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Heartworm Disease in Dogs

ON THIS PAGE: Also visit our Heartworm Disease in Cats page.


What is canine heartworm disease, and what causes it?

Heartworm disease is a dangerous and potentially fatal health condition that can afflict dogs, cats, and ferrets (as well as non-domestic species like coyotes, wolves, foxes, etc.) and can result in their death. It is characterized by parasitic worms that infest and damage the arteries, heart, and other vital organs of a host body, like a pet dog. Adult heartworms resemble thin, cooked spaghetti strands.

Here are a few facts about heartworms and the disease they cause in dogs:

  • Heartworms are long-stranded parasites that infect canine and feline species
  • They are called “heartworms” because they live mainly in the blood vessels around the heart and lungs and can find their way into the heart
  • Heartworms can restrict the flow of blood, damage critical organs, and cause death
  • Heartworm disease is spread only through the bites of carrier mosquitoes

Dogs are more susceptible to heartworm disease than cats. The disease in dogs is caused by the introduction of larval heartworms into a dog’s bloodstream. It is not contagious between dogs. Rather, an intermediary host—a mosquito—is needed to transmit the disease. This means that diseased animals cannot infect other pets with heartworms, nor can people contract the disease from their pets. In other words, mosquitos are the only transmitters of the disease.

People can contract heartworms if they get bitten by an infected mosquito, but this is rare. Moreover, heartworms do not thrive very long in the human body, so the parasites do not pose a substantial threat to humans. Dogs, on the other hand, are more susceptible to heartworm disease than other creatures; they are a natural host to heartworms.

Adult heartworms can live inside a dog for up to 7 years. Male worms can measure an average of 5 inches long, while female worms can measure an average of 11 inches. Generally, about 15 worms may live inside an infected dog’s organs at any given time. However, the number of worms infecting a dog can reach up to 250!

What risk factors contribute to heartworms in dogs?

The following factors, explained in greater depth below, can increase the risk of your dog contracting heartworm disease:

  • No heartworm prevention, or gaps in prevention
  • Outdoor dogs; indoor dogs where mosquitoes can come indoors
  • Residing or vacationing within the high-risk areas mentioned below
  • Areas that harbor stray dogs, feral cats, coyotes, or other wild canines and felines that may act as a reservoir of heartworms to infect local mosquitoes
  • Areas with hot, wet, and humid climates; long summers and rainy seasons (mosquito heaven)
  • Areas near water or wet grounds; any areas with standing water (perfect for mosquitoes)

According to the American Heartworm Association, over 1 million dogs in the U.S. have heartworm disease. Since the only way pets can be infected with heartworm disease is through mosquitoes, then naturally the disease is more common in areas where mosquitoes are more abundant. Climate, and climate change, are significant factors in the population of mosquitoes and the spread of the disease.

People used to think that heartworm disease in the U.S. was found only in the Southeastern states because other areas of the country are generally dryer or cooler. But dogs have been diagnosed with the disease in every U.S. state, including Alaska! Truth be told, heartworms exist wherever mosquitoes exist. (Alaskan mosquitoes are some of the biggest!)

Home relocation and travel, therefore, have inherent risks. Owners who travel with their dogs may unwittingly visit an area where heartworm disease is prolific, increasing the chances of transmission. In addition, moving to or from a location with such wildlife as the wolf, coyote, fox, bobcat, panther, or mountain lion—all of whom can carry heartworms—has a chance of spreading the disease, including to areas where it is currently uncommon.

Dogs that live an outdoor life are at higher risk of heartworm disease than indoor-only dogs. However, indoor-only dogs are not immune to being bitten by mosquitoes that make their way inside the house. So, both outside and inside dogs are at risk.

The risk to dogs increases the longer they are not on heartworm preventatives. Neglecting to start new puppies on heartworm prevention, forgetting to give dogs their regular monthly dose, switching heartworm medications, and not remembering the last time prevention was administered—these are all risk factors related to heartworm disease.

How does heartworm disease develop in dogs?

As mentioned earlier, mosquitoes are the carriers of heartworm disease. When a mosquito bites and ingests the blood of a dog that is already a host to adult heartworms (named Dirofilaria immitis), microscopic baby heartworms (called microfilariae) that circulate in the dog’s bloodstream are ingested by the mosquito as well.

These heartworm microfilariae must then incubate inside the mosquito for 10 to 14 days to become infective larvae. That means a mosquito can cause heartworm disease after about 2 weeks of biting an infected dog. In this way, the mosquito becomes the intermediate host of heartworms. Rather than suffer the disease itself, the insect simply carries it to another animal, the definitive host.

When a carrier mosquito subsequently bites a healthy dog, the heartworm larvae inside the insect pass through the bite wound into the canine’s bloodstream. Then, in 6 to 7 months, they mature into adults that find a home in the dog’s heart, lungs, or major arteries. Thus, the dog becomes infected with heartworms and can go on to develop heartworm disease.

Development of heartworm disease progresses as the mature heartworms can live up to 7 years inside dogs. During that time, they reproduce again and again, and the females release new microfilariae into the bloodstream of the dog. The microfilaria can then be transferred to another animal via mosquito bite. And since the worms live so long, each mosquito season can increase the number of worms present in an infected dog, as each exposure to a carrier mosquito can mean more worms.

Here is a summary of the heartworm life cycle in dogs:

  1. A mosquito bites a heartworm-infected animal and ingests microscopic baby heartworms  (microfilariae).
  2. In the mosquito, the microfilariae become infective (disease-causing) in about 10 to 14 days.
  3. The mosquito then bites a dog, and the larvae enter its bloodstream through the wound.
  4. In the now-infected dog, the larvae mature into adult heartworms in about 6 to 7 months.
  5. The adult heartworms mate and generate new microfilariae within the dog’s bloodstream.
  6. A mosquito bites the infected dog, picking up those microfilariae, and starting the cycle again.

A single bite of just one infected mosquito is all it takes to spread the disease to a healthy dog. If left untreated, the burden of these worms will cause inflammation and permanent damage to the dog’s critical organs. This happens when the parasites feed on the tissues of arteries and the heart, or more often when the parasites restrict the flow of blood to the heart and lungs by blocking the pulmonary artery and smaller vessels. Lung and/or heart disease follow, and eventually death.

What are the symptoms of dog heartworm disease?

Owners may not notice symptoms of heartworm disease in their dog for months or years after infection. By the time dogs show signs of heartworms, there is often advanced heartworm disease present. Much of the damage caused by heartworm infection is not reversible by the time signs appear. If the disease has progressed far enough, it may be too late to start life-saving treatment.

Symptoms of dog heartworm disease, depending on the severity, can include:

  • Coughing
  • Fatigue
  • Intolerance to exercise
  • Loss of appetite
  • Loss of weight
  • Loss of muscle
  • Labored breathing
  • Irregular sounds in the chest
  • Weak pulse
  • Fainting
  • Bloated stomach
  • Dark, bloody urine
  • Pale gums

The intensity of symptoms, and the timing of their appearance during the development of the disease, generally depend on several factors. These include the length of time the dog has been infected with heartworms, the number and size of worms in the dog’s system, whether the dog has an active lifestyle, and whether the dog has existing health conditions.

Symptoms of heartworm disease, and its severity, are classified as follows:

Class 1 heartworm disease is characterized by no visible symptoms at all, or by only minimal symptoms like occasional coughing. This is the best time to diagnose and start treatment for heartworm disease.

Class 2 heartworm disease is indicated by increased coughing and by fatigue after moderate exercise. X-rays of the dog’s chest may begin to reveal abnormalities in the heart and lungs.

Class 3 of the disease manifests when dogs begin to appear sickly, with loss of weight and muscle. Coughing is persistent and fatigue occurs after even mild activity. Breathing is labored and heart failure can occur. Class 3 can be fatal if left untreated.

Class 4 heartworm disease has its name: caval syndrome. The population of heartworms has grown so large that blood can no longer flow freely into the heart and brain. The only clinical option is the surgical removal of the worms, a risky procedure that cannot undo the damage to the heart, lungs, liver, and/or kidneys. Most dogs do not survive Class 4 of the disease even after surgery. The main goal of treatment for caval syndrome is to provide comfort for the pet.

It bears mentioning that there is not always a direct correlation between the severity of the symptoms and the severity of the disease. Dogs with only 1 or 2 worms could have intense symptoms from the start—even collapse without warning. Conversely, dogs with many worms could have no symptoms at all. So how can we make sure our dogs are worm-free?

How is dog heartworm disease tested for and diagnosed?

Adult dogs should always be tested for heartworm disease before starting heartworm prevention, and then regularly tested even while on prevention. When and how often your dog is tested must coincide with the administering of preventatives, because certain preventatives should not be given if your dog currently has heartworms. Without testing to determine if your dog is already infected with heartworms, preventatives could be harmful and possibly fatal.

If your dog is administered heartworm prevention while infected with immature worms, the medication could kill microfilariae and cause a life-threatening reaction. If given prevention while infected with adult heartworms, your dog will remain infected because preventatives do not kill adult worms. In this case, the infection will continue to spread, and symptoms of the disease will eventually appear. Therefore, testing is critical to the prevention of heartworm disease.

In general, your vet will follow these guidelines for the timing and frequency of testing:

  • Puppies aged 8-weeks to 7-months can start heartworm prevention even without testing. This is because the infection is not detectable until at least 6 months after its inception.
  • Dogs aged 7-months and older, and not already on heartworm prevention, should be tested before starting a heartworm prevention regimen.
  • All dogs (puppies and adults) should be tested 6 months after their first preventive dose, then again 6 months later. Thereafter, they should be tested annually.
  • Any time a preventive dose is missed or is late, and the heartworm prevention regimen must be restarted, dogs should be tested 6 months after the restart. If more than 6 months of dosing are missed, dogs should be tested before restarting and again 6 months after restarting.
  • Dogs that change heartworm preventatives, have recently been in a high-risk area for heartworm disease, or have no prevention history, should restart a testing schedule.

As effective as heartworm prevention is, it is not guaranteed to work for all dogs. Dogs may spit out (or vomit up) an oral preventative, or rub off a topical one. Testing for heartworm disease should therefore occur annually to make sure the preventatives are working for your dog, and that he or she remains free of these dreaded parasites. Tests are typically performed during your dog’s routine wellness exams. Talk to your veterinarian to determine the best timing for your dog’s regular heartworm tests.

There are two kinds of heartworm tests, both of which may indicate the presence of adult heartworms in your dog:

  • A blood test detects specific antigens (or proteins) released into a dog’s bloodstream by adult female heartworms; this is the most common test. If one or more female worms are infecting the patient, the antigen test will—with a high degree of accuracy—return a positive result. However, the test cannot detect these antigens earlier than about five months after the dog was infected via mosquito.
  • A blood test to detect microfilariae in a dog’s bloodstream. Being the larval offspring of adult heartworms, the detection of microfilariae indicates the presence of both male and female adult heartworms. In most cases, this test will not detect very young worms, such as those that more recently infected a dog via mosquito bite. If the test detects worm larvae, it will typically be after six months following infection by a mosquito.

If either type of test returns a positive result, an additional heartworm test or tests will be performed to verify the result. If a positive result is confirmed, additional testing will likely be recommended to determine the stage of heartworm disease. These may include chest X-ray, EKG, ultrasound, or even echocardiogram. Additional bloodwork and urine testing will also likely be recommended to determine if there is additional organ damage (such as kidneys or liver). These other screenings will help your veterinarian determine the best treatment strategy that ensures your dog’s safety and recovery.

If the diagnosis of heartworm infection is confirmed, preventive measures may be started but should be directed by your veterinarian. As mentioned above, certain preventatives can cause complications in an already-infected pet. It is critical to your dog’s well-being to get a diagnosis as early as possible. Treatment for heartworm disease is difficult on pets; but the sooner it begins, the less damage is done by the worms, and the better the outcome. Most dogs in Class 1, 2, or 3 of heartworm disease have a good to excellent prognosis with treatment.

If test results are negative, your dog may begin heartworm prevention. However, a cautionary reminder is in order: Preventatives do not kill adult worms, and very young worms may go undetected in tests. That is why routine testing for heartworms is so vital to your dog’s health. Regular testing and prevention should go hand in hand.

What is the treatment for dog heartworm disease?

Learning that your dog has heartworm disease will no doubt distress your family, but there is good news. Most heartworm-positive dogs—in particular, those with only mild symptoms (Class 1 or 2)—can successfully undergo treatment to remove the infection. With your help and input, your veterinarian will devise a treatment and recovery strategy that will take into account the unique needs and circumstances of you and your dog.

Since treatment for heartworm disease places a great burden on pets, the vet will perform a pre-treatment evaluation to determine if your dog is healthy enough to receive treatment. The doctor will use the results of the diagnostic screenings described above—plus a complete physical exam with X-rays, total blood work, and other tests specific to your dog’s situation—to assess your dog’s ability to undergo the rigors of heartworm eradication.

These tests will also establish the extent and severity of the infection and any abnormalities in the internal organs, which will help your vet strategize a treatment plan. Your veterinarian will follow these general steps for the treatment of your dog’s heartworms:

  1. Consider, together with you, the risks involved with heartworm treatment
  2. Restrict the activity and exercise of your dog
  3. Stabilize the health condition of your dog
  4. Administer medication to kill adult heartworms and/or microfilariae
  5. Perform surgery to remove adult heartworms, if necessary
  6. Evaluate the treatment’s effectiveness, and start the prevention

Consider the risks

Treatment for heartworm disease is hard on pets, and they may struggle to endure it. Heartworms must be killed or extracted before they cause more life-threatening damage. Severe cases of heartworm disease are treatable, but the chance of complications resulting from treatment is higher than in mild cases.

Medications used to treat heartworms can create additional issues. When worms die because of treatment, fragments of them can become lodged in the arterial pathways and capillaries. This can cause inflammation and blockages in the flow of blood, especially in the lungs. Respiratory failure is also a risk if there is an excessive number of dead heartworms.

In addition, it is expensive to treat heartworm disease, especially if surgery is required. The standard treatment will involve several visits to the vet, a schedule of injections, and hospitalization. Post-treatment will require testing and imaging that are in addition to those used to make the original diagnosis and pre-treatment assessment.

Some owners may be tempted to decline treatment for their dog and simply start them on heartworm prevention, maybe under the belief that it will kill the infection. In reality, the preventative may indeed kill heartworms, but not before they cause irreversible damage to their pet’s vital systems. It can take years, if at all, for preventatives to kill off all worms.

Bear in mind that, even after treatment, any damage to internal organs may be irreversible. If treatment is required, it should begin as soon as possible after the diagnosis is confirmed to mitigate as much hardship as possible. Our veterinarians undertake treatment for heartworms with conscientious judgment and resolve, and only when you and they believe that the benefits to your dog will exceed the risks.

Restrict the activity

We might be tempted to think that the number of heartworms alone determines the severity of heartworm disease in a dog. But just as crucial, if not more so, is the dog’s activity level. The disease progresses faster the more exertion a dog displays. This is because exercise—however moderate—increases the respiratory rate (panting), heart rate, and blood flow. Since heartworms restrict the flow of blood, this higher heart rate will tax the pet’s circulatory and respiratory systems even more. In addition, panting is much more likely to dislodge worms in the blood vessels, creating life-threatening clots.

The treatment of heartworm disease will escalate the dangers associated with activity. Heartworms die and break apart because of medications used to treat the disease. Whatever blockages in the bloodstream existed before, more obstructions will likely develop from the remains and remnants of dead heartworms. So, during treatment, the activity level of your dog must be restricted.

Reduction of your dog’s activity must occur from the moment of diagnosis, throughout treatment, until complete recovery. The amount of inactivity depends on the severity of the disease. In Class 3 or 4 of heartworm disease, or at the very start of treatment, dogs will likely need to be confined to a crate—sometimes for three months or more. Restriction will be especially hard for dogs with an active lifestyle, but their recovery will depend upon it. Many dogs who die during or after treatment do so because of too much exercise.

Stabilize the condition

As stated above, heartworms cause damage to a dog’s heart, pulmonary arteries, and lungs. Another symptom of heartworm disease is abdominal bloating from an accumulation of excess fluid. These issues should be given attention by your veterinarian before treatment commences; otherwise, more complications could arise.

Stabilizing your pet’s condition will address any of these three aspects of disease progression:

  • Inflammation of the pulmonary arteries and lungs. The drug prednisone can reduce inflammation caused by substances released by dead heartworms.
  • Right-sided congestive heart failure. The drug furosemide and enzyme inhibitors can treat arterial scarring and blood clots that may lead to heart failure.
  • Distended abdomen. If necessary, a swollen abdomen will be treated by draining it of excess fluids during the first two months of treatment.

These pre-treatment therapies can take several months to complete, depending on the severity of the disease. After your dog’s condition is stabilized, administering the treatment medications that kill heartworms will be a safer undertaking.

Administer the medication

Your veterinarian will follow guidelines established by the American Heartworm Society to develop a series of treatment protocols that will be specific to your pet. Treatment for heartworm disease is possible through drugs administered by your pet’s doctor. The treatment plan may include a combination of a few drugs that either kill heartworms or keep the effects of dead worms to a minimum, such as inflammation.

One class of medications, called macrocyclic lactones, kills microfilariae and can be used in combination with doxycycline to speed up the elimination of larval worms and disrupt the transmission of the disease. Additionally, doxycycline and ivermectin are effective at reducing the risk of anaphylaxis and other side effects. This latter combination is typically administered before the doctor targets adult heartworms.

Another medication contains melarsomine, a chemotherapeutic drug related to arsenic, and is the only drug approved by the FDA to kill adult worms that infect dogs. Three injections are given into the muscles over several weeks. Afterward, prednisone may be prescribed to help overcome the effects of inflammation from the worms as they die off. All of the above treatment protocols apply to dogs with Class 1, 2, or 3 heartworm disease.

Perform the surgery

Dogs with Class 4 heartworm disease, or caval syndrome, have no alternative but to undergo surgery to remove the large number of worms damaging their heart. (Surgery may also be needed for Class 3 dogs who are not able to tolerate melarsomine treatment or who require immediate care.) The killing of so many worms so quickly, by the drug therapies described above, would almost certainly be fatal to a Class 4 patient.

At the same time, however, surgical treatment can only address worms lodging within the heart; it cannot remove the worms in the pulmonary arteries. After surgery, the remaining adult heartworms must be treated with melarsomine to kill them. Pet parents should also bear in mind that full recovery from caval syndrome (Class 4) is rare because serious damage to internal organs has already occurred.

Fortunately, due to the wide accessibility of heartworm testing and prevention, Class 4 heartworm disease is much rarer now than it was just a few decades ago.

Evaluate the treatment

During treatment, your dog’s condition will be regularly monitored to look for any changes in the infection, damage to organs, and in side effects from treatment. When the series of injections that target heartworms is complete, the recovery period will begin. This phase will last one month or more, depending on the severity of the infection.

During recovery, the injected medications will still be working to kill the parasites, so your dog will remain under evaluation and restricted activity. Since living heartworms will still be inside your dog’s system—albeit targeted for elimination—your vet will also address the possibility that more microfilariae may be produced. For this, the doctor will give your dog a specifically chosen heartworm preventative as a temporary measure.

Six months after the injection series began, the vet will perform the diagnostic tests again to determine if all the heartworms have been eradicated from your dog’s bloodstream, heart, and lungs. If tests are positive, your dog may need to go through treatment again to address the remaining heartworms. If tests are negative, the doctor will start your dog on regular heartworm prevention to keep your pet protected from future heartworm infections.

Many medications are approved to prevent heartworms in dogs. Options include topical, oral, and injection; all must be either prescribed or administered by a veterinarian. If you want to prevent more than just heartworms, some preventatives also protect against fleas, ticks, and intestinal worms. Please talk to your veterinarian about which option is best for your dog.

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