Fleas!!
Fleas are more than just an annoyance, heavy infestations can compromise the health of a pet through three routes:
1. Anemia: Blood sucking by fleas can cause anemia, especially in very young puppies or kittens, which can be fatal.
2. Disease: Fleas carry other parasites, such as roundworms, and diseases such as bubonic plague, which are transmittable to both the pet and the owner.
3. Allergic response: Fleas inject a protein into the blood when they bite to keep the blood from clotting as they feed. Sensitive pets can become allergic to this protein causing intensive itching at the site of both old and new flea bites. Some pets will scratch flea bites to the point of causing open wounds which can become infected.