Some might also be allergic to the fleas' saliva which gets mixed in when they pierce the pores and skin and suck their hosts' blood. They can also absorb and get rid of bile acids, cholesterol, sugar, and the poisonous materials discovered all through the bowel. If you don't understand the wording, ask your veterinarian or call the manufacturer. There are 5 kinds of fleas that feed on blood and can be difficult to get rid of.