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Invermectin, selamectin, moxidectin, and milbemycin oxime may be used in dogs for the prevention of heartworm disease and control of Gl round worms. Many canine parasites are susceptible to invermectin at the dosages used in other animals; however, because some dogs are adversely affected at these levels, invermectin is used in dogs at only 6 mg/kg body wt, Po, given at 1-mo intervals, to prevent development of Dirofilaria immitis, the cause of heartworm disease. At higher dosages (>100 mg/kg), some Collies and individual dogs of other breeds are adversely affected by ivermectin