Weather changes.

Wildlife moves through communities.

Risk isn't static.

Good leadership means planning for the problem before your dog becomes the statistic.

 

A Special Note for Herding Breeds

Some dogs—particularly certain herding breeds and related dogs—can carry a mutation in the ABCB1 gene, commonly called MDR1.

Affected breeds can include:

• Collies
• Australian Shepherds
• Shetland Sheepdogs
• Related breeds and mixes

Dogs with this mutation can be unusually sensitive to certain medications at particular doses, including ivermectin.

But please don't misunderstand this.

MDR1 is not a reason to skip heartworm prevention.

Veterinary-approved heartworm preventatives containing macrocyclic lactones are formulated and labeled at specific preventive doses, and your veterinarian can help determine which product is appropriate for your individual dog.

If you're concerned about MDR1 sensitivity, talk with your veterinarian about testing and medication selection.

Choose wisely.

Don't choose nothing.

 

Aly’s Bottom Line

Heartworms in dogs don't usually start dramatically.

That's precisely what makes them dangerous.

If you're waiting for unmistakable symptoms of heartworm in dogs, you're waiting too long.

If you want your beloved companion:

• Hiking beside you
• Running through the yard
• Playing with the family
• Exploring the world
• Breathing easily
• Living a strong, healthy life for as long as possible

…then heartworm prevention for dogs matters.

Know how dogs get heartworm.

Understand the early signs of heartworm in dogs.

Recognize that the risk of heartworms in dogs exists wherever infected mosquitoes and susceptible animals overlap.

And work with your veterinarian to protect your dog appropriately.

Test yearly.

Prevent year-round.

Pay attention always.

That's not fear-based care.

That's informed, thoughtful leadership.

 

FAQs: Heartworms in Dogs

1. What are heartworms in dogs?

Heartworms in dogs are parasitic worms (Dirofilaria immitis) transmitted through infected mosquitoes. Adult worms primarily live in the pulmonary arteries and can cause serious damage to the lungs, blood vessels, and heart.

2. How do dogs get heartworm?

If you're wondering how do dogs get heartworm, there is one required player: an infected mosquito.

Dogs do not simply catch heartworms by being near another infected dog. A mosquito picks up immature parasites from an infected animal and later transmits infective larvae when it bites another dog.

3. What are the first signs of heartworms in dogs?

What are the first signs of heartworms in dogs?

Sometimes there aren't any.

When early signs of heartworm in dogs occur, they can include:

• Mild persistent coughing
• Reduced stamina
• Exercise intolerance
• Fatigue
• Taking longer to recover after activity

Because these signs can be subtle, routine testing is essential.

4. What are the more serious symptoms of heartworm in dogs?

More advanced symptoms of heartworm in dogs may include:

• Persistent coughing
• Significant exercise intolerance
• Difficulty breathing
• Weight loss
• Weakness
• Swollen abdomen
• Collapse

Severe symptoms require prompt veterinary attention.

5. Can indoor dogs get heartworms?

Yes.

Mosquitoes can enter houses, garages, screened spaces, and other indoor environments.

Indoor living may reduce some exposure, but it doesn't eliminate the risk of heartworms in dogs.

6. Do dogs need heartworm prevention all year?

Current veterinary guidance recommends year-round heartworm prevention for dogs, rather than stopping and restarting based solely on mosquito season.

Consistency matters.

7. Does my dog need a heartworm test if they're already on prevention?

Yes.

Annual testing is recommended even for dogs receiving prevention consistently.

Current American Heartworm Society guidance recommends annual antigen and microfilaria screening.

8. What happens if I miss a heartworm prevention dose?

Contact your veterinarian.

Don't simply double a dose or make up your own schedule.

Your veterinarian can advise you about restarting prevention and the appropriate timing for follow-up testing.

9. Can heartworms in dogs be cured?

Many infected dogs can be successfully treated.

However, eliminating the worms doesn't necessarily erase damage that has already occurred to the heart, lungs, and blood vessels.

That's one reason prevention is so important.

10. Is heartworm treatment dangerous?

Heartworm treatment carries risks and requires careful veterinary supervision.

Dogs generally need significant exercise restriction during treatment because dying worms can contribute to dangerous pulmonary complications.

11. Are natural remedies effective for heartworm prevention?

There is no proven natural substitute for FDA-approved heartworm prevention for dogs.

Mosquito-control measures can be helpful as an additional layer, but they should not replace proven preventive medication.

12. Does heartworm prevention also protect against fleas and ticks?

Some products protect against additional parasites and some don't.

Check the exact product label and work with your veterinarian to determine what protection your dog needs.

13. Can puppies get heartworms?

Yes.

Puppies can become infected through mosquito bites. Prevention can generally begin when puppies are young, depending on the specific product's approved age and weight requirements.

Your veterinarian can establish the appropriate schedule.

14. What is the biggest thing owners should remember about heartworms in dogs?

Don't wait for symptoms.

That's the whole lesson.

The early signs of heartworm in dogs may be subtle—or completely absent.

Heartworm prevention for dogs combined with routine testing gives us the chance to stay ahead of the disease instead of chasing it after damage has already begun.

Prevent the problem you don't ever want to have to treat.