Buy Activyl Spot-On Flea Treatment for cats online! Activyl for cats controls existing flea infestations and provides control of new infestations for at least 4 weeks. It controls flea allergy dermatitis and immature stages of fleas. This treatment can be used on kittens from 8 weeks of age. Activyl is manufactured by Merck Animal Health (MSD) and uses the active ingredient Indoxacarb (S-isomer). We sell Activyl for cats in single doses, 6 packs and 12 packs. Each Spot-On applicator will treat your cat for 4 weeks. It is available for small cats and kittens to large cats.