Easily preventable, heartworm disease can kill a dog if left untreated. Passed to your dog in the bite of a mosquito, tiny larva called microfilaria grow into large adult worms that pass through your pet's circulatory system and mature in the left ventricle of his heart. The U.S. Food and Drug Administration approves the use of certain preventive and treatment medications in veterinary doses; however, adverse side effects, including some deaths, have been noted when dogs are inadvertently given an overdose.
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Preventive Medications
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Many veterinarians recommend using a monthly dose of ivermectin (trade name Heart Guard) to prevent heartworms in your dog. Ivermectin works by destroying the central nervous system of the adult worms and the microfilaria (developing worms). It has also proven effective against certain intestinal parasites (both developing larvae and adults) and mites that cause mange.
Ivermectin toxicity occurs at extremely high doses and in dog breeds where it crosses the cellular barrier into the dog's brain, states Dr. Mark Papich in "Saunder's Handbook of Veterinary Drugs." Sensitive breeds include collies and collie-mixes, Australian shepherds, old English sheepdogs and shelties. Adverse reactions consist of difficulty with vision, depression, ataxia (inability to walk without stumbling), coma and death. Because the typical preventive dosage of ivermectin is so low, the U. S. Food and Drug Administration (FDA) approves it for use with a veterinary prescription.
Ivermectin is not recommended for dogs with a high count of microfilaria. Because the drug works fairly quickly, dead microfilaria can become lodged in the animal's heart, lungs and circulatory system. Veterinarians routinely suggest your dog receive a blood test to check for heartworms prior to prescribing any preventive medication.
Milbemycin oxime (trade names: Interceptor, Flavor Tabs, Sentinel) works similarly to ivermectin as a prevention and treatment of heartworms and intestinal parasites and mites. It also demonstrates adverse central nervous system side effects in high dosages and in the collie breeds. FDA approved at the veterinary dosing rate, your dog needs a heartworm test and veterinary prescription to be administered this drug.
In 2001, Fort Dodge Animal Health pulled the preventive moxidectin (trade name ProHeart 6) from the market based on concerns of the FDA about reports of serious side effects, including death. According to the FDA, they "concurred with Fort Dodge's decision to reintroduce ProHeart 6 to the U. S. market under a special program to manage the risks and restrict distribution" in June 2008. Moxidectin injections are designed as sustained release medications to be administered every six months. The documented side effects include neurotoxicity (ataxia, loss of vision, depression) as well as lethargy, depressed appetite, vomiting and lesions at the injection site. Because of the drug's previously documented high mortality rate, you must sign an FDA release form for your vet to prescribe this medication.
Treatment Medication
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Once your dog receives a positive heartworm diagnosis, your veterinarian may recommend treatment with melarsomine (trade name Immiticide), an arsenical compound. Melarsomine kills the adult heartworm by altering its ability to ingest glucose and metabolize it into energy.
Two melarsomine injections are performed 24 hours apart deep into the epaxial muscles (lower back) on either side of your dog's spine. Dr. Robert Bill states that, "in the weeks following adulticide treatment, the dead adult heartworms degenerate, forming emboli that can travel along the pulmonary arteries to lodge in the smallest arterioles in the lung where they can cause an inflammatory reaction" that leads to respiratory distress. In his book "Pharmacology for Veterinary Technicians," Dr. Bill suggests that your dog not be allowed to exercise or become excited for six weeks after treatment because of this risk, and recommends using glucocorticoids to combat any inflammation and ease any pain at the injection site.
Other adverse side effects include anorexia (inability or inattention to eating food), lethargy, depression and elevation of liver enzymes.
Topical Preventives
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Moxidectin and selamectin can be administered monthly to your pet. The manufacturers state that the "parameters for treatment with topical products are the same for the monthly oral" preventives, including having your dog tested for heartworms prior to using the drugs and needing a veterinary prescription. Overdosing your dog leads to the same neurotoxic side effects as other preventive medications.
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