Heartworm Disease in Dogs – A Definitive Guide

Parasitic worms are a common health issue in dogs. Heartworm disease is not considered among the most common dog diseases. However recent studies indicate that heartworm disease in dogs is increasing significantly. 

Heartworm disease in dogs is a serious threat to your dog’s health and life. As such every dog owner should know how to protect their furry friend from the disease. Understanding how to tell its symptoms, and what action to take in of infection is a good place to start.

The goal of this article is to tell you everything you need to know about heartworm disease. We give you complete details about the nature, mode of spread, symptoms, diagnosis, treatment, and post-treatment risks of heartworm disease. Read on to find the details!

What is Heartworm Disease in Dogs?

Heartworm disease is a dangerous and potentially deadly disease caused by a filarial nematode worm known as Dirofilaria immitis that infests the hearts of dogs. 

Heartworms  present in the heart of a dog.
Photo Credit: houstonspca

Heartworms also live in your dog’s lungs and other related blood vessels, which explains why dogs with heartworms will suffer severe lung disease, damage to other body organs, and even heart failure.

But how does your dog get heartworm disease? Find out in the next section.

How Do Dogs Get Heartworm Disease?

Dirofilaria immitis, the parasite that causes heartworm disease, is a blood-borne parasite that is temporarily hosted in mosquitoes.  In other words, mosquitoes serve as intermediate hosts of the parasite until it becomes infective and is then passed to dogs through a mosquito bite.

Dogs are the final host of heartworms. The larvae deposited in your dog’s body through mosquito bites matures into adult heartworms. Consequently, adult heartworms mate and reproduce, increasing the number of heartworms inhabiting your dog’s body and the risk of severe heartworm disease.

To send heartworm larvae into your dog’s body through their bite, mosquitoes pick up heartworm parasites from other infected dogs and carrier animals. Cats, other ferrets, and mammals like coyotes, wolves, foxes, and sea lions are all possible hosts of heartworms. 

Wild species that live close to cities and are potential heartworms host are considered significant heartworm disease carriers. 

See this diagrammatical representation of how dogs get heartworm disease.

Note, though, that the heartworm life cycle is a lot more complex than this, as you’ll read in the following section.

Heartworm Life Cycle in Dogs

Heartworms have a complex life cycle that initiates with an infected dog or another animal, through the mosquito to another dog. The mosquito plays a crucial role in the spread of heartworms as it acts as the medium for spreading the disease.

This is how the lifecycle of heartworms in dogs looks like in stages:

Stage 1

Adult female heartworms in an infected dog or other animal produces young heartworms, known as microfilaria (plural microfilariae). These young heartworms circulate in the blood of the host animal.

Stage 2

A mosquito bites the infected animal and picks up the young microfilariae in the sucked blood. The microfilariae inside the mosquito develop into infective larvae within 10-14 days, then move into the mosquito’s mouthparts.

Stage 3

The mosquito bearing the heartworm infective larvae bites a dog and deposits the larvae on the dog’s skin. The infective larva enters the dog’s blood system through the mosquito bite wound and into the heart and adjoining blood vessels.

Stage 4

The heartworm larvae in the new host dog take around 6 months to mature and start reproducing. Mature heartworms (macrofilaria) can live for 5-7 years in your dog.

Stage 5

The heartworm cycle recommences with mosquitoes picking young heartworms from the new host to other dogs and animals.

Mosquitoes are the culprit

It’s important to restate that mosquitoes are the major intermediaries in the spread of heartworms. According to the heartworm society, around 20 species of mosquitoes spread heartworm disease in the US. What’s more, the number keeps increasing and is even higher in other parts of the globe. 

The difference between heartworm mosquito spreaders and non-spreaders is in the species’ feeding and traveling behavior.

Also, mosquitoes are resistant to unfavorable weather situations, explaining the growing heartworms prevalence.

So, in which places are dogs most prone to heartworms in the US? Find out in the next section if you live in a heartworm-prone area.

Where Can Dogs Get Heartworm in the USA?

According to the American Heartworm Society, incidences of heartworm disease have been reported in all 50 states of the USA. Nonetheless, the south and southern regions have reported the highest number of cases in the past. These states are around the Atlantic Ocean coastlines, the Gulf of Mexico, and the Mississippi River.

A study from 2012 to 2018 also found that the area with the highest increase in heartworm prevalence in the USA was the Lower Mississippi River Valley. Heartworm disease prevalence at regional levels was recorded northward into Indiana and Illinois and in the southeastern states.

At the local level, increased heartworm prevalence was recorded along the Atlantic coast, the western states, and the central US.

Instead, a decreased prevalence was noted in the traditional endemic areas along the Mississippi Alluvial plain, Florida, Kansas, and Oklahoma

You can check the American Heartworm Society’s latest heartworm incidence map here.

Common risk factors predisposing dogs to heartworm disease

  • The movement of mosquitoes. The wind can blow mosquitoes to miles of distances. If these mosquitoes are carriers of the heartworm microfilariae, they will spread the disease to pets in their new habitat.
  • Relocation of carrier pets (dogs, cats, coyotes, foxes, etc.) from one location to another.
  • Abandoned and stray dogs.

Dog owners should know that, even though heartworm disease may not currently be a problem in their state, there may be more incidences of the disease than diagnosed. And, as you’ll learn in the section on heartworm disease diagnosis, it is difficult to tell the disease in its early stages.

Traveling with your dog to a place where the disease is common also puts your pet at risk of heartworm disease.

Most significantly, heartworm disease is being recorded in new regions each year, and that is enough reason to take preventative measures against heartworms. We’ll talk about the heartworm disease preventive measures a little later in the article, but first, we focus on the symptoms of heartworm disease.

Symptoms of Heartworm in Dogs

The severity of heartworm disease in an infected dog will depend on how far the disease has progressed. Each stage (class) of heartworm disease will show varying symptoms. These stages are defined by:

  • The worm burden or the number of worms living in the dog.
  • How long heartworms have infected the dog.
  • The dog’s capacity to resist the disease and its effects.

The symptoms of heartworm disease are not obvious in the beginning when the worm burden is low. As the worm burden increases, the dog will show clear signs of heartworm disease.

According to the U.S. Food & Drug Administration (FDA), the symptoms of heartworm disease in dogs can be divided into four stages:

First stage symptoms

The worm burden is low, and there are no noticeable symptoms or very mild symptoms that are hard to detect. The only possible clinical sign at this stage is an occasional cough that may seem harmless to the dog’s owner.

Second stage symptoms

The worm burden is still low but begins to be felt by your dog. Stage 2 has mild to moderate symptoms that include:

  • An occasional cough.
  • Fatigue after moderate activity.

Third stage symptoms

The worm burden has increased significantly, and your dog will begin to show severe symptoms, including:

  • A general sickly appearance.
  • A persistent cough.
  • Extreme fatigue after a little exercise.
  • Difficulty in breathing.
  • Signs of heart failure.
  • Changes in the heart and lungs are seen on a chest x-ray.

Fourth stage symptoms

The worm burden is extremely high. This stage is often described as the Caval Syndrome stage because the large number of heartworms block blood flow into the heart.

Heartworm disease is life-threatening in the Caval Syndrome stage. Your dog will need a surgical operation to remove the heartworms to survive, and the survival rate after an operation is extremely low.

While not every dog with heartworm disease develops Caval Syndrome, untreated heartworm disease can progress quickly to stage 4, causing damage to the heart and other vital organs like the lungs, kidneys, and the liver, and often turning fatal.

All these complications underscore the severity of what heartworm disease does to your dog. 

What does Heartworm Disease Do to Your Dog? 

Because the symptoms of heartworm disease do not manifest immediately after infection, heartworm disease in dogs is usually diagnosed when already advanced. In the meantime, the disease still causes damage to your dog’s body and vital organs.

Adult heartworms (Microfilariae) cause heartworm disease by clogging the heart and other key blood vessels like the pulmonary artery. In addition, they interfere with the function of the heart valves. All this damage compromises blood flow into the heart and out of the heart into other body organs. As a consequence, there’s low oxygen supply and possible organ malfunction.

Young heartworms (Microfilariae) are mainly found in the small blood vessels, but they also circulate in the blood. Considering that microfilariae are about the size of the small blood vessels, they can block blood flow into body cells. As a result, the cells that receive blood through these vessels do not get the necessary oxygen and nutrients and will be damaged. 

Apart from the heart, the organs most affected by heartworm microfilariae are the liver and the lungs. This is why your dog may show clinical signs such as coughing, jaundice, live cirrhosis, anemia, and general weakness. Your dog’s kidneys may also be affected, causing accumulated toxins in the body. 

So, how is heartworm disease diagnosed, and when should you test your dog for heartworms? Read the details in the next sections.

Heartworm Disease in Dogs Diagnosis

A dog undergoing a test by a vet.

There are several options used in the diagnosis of heartworm disease in dogs. They include:

Microfilariae Blood Test

Heartworm disease in dogs can be diagnosed with a simple blood test to detect the presence of young heartworms (microfilariae) in the dog’s bloodstream. The presence of microfilariae points to the existence of adult heartworms in the dog’s body, as only these can produce young heartworms. 

This test will show the presence of microfilariae in the dog’s bloodstream only 6 months after your dog is bitten by an infected mosquito. A positive heartworm blood test is followed by other diagnostic tests to determine if the dog can be safely treated.

Serological (Antigen) Test

A serological test is done on a dog’s blood sample to detect antigens or heartworm proteins released by adult female heartworms into your dog’s bloodstream.

Chest X-rays (Radiograph)

Chest X-rays are done when a dog has already tested positive for heartworm disease from a blood test. These radiographs are used to tell the extent of the disease on body organs, including the heart and lungs. 

Complete Bloodwork

Before initiating the treatment of heartworms in a dog that has tested positive for the disease, a complete blood cell count and serum biochemistry are done. This test is used to determine overall heartworm-related organ damage in the dog.  

When should you Test your Dog for Heartworm Disease?

All dogs should be tested every 12 months for heartworm infection, even those under preventative medication, to ascertain the effectiveness of the meds. Your dog’s vet should recommend a heartworm test during one of the scheduled annual vet visits.

Away from the routine annual testing, heartworm disease test on your dog should also be considered in the following situations and factors:

1. The age at which heartworm treatment is started

Puppies below 7 months of age should be given heartworm prevention medication without testing, tested 6 months after their first vet visit, then 6 months later, and annually after that.

Dogs 7 months and older who have not been started on heartworm preventive meds should be tested before being put on the prevention medication, then tested 6 months later and consequently every year. 

Note: The six-month testing after putting your puppy or dog on preventative heartworm medication is established on the fact that heartworm larvae mature around 6 months after your dog or puppy is bitten by a mosquito.

2. Omitted heartworm preventative meds

Suppose you have missed giving your dog the usual heartworm preventative meds. In that case, your dog should be tested for heartworm disease immediately after you realize it, tested again after six months and annually after that.

3. Changing the type of heartworm prevention meds

If your dog has changed the type of heartworm prevention medications, it should be tested 6 months later and annually after that. 

Note that changing your dog’s heartworm meds should only be authorized by a vet after a negative heartworm test.

4. Your dog has traveled recently to a high-risk area

If you have recently traveled with your dog to areas with high heartworm prevalence, your dog should be tested 6 months after the visit and annually after that.

5. length of the heartworms season in your geographical area

The heartworm season is usually in spring and summer when mosquitoes are buzzing all over. The length of the heartworm season varies geographically but is known to last 6 months or a little less in most US states. However, some states in the Gulf and southeastern regions have longer mosquito seasons.

What Happens if My Dog Tests Positive for Heartworms?

If your dog tests positive for heartworm disease, the next thing is treatment and care, which should be given in the following steps:  

1. Confirm Diagnosis

If your dog has tested positive for heartworms, confirm the diagnosis with a different test. Heartworm treatment is quite expensive, and you do not want to spend your buck on treatment that your dog does not need. Besides, treatment for wrong diagnosis is detrimental to your dog’s overall health and wellbeing.

2. Restrict your Dog’s Exercise

With a confirmed heartworm diagnosis, you should limit your dog’s movement and exercise. The physical strain increases the rate at which heartworms cause damage to your dog’s heart and lungs and should be minimized. 

3. Stabilize your Dog’s Heartworm Disease

Especially if your dog is already showing signs of heartworm disease, it is important to stabilize its condition with prescribed therapy before commencing treatment. This is especially crucial if your dog has other health conditions.

4. Commence Treatment 

Your dog’s vet will confirm when your dog is stable and can begin treatment for heartworm disease. While even dogs with severe heartworm disease can be successfully treated, those with mild signs have higher chances of healing. Severe heartworm disease comes with greater risks for complications.

5. Retest and Prevent

Around 6 months after heartworm treatment, your dog’s vet will take a heartworm test to assess the success of the treatment. 

Continue to administer preventative heartworm medication after your dog is cured and perform the recommended annual tests.

You will not need to follow these steps if your dog has a negative heartworm test.

What Does a Negative Heartworm Test Mean?

A negative heartworm test can point to one of these two situations:

  1. Your dog is indeed not infected with heartworms, which is every dog owner’s wish.
  2. The worm burden is below detectable limits, or no antigens have been detected.

The first case is good news for you and your dog. In the second case instead, your dog may test negative for heartworms even with infection. This happens if:

  • The heartworms in your dog’s body are young and sexually immature.
  • All the heartworms infecting your dog are of the same sex (female or male).
  • Your dog’s immune system is killing the heartworm microfilariae as soon as they are produced. 
  • The immune system is obstructing the detection of the heartworm proteins (antigens).
  • Other medication your dog is on kills the microfilariae but not adult heartworms.
  • Your dog is using meds that cause infertility in female heartworms but do not kill them.

To preempt these negative heartworm test situations when your dog is indeed infected with heartworms, the American Heartworm Society advises that dogs be tested for heartworm infection using both the antigen and the microfilaria blood tests

Heartworm Treatment for Dogs

Heartworm treatment is complex and entails killing the microfilariae and adult heartworms, plenty of rest for your dog, and surgical intervention in Caval Syndrome cases. Other supporting treatments may also be required before, during, and after actual heartworm treatment.

A dog getting checked up by a vet.

Killing Heartworm Larvae (Microfilariae)

It’s important to target any young heartworms in your dog’s blood as these will re-infect your dog even when the adult heartworms have been killed.

Advantage Multi for Dogs (Imidacloprid & Moxidectin) is an FDA-approved drug for treating heartworm microfilariae. The drug is topically applied to the dog’s skin to kill the larvae in the bloodstream.

The treatment can be administered before or after the injection to kill adult heartworms. Your dog needs hospitalization on the day the med is administered.

Newer drugs for killing heartworm microfilariae are entering the market and should only be used with the recommendation of your dog’s vet.

Killing Adult Heartworms (Macrofilariae)

Melarsomine (Immiticide® & Diroban®) is the FDA-approved injectable drug given to dogs with stable stage 1, 2, and 3 heartworm disease. The drug kills adult heartworms in the heart and the adjacent blood vessels.

The treatment is given at the back muscles of your dog in a series of injections, depending on the severity of your dog’s disease. Generally, after the first injection is administered, your dog’s vet prescribes 30 days of rest, then a second injection is given, followed by a third one within 24 hours. 

Your dog needs complete rest after the treatment is done.

Complete Rest after Heartworm Treatment

Rest is an essential part of heartworm treatment in dogs. Once the treatment for adult worms is done, the worms die and start to decompose. The fragments of dead heartworms are carried into the lungs. They lodge into the small vessels and are gradually reabsorbed into the body. 

This process takes weeks or even months, and your dog needs to be in complete rest all this while, without any exercise. You may notice a cough in this period, especially in dogs with a high worm burden. 

A severe cough, shortness of breath, depression, or coughing up blood should all be addressed by a vet immediately.

Treatment for Advanced Heartworm Disease

Most dogs have advanced heartworm disease at the time of diagnosis. This also means advanced damage to the heart and other vital organs like the kidneys, liver, and lungs. In extremely advanced cases, a vet may choose to keep your dog comfortable rather than risk the adverse effects of heartworm treatment, among which death. Dogs in this state do not live beyond a few weeks or months.

Also, dogs with stage 4 heartworm disease (Caval disease) require a surgical operation to remove heartworms. This is the only treatment for heartworm disease at this stage, as the worm burden is extremely high. 

The surgical operation to remove adult heartworms in a dog’s heart is extremely risky and often results in the dog’s death.

Other Supporting Treatment for Heartworm Disease in Dogs

Depending on your dog’s case and at the veterinarian’s decision, additional treatments may be given together with heartworm treatment before, during, and after the treatment. These include:

  • Antibiotics (usually Doxycycline) prevent or cure Wolbachia, a bacterial infection that lives in heartworms.
  • Anti-inflammatories, especially in the period of decomposition and reabsorption of dead adult heartworms.
  • Intravenous fluids for dogs with severe disease or those that react to the treatment.
  • Pain relievers.
  • Diuretics to counter fluid accumulation in the lungs and enhance heart function before and/or after treatment. 
  • Lifetime treatment to preempt heart failure in some dogs. These could be beta-blockers (cardiac glycosides), ACE inhibitors, and a low-salt diet.

Risk Factors for Dog Heartworm Treatment

Just like the disease, heartworm treatment is heavy on dogs and can trigger some risk factors.

First, if your adult dog is put on preventive medication before being tested and it happens that the dog already has heartworm infection, the adult heartworms will stay in your dog’s body until your dog begins to show signs of heartworm disease. This could be an already advanced disease stage.

Second, according to the FDA

  • Administering heartworm preventive treatment to a dog already infected with adult heartworms can be both harmful and potentially fatal. 
  • If a dog has microfilariae in the bloodstream and is given heartworm preventative meds, the microfilariae can suddenly die and cause a shock-like reaction in your dog, which could kill your dog. Note, though, that using preventive heartworm medication in an infected dog can also reduce the impact of the disease on your dog’s health.
  • Overall, heartworm treatment can be toxic to your dog’s body and trigger complications like blood clots in the lungs. These clots can lead to death.

Prevention and successful treatment opportunities

If these risk factors sound scary to you, knowing that heartworm disease in dogs can be successfully treated should be comforting. In fact, many dogs recover their appetite, weight, vigor, and vitality following effective heartworms treatment. 

Additionally, you can take precautions by learning how to prevent your dog from heartworm infection.

After heartworm treatment, the life expectancy of a dog who’s cured of heartworm disease can be short or long depending on:

  • How long the dog was infected with heartworm disease.
  • The extent of the worm burden.
  • The extent of damage caused to the heart and other vital organs.
  • The effectiveness of the treatment.
  • The age of the dog (older dogs have slimmer chances of a longer after-heartworm life.
  • If the dog has other health conditions.

How Can I Prevent My Dog From Getting Heartworm Disease?

Heartworm disease prevention is primarily done by faithfully giving your dog preventive heartworm meds every month. This applies to adult dogs who have tested negative for heartworm infection, those who have been successfully treated for the disease, and puppies below 7 months of age.

You can check the comprehensive list of heartworm preventives for dogs that are approved by the FDA.

In addition, your dog should be tested for heartworm disease infection every 12 months.

Note that a dog on preventative meds can be infected with heartworm disease if the meds are missed for some months, your dog spits out a pill without you noticing, or the topical medications are wrongly applied. This is why faithfully giving heartworm preventive meds is your dog’s salvation from the potentially deadly disease.

Heartworm in Dogs FAQs

To conclude the extensive info about heartworm disease, a quick focus on questions frequently asked by dog owners is essential. 

There are no known natural remedies for treating heartworm disease in dogs. Being a severe, dangerous, and potentially fatal parasitic disease, you are advised to always work with a veterinarian to test, diagnose, and treat heartworm disease in dogs. In addition, only heartworm prevention medications tested and approved by the U.S. Food and Drug Administration (FDA) should be administered.

No. Heartworm is not contagious, and dogs cannot spread the disease from one dog to the other. The disease is exclusively spread through the bite of an infected mosquito.

Humans cannot get heartworm disease from dogs or other pets. This is because the disease is solely spread by infected mosquitoes.

However, according to the FDA, there are rare cases of people infected with heartworm disease from a mosquito bite. Because humans are not natural hosts for heartworms, the heartworm larvae that migrate in the arteries of the human heart and lungs die shortly after and cannot thrive into adult heartworms.

Key Takeaways

Heartworm disease in dogs is an extremely dangerous and potentially fatal disease affecting dogs. 

It is transmitted from infected dogs and other carrier animals to healthy dogs through the bite of an infected mosquito.

The lifecycle of heartworms is complex, commencing in an infected dog or other carrier animals, through the mosquito, which acts as the intermediary host, and into other dogs.

Heartworm larvae deposited by a mosquito bite into your dog are carried to your dog’s heart through the bloodstream and develop into adult heartworms in about 6 months.

Heartworm disease is preventable through the administration of monthly meds and annual testing. The disease can be successfully treated if diagnosed early. Advanced heartworm disease often leads to death. 

As a rule, the most effective treatment for heartworm disease in dogs is prevention!