Stronghold (selamectin) is used to treat parasitic infections in puppies weighing less than 2.5kg. It is a spot-on solution applied to the skin that works by killing or paralyzing parasites such as fleas, ticks, mites, biting lice, ear mites, heartworms, roundworms and hookworms. Active ingredient, Selamectin, is an antiparasitic medicine that affects the nervous system of parasites, leading to paralysis or death.