it dont work like that mate, fleas and ticks also attach themselves to other mammels fox,badger, squirrels,hedgehogs,mink,rats,mice etc and birds so the desease is spread, also if a buzzard or fox picks up an infected one thats full of fleas and takes it away to another area to eat it then there will be a good chance that the infected fleas will jump onto another hoast in that area so the desease spreads. its also tends to be cyclic aswell.