Heartworm Disease In Dogs: Long Time Vet Breaks It All Down

A dog at home with Heartworm disease

Heartworm disease is a severe and potentially deadly condition that affects dogs of all ages, breeds, and sizes. According to the American Heartworm Society, heartworm disease is “the most common and dangerous parasitic infection in dogs in the United States.”

In this blog post, we will discuss heartworm disease, how common it is, whether or not it is deadly, and how you can prevent your dog from getting it.

What Is Heartworm Disease?

Dirofilaria immitis, better known as heartworm disease, is a common parasitic disease spread by mosquitoes contracted most commonly by dogs, cats, and ferrets.  

Heartworms are foot-long worms that, once mature, live in the heart itself. In dogs, this could be hundreds of worms in one animal.

These parasites can be picked up by other non-domestic species such as wolves, coyotes, foxes, sea lions, and in rare instances, humans.

Warm wet areas are more likely to have endemic heartworm disease present, as they are areas where mosquitoes flourish. Heartworm has been diagnosed in all 50 states of the U.S.A. but is more concentrated in the Southeast.

Even if a pet doesn’t spend much time outside, mosquitoes are still good at getting inside our homes, so indoor pets can be affected just the same. 

What is the Life Cycle of Heartworm Disease?

A mosquito bites an animal infected with heartworm and ingests the microfilariae during a blood meal.

They then develop over 10-30 days into an infective larval stage which can be transmitted through the mosquito’s bite to unsuspecting pets.

After migrating through the tissues into the bloodstream, they will mature over 6-8 months and end up in the heart as adults, where the process starts over.

Heartworms can inadvertently migrate into different tissues of the body, such as the eyes, central nervous system, abdominal cavity, or more external tissues or vessels.

We will focus mainly on dogs for this article but include some information on cats and ferrets. 

What Does Heartworm Disease Look Like?

There may be no signs in the early stages of the disease in dogs. It’s not until the disease is well advanced that you may see the first signs.  

Once the worm burden is high, it can cause the heart muscle to stretch and overall negatively affect the cardiovascular system. You may see exercise intolerance, lethargy, or coughing, to name a few.  

A new heart murmur may prompt your veterinarian to test for heartworm. 

Ferrets may have the most severe issues with heartworm, with similar clinical signs to canines; they are more quickly affected due to the size of their heart. For them, just one heartworm can cause severe disease. 

Cats with heartworm disease generally only have one or a few worms present, so they may not have any clinical signs at all. Some cats may even experience sudden death without any prior illness and later be discovered to be due to heartworm disease.  

Common clinical signs are asthma-like symptoms, heart murmur, intermittent vomiting, lack of appetite, or weight loss, all considered non-specific. 

How Do You Diagnose Heartworm Disease?

Canines, in particular, should be checked every year for heartworm for the duration of their lifetime. This is done with a blood test. If your pet has a positive test, your veterinarian will likely recommend follow-up testing to confirm the infection.

Most heartworm tests can be done in the hospital during your appointment, but confirmatory blood screening is generally sent to the lab for testing. 

Felines are a little bit different. Our heartworm tests are checking for female worms only.

With generally only a couple of worms present, if all male, will not be positive on the test.

In rare cases, they may only be discovered with an ultrasound of the heart. 

How to Treat Heartworm Disease

Unfortunately, dogs are the only species that can receive treatment among our companion animals.

Cats and ferrets will not tolerate the medication used for treatment.

The entire course of treatment is long and is not considered successful until a negative heartworm test nine months after their last treatment date. 

Staging is essential to determine how to proceed with treatment for your pet. Standard pre-treatment testing consists of chest x-rays to evaluate the heart and lungs and vital blood work to assess other underlying conditions. 

Mild: asymptomatic or cough

Moderate: Cough, exercise intolerance, abnormal lung sounds

Severe: as above and may include trouble breathing, abnormal heart sounds, enlarged liver, syncope (temporary loss of consciousness from decreased blood flow), fluid accumulation in the abdominal cavity (ascites), death 

The first thing you should do if your dog is diagnosed is to restrict its activity.

This will be for several months during treatment and may be the most frustrating part of treatment for asymptomatic pets.

Your veterinarian may also change your pet’s heartworm prevention as part of their treatment.

Before their next dose of prevention, your veterinarian may also recommend giving antihistamines or steroids to try and prevent the risk of a reaction. 

In general, they are started on antibiotics to treat for the possibility of Wolbachia, a bacteria that can live inside the worms. This bacteria can cause an inflammatory response that can further complicate treatment.

In most cases, they are prescribed a 4-week course of antibiotics before their first injection.

The treatment, melarsomine, is generally given as a course of 3 injections.

The initial injection is given, and then the last two doses are given 24 hours apart, one month later. These are delivered in thick muscle layers along their lower spine.

One possible complication comes from the body’s reaction to the dying heartworms during treatment.

This can acutely cause trouble breathing, and if seen, your pet should be taken to a veterinarian immediately.

Steroids are given over two months during treatment to try and prevent complications. 

Preventing Heartworm Disease

Heartworm disease and its treatment both have the potential to be life-threatening.

Prevention is the best measure to reduce their risk and should be given year-round, every year, for the life of your pet.

We will focus primarily on canine prevention here but will notate where products are available for cats with an *.

As a general rule, prescription products should be purchased through or with guidance from your veterinarian as there are many options and increasing instances of counterfeit medication online. 

Oral Prevention

The most prevalent are oral preventions, given monthly.

Some formulations have flea prevention included as well. All heartworm products are prescription and must be obtained directly from your veterinarian or with a prescription.

Below we’ve listed a few familiar brands but check with your vet for the best option for your pet. 

Oral prevention (heartworm only): Heartgard*, Interceptor, Tri-heart, Iverhart

Heartworm plus flea prevention: Trifexis, Sentinel

Heartworm, flea, and tick prevention: Simparica Trio

Topical Prevention

Topical preventions are used most commonly for cats, but two products are available for dogs.

If your pet is difficult to give oral prevention or cannot have oral products, topical prevention may work for you.

Topical products are issued monthly, and as a general rule, you should not bathe your pet a few days before or after application as they use natural skin oils for absorption.

Common brands are Revolution* and Advantage Multi (heartworm + flea protection). 


Proheart is an injectable product that, when given, lasts for either 6 or 12 months, depending on which formulation.

The active ingredient, moxidectin, is slowly absorbed under the skin over that time.

As with all prevention products, there is a small risk of a reaction. With Proheart, this would most likely be similar to a vaccine reaction if present.

Make sure to ask your veterinarian what to look out for.

If they have a history of vaccine reactions or reactions to other products that contain moxidectin, this should be discussed prior with your veterinarian. 

Active Ingredients in Medications

The most common medications used for these products are ivermectin, selamectin, moxidectin, and milbemycin. Many oral products may also include intestinal parasite dewormers such as pyrantel and praziquantel. 

MDR1 Gene and Ivermectin Sensitivity

Some dogs may be overly sensitive to ivermectin and several other medications if they have the mutated MDR1 gene.  

Australian Shepherds and Collie breeds are more likely to have this gene. A simple blood test can be used to determine if the gene is present, or your veterinarian may recommend avoiding these medications to be on the safe side.  

For most pets with this gene, a single dose of heartworm prevention for their size is unlikely to cause problems. If they eat several doses, they may likely experience side effects. 

Final Thoughts

Heartworm disease is a serious problem for dogs, but it can be prevented. Make sure your dog is on heartworm prevention medication, and check with your veterinarian to make sure you are using the right product for your pet. If you think your dog has been infected with heartworms, take them to the vet as soon as possible.

References & Additional Resources

Below we’ve listed some reputable websites you can explore if you have more questions regarding heartworm disease with your pet. When selecting a product, include your veterinarian so that you can pick the prevention that’s best for your pet. When choosing a resource online, it’s best to stick to those associated with a veterinary school or other veterinary-based sites. 

American Heartworm Society: https://www.heartwormsociety.org/

This is an in-depth and the most up-to-date resource for heartworm disease in the United States for veterinarians and clients alike. 

Companion Animal Parasite Council (CAPC): https://capcvet.org/

Broad database for the majority of companion animal parasites. 

American Veterinary Medical Association (AVMA) page on heartworm:https://www.avma.org/resources/pet-owners/petcare/heartworm-disease

Cornell Veterinary School website on feline heartworm: https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/heartworm-cats

MDR1 gene information: https://vcpl.vetmed.wsu.edu/affected-breeds

Laura Crawley

Dr. Laura Crawley is a writer, avid traveler, and companion animal veterinarian. With over 10 years of experience, she currently does relief work at general practices in New England. She and her fiancé enjoy living on the Long Island Sound with their Scottish Terrier and traveling to exotic places whenever possible. Now that her writing hobby has turned into a passion, she has become a freelance writer in her spare time. She also enjoys the Arts, scuba diving, and travel photography.

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