Animals get anemia for many different reasons. The most common ones are drug or toxin reaction, disease, and blood loss. Many common household products pose a serious toxic threat; acetaminophen, the active ingredient in Tylenol, is among the deadliest. Onions, whether cooked, raw, or dehydrated, are also especially dangerous. Other anemia-inducing substances include anti-inflammatory medications like aspirin, zinc, and propylene glycol, which is sometimes found in canned food.
Certain infectious diseases that animals contract are immune-mediated -- that is, the body begins killing its own red blood cells, resulting in hemolytic anemia. These diseases are more common among dogs. Other infectious diseases that can lead to anemia include the feline leukemia virus, feline immunodeficiency virus and blood parasites such as Haemobartonella and Babesia. Cancer and kidney failure, as well, can make a cat or dog anemic.
Blood loss, of course, means fewer red blood cells, so animals that have lost blood through trauma often become anemic. Fleas are literally bloodsuckers and are especially threatening to kittens and puppies, which are smaller and have less blood than adult animals.
