Stronghold (selamectin & sarolaner) is used to treat parasitic infections in cats. It is a spot-on solution applied to the skin, and it works by killing or paralyzing parasites such as fleas, ticks, mites, biting lice, ear mites, heartworms, roundworms and hookworms. Affects the nervous system of parasites, leading to paralysis or death. Contains Sarolaner, a member of the isoxazoline class.