For most dogs that live in the United States, heartworm prevention medication is a way of life. However, for some dogs, particularly white-footed herding dogs with the genetic mutation for ivermectin sensitivity, the medication Heartgard is sometimes considered to be not an option.
Common Adverse Reactions (Mild)
Heartgard is the trademarked name of the generic drug ivermectin. According to the Heartgard information sheet provided by the manufacturer (Merial), it is given to dogs 6 weeks of age and over. Some of the milder adverse reactions include nausea, vomiting, diarrhea, depression, hypersalivation and excitability. According to Merial, although Heartgard is largely a safe product, some dogs may show some “mild hypersensitivity” if they have immature roundworms circulating in their bloodstream.
Common Adverse Reactions (Severe)
Some of the more severe reactions to Heartgard include mydriasis (severe pupil dilation), ataxia (loss of muscular control), paresis (partial paralysis), and convulsions. These symptoms are not common in response to the prescribed dosage recommended by Merial.
Adverse Reactions in Collies and White-Footed Herding Dogs
Many collies are sensitive to Ivermectin; however, according to Merial, Heartgard use has proven safe for collies when used even in elevated doses up to 10 times the recommended dose. However, collies administered elevated doses of 16 times the recommended dose of Heartgard were prone to all adverse reactions to the drug, as well as to stupor, coma and death. Other white-footed herding breeds may also be affected by the genetic mutation that causes this sensitivity. Washington State University research indicates that on rare occasions, dogs with the genetic mutation may show adverse reactions at doses "that are 1/200th of the dose required to cause toxicity in other dogs."