How Do Dogs Get Heartworms? The Complete Guide Every Dog Owner Should

How do dogs get heartworms? It's one of the most important questions every dog owner should ask because heartworm disease is a serious and potentially life-threatening condition that often develops silently. Heartworms are just one of many illnesses that can affect dogs. Learn about other common canine health conditions in our guide to 25 Most Common Dog Diseases Every Owner Should Know.

Many people mistakenly believe dogs catch heartworms directly from infected dogs, but the truth is far different. In reality, mosquitoes are responsible for spreading heartworms, making even indoor dogs vulnerable. Understanding how heartworms are transmitted, recognizing the risk factors, and learning how to protect your dog can prevent costly treatments and save your pet's life. In this guide, you'll discover everything you need to know about heartworms, including how dogs become infected, the parasite's life cycle, and which dogs are most at risk.

What Are Heartworms?

Heartworms are long, thin parasitic worms scientifically known as Dirofilaria immitis. Unlike intestinal worms that live in a dog's digestive tract, adult heartworms inhabit the heart, lungs, and surrounding blood vessels. Over time, these parasites interfere with normal blood circulation, forcing the heart to work harder and damaging vital organs.

Heartworm disease develops gradually. A dog may appear perfectly healthy for months—or even years—before obvious symptoms become noticeable. Unfortunately, by the time many dogs begin coughing or showing signs of fatigue, the worms may already have caused permanent damage to the heart and lungs.

Adult heartworms resemble strands of cooked spaghetti and can grow up to 12 inches long. According to the Merck Veterinary Manual, adult heartworms live primarily in the pulmonary arteries and heart, where they can cause serious cardiovascular and respiratory damage if left untreated.

In severe infections, a single dog may harbor dozens of adult worms. These worms can survive for five to seven years inside a dog, continuously reproducing and increasing the severity of the disease if left untreated.

Heartworm disease occurs worldwide but is especially common in warm, humid regions where mosquitoes thrive. However, no location is completely safe. Even areas with cold winters experience mosquito activity during warmer months, meaning dogs everywhere can be exposed.

It's also important to understand that heartworms are completely different from common intestinal parasites such as:

• ·        Roundworms
• ·        Hookworms
• ·        Whipworms
• ·        Tapeworms

These parasites spread through contaminated soil, feces, or infected prey, whereas heartworms require mosquitoes to complete their life cycle.

Because early infections often produce no symptoms, annual heartworm testing and year-round prevention remain the most effective ways to protect your dog.

How Do Dogs Get Heartworms?

The answer is surprisingly simple: dogs get heartworms through the bite of an infected mosquito.

A dog cannot catch heartworms by playing with another infected dog, sharing food or water bowls, sleeping together, or licking another dog's face. Instead, mosquitoes act as the essential carrier that transfers heartworm larvae from one animal to another.

Here's how the process works:

Step 1: A Mosquito Bites an Infected Animal

When a mosquito feeds on an infected dog, coyote, fox, or wild canid, it ingests microscopic baby heartworms known as microfilariae. These tiny parasites circulate in the infected animal's bloodstream.

Step 2: The Larvae Develop Inside the Mosquito

Over the next 10 to 14 days (depending on environmental temperatures), the microfilariae mature into infective larvae inside the mosquito. Without this development stage, the parasite cannot infect another dog.

Step 3: The Mosquito Bites Another Dog

When the infected mosquito bites another dog, the infective larvae leave the mosquito and enter the dog's skin through the bite wound.

Contrary to popular belief, the larvae are not injected directly into the bloodstream. Instead, they enter through the tiny puncture left by the mosquito.

Step 4: The Larvae Begin Their Journey

After entering the body, the larvae migrate through muscles and tissues for several months. During this time, they continue to grow and mature while remaining largely undetectable.

Eventually, they enter the bloodstream and travel to the heart and pulmonary arteries, where they become adult worms capable of reproducing.

Because the disease develops so slowly, infected dogs often show no immediate signs of illness. This silent progression is one of the reasons heartworm disease is considered so dangerous.

Fortunately, monthly heartworm preventives kill these immature larvae before they develop into adult worms, which is why prevention is far easier, safer, and less expensive than treatment.

The Heartworm Life Cycle Explained

Understanding the heartworm life cycle helps explain why consistent prevention is essential.

Stage 1: Microfilariae

Adult female heartworms living inside an infected dog release microscopic offspring called microfilariae into the bloodstream. These baby worms cannot infect another dog directly.

Stage 2: Mosquito Transmission

When a mosquito bites an infected dog, it ingests the microfilariae along with the blood meal.

Inside the mosquito, the parasites undergo several developmental stages until they become infective larvae.

Stage 3: Infection of a New Dog

The infected mosquito bites another dog, depositing infective larvae onto the skin. The larvae enter through the bite wound and begin migrating through body tissues.

Stage 4: Migration

For approximately two to three months, the larvae develop beneath the skin and within muscle tissue. During this stage, the dog usually appears completely healthy.

Stage 5: Arrival at the Heart

Around four to six months after infection, the immature worms enter the bloodstream and eventually reach the pulmonary arteries and heart. Here, they mature into adult heartworms.

Stage 6: Reproduction

Adult worms begin reproducing, releasing thousands of microfilariae into the bloodstream. Mosquitoes feeding on the infected dog then continue the cycle by transmitting the parasites to other animals.

This life cycle highlights why missing even a few doses of preventive medication can leave your dog vulnerable.

Can Dogs Catch Heartworms From Other Dogs?

This is one of the biggest myths surrounding heartworm disease. The answer is no.

Dogs cannot transmit heartworms directly to one another.

Heartworms require a mosquito to complete their life cycle, meaning direct contact between dogs poses no risk.

Your dog cannot become infected by:

• ·        Playing at the dog park
• ·        Sharing toys
• ·        Sharing food bowls
• ·        Sharing water bowls
• ·        Sleeping beside another dog
• ·        Grooming or licking another dog
• ·        Visiting kennels or boarding facilities

Without mosquitoes, the disease cannot spread. However, if an infected dog lives in an area with mosquitoes, those mosquitoes can become carriers after feeding on the infected dog's blood. They may later bite another dog, transmitting infective larvae.

This is one reason veterinarians recommend year-round heartworm prevention—even for dogs that rarely leave home. Indoor dogs are still exposed whenever mosquitoes enter the house through open doors, windows, garages, or screened patios.

Which Dogs Are Most at Risk?

Every dog is potentially at risk of heartworm disease, regardless of breed, age, or lifestyle. However, some dogs have a greater chance of exposure than others.

Dogs Living in Warm, Humid Regions

Mosquito populations flourish in warm climates, making heartworm transmission more common in tropical, subtropical, and coastal areas.

Dogs That Spend Time Outdoors

Dogs that frequently:

• ·        Go hiking
• ·        Visit parks
• ·        Camp
• ·        Hunt
• ·        Swim
• ·        Spend long hours in backyards

are naturally exposed to more mosquitoes.

Dogs Without Preventive Medication

The greatest risk factor is simply not receiving year-round heartworm prevention. Even one missed month during mosquito season can increase the likelihood of infection.

Rescue Dogs

Dogs adopted from shelters or rescue organizations may have unknown medical histories and could already be infected before adoption. A veterinary examination and heartworm test are essential soon after bringing a rescue dog home.

Puppies

Although puppies are born free of heartworms, they become vulnerable once mosquitoes begin biting them. Veterinarians typically recommend starting heartworm prevention during puppyhood according to age and weight guidelines.

Senior Dogs

Older dogs are not naturally resistant. As dogs age, regular veterinary care becomes even more important. Our guide on How to Keep Elderly Dogs Healthy covers practical tips for supporting your senior dog's heart, joints, and overall well-being.

In fact, long-term exposure to mosquitoes over many years may increase the likelihood of infection if prevention has been inconsistent. Even dogs that spend most of their lives indoors should never be considered completely safe. Mosquitoes are surprisingly adept at finding their way into homes, making year-round prevention the safest strategy for every dog.

Signs and Symptoms of Heartworms in Dogs

One of the most dangerous aspects of heartworm disease is that infected dogs often show no symptoms during the early stages. Since heartworms take several months to mature, many dogs appear healthy while the parasites silently damage the heart, lungs, and blood vessels.

As the worms grow and multiply, symptoms gradually become more noticeable. The severity usually depends on the number of worms present, how long the dog has been infected, and the dog's overall health.

Early Signs of Heartworm Disease

During the initial stages, you may notice only subtle changes in your dog's behavior, including:

• ·        Occasional soft cough
• ·        Reduced stamina during walks
• ·        Mild fatigue after exercise
• ·        Less enthusiasm for playing
• ·        Slight decrease in appetite

These symptoms are often mistaken for normal aging or minor illnesses, which is why regular heartworm testing is so important.

Moderate Heartworm Disease

As more adult worms accumulate in the heart and pulmonary arteries, symptoms become more obvious.

Common signs include:

• ·        Persistent coughing
• ·        Difficulty exercising
• ·        Heavy panting
• ·        Weight loss
• ·        Decreased appetite
• ·        Lethargy
• ·        Labored breathing

At this stage, permanent damage to the lungs and heart may already be occurring.

Advanced Heartworm Disease

Without treatment, heartworm disease can become life-threatening.

Severely infected dogs may develop:

• ·        Swollen abdomen caused by fluid buildup
• ·        Collapse after activity
• ·        Pale gums
• ·        Difficulty breathing even while resting
• ·        Fainting episodes
• ·        Heart failure

In rare cases, dogs develop caval syndrome, a medical emergency in which a large number of worms block blood flow through the heart. Dogs with caval syndrome may suddenly collapse and require immediate surgery to remove the worms. Without emergency treatment, survival is unlikely.

If your dog develops any of these symptoms, schedule a veterinary examination immediately. Early diagnosis greatly improves the chances of successful treatment.

How Veterinarians Diagnose Heartworm Disease

Heartworm disease cannot be diagnosed based on symptoms alone. Because many infected dogs appear perfectly healthy during the early stages, veterinarians rely on specific diagnostic tests.

Heartworm Antigen Test

The most common screening method is the heartworm antigen test. This simple blood test detects proteins produced by adult female heartworms. Results are usually available within minutes and are highly accurate for identifying mature infections. Most veterinarians recommend testing:

• ·        Once a year
• ·        Before starting heartworm prevention
• ·        Whenever prevention has been missed for several months
• ·        Before changing preventive medications

Microfilaria Test

Your veterinarian may also examine a blood sample under a microscope to look for microfilariae, the tiny offspring produced by adult worms.

Some infected dogs test positive for adult worms but have no circulating microfilariae, so this test is often combined with the antigen test.

Chest X-rays

X-rays help evaluate:

• ·        Heart enlargement
• ·        Lung damage
• ·        Enlarged pulmonary arteries
• ·        Severity of infection

This information helps veterinarians determine the safest treatment plan.

Ultrasound (Echocardiogram)

For severe cases, an ultrasound may allow veterinarians to visualize adult worms inside the heart or nearby blood vessels.

Additional Blood Tests

Your veterinarian may recommend:

• ·        Complete blood count (CBC)
• ·        Blood chemistry profile
• ·        Urinalysis

These tests assess liver and kidney function before treatment begins and help identify any complications.

Can Heartworms Be Treated?

Yes—but treatment is significantly more difficult, expensive, and risky than preventing the disease. Heartworm treatment involves killing adult worms while minimizing damage to the dog's heart and lungs. The American Veterinary Medical Association (AVMA) emphasizes that heartworm treatment should always be performed under veterinary supervision because improper treatment can lead to serious complications.

Step 1: Stabilize the Dog

Dogs with advanced infections may require medication to reduce inflammation and stabilize heart function before treatment begins.

Step 2: Kill Immature Worms

Your veterinarian may prescribe medications to eliminate immature heartworms before targeting adult worms.

Step 3: Adult Heartworm Treatment

The only FDA-approved medication for eliminating adult heartworms is an injectable drug administered by a veterinarian. Treatment usually involves multiple injections over several months.

Step 4: Strict Exercise Restriction

This is one of the most important parts of treatment. As adult worms die, they begin breaking apart inside the blood vessels. Excessive activity can increase the risk of life-threatening complications such as pulmonary embolism. Dogs often require 6–8 weeks of strict rest after treatment.

Step 5: Follow-Up Testing

Approximately six months after treatment, veterinarians perform another heartworm test to confirm that all adult worms have been eliminated. Even after successful treatment, some dogs are left with permanent heart or lung damage, making prevention the far better option.

How to Prevent Heartworms in Dogs

Fortunately, heartworm disease is almost entirely preventable. Modern preventive medications are extremely effective when used consistently. The American Heartworm Society recommends year-round heartworm prevention and annual testing for dogs, even if they spend most of their time indoors, because mosquitoes can enter homes and transmit the disease.

Give Preventive Medication Year-Round

Many owners mistakenly stop prevention during colder months. However, mosquitoes can survive indoors or emerge unexpectedly during warm weather, making year-round protection the safest choice. Always follow your veterinarian's recommended schedule.

Never Skip Monthly Doses

Missing even one monthly treatment may allow developing larvae to mature into adult worms. Set reminders on your phone or mark doses on a calendar to avoid missed treatments.

Reduce Mosquito Exposure

Although prevention medication remains essential, reducing mosquito exposure can provide additional protection. Simple measures include:

• ·        Eliminating standing water around your home
• ·        Keeping grass trimmed
• ·        Repairing window screens
• ·        Avoiding outdoor activities during peak mosquito hours
• ·        Using pet-safe mosquito control methods
• ·        Schedule Annual Heartworm Tests

Even dogs receiving monthly preventives should be tested annually. Testing confirms that prevention is working and detects infections before severe damage develops. Preventive care also includes proper grooming. Our article on Important Areas You Can't Miss When Grooming Your Dog explains how routine grooming helps you detect lumps, skin changes, and other health concerns before they become serious.

Best Heartworm Prevention Products (Amazon Affiliate-Ready)

While prescription heartworm medications must be obtained through your veterinarian, several products available on Amazon can help reduce mosquito exposure and support your dog's overall health. These products complement—but do not replace—veterinary-prescribed heartworm prevention.

1. Vet's Best Mosquito Repellent for Dogs 

This plant-based mosquito repellent uses natural essential oils to help repel mosquitoes during walks and outdoor adventures.

Why it's recommended:

• ·        Helps reduce mosquito exposure
• ·        Easy spray application
• ·        Suitable for outdoor activities
• ·        Pleasant natural scent

👉 Protect your dog from mosquitoes during outdoor adventures. Check today's price on Amazon.

2. K&H Pet Cot Elevated Dog Bed 

Keeping dogs off damp ground can reduce exposure to insects while camping or relaxing outdoors.

Benefits:

• ·        Breathable mesh design
• ·        Elevated sleeping surface
• ·        Durable construction
• ·        Ideal for patios and backyards

👉 See why thousands of dog owners recommend this elevated outdoor dog bed on Amazon.

3. Furhaven Orthopedic Dog Bed 

Dogs recovering from heartworm treatment require weeks of strict rest. A supportive orthopedic bed helps keep them comfortable during recovery.

Benefits:

• ·        Supports joints
• ·        Comfortable memory foam
• ·        Machine-washable cover
• ·        Available in multiple sizes

👉 Give your recovering dog the comfort they deserve with this highly rated orthopedic bed.

4. Outward Hound Interactive Puzzle Toy 

Dogs recovering from heartworm treatment must avoid vigorous exercise. Interactive puzzle toys provide mental stimulation while keeping physical activity to a minimum.

Benefits:

• ·        Reduces boredom
• ·        Encourages problem-solving
• ·        Slows treat consumption
• ·        Ideal during recovery periods

👉 Keep your dog mentally engaged during recovery with this bestselling puzzle toy.

Important: Prescription heartworm preventives such as Heartgard Plus®, Simparica Trio®, Interceptor® Plus, and NexGard® PLUS should only be obtained through your veterinarian or a licensed veterinary pharmacy.

Common Mistakes Dog Owners Make

Heartworm disease is largely preventable, yet many infections occur because of avoidable mistakes.

Believing Indoor Dogs Are Safe

Mosquitoes frequently enter homes through open doors, windows, garages, and patios. Indoor dogs still need year-round prevention.

Skipping Preventive Medication During Winter

Mosquito activity depends on local temperatures, not the calendar. Stopping prevention too early can leave dogs vulnerable.

Missing Monthly Doses

One missed dose may allow larvae to mature beyond the stage where preventive medications are effective.

Ignoring Annual Testing

No preventive medication is 100% foolproof. Annual testing ensures infections are detected early.

Using Over-the-Counter Alternatives

Many products claiming to prevent heartworms do not kill heartworm larvae. Only veterinarian-approved prescription preventives have been proven effective.

Waiting for Symptoms

By the time coughing, fatigue, or breathing difficulties appear, heartworms may have already caused irreversible damage.

The best approach is simple: prevent the disease rather than treat it. Consistent preventive medication, annual veterinary testing, and mosquito control remain the most effective ways to protect your dog's heart and overall health.

Frequently Asked Questions

1. Can humans get heartworms from dogs?

No. Heartworms are not contagious between dogs and humans. People cannot catch heartworms by touching, petting, or living with an infected dog. In rare cases, a mosquito carrying heartworm larvae may infect a person, but the worms usually die before maturing and do not cause heartworm disease as they do in dogs.

2. Can indoor dogs get heartworms?

Yes. Many owners assume indoor dogs are safe, but mosquitoes can easily enter homes through open doors, windows, garages, or patios. Since mosquitoes are responsible for transmitting heartworms, both indoor and outdoor dogs should receive year-round heartworm prevention.

3. Can puppies get heartworms?

Yes. Puppies can become infected after being bitten by an infected mosquito. Veterinarians typically recommend starting heartworm prevention at 6–8 weeks of age, depending on the medication used and your veterinarian's advice.

4. How long does it take for heartworms to affect a dog?

Heartworms develop slowly. After infection, it usually takes about six months for the larvae to mature into adult worms capable of causing serious damage. Many infected dogs show no symptoms during this period, which is why annual heartworm testing is essential.

5. Is heartworm treatment painful?

Heartworm treatment involves a series of injections administered by a veterinarian. Some dogs experience soreness at the injection site and may feel tired for a few days. However, veterinarians provide pain management and closely monitor dogs throughout treatment to ensure their comfort and safety.

6. Can heartworms be completely cured?

Yes, many dogs recover successfully when diagnosed early and treated properly. However, heartworms can leave permanent damage to the heart, lungs, and blood vessels, especially in advanced cases. Prevention remains much safer and less expensive than treatment.

7. Do mosquitoes spread heartworms year-round?

That depends on your climate. In warm regions, mosquitoes may remain active throughout the year. Even in colder areas, unexpected warm weather can allow mosquitoes to survive. For this reason, most veterinarians recommend year-round heartworm prevention rather than stopping during winter.

8. What is the best way to protect my dog from heartworms?

·        The most effective approach combines:

• ·        Monthly or long-acting prescription heartworm prevention
• ·        Annual heartworm testing
• ·        Routine veterinary checkups
• ·        Reducing mosquito exposure around your home

Using these methods together offers the best protection against heartworm disease.

Conclusion

Heartworm disease is one of the most serious yet preventable illnesses affecting dogs today. Although many owners ask, "How do dogs get heartworms?", the answer is straightforward: dogs become infected through the bite of an infected mosquito—not from direct contact with other dogs.

Because heartworms can silently damage the heart and lungs for months before symptoms appear, prevention should always be your first line of defense. Consistent use of veterinarian-prescribed preventive medication, annual heartworm testing, and minimizing mosquito exposure can dramatically reduce your dog's risk of infection.

If you notice persistent coughing, reduced energy, difficulty breathing, or unexplained weight loss, don't wait. Schedule an appointment with your veterinarian as soon as possible. Early diagnosis and treatment greatly improve your dog's chances of a full recovery.

Ultimately, protecting your dog from heartworms is far easier—and far less expensive—than treating the disease. By staying informed and proactive, you can help your furry companion enjoy a longer, healthier, and happier life.