It provides a method of treating an animal infested with lice that characterized by the step of administering to the animal an internally pharmaceutically effective amount at least one spinosyn, such that the spinosyn systemically is present within the animal.Se proporciona un método de tratamiento de un animal infestado con piojos mordedores caracterizado por el paso de administrar internamente al animal una cantidad farmacéuticamente eficaz de -al menos- una espinosina, de modo tal que la espinosina esté presente sistémicamente dentro del animal.