Treatment for Canine Heartworm Infections

Treatment for heartworm infection is critical, though risky.


by Mary Straus

Editor’s note: In last month’s article “Heartworm: Don’t Take It Lightly,” Mary Straus discussed the life cycle of the heartworm, and how it infects dogs. She also discussed heartworm prevention strategies. This month, she explains treatment for dogs who are already infected with heartworm.


Whether or not you choose to use conventional heartworm preventatives, there is always the possibility that you may one day find yourself with a heartworm-positive dog, and be faced with the decision of how to treat her. Unfortunately, there is no one right answer when it comes to heartworm treatment. Decisions must be tailored to the individual dog, taking into account both the extent of the heartworm infestation, and the dog’s overall age, activity level, and health.

There are a number of reasons why someone may have a dog who is heartworm-positive. One of the most common is adopting a dog from a rescue organization. Especially in the South where heartworm is ubiquitous, most dogs that are not given heartworm preventative regularly will test positive for heartworm.

There is also a small chance that your dog could become infected with heartworms if you choose to extend the time between doses of the heartworm preventative you give your dog beyond what is recommended on the label, if you give less than the recommended dose, or if your dog should vomit up the pill without your knowledge. Your dog has a much higher risk of becoming infected with heartworms if you choose not to use preventatives or if you use alternative methods that have not been shown to be effective.

It is important to understand that heartworm infections are not detectable until about six months after a dog has been bitten by a heartworm-infected mosquito. (This is why it’s not possible to have a heartworm test performed monthly and give the preventative only if an infection is found.) Blood tests will not detect heartworms in a dog until the larvae have matured into adult worms, which takes about six months following initial infection. Symptoms, such as coughing, lethargy, and difficulty breathing, will not show up until the infection is advanced.

Judy Wilds, of Corpus Christi, Texas, adopted her dog, Taffy, in September 2004. Taffy tested negative for heartworms when adopted, and was started on a regular heartworm preventative schedule. Six months later, a routine heartworm test showed that Taffy was heartworm-positive. “Thankfully, a holistic vet had suggested that I have her rechecked six months after her first test, as it was possible (though unlikely) that she could have heartworms,” says Judy. In this case, Taffy was obviously infected when she was adopted, but had not had the infection long enough for it to be detectable.

It’s a good idea to run a heartworm test on an adopted dog with an unknown medical history immediately upon adoption and then again about six months later. The same would apply if there was a lapse in heartworm preventative dosing for more than two months during the heartworm “season.”

Staging the infection
A heartworm antigen test is the most commonly used procedure for detecting heartworm infection. These tests are extremely specific and have almost no false positives, but it is still a good idea to confirm any positive test results with a second test. There are a number of different heartworm antigen tests, so it is best to confirm using a different test than the one that gave the initial positive result.

If the dog has not been given heartworm preventative recently, then a test for circulating microfilariae can also be done, which will confirm the presence of adult, breeding heartworms in the body. However, a negative microfilariae test cannot be used to rule out heartworm infection for a number of reasons.

All heartworm preventatives kill the microfilariae that are in the dog’s blood, so any dog who has been given heartworm preventatives recently may be negative for microfilariae but could still have an infection. There are also cases of single-sex heartworm infection where no microfilariae are produced. A population of all male or all female worms cannot produce microfilariae. And, finally, in any dog with a very low heartworm burden – say, just a couple of worms – the microfilariae may not be detected (this can also cause a false negative on the antigen test).

Once heartworm infection has been confirmed, additional tests should be done to try to determine how extensive the infestation is. Radiographs can reveal inflammation and damage to the arteries and the heart, and blood tests will show whether the liver and kidneys have been affected.

Heartworm infection is divided into four or five stages (depending on the model used), based on the severity of the infestation and the age and health of the dog.

Stage 1 (mild) consists of young, healthy dogs with no symptoms and minimal changes evident on X-rays.

Moderate (stage 2) infection will show heartworm disease that is evident on X-rays, but symptoms are minimal, mostly coughing. Stage 3 is a severe infection, with weight loss, coughing, difficulty breathing, more damage visible on X-rays, along with liver and/or kidney damage.

Stages 4 and 5 are considered critical, with the dog often collapsing in shock. These dogs will not survive ordinary heartworm treatment, and must have the worms surgically removed if they are to have any hope of survival.

Heartworm treatments
There are three conventional methods of treating heartworm: a “fast kill” method using Immiticide (melarsomine); a “slow kill” method employing Heartgard (iver-mectin); and a surgical method in which the worms are surgically removed from the arteries. In addition, there are so-called holistic treatments such as Paratox homeopathic or herbal preparations.

In each case, and indeed even if no treatment is done, there is a risk of the dog dying from a pulmonary embolism caused by worm die-off. In addition, there is risk while the worms are present of damage to the heart, the arteries, and the rest of the body, due to inflammation and immune reaction.

When deciding which method to use, you need to take into account the age of your dog, his level of activity, and the severity of the infestation.

Immiticide (fast kill)
Standard treatment with Immiticide consists of giving two injections 24 hours apart, then keeping the dog strictly confined for the next four to six weeks. The injections must be given in a painful location – the muscle close to the dog’s spine in the lumbar (lower back) area. The worms start to die immediately. As their bodies begin to decompose, pieces are “shed” into the dog’s bloodstream and filtered out through the dog’s lungs. This can cause the dog to cough and gag, or lead to a fatal pulmonary embolism.

The dog must be kept confined and his physical exertion kept to an absolute minimum, in order to prevent pieces of the dead worms from being forced by a rapid heart rate and/or increased blood pressure into clogging the tiny blood vessels in his lungs, causing embolisms. This generally means that the dog must be kept crated or penned and allowed out to potty only on a leash. Aspirin may be prescribed to lower the risk of blood clots, though this is controversial. Remember that it’s dangerous to combine aspirin with any other nonsteroidal anti-inflammatory drug (NSAID) or with prednisone, and to give it only with food.

A safer approach, sometimes called a “split-dose,” “staged-kill,” or “three-dose” protocol, consists of giving one injection, waiting one month or more, then giving two more injections 24 hours apart. This has the benefit of reducing the worm burden by about 30 to 50 percent with the initial treatment, before the balance are killed by the second set of injections.

This protocol is more expensive, requiring three injections of the drug instead of two. Also, the dog must be kept strictly confined for a longer period of time. Nevertheless, this split-dose protocol over two months is recommended for dogs with heavy worm burdens or other health problems (Stage 3), and is also considered safer for dogs with a lower worm burden.

Judy Wilds treated Taffy (described above) using the fast kill method with Immiticide. Although it is recommended to keep treated dogs in a crate to limit their exertion, Taffy was unused to being confined, and her vet was concerned that crating her could be dangerous, as excess stress and barking could cause lung problems. Wilds used a small outdoor pen for Taffy, instead.

“Taffy handled the heartworm treatment quite well. One evening, however, she gave me quite a scare when she began gagging. Later, I realized that this was related to her heartworm treatment,” Wilds says.

Some coughing or gagging is considered normal in dogs being treated for heartworm infection. However, it is important to understand that, whatever method of heartworm treatment is used, any respiratory difficulty in dogs receiving therapy should be considered a life-threatening emergency.

If the coughing or gagging is very heavy, seems uncontrollable, or causes the dog distress, get her to a vet right away. Vomiting or any bloody discharge combined with lethargy, fever, and/or pale gums should also be considered an emergency, and the dog should be taken to the veterinary or emergency clinic immediately. Corticosteroids, fluids, and oxygen may be needed at this time to help the dog survive.

Immiticide (melarsomine) has a much lower risk of complications than its historical predecessor, another arsenic compound called Caparsolate (thiacetarsemide sodium). Unlike Caparsolate, Immiticide does not damage the liver and kidneys, and kills a higher percentage of worms, so that fewer treatments are needed.

However, the Immiticide injections will cause muscular pain and soreness for a few days. It’s critical that the injections be done in an exacting fashion in order to minimize this effect. This includes changing needles after filling the syringe before injecting, choosing the site with care, putting pressure on the site after injection, and alternating sites for future injections.

Corticosteroids (e.g., dexamethasone) can be given at the same time as the Immiticide injection to lessen the reaction. Combination painkiller and sedative drugs, such as xylazine, may also be used to reduce the pain of the injection. Pain medications, such as NSAIDs, are often prescribed for a few days.

Christie Keith, a Scottish Deerhound breeder from Northern California, had two dogs who were heartworm-positive. “Raven had some obvious clinical changes, and was showing some symptoms (coughing). Due to her hyperreactive immune system, I was concerned that the presence of the worms in her pulmonary arteries, with their known ability to stimulate autoimmune reactions, would further worsen her existing allergies. So I opted to do the conventional Immiticide heartworm treatment, with the thought that it would be best to get the worms out of her more quickly.”

Christie intended to treat Raven with the split-dose schedule, but a couple of weeks after the first dose, Raven, despite having been completely confined and kept from activity more strenuous than going out into a tiny potty yard on a leash, developed a series of pulmonary emboli (clots), and nearly died. “We rushed her to the ER and when we got there, my mom, who was with her in the back of the van, thought she had died. She was blue,” recalls Keith. Emergency treatment saved Raven’s life. Tests conducted after the first treatment revealed no trace of heartworms, so Raven didn’t have to undergo another round of treatment.

Heartgard (slow kill)
The “slow kill” method, which is a newer approach, consists of giving the dog Heartgard on a monthly basis. This heartworm preventative medication has some effect against the adult worms and should gradually eliminate them over a period of one to two years; without treatment, the worms can live up to five years. The earlier the treatment is started after infection, the more quickly it will work to eliminate the adult worms. Note that only Heartgard (ivermectin) should be used, as Revolution (selamectin) affects far fewer adult worms, and Interceptor (milbemycin oxime) almost none at all.

Although this method is gentler than the use of Immiticide, the danger from the dying worms is still present, and for a much longer period. A recent Italian study showed that pet dogs (as opposed to the caged laboratory dogs this method had been tested on before) did get pulmonary emboli and some of the dogs died of it. The more active the dog, the higher the risk.

In addition, damage is being done to the arteries leading to the heart, and possibly to other areas of the body due to inflammation and immune response as long as the adult worms are present in the body. Most vets recommend using the fast kill method to treat heartworms, unless the dog’s health will not permit it or the owners cannot afford it. In those cases, or in the case of a very mild infection, the slow kill method may be used instead.

Janice Adams adopted Pepper, a Border Collie/Chow-mix, in June 2000. A blood smear in the vet’s office at the time of adoption was positive for microfilariae. “Pepper didn’t seem to have any symptoms of heartworm disease, no coughing or shortness of breath, so, at the recommendation of my regular vet, I elected to start her on Heartgard monthly,” says Adams.

“Pepper is a very calm dog, and didn’t have any activity restrictions. I kept her on Heartgard monthly for about 18 months, then switched her to a low dose of Interceptor on a 45-day schedule. We live in Florida, so I give heartworm preventatives year-round. All subsequent heartworm tests have been negative. She didn’t seem to have any problems related to the heartworm disease or treatment.”

Christie Keith’s second heartworm-positive dog, Bran, had no radiographic changes and no allergies, so Keith opted not to do the Immiticide treatment on him. “I put Bran on Heartgard monthly. At the time, there was less research on the ‘slow kill’ method with Heartgard than there is today, but there was enough that I felt confident it would be effective. And it was; within a few months or a year, he tested heartworm-negative.”

Other heartworm treatments
Surgical methods of heartworm removal require specialized training and instrumentation, and are generally reserved for high-risk patients who would not otherwise be expected to survive. The surgery is followed by one of the standard treatments a few weeks later to kill any remaining worms.

So-called alternative methods to kill heartworms, such as Paratox, are no safer than conventional drugs, since they rely on the exact same action – they kill the larvae/worms in the bloodstream. It is the death of the worms that causes the greatest danger to dogs during treatment.

In addition, no studies have been done to show that alternative treatments are effective. If these treatments do have any effect, they would be comparable to the slow kill method, with the same drawback of continued damage to the body while the worms remain present.

Finally, some of the herbs used to treat heartworm are considered dangerous and may be toxic in the amounts used to try to kill the worms.

Additional information
Once the adult worms have been killed, there may still be circulating microfilariae in the bloodstream. Although these microfilariae will not develop into adult worms (they mature further only inside a mosquito), they can be a source of transmission of heartworm disease to other dogs, so it is best to treat the dog with heartworm preventative four to six weeks after heartworm treatment in order to kill them.

Normal monthly preventative doses of Interceptor (0.5 mg/kg) or high doses of Heartgard (50 mcg/kg, approximately eight times the preventative dose) will eliminate most microfilariae immediately. Normal monthly doses of Heartgard or Revolution will also work for this task, but more slowly, over a period of several months.

Because heartworm treatments can cause an allergic reaction due to the death of the worms and the microfilariae, veterinarians will usually give dogs Benadryl and/or cortico-steroids prior to each treatment. Monthly Heartgard (ivermectin), at preventative doses, should not be strong enough to cause this kind of reaction, although it’s a good idea to stay home the day you give it to your dog.

If Interceptor (milbemycin oxime) at normal doses, or Heartgard at high doses, is used to kill microfilariae following heartworm treatment, anaphylactic shock can occur, especially in dogs with high microfilariae counts. This treatment is best done at the veterinarian’s office under close observation for any adverse reaction.

The damage done
Although heartworm treatment can be dangerous, so are the heartworms themselves. Adult heartworms are large, growing up to 12 inches in length and living as long as five years. They can plug up the pulmonary arteries, and when the infestation becomes severe, they will start to back up into the heart and eventually fill it. They can cause blood clots, and force the heart to work abnormally hard to pump blood through the clogged arteries. In addition, heartworms cause an extreme inflammatory response in the arteries that can affect other parts of the body, especially the kidneys and liver.

Treatment for heartworm infection is one area where conventional veterinary medicine offers valuable options. Whether you elect to do the fast-kill method using Immiticide, or the slow-kill method using monthly Heartgard, either is preferable to leaving the dog untreated, or using unproven, alternative methods that may have no effect or even be harmful.

This is also a situation where steroids and antibiotics can be lifesavers, when used judiciously during treatment.

The decision of how to treat a heartworm-infected dog is not an easy one, and is best made after consulting with your veterinarian regarding the safest method to use for your dog.

Also With This Article
Click here to view “Heartworm Treatment Options”
Click here to view “Heartworm: Don’t Take it Lightly”

-Mary Straus does research on canine health and nutrition topics as an avocation. She is the owner of the Web site. She lives in the San Francisco Bay Area with her dog Piglet.

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Mary Straus has been a regular contributor to Whole Dog Journal since 2006. Mary first became interested in dog training and behavior in the 1980s. In 1997, Mary attended a seminar on wolf behavior at Wolf Park in Indiana. There, she was introduced to clicker training for the first time, and began to consider the question of how we feed our dogs after watching the wolves eat whole deer carcasses. Mary maintains and operates her own site,, which offers information and research on canine nutrition and health. has been created to help make people more "aware" of how to make the best decisions for their dogs. It's designed for people who like to ask questions and understand the reasoning behind decisions, rather than just being told what to do.  Mary has spent years doing research for people whose dogs have health problems, or who just want to learn how to feed them a better diet. Over this time, she has learned a great deal about dog nutrition and health, including the role of diet, supplements and nutraceuticals.  In 2007, she was asked by The Ivy Group to contribute to The Healthy Dog Cookbook. She previously also wrote a column for Dog World.