Wendy C. Brooks, DVM, DABVP
American Heartworm Society

There are several strategies that can be used depending on the dog's medical condition, including the option of not treating at all. The important concept to realize is that very harsh arsenic based drugs are necessary to kill adult heartworms and that treating for heartworm infection is neither simple nor safe in itself.

Patient Evaluation and Staging

Prior to therapy, the heartworm patient is assessed and rated for risk into one of four categories. Important factors include: how many worms are thought to be present based upon the tests performed, the size of the dog, the age of the dog, concurrent health factors, severity of the heart disease, and the degree to which exercise can be restricted in the recovery period. The categories into which patients are grouped are as follows:

  • Class I: Lowest Risk. These dogs are typically without symptoms and their infection is a surprise discovery during a routine annual health examination when a positive test comes up. Other blood tests are normal and radiographs show mild changes if any change at all.
  • Class II: Moderately Affected. Healthy dogs with minimal signs as above, occasional coughing, fatigue only with exercise but with radiographs that show definite evidence of heart disease. Lab testing shows mild anemia, urine dipsticks show some protein present but not severe urinary protein loss.
  • Class III: Severely Affected. Dog is suffering from weight loss, cough, difficulty breathing, blatant damage to the vasculature is apparent on radiographs, lab work reveals a more severe anemia and marked urinary protein loss. The damage to the lung blood vessels from the worms, creates extra resistance for the heart to pump against and often episodes of collapse occur with exercise. If the damage is severe enough, the heart can actually fail trying to pump through all the clogged up blood vessels. Class III dogs are expected to die without treatment but are, unfortunately, sick enough that treatment itself is not without risk.
  • Class IV: Caval Syndrome. Dog is collapsing in shock with dark brown urine evident. Heartworms visible by ultrasound in the AV valve of the right side of the heart, very abnormal bloodwork. These dogs are dying and can only be saved by the physical removal of adult heartworms via an incision through the jugular vein. If such a dog can be saved from this crisis, further heartworm infection treatment cannot be contemplated until the dog is stable enough to fit into one of the other categories above.

Heartworm Stages

Dogs have three stages of heartworms in their body, we need to understand them in order to understand the treatment.

  • The microfilariae- These are the newborn children of the adult worms living in the heart and pulmonary arteries. The microfilariae are swimming freely in the bloodstream, possibly in large numbers, and it is the microfilariae that can spread to other dogs through a mosquito. The microfilarias are killed so as to keep the dog from spreading the infection.
  • L3 & L4 larvae - These are the new arrival heartworm larvae, which are delivered from mosquito bites. These are L3 & L4 larvae living in the skin (having arrived within the last three months). These will continue their maturation and repopulate the heart and pulmonary arteries if they are not killed before the adult worms are.
  • L5 larvae and adult worms - These are living inside the heart and pulmonary arteries. This group requires the arsenic compounds for destruction while the other two groups can be killed with less toxic products.

Treatment Guidelines

The goal is to get rid of all the worms with as little inflammatory reaction as possible. To achieve this, treatment is approached in stages with the goal being to minimize the number of adult worms to be killed and having the patient as healthy as possible going into what is called adulticide treatment.

Initial Treatment (Day 0-30)

From the moment your dog is diagnosed with heartworm disease your dog must be kept strictly confined! That means your dog should be restricted to a very small area at all times throughout the entire treatment period (the use of a small cage or crate may be necessary), and continuing for 6 to 8 weeks after the last injection of melarsomine (the drug that kills adult heartworms). To ensure exercise restriction, your dog must be kept on a leash when going outside.

Your dog will be started on a once-monthly medication called a “preventive,” to kill immature heartworms and prevent new infections from developing. You should continue to give it to your pet throughout treatment as well as for the rest of his/her life.

Your dog will also be given the antibiotic doxycycline for 30 days. This medication kills bacteria which live in the heartworms. These bacteria help the worms survive and reproduce. They also may cause worsening inflammation when adult heartworms die. By giving this antibiotic prior to the medication that will kill your dog’s heartworms, we decrease the likelihood of complications from the treatment and optimize the chance for complete elimination of the infection.

First Adulticide Treatment (Day 60)

Two months after starting heartworm preventive, you will need to bring your dog to the clinic for the first injection of melarsomine, the drug that kills adult heartworms. Because complications can occur, your pet needs to stay in the clinic for observation. The patient receives an intramuscular injection deep in the lower back muscles. This is a painful injection, and it is common for the patient to be quite sore at home afterwards. Be careful and gentle around the injection site.   Upon discharge, we may also prescribe prednisone or another anti-inflammatory medication for your dog to reduce side effects.

In special cases and with patients in Class I a Doctor may choose to skip this step and go directly to the next step saving the pet 30 days of confinement.

Second Adulticide Treatment (Day 90)

One month after the first injection, two additional injections of melarsomine must be given 24 hours apart. Once again, your dog will need to stay at the clinic during this treatment period. The patient receives two intramuscular injections deep in the lower back muscles. These are painful injections, and it is common for the patient to be quite sore at home afterwards. Be careful and gentle around the injection sites.   Upon discharge, we may prescribe prednisone or another anti-inflammatory medication for your dog to reduce side effects.

Interim Assessment of Health and Microfilaria Status (Day 120)

We will examine your dog to determine his/her status following treatment. This examination will include a careful physical examination with an emphasis on the heart and lungs as well as testing for circulating immature stages of heartworm (microfilaria). This interim assessment will us determine when your pet can gradually resume more vigorous physical activity. If microfilaria are still present, we may prescribe an additional medication to help eliminate them and then retest at Day 150 to assess effectiveness.

Final Assessment of Efficacy (Day 270)

We will test your dog’s blood to assess the success of adulticide treatment. The goal is to eliminate all stages of heartworms from your dog.

Home Care – Your Responsibility

Throughout the recovery period, please watch your pet for coughing, gagging, vomiting, diarrhea, or loss of appetite. Excessive sluggishness, respiratory distress, and coughing up blood are signs of a serious problem that require immediate veterinary attention.

The most important thing you can do to minimize complications during treatment is to restrict your pet’s exercise before, during and after the melarsomine injections. When adult worms die, they collapse and are pushed deeper into the smaller branches of the vessels that supply blood to the lungs. Because exercise increases blood flow to the lungs, it increases the likelihood that dead worms will block blood flow (embolism). This can result in severe complications and possibly death. The importance of restricting your dog’s activity cannot be overemphasized.

What if I don't treat?

Ivermectin Only

Melarsomine treatment is expensive and often out of reach for rescue groups, shelters, and many individuals. If the dog is stable (Class I) one option is to simply leave the dog on an ivermectin-based preventive. This option has led to a great deal of misconception about the ability of ivermectin to kill adult heartworms. Let us lay the rumors to rest now:

  • Ivermectin does not kill adult heartworms.
  • Ivermectin does shorten the lifespan of adult heartworms.
  • Ivermectin does sterilize adult heartworms.
  • Ivermectin does kill microfilaria (keeping the dog from being a source of infection)
  • Ivermectin does kill L3 and L4 larvae (preventing new infections).

This means that if you opt to treat a heartworm positive dog only with an ivermectin heartworm preventive, you can expect the dog to remain heartworm positive for as long as two years and the heartworm disease will be progressing during those two years. This is not good for the dog but certainly beats getting no treatment of any kind.

There is another aspect to this treatment that is important to mention and that is the development of resistant heartworm strains. Using heartworm preventives long term in dogs with adult heartworms leads to the production of heartworm preventive-resistant microfiliaria.  In other words, this slow kill method of treating heartworm infection leads to strains of heartworm that cannot be stopped by any of the heartworm preventives available on the market.  Heartworm preventive resistance has now been documented in some of the heartworm strains of the Mississippi river delta area and inappropriate use of heartworm preventives is felt to be the cause. Previously, the slow kill method of heartworm treatment was deemed acceptable for dogs in the class I (early) stage of heart disease but this has changed given confirmation of resistant strains. The Companion Animal Parasite Council now recommends that the slow kill protocol using heartworm preventives NEVER be used regardless of whether the patient is in class I stage of infection or not. Heartworm infection should be dealt with definitively and promptly so as to preserve the effectiveness of the medications we have.

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