Treatment of Heartworm Disease in Dogs

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Treatment of heartworm disease refers to dogs that have heartworm infections or heartworm disease. These dogs may be asymptomatic (testing positive for heartworms but not sick) or they may actually be suffering from a disease caused by the heartworms. This differs from dogs that are receiving heartworm medication to prevent infection.

Principles of Treating Heartworm Disease

Treating canine heartworm disease involves killing the adult worms that live in the heart and pulmonary arteries, as well as those in the larval stages (called microfilaria) that circulate in the bloodstream of the dog.

  • If the larval stages are not killed, they will develop into adult heartworms and the cycle of heartworm disease will continue.
  • The adult worms that live in the heart and pulmonary arteries are the worms that cause the damage to the heart and lungs. This is the life stage that actually causes the signs of heartworm disease to develop. As a result, they must be killed to be effective in treating heartworm disease.

In addition, there is a rickettsial organism known as Wolbachia that is a parasite of the heartworm. It is believed that Wolbachia may provide some sort of protection for the adult heartworm and there is concern that it may also contribute to the inflammation in the lungs that results as the heartworms die.

As with any other disease, formulating a treatment plan begins with a thorough examination of the dog's overall health and condition. The findings of this examination can be used to formulate a treatment plan.

Medications Used in the Treatment of Heartworm Disease

Ivermectin is the medication most commonly used to kill the microfilaria (larval stage). There are other medications that will kill them (such as milbemycin), but ivermectin kills more slowly. When too many microfilaria die at once, it can cause shock and collapse for the dog. Thus, ivermectin is preferred because of the slower kill rate.

Other products like selamectin and moxidectin do not kill the microfilaria efficiently enough to clear them reliably. Fortunately, ivermectin is available in several monthly heartworm preventive medications. Examples are Heartgard ®, Tri-Heart®, and others.

The only medication currently available to kill the adult heartworms is melarsomine (Immiticide®). Melarsomine has to be injected deep into the muscles of the back and the injections can be quite painful. Often, pain medications are given concurrently with the melarsomine injections to reduce the level of discomfort for the dog.

Heartworm Treatment Protocol

There are two protocols that can be used for treating heartworm-infected dogs with melarsomine.

  • The first protocol is reserved for dogs that are relatively healthy and not showing significant signs of heartworm disease. In this protocol, two injections of melarsomine are administered 24 hours apart.
  • The second protocol is the one most commonly used and many veterinarians recommend this protocol for all dogs, regardless of the stage of disease. This protocol involves three injections of melarsomine. A single injection of melarsomine is given, followed a month later by two additional injections given 24 hours apart.

The second protocol (3 injections) is generally safer because the adult worms die more slowly with this protocol. Fewer numbers of dead heartworms mean less chance of adverse effects from the treatment. However, it should be realized that treatment for heartworm disease is risky and there is always a chance of complications.

Doxycycline Usage

Doxycycline is often used also as part of the treatment of heartworm infection/heartworm disease. This is because doxycycline kills the Wolbachia organism that many veterinarians believe may make adverse reactions to the heartworm more severe.

There is disagreement about the role that Wolbachia plays in the development of heartworm disease and treatment. Not all veterinarians endorse using doxycycline as part of the heartworm treatment.

Home Care for Dogs Being Treated for Heartworm Disease

The most important thing for a dog being treated for heartworm disease is complete rest. Strict confinement is essential during the heartworm treatment period and for at least one month following the last melarsomine injection.

During the recovery period, embolism of dying worms is a major concern. This means that the dying worms cause obstructions in the blood vessels in the lungs. Exercise increases the risk of embolism, thereby increasing the risk of serious side effects.

Any medications that your veterinarian has sent home with you should be given as directed. A monthly heartworm preventive should be continued.

If your dog starts to cough, has nosebleeds, develops a fever or is acting abnormally otherwise, you should consult your veterinarian immediately.

Treatment of Caval Syndrome

Caval syndrome is a particularly severe form of heartworm disease in which the heartworms fill the entire right side of the heart and spill out of the heart into the vessel that leads to the heart. Dogs with caval syndrome are generally very ill and the only successful treatment is the physical removal of the worms from the heart.

Slow Kill Heartworm Treatment Method

The so-called "slow kill" method of heartworm treatment involves administering monthly heartworm preventive medications (usually ivermectin-based) and waiting for the adult worms in the heart to die a natural death. This is only recommended in cases where the dog is not a candidate for treatment with melarsomine. In this scenario, the adult worms can take up to two years to die and are still capable of causing damage to the heart and lungs during that period.

Please note: this article has been provided for informational purposes only. If your pet is showing any signs of illness, please consult a veterinarian as quickly as possible.-