Fleas and ticks are common parasites of cats. While some cats don’t exhibit adverse reactions to fleas, flea bites can trigger an allergic reaction in hypersensitive cats, manifested by severe itching. As the cat scratches persistently all over his body, the irritation can give rise to small crusty sores. Hair loss may also be a consequence of a heavy flea population.
Fleas are bloodsuckers, and a heavy population can result in the loss of substantial amounts of blood, increasing the cat’s risk of developing anemia. This can be quite serious among very young kittens; without proper veterinary attention, these kittens can die from the infestation.
Fleas can transmit tapeworms in cats. Tapeworms are common internal parasites, which, when present in high numbers can compete with the host for essential nutrients in the cat’s digestive system.
Source: Fleas Infestation In Cats