The disclosure relates to a method of treating a non-human animal, particularly sheep infested with biting lice of the species Bovicola ovis, wherein the method comprises the step of administering internally to the animal a pharmaceutically effective amount of spinosyn such that the spinosyn reaches multiple areas of a body of the non-human animal, at least in part via the bloodstream such that spinosyn is present systemically within the animal, wherein the spinosyn is spinosad or spinetoram, and wherein the spinosyn is administered in a total dosage of equal to or less than 40 milligrams per kilogram of the animal such that the spinosyn is administered to the animal in the order of two or more 1 to 6 milligrams per kilogram dosages spaced apart by a day or more.
