Cover Image for Understanding Heartworm in Dogs: A Comprehensive Guide
Pet Health

Understanding Heartworm in Dogs: A Comprehensive Guide

The best way to tackle heartworm disease is through prevention. Several highly effective heartworm preventatives are available and should be utilized as the first line of defense against this disease.

Dr. Karen Whala

August 01, 2023

Every pet owner's nightmare is the thought of their beloved friend falling ill. When it comes to heartworm disease, this fear can feel all too real. But fear not, we at CodaPet are here to guide you through this challenging topic. This comprehensive guide will arm you with knowledge about heartworm disease in dogs - from understanding its nature, identifying symptoms, through diagnostic testing, treatment, and most importantly, prevention. By investing your time in understanding this disease, you will be well-equipped to protect your pet and ensure their health and happiness.

The Nature of Heartworm Disease

What is Heartworm?

Heartworm disease is a severe and potentially fatal condition, predominantly affecting dogs. It is caused by parasitic worms, Dirofilaria immitis or D. immitis, which reside in the heart, lungs, and associated blood vessels of infected animals. The adult worms can grow to a foot long and inflict significant damage to internal organs, impacting the pet's overall health and quality of life. Dogs, as natural hosts for heartworms, are needed for the parasites to mature and reproduce, which can lead to lung damage, heart failure, and other organ damage.

The severity of the disease caused by these parasites, along with their capacity to thrive and multiply dramatically if untreated, makes heartworm disease a significant concern for pet owners. The disease is pervasive, affecting even indoor pets.

Transmission of Heartworm

Heartworm is transmitted through the bite of an infected mosquito. Heartworms cannot be transmitted directly from one dog to another without such a bite from an infected mosquito. Dogs (and sometimes other canids) are the primary host for heartworm, while mosquitoes are an intermediate or secondary host. Both are crucial for the completion of the life cycle but the worms spend a majority of their life in the dog, where they reach sexual maturity. To further understand the transmission of heartworm disease we must look into the life cycle of a heartworm.

The Life Cycle of Heartworm

Understanding the life cycle of a heartworm is essential to comprehend the nature of heartworm disease testing and prevention. The cycle begins when an adult female heartworm in an infected dog releases offspring, called microfilariae, into the dog's bloodstream. A mosquito taking a blood meal from the infected dog picks up these microfilariae, which then develop into infective larvae within the mosquito.

The maturation of the microfilariae into larvae within the mosquito takes approximately two weeks, then the larvae are ready to infect a new dog. Upon biting, the larvae within the mosquito’s mouth-parts enter the bitten dog's bloodstream. Over the period of about six months, these larvae develop into adult heartworms, mate, and release new microfilarial offspring into the infected dog’s bloodstream ready for a mosquito to pick them up and continue the cycle.

Life Cycle of Heartworm

Common Misconceptions about Heartworm

There are a handful of misconceptions surrounding heartworm disease, so let’s set the record straight and keep your beloved pet safe.

Misconception #1: Heartworms affect only outdoor dogs.

I don’t know about you, but I have often encountered mosquitoes indoors. All it takes is one bite from an infected mosquito to set off a heartworm infection in your dog, regardless of whether or not your dog is outdoors.

Misconception #2: Heartworm disease is not a risk in colder climates.

The incidence of heartworm disease is traditionally thought of in warmer climates, especially the Atlantic and Gulf coasts, along the Mississippi River, and its major tributaries. While it is more prevalent in these areas, heartworm disease has been diagnosed in all 50 states. This widespread incidence can be attributed to factors like climate variations, traveling pets, and populations of carrier animals.The Companion Animal Parasite Council puts out clickable prevalence maps showing the incidence of heartworm disease throughout the US.

Misconception #3: Heartworm disease is not life-threatening.

This is a dangerous misconception. In truth, heartworm disease can cause lung disease, heart failure, and major damage to other organs, or even death if left untreated. Furthermore, it is potentially fatal if an infected dog is given regular heartworm preventatives, more on that later as we discuss treatments.

Recognizing the Symptoms

Prevention is always the best policy but identifying early signs of heartworm disease in dogs is important. The clinical signs can vary based on the stage of the disease, activity level of the dog, and the number of adult worms. Symptoms range from none at all to severe, or even death. Here, we've divided the symptoms into three categories: early, progressing, and severe symptoms.

Additionally, we provide a list of warning signs to look out for. While these symptoms can be indicative of heartworm, they can also suggest other health problems. It's important to have any changes in your dog's health or behavior checked by a veterinarian.

Early Symptoms

In the early stages of heartworm disease, dogs may show few to no symptoms at all. However, that does not mean the disease is not present. Once the dog becomes infected, it takes 5-7 months for the heartworm to migrate to the heart and mature into an adult. During this timeframe, the dog will not show outward signs of illness.

As the adult grows and reproduces some possible early signs may include a mild persistent cough, fatigue after moderate activity, or a decrease in appetite. Weight loss might also be seen but usually isn't common until the disease has progressed.

These symptoms might be subtle and easy to overlook as normal behaviors or slight health issues. If you notice any such changes in your pet, it's always wise to consult your veterinarian.

Progressing Symptoms

As the disease progresses, the symptoms become more severe and evident. The worms continue to grow and lengthen, females can grow to a foot long, and to reproduce. The presence of these foreign “objects” cause the dog’s immune system to launch a response progressing through inflammation and scarring.

As this occurs, your dog may experience increasing reluctance to exercise, fatigue after light activity, and a more noticeable loss of appetite. There may also be a worsening cough. You may notice that your dog begins to look unwell or 'off.' These signs may not be specific to heartworm disease but are always reasons to seek help from your veterinarian.

Severe Symptoms

In the severe stages, dogs may exhibit signs of severe cardiac distress due to heart failure or blockage. Symptoms include a swollen belly due to excess fluid in the abdomen, labored breathing, pale gums, and dark, bloody, coffee-colored urine, or acute collapse and death. At this stage, the disease is life-threatening and requires immediate medical attention. Even with intervention dogs may not survive.

Warning Signs to Look Out For

Because heartworm disease will ultimately lead to death if left untreated it is important to not brush off symptoms that feel minor or insignificant. Please seek veterinary evaluation if your dog seems easily fatigued with light or moderate activity, or has a persistent cough even if the cough seems soft and dry.

Diagnosing Heartworm in Dogs

As with many diseases, early detection can significantly increase the chances of recovery and can minimize organ damage from a heartworm infection. Let’s turn our attention to how the disease is diagnosed.

Consultation and Physical Examination

The initial step towards diagnosing heartworm in dogs involves a thorough consultation and physical examination by your trusted veterinarian. During this consultation, the veterinarian will ask about your dog's health history, any noticeable changes in behavior, and observe closely for clinical signs that could indicate disease.

The vet may be particularly interested in any recent travel and the home environment especially as it relates to possible mosquito exposure. A detailed physical examination can reveal signs such as a heart murmur or unusual lung sounds which are often associated with heartworm (or other) disease. The presence of a swollen belly, due to fluid accumulation, is indicative of a serious condition and may be due to advanced stages of heartworm disease.

Blood Tests

Blood tests are another vital part of the diagnostic process for finding heartworms in dogs. These are typically administered by your vet using a small blood sample from your pet. Most frequently, the blood is screened for the presence of heartworm proteins (antigens), produced by adult female heartworms.

It is a reliable method used to identify the presence of adult heartworms. Another blood test traces microfilariae in your dog's bloodstream, indicative of a heartworm infection. The dog’s history will play a crucial role in testing and interpretation. For example a dog who was potentially exposed to heartworm a month ago will not return a positive test for adult antigen as it takes 5-7 months for adults to mature.

Therefore it is often necessary to repeat testing at 6 month intervals to establish a true negative. As you can see, heartworm infections can be present even in dogs appearing outwardly healthy, making proper use of these blood tests critical in the detection process.

Imaging Tests

Imaging tests are often used in cases where the disease has gone unnoticed for a longer period of time. In such instances, determining the extent of damage is as important as definitively diagnosing the cause. Diagnostic imaging may include x-rays and ultrasound of the heart (echocardiogram).

Radiographs or X-rays can be taken somewhat quickly and provide an image of your dog's heart and lungs. The images help the veterinarian detect any changes in the size or shape of the heart or anomalies in the lung's vascular pattern that might signal heartworm disease. Ultrasonography of the heart, or echocardiography, can show real-time moving images of the interior of the heart, and can show the adult heartworms moving.

This imaging test requires the patient to sit still for a longer period of time but allows for a precise location and estimation of the worm burden, helping guide treatment plans.

Confirmatory Tests

Confirmatory tests are done to clarify any ambiguity stemming from earlier testing. These can be particularly useful in cases where a dog’s infection is hard to detect, dogs with low worm burdens, or in the early stages of the disease (where microfilariae are present but adults are not yet).These tests use labor-intensive methods to concentrate the blood samples and detect microfilariae.

Confirmatory tests are important to assure the correct diagnosis before your dog embarks on specific treatment plans. As heartworm treatments can be challenging and require hospitalization, the accuracy in diagnosis is extremely important.

Treatment for Heartworm in Dogs

Addressing a heartworm diagnosis requires knowledge, understanding, and patience. Treatment protocols may vary depending on the severity of the infection and the overall health of your pet. The goal is to rid your beloved pet of adult and immature worms while avoiding a shock to your pet’s system.

Preparation for Treatment

Upon a heartworm diagnosis, your veterinarian will need to carry out a thorough evaluation of your dog's health to craft the most appropriate treatment plan. Comprehensive examinations might include blood work, x-rays, urine analysis, and specific heartworm tests. These evaluations help determine the severity of the infection and the safest treatment approach.

During the preparatory phase, the focus is on improving your pet's physical condition to better tolerate the primary treatment. Your dog may be put on a strict regimen of exercise restriction. As dying heartworm larvae can cause blockages in your pet's blood vessels and lungs, reducing physical activity helps lower the risk of life-threatening complications.

Stabilizing your dog's health before initiating treatment for heartworm infection is critical. It ensures that your pet can withstand the therapy and recover safely.

Medication and Procedures

There are a few different protocols for treating heartworm in pets based on your pet’s condition and their specific case of heartworm disease. Generally speaking, the medication and procedures involved aim to kill adult heartworms and their offspring and ensure that the dead worms are safely eliminated from your pet's body.

Killing the parasites too quickly can lead to anaphylactic-like reactions, embolism, and shock. Your veterinarian will tailor the exact treatment plan to meet your pet’s needs, regardless of the protocol selected, you can expect that this will be a process lasting at least a few months.

Initially, your pet may receive a dose of regular heartworm preventative along with antibiotics like doxycycline. The heartworm preventative will kill the immature worms and the doxycycline helps to weaken the heartworms, making them easier to destroy. Steroids may also be prescribed to manage the inflammation and potential adverse drug reactions.

Once your veterinarian believes your pet is ready, the treatment phase directed at killing the adult worms can start and Melarsomine dihydrochloride is administered. It is essential to prevent your dog from strenuous activity during this stage as physical exertion could cause the dead worms to travel to the lungs, leading to emergency and life-threatening complications.

Post-Treatment Care

We understand that providing optimal post-treatment care can be stressful for pet parents. After your pet undergoes heartworm treatment, they will require ample time to recover. This period is crucial as it allows your dog's body to absorb and remove the dead heartworms, a process that can take several weeks to months.

During this time, exercise restriction continues to be vital. Too much activity can cause dead heartworm remnants to catch in smaller blood vessels, leading to potentially fatal blockages.

Follow-up testing is part of post-treatment care. Your veterinarian may perform additional tests to confirm that all heartworms of all life-stages were eliminated. Vet-recommended heartworm preventatives should be continued as prescribed to prevent future infections.

Potential Risks and Complications

It’s never easy to discuss potential risks, especially when it comes to our cherished pets. Despite our best efforts and the expertise of veterinary professionals, the treatment of heartworm can have complications.

As we have already discussed, one major risk is related to the dead worms creating an embolism, blocking small blood vessels and preventing blood from reaching important tissues and organs. Again, this is why exercise restriction is so crucial during treatment and recovery.

Reactions to the heartworm medications themselves can also occur, although these are much less common. Side effects may be mild, such as lethargy or tender skin at the injection site, or more severe such as fever, coughing, vomiting, and shock.

The risks associated with heartworm treatment are one reason to stress the importance of prevention. Giving your pet vet-prescribed heartworm preventatives year-round reduces the risk of such a complex and stressful process and spares your pet the long-term damage heartworms can cause.

Preventing Heartworm Disease

The best way to tackle heartworm disease is through prevention. Several highly effective heartworm preventatives are available and should be utilized as the first line of defense against this potentially life-threatening disease. Your veterinarian will assess your pet and determine which preventative medication is best suited to your pet’s lifestyle, environment, desired application, and other risk factors.

Heartworm preventatives are often combined with medications to control other parasites and come in a variety of applications ranging from topicals, to oral medications and chews, or even injections given by a veterinarian.. Most heartworm preventatives are administered on a monthly basis.

These preventative medicines function by eliminating any heartworm larvae, which have been transmitted by mosquitoes, before they can mature into adult heartworms. It's critical to understand that these preventatives do not kill adult heartworms. They are designed for consistent use, which means you should administer these medications all year round for effective heartworm prevention.

Environmental Control

Mosquitoes are key players in the life cycle of heartworms and by reducing your pet's exposure to these insects; you significantly lower the risk of heartworm infection. There are a few measures you can take to impact the number of mosquitoes in your pet’s environment.

Firstly, it's important to reduce mosquito-breeding sites in your yard. Mosquitoes breed in standing water, so removing these from your outdoor space can help decrease the mosquito population. Regularly emptying water-filled containers and cleaning pet dishes goes a long way in contributing to mosquito population control.

Secondly, consider using mosquito control products. Various sprays and foggers are available that can reduce mosquito populations in and around your house. Where possible, keep your pet indoors during peak mosquito activity times, specifically during dawn and dusk.

Regular Vet Check-ups

Regular check-ups with your vet are an essential part of preventing heartworm in dogs. Most dogs show no signs of disease in the early stages of a heartworm infection, which make routine vet visits all the more beneficial for early detection. Your veterinarian will typically test your dog for heartworm infection as part of their annual health check-up. The American Heartworm Society recommends testing for both adult heartworm antigen as well as circulating microfilariae.

Regular vet check-ups also provide an opportunity for maintaining your pet's preventative medicine regimen. Your vet will remind you of when the next dose is due and will help you refill your prescription in a timely manner

The Importance of Early Detection

Heartworm disease can cause serious harm to your dog's health, and in severe cases, it can be fatal. This is why catching the disease in its early stages is absolutely essential. Acute symptoms are hardly noticeable, and therefore, regular testing becomes a necessity.

Early detection of heartworm disease can dramatically increase your dog's chances of a full recovery. The treatment for heartworms is much safer and more effective in the early stages of the disease. It is also less taxing for the dog and less expensive for the pet parent.

Remember, it is always easier to prevent heartworm disease than to treat it.

Is Heartworm Contagious to Other Pets?

As many pet parents do, you may worry about the transmission of illnesses between pets; especially when it comes to something as serious as heartworm disease. Fortunately, heartworm disease is not directly contagious from one pet to another, recall that the disease is transmitted by the bite of an infected mosquito. When one of your pets is diagnosed with heartworm, it doesn't necessarily mean that your other pets have it too.

However, the unfortunate reality is that where one pet is infected, there's a higher risk for other pets in the same environment. On one hand, your pets have likely been exposed to the same conditions and may have both been bitten by infected mosquitoes. Furthermore, an infected pet can be a source of heartworms to non-infected mosquitoes in the area, although the bite of a newly infected mosquito will not be infectious for about 10-14 days.

If you find yourself in a scenario where one of your pets is heartworm positive, it is absolutely crucial to continue heartworm preventatives for your other pets, as it can prevent the disease from taking hold even if the pet is bitten by an infected mosquito. Your veterinarian will give you further instructions for keeping other pets safe in the face of a heartworm diagnosis.

Cats and ferrets may also contract heartworm disease. While these species are not the primary host for heartworms they can become infected. Fortunately, heartworms do not thrive as well in these species- worms do not grow as big nor live as long as they might otherwise in a dog. However, this can mean that the already subtle early signs of heartworm disease go unnoticed. Speak to your veterinarian if you have reason to suspect heartworm disease in any of your pets.

Can Humans Get Heartworms from Dogs?

When it comes to heartworm disease, another common concern is whether humans are at risk of contracting it from their pets. The answer to this is largely reassuring: human infection with heartworms (Dirofilaria immitis) is very rare. When it does occur, it's typically due to being bitten by an infected mosquito, it doesn’t come from contact with an infected animal.

Humans are not a natural host for heartworms, meaning the worms don’t thrive in the human body. Very rarely, a heartworm can migrate to a human's lung, heart, or other area and cause a granuloma, or walled off inflammation of tissue. However, these cases are infrequent, and the infection typically remains asymptomatic and self-limiting.

That being said, maintaining your pet's heartworm preventative regimen is something that benefits not just your animals but also your family. It keeps the overall heartworm prevalence in your immediate environment lower, which decreases the likelihood of either pets or people encountering an infected mosquito.

How Often Should I Test My Dog for Heartworm?

Regular veterinary checkups are crucial in maintaining the overall health and well-being of your pet. Among other things, these checkups often include testing for heartworm. In general, the American Heartworm Society recommends that dogs should be tested annually for adult and microfilarial heartworms.

Annual testing is important even when your dog is on heartworm prevention year-round. No preventative is 100 percent effective, and there are ways the infection could take hold. For example, pets may not receive their full dose of medication either due to topical applications rubbing off or oral formulations being spit out rather than swallowed, etc. Regular testing ensures that if there is an infection, it gets detected early, leading to better treatment outcomes and avoiding an occult infection.

For dogs older than seven months, a screening test is usually recommended before starting heartworm preventatives, as a dog might appear healthy while heartworms are silently thriving within. If an infected dog is not tested before starting a heartworm preventive, the medication could potentially lead to a rapid die-off of microfilaria, leading to a shock-like reaction.

What Happens If Heartworm Disease is Left Untreated?

If a heartworm infection in dogs is left untreated, the situation can become dire. Over time, as the heartworms multiply and grow, they cause the immune system to react in an attempt to clear the infection. The antibodies produced to fight the worms can collect in small delicate tissues such as those of the kidney and cause irreparable harm. Additionally, growing adult worms can block the heart's blood vessels.

With the adult males growing up to 6 inches and the adult females potentially reaching a foot long, each worm can take up a significant amount of space inside a dog’s heart. When worm burdens are particularly high the dog is at risk for Caval Syndrome a critical condition in which blood cannot flow through the right side of the heart nor the vessel that feeds it (vena cava)

Clearly, the disease can lead to serious complications, and if left untreated, it will ultimately result in death. Therefore, if your pet tests positive for heartworms, prompt treatment is essential to manage the disease effectively and prevent further damage to delicate organs.

Treatment is, inevitably, a significantly longer, more complex, and more expensive process than prevention, often requiring several interventions, including multiple injections and hospitalization. Effective, ongoing preventative measures and regular testing remain the best approach to keeping your pet heartworm-free.

Seeking Help and Support

At CodaPet, we understand that your pet's health and wellbeing is a matter close to your heart. From understanding the heartworm life cycle to recognizing symptoms, pursuing diagnosis, understanding treatments, and recognizing the importance of prevention, the journey to keeping your pet heartworm-free can feel like a daunting task. But, remember, each step you take is a significant stride towards helping your beloved pet enjoy a healthy and fulfilling life.

We hope this comprehensive guide has provided you with valuable insights into heartworm disease in dogs. These are no easy topics to grapple with, but being armed with the right knowledge can be a powerful tool against this life-threatening condition. Remember, heartworm disease is preventable, and through consistent preventative measures and regular vet check-ups, you can significantly lower the risk of this disease in your pet.Together, we can make a difference in the lives of our cherished pets.

This is not intended as medical advice nor for diagnostic purposes. Consult your veterinarian with any questions.


Dr. Karen Whala

Fresno, CA


Dr. Karen Whala has always had a soft spot for the older pets she’s treated in practice and saw a need for these beloved pets to pass peacefully at home rather than in a clinic setting. To that end, she started Peaceful Passing in 2018 to help families assist their terminally ill, injured, or suffering pets as they transitioned from life in the Fresno and Clovis areas.  Read More