Truth about the Treatment of Canine Heartworm Disease

Truth about the Treatment of Canine Heartworm Disease

By Dr. Ed Mapes
Stonebridge Animal Hospital
McKinney, Tx

A commercial promoting heartworm preventative medication for dogs is currently airing on local television. Some statements made sensationalize the treatment of heartworm disease, while some other things said are actually untrue or misleading. Patients are said to undergo “the punishment of this deadly treatment” with “painful arsenic-based injections” and “weeks of confinement with little human interaction.” I hope this article will clarify some confusion that exists about heartworm disease in dogs, and provide insight into modern protocols that are safe and efficacious with minimal discomfort for our patients.

Dogs contract heartworms (Dirofilaria immitis) while being bitten by an infected mosquito. The insects become exposed to immature forms of the worm when taking a blood meal from another dog or wild carnivore such as a fox, coyote, or wolf. The larval heartworms aspirated into the mosquito develop and mature in its salivary glands, and eventually reach a stage that is infective to the mosquito’s next bite victim when injected along with the saliva.

Further development and maturation of the larvae takes place over the next six to nine months in this next host. The process leads to adult heartworms, which take up residence in the dog’s heart and pulmonary arteries. These worms – ranging in length from 14 to 27 centimeters – cause harm by their physical presence and by the dog’s own immune system reaction against the parasites.

Pathology can be severe, even leading to death, in some animals with large worm loads and/or exaggerated immune response that attempts to fight off the worms.
I agree wholeheartedly with any attempts to increase pet owner awareness of heartworm disease and how to prevent it. I disagree, however, with the use of overzealous descriptions and falsehoods in the interest of increasing product sales.

We diagnose the presence of adult heartworms in canine patients with blood tests that detect the dog’s antibodies directed against the worms. Once a patient is identified as heartworm positive, we next perform tests to assess the degree of damage to heart, lungs, and other organ systems in the body. This usually includes blood tests (serum chemistries and CBC), chest X-rays, and EKG.

Results of these diagnostics allow veterinarians to categorize patients from I to IV according to the severity of infection. Stage I patients have minimal signs of disease while Stage IV includes dogs with severe symptoms and serious systemic disorders that require other therapies and may even preclude treating to eradicate the adult heartworms. Most patients fall into category II; those with evidence of heartworms in the heart and lungs but without concomitant systemic disease; these make excellent candidates for treatment.

Most veterinarians begin therapy by administering Doxycycline, an antibiotic, for one month. This drug is effective in eradicating an organism called Walbachia – a type of bacteria that actually infects the heartworm itself. These organisms are thought to cause increased inflammation in the dog’s lungs when the heartworm treatment begins to kill adult heartworms. Studies have shown that killing Walbachia leads to the death of some heartworms, and also eliminates the Walbachia organisms directly. Taking this step makes the actual heartworm treatment easier on the patient.

Thiacetarsemide sodium was the drug used thirty years ago to treat heartworm cases. This drug did in fact contain minute amounts of arsenic as the active ingredient. It was given by intravenous injection that was painful if the drug escaped from the blood vessel – which almost never happened because we injected into intravenous catheters. Thiacetarsemide must be the “painful, arsenic-based injections” referred to in the commercial, but this drug hasn’t been used to treat dogs for heartworm in decades.

After one month of receiving Doxycycline, the dog is hospitalized and given a thorough physical exam. If everything checks out, most protocols call for starting with one injection of the modern drug used to kill adult heartworms – Melarsemine dihydrochloride, or Immiticide. This injection is administered into heavy muscles of the dogs back after clipping the hair and scrubbing the area in the same way that we prep for sterile surgery. The drug itself causes no pain when injected; patient reactions are usually no more than seen with routine vaccination.

We hospitalize the dogs that day and overnight, observing for signs of overzealous immune system reactions to the dying heartworms – fever, labored breathing, cough, lethargy. Since our patients are pre-screened and medicated as indicated before treatment is authorized, these symptoms are rare in modern day heartworm cases.

Patients are usually released the following day, with medications prescribed as indicated on a per case basis. This often includes antiinflammatories and antibiotics; some doctors also prescribe medication to diminish the possibility of blood clots. Owners are advised to curtail vigorous activity – since dying worms are present in the heart and lungs – and to observe their dogs for one month’s time. There has never been a recommendation to “confine patients with little human interaction”. I have no idea where this statement came from, but it is patently untrue since we encourage monitoring of the dogs for development of any symptoms – cough, decreased exercise tolerance, loss of appetite, etc.

The second phase of treatment commences after one month. Two injections of Immiticide are usually given this time, one on the morning of admittance followed by a second dose the next morning. The same methods of injection site preparation and patient monitoring are employed.

The patient is released on the third day, again with instructions to discourage vigorous activity and to watch for any symptoms. Some protocols call for initiating heartworm preventative medication before treatment; some begin preventatives afterward – that’s up to individual practitioners’ preference.
Preventing heartworm infection is always preferable to treating the disease. Modern methods and medications, though, have made the treatment safer than ever, involving minimal discomfort from the injections, with recovery periods that are usually without incident. Do not be mislead by inflammatory language in the commercial; our canine patients with heartworm disease are not subjected to “the punishment of deadly treatment”.

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