I would probably say no that they cannot become immune but I'm not 100% certain. At the rate that fleas can reproduce we would have all manners of medicine resistant fleas! I find a flea or two from time to time on my puppy and he is on the skin drops and his heartworm meds have flea stuff in them. He gets them sometimes when we play in the yard for a long time or come from the dog park but never an infestation or anything. I wouldn't worry about them too much... |