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Heartworm Disease in Dogs



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 may result in death. The disease is often 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

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 may 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.

Pet parents should be aware that outdoor dogs experience a higher risk of falling victim to heartworm disease, compared to the risk of that with indoor dogs. It's important to also keep in mind that indoor dogs may experience the risk if bitten by an effected mosquito. 

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.

Below 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.

What are the symptoms of dog heartworm disease?

Pet parents 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 own 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 diagnosed?

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.

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.

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

It's 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.

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.

Stabilize the condition

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.

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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Treating heartworm disease in dogs

Northeast Animal Hospital