We don't see a whole lot of blood or bone marrow disorders in cats. By far, the two most common are feline leukemia, and feline infectious anemia. There are many other blood disorders, but they are rare.
Feline leukemia. This is caused by a virus that is spread through bodily fluids and requires direct contact to be exposed. This is most common in young cats as kittens are very susceptible, and occasionally in older cats. Most older cats, even if not vaccinated, if they have a healthy immune system, only have a 5% chance of contracting feline leukemia if directly exposed. Feline leukemia primarily affects the bone marrow, and in final stages can cause anemia by stopping the production of red blood cells, or extremely high or low white blood cells by affecting white blood cell production in the bone marrow. There is no cure for this disease, and we will just try to keep your cat's quality of life good for as long as possible. Most cats will only live 1-5 years after contracting this disease. Very rarely, a cat can clear this virus themselves and eliminate it, but that is the exception, not the rule.
Feline Infectious Anemia, or FIA. This disease has had many names over the years and is just starting to be recognized as a common cause of disease in cats. FIA used to be called hemobartonella felis, then a few years ago it was changed to mycoplasma haemofelis. It is also called hemoplasma disease. Regardless of the name, this is a bacterial infection of the blood that is spread through eating infected fleas. Cats can have this infection simmering in their blood stream for many years without showing clinical signs, until they finally get sick. The way this bacterial infection causes anemia is the bacteria adhere to the red blood cells. As the red blood cells travel through the spleen, the spleen sees them as abnormal due to the bacteria present and takes them out of circulation. The bone marrow cannot keep up with the destruction of the red blood cells by the spleen, and we see progressive anemia.
There are three different strains of FIA, two of which are more likely to cause illness in your cat, one of which tends to just hang out without causing clinical illness. All three can be tested for using PCR. Clinical signs can include a fever that may respond to most antibiotics, but it always comes back, elevated white count, anemia, or elevated bilirubin on blood work, jaundice from severe disease, and just all over not feeling good. Some cats never show any clinical signs, however, and it is found incidentally.
FIA requires a very specific antibiotic called doxycycline. However even doxycycline can have problems eradicating FIA, so we will often treat with other antibiotics that will boost the effectiveness of doxycycline. In severe cases of illness, we may also briefly even treat with an immunosuppressant to get the spleen to stop destroying red blood cells, and some cats will need blood transfusions to stay alive long enough to respond to the treatment.