Heartworms are common parasites in dogs and have been found in all 50 states. Medications prevent heartworm infection by killing larvae before they can mature. The most common heartworm medications are ivermectin (Heartgard) and milbemycin oxime (Interceptor).
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Potential Adverse Reactions
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Reported adverse reactions in dogs given heartworm medication include loss of appetite, lethargy, depression, vomiting, diarrhea, excessive salivation, loss of coordination and convulsions. These reactions are generally mild and transient.
Sensitive Breeds
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Some breeds, including collies, Shetland sheepdogs and Australian shepherds, have a genetic mutation that makes them sensitive to various drugs. There is a DNA test for this mutation available.
Clinical Trials
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Heartworm medications have a wide margin of safety in clinical trials at normal dosages, even for sensitive breeds. Adverse reactions were seen with exaggerated doses (16 to 25 times the normal dose, or daily administration of a monthly product at three times the normal dose).
Seasonal Prevention
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To minimize the risk of adverse reactions, some vets recommend a seasonal medication schedule. Dogs living north of the Virginia/North Carolina state line should be adequately protected when monthly pills are given from June through November.
Other Parasites
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Some heartworm medications also control intestinal parasites. Heartgard Plus controls roundworms and hookworms; Interceptor controls these as well as whipworms.
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