The Dangers of Heartworm Medicine

By Jennifer Gittins
Heartworm disease occurs when a mosquito carrying the microfilariae comes into contact with your pet. The microfilariae are baby heartworms that enter the bloodstream, travel to the heart and mature. After approximately six months, the mature heartworms begin to produce new microfilariae (see references 1, 3 for everything above). Heartworms can be deadly if not effectively treated, however they are not always a death sentence.

Preventive Medications

Heartworm preventives are topical applications or oral medications that are administered on a monthly basis. Veterinarians usually will recommend that most pets be on a heartworm preventive. These medications kill the initial stages of heartworm, specifically the microfilariae, before they fully mature and reproduce (this paragraph, references 1, 2, 3).

Today, many heartworm preventives also treat or prevent various species of roundworms, hookworms, tapeworms and whipworms. Some medications will also treat fleas and their eggs, as well as ear mites and even mites that cause mange (reference 2).

Active Ingredient: Ivermectin

Ivermectin is one of the most common ingredients in many heartworm preventives. It is considered to be one of the most effective because it covers several different species of parasites that can affect pets, including many of the parasites mentioned above. For most pets, this ingredient is safe when proper dosage is followed (reference 1, 2 for all).

Unfortunately, certain herding breeds such as Collies and Shetland Sheepdogs have been known to show mild to severe reactions to ivermectin. Reactions include vomiting, whining or abnormal vocalization, seizures, difficulty breathing and in severe cases, death (resource 1, all).

Ivermectin is the active ingredient in Heartgard, Iverhart and Tri-Heart Plus, all of which are orally ingested (references 1, 2, 3).

Alternative Ingredients

There are other heartworm preventives that are just as effective as medications containing ivermectin. Milbemycin oxime is another effective active ingredient found in Sentinel for dogs and Interceptor for both dogs and cats. Both of these medications are administered orally, often with food.

Selamectin is another active ingredient found in popular heartworm preventives such as Revolution, for both dogs and cats. This form is a topical preventive that is applied between the shoulder blades of the animal in order to prevent ingestion.

Lastly, moxidectin combined with imidacloprid is found in Advantage Multi, and used with both dogs and cats. Just like milbemycin oxime, this medication is topical and applied between the shoulder blades (references 1, 2, 3 for whole section).

Arsenic Treatment

In the past, medications containing arsenic were administered in the case of a heartworm infestation (reference 1, 3). Thiacetarsamide sodium, also known as caparsolate, was provided to the pet through a vein using a catheter. Any medication that escaped the vein could cause severe damage to the surrounding tissue. Unfortunately, many animals became extremely ill from this form of medication. Tragedy was not uncommon due to the dangerous nature of this arsenic-based medication (resource 3 for all above).

Melarsomine Dihydrochloride Treatment

Today, arsenic-based medications are no longer the standard heartworm treatment and a safer alternative exists. Melarsomine dihydrochloride, also known as "Immiticide," is injected into two deep muscles within the pet. Following the injections, your pet will be hospitalized and closely watched for side effects (references 1, 3, all above). For a minimum of two months following the injections, your pet will be restricted from exercise, stair climbing, jumping and even walking (references 1, 3; resources 2, 3).

Three to four weeks after the initial injections, additional medication may be given to your pet. Administration of the second medication is based on your veterinarian’s assessment of your pet’s heartworm infestation (resource 3, above). The second round of medications is meant to kill off any remaining microfilariae that may still be remaining in the blood stream (resource 2, 3).
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