Heartworm Treatment for Dogs: An Expert Guide

Heartworm treatment for dogs is not pleasant for the pup, the people or the veterinarian — so prevention is the name of the game when it comes to heartworms.

“But can a dog with heartworm be cured?” you might ask.

Yes — when a dog contracts heartworm, we can treat it. Here’s how.

Heartworm Diagnosis and Staging

Dogs should get tested for heartworm once a year.

We previously discussed dog heartworm symptoms and what causes heartworms in dogs and cats in our article “7 Symptoms of Heartworm in Dogs.” Now let’s talk about how we diagnose an infection.

Diagnosis

Your veterinarian can usually easily diagnose heartworm with a simple blood test.

The most common test is a quick test called the SNAP 4Dx Test, completed within a few minutes at the vet’s office.

When dogs test positive for heartworm, many vets will send out a blood sample to a laboratory to verify the result, since heartworm treatment cost is expensive — and quite involved.

Staging

Once it’s verified that the dog is positive for heartworm, we “stage” the disease to guide the heartworm treatment, calculate the risks, determine the outlook, etc. Staging involves:

• Chest X-rays
• Complete blood tests
• Urinalysis
• More of a cardiac workup if X-rays and clinical signs determine that the heartworms have already caused severe damage

Can Heartworm Treatment Be Dangerous?

Yes. Treating for heartworm means killing the worms in the dog’s heart.

The drug melarsomine is what will kill heartworms in dogs. The vet gives this drug in a series of deep intramuscular injections, and it’s very effective against adult heartworms. Melarsomine is called an adulticide.

As the heartworms die off, the dog may suffer a thromboembolism (like a stroke) or a severe allergic reaction.

A large part of a veterinarian’s role in treating heartworm is trying to do everything possible to avoid or lessen these serious, potentially fatal complications.

The Goals of Heartworm Treatment for Dogs

The goal of any heartworm treatment is to:

• Improve the clinical condition of the animal.
• Eliminate all life stages of the heartworms (microfilariae, larval stages, juvenile and adult worms).
• Minimize complications during and after treatment.

Stages of Heartworm Treatment for Dogs

Pre-Treatment: Killing the Larvae

First, we stabilize any very sick dogs or dogs with clinical signs of heartworm such as cough, changes in breathing or heart disease.

This can include:

• Giving medications such as glucocorticoids, diuretics or cardiac medications
• Treating other diseases
• Cautious use of intravenous fluids

Then, for 2 months before the adulticide injections, we give heartworm preventive (Heartgard or Interceptor, for example) to reduce new infections and eliminate existing susceptible larvae.

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Finally, for 1 month before the adulticide treatment, we give adjunct therapy with doxycycline.

This eliminates a bacteria called Wolbachia, reducing the clinical complications associated with dead heartworms.

Adulticide Treatment: The 3-Injection Protocol

The first injection of melarsomine, the only approved adulticidal drug, should be given after the 2 months of Heartgard.

This is a deep intramuscular injection given in the back muscles, specifically between the third and fifth lumbar vertebrae.

After the first injection, the dog is sent home to rest for 1 month. Then, after the month of rest where some of the adult worms have been eliminated, 2 more melarsomine injections are given 24 hours apart, totaling 3 injections in all:

• The first injection primarily kills the smaller male worms.
• The second set of injections kills the remaining larger female worms.

You’ll need to put your dog on strict exercise restriction during the entire treatment and recovery period to lessen the risk of heart or lung complications.

This should help prevent dangerous heartworm treatment side effects such as fever, cough, breathing difficulties and acute death. The 3-injection method has been proven to help dogs with advanced heartworm disease, so the theory is that it benefits dogs with lesser disease as well.

Administering the Injections

Many people ask, “Is heartworm treatment painful for the dog?”

There is a specific way to administer the melarsomine injections to cause as little pain as possible and lower the chances of post-injection pain and swelling. This requires technical skill.

Even if the veterinarian has experience giving these injections, uses the appropriate needle and injects in the prescribed place in the epaxial muscles of the dog, there can still be some pain on injection and local swelling and pain.

These reactions usually get better on their own, and we can use pain medications to lessen any heartworm treatment side effects.

If a dog is young and jumpy or difficult to gently restrain for about 10 seconds, we can use a mild tranquilizer or anxiolytic to make the injection process go more smoothly.

Heartworm Treatment Exercise Restriction

When it comes to caring for a dog after heartworm treatment, rest and exercise restriction is extremely important in order to lessen the possibility of complications.

And what’s the biggest risk of complications? That you allow your dog to be highly active during treatment and for 6–8 weeks after the last melarsomine injection, according to the American Heartworm Society. “The importance of restricting your dog’s activity cannot be overemphasized,” the group says.

The more severe the heartworm disease, the more likely there will be complications during treatment.

Some dogs will even die of heartworm-related illness if not treated. These dogs must be handled with extreme care.

Heartworm treatment exercise restriction may mean:

• Crating your dog
• Not letting your dog get overly excited or play with other dogs
• Leash walks in the backyard only or on a quiet street

It’s not wrong to use calming medications if you are trying to get a crazy, fun-loving dog through heartworm treatment without dropping dead of a stroke.

We’ve learned over the years that diagnostics don’t always correlate with worm burden (how many heartworms are actually in the heart) or dictate which dogs may suffer a violent immune reaction to the dead and dying worms.

That’s why we aim to follow the safest protocol — including strictly enforced exercise restriction — regardless of what stage of heartworm disease the dog may have.

Steroids and Antihistamines

Glucocorticoids (prednisone) are recommended on a decreasing dose to control clinical signs of pulmonary thromboembolism. These steroids also help with post-injection pain and swelling.

Antihistamines and additional steroids are recommended in dogs with microfilariae to prevent allergic reactions.

Caval Syndrome

In severely affected dogs, where there are many worms in the right heart, worms can be surgically removed from the heart. The condition is called caval syndrome, and it can be rapidly fatal.

Learn more about heartworm risk and prevention in this video:

The Slow-Kill Method

Another form of treatment for heartworm is called the slow-kill method (SKM).

Although vets have found the SKM successful in eventually eliminating heartworm in their patients, this method is not recommended by the American Heartworm Society. The SKM, in other words, is controversial.

The SKM involves giving the dog heartworm preventive 1–2 times per month in hopes of slowly killing off the heartworms over many months.

There have been no conclusive studies on how successful or safe this method is. One study published using Advantage Multi in conjunction with doxycycline had promising results, but this protocol is still not recommended by the American Heartworm Society.

Pros of the SKM

• Clearly, the biggest advantage of the SKM is cost. There are no other costs except for the monthly heartworm medication and the doxycycline.
• For rescue groups or people with limited funds, it is certainly better to use the SKM than no treatment.
• Some vets believe the melarsomine injections might be too harsh on certain patients or they have had bad experiences giving the injections.
• If a dog has other illnesses and it’s likely the dog may die from causes other than heartworm, it may not be to their advantage to put them through the melarsomine injections.
• Sometimes melarsomine is not available. The SKM is certainly better than nothing in this instance.

Cons of the SKM

• It can take up to 2 years for a dog to clear 95% of heartworms with the SKM. Some dogs remain heartworm positive. During this time, heartworms continue to cause damage in the lungs and the heart.
• The dog is a reservoir for transmitting heartworm infection to other dogs while being treated.
• It’s unclear how much exercise restriction is needed to prevent a thromboembolism during the slow-kill period.
• The SKM may lead to drug resistance.

Common Questions

If your dog develops a heartworm infection or if you adopt a dog who turns out to be positive, then go into treatment with your eyes wide open.

How much does it cost to treat heartworms in a dog?

Heartworm treatment for an average-size to large-breed dog will probably cost $1,000–$1,500.

Is heartworm treatment painful for the dog?

Yes, there may be some painful days for your dog around the injections — but pain medication will alleviate the pain. Most dogs recover completely within a few days.

What else should I know about caring for a dog after heartworm treatment?

Again, exercise restriction is extremely important for several months. Plan early on how to cope with this big lifestyle change for you and your dog.

Realize that after the last injection, your dog should be feeling really good, but you still have to keep the dog quiet for several weeks.

References

This pet health content was written by a veterinarian, Dr. Debora Lichtenberg, VMD. It was last reviewed Jan. 15, 2019.

If you have questions or concerns, call your vet, who is best equipped to ensure the health and well-being of your pet. This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis or treatment. See additional information.