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Cats can be infected by tiny organisms that attack the surface of red blood cells, leading to disease or death due to anemia. This disease is transmitted via bites of fleas, but the route of natural infection has not been confirmed.

The disease previously was referred to as feline infectious anemia (FIA) or Hemobartonellosis, but recent genetic sequencing has now classified the bacteria within the genus Mycoplasma, so the preferred term is hemotropic mycoplasmosis. The organisms responsible for the disease have been re-named Candidatus Mycoplasma haemominutum (CMhm) for the small-sized strain, and Mycoplasma haemofelis for the large sized strain.

Differentiation is important because these two subtypes produce different clinical signs and severity of disease.

Acute M. haemofelis infection causes a moderate to severe hemolytic anemia. Fever, weight loss, poor appetite, pale or yellow mucous membranes and lethargy are signs that are commonly noted.  The term "hemolytic" refers to a low red blood cell count or anemia that results from red blood cell breakage (termed cell lysis). The organisms attached to the red blood cells are recognized by the immune system as invaders, and the spleen filters out the infected red cells, destroying them. The CMhm strain may be asymptomatic (silent) or cause a mild regenerative (responsive) anemia.  If both strains are present, or they are present along with a feline leukemia virus infection, more severe disease may occur.

The route of transmission between cats is not fully understood, but we know that blood transfusions can transfer the parasite, and queens have been known to infect their kittens. Fleas have been considered as vectors, biting first an infected cat, and subsequently spreading the mycoplasma to a cat subsequently bitten. Ticks, mosquitos and lice have also been considered as possible vectors that could transmit the organism cat-to-cat. Cat bite wounds have been recognized preceding infection, and may serve as a route of transmission. The cat will often develop signs of illness about one month after infection. Cats that recover from active signs of infection generally remain carriers that appear healthy, with variable numbers of organisms staying in the bloodstream. 

The Mycoplasma bacteria may be viewed in blood smears under a microscope. A PCR laboratory test to identify Mycoplasma DNA may be performed. This PCR test may be necessary for a diagnosis because the bacteria attached to red blood cells are not always found in the blood stream. This cyclic presence of the mycoplasma may also lead your veterinarian to suggest serial blood samples from different times of day to be sampled.

The more pathogenic M. haemofelis strain can produce death rates of up to one in every three of untreated sick cats, so prompt treatment is important. Cats with concurrent feline leukemia virus will usually have more severe anemia, and co-infection with these viruses is common in afflicted feral and outdoor cats. 

Prevention consists of avoiding exposure to infected cats and the vectors (insects).

Antibiotics, and potentially steroid anti-inflammatory therapy, intravenous fluids and blood transfusions may be required. Therapy does not completely clear the parasite and the cat will continue to carry the parasite, but relapse of sings of illness is uncommon. 

If your cat is showing signs of illness such as depression, decreased appetite, weight loss, weakness or pallor, promptly contact your veterinarian. With prompt diagnosis and therapy, the prognosis for recovery is good.