Hemobartonella felis
Rod-shaped forms are seen on the edges of many RBC in this field. A faint ring form is visible on the face of one cell in the lower left quadrant. Increased polychromatophilic RBCs reflect the marrow response to the hemolytic anemia. One RBC in this field contains a medium sized Heinz body.... can you find it?

Mycoplasma haemofelis (formerly Haemobartonella felis) is an epicellular bacterial parasite of feline red cells that can cause hemolytic anemia. In blood smears stained with polychrome stains, the organisms are recognized as small blue cocci, rings, or rods on the edges or across the faces of red cells. Stain precipitate is often mistaken for organisms, resulting in unnecessary tetracycline administration.
     The hemolytic anemia caused by M. haemofelis is called feline infectious anemia (FIA) and is usually regenerative in nature (unless underlying disease suppresses the regenerative response, e.g. Feline Leukemia Virus infection). Therefore, a non-regenerative anemia in a cat with H. felis should not be attributed only to M. haemofelis infection and the cat should be further evaluated for another disease process. In this situation, the underlying disease has likely resulted in recrudescence of the M. haemofelis in an asymptomatic carrier due to stress.
      In feline infectious anemia, the parasitemia is episodic, with decreasing hematocrits at times of parasitemia. Because of the cyclic parasitemia, organisms may be numerous, scarce or not found in a given blood sample as illustrated in the graphs below.

Changes in packed cell volume (PCV), temperature and blood parasites in a cat after inoculation of Mycoplasma haemofelis on day 0. Phase A: preparasitemic phase; Phase B: acute phase; Phase C: recovery phase; Phase D: carrier phase.

Reproduced with permission from: Hemotrophic mycoplasmosis (Hemobartonellosis) by J Harvey in Infectious Diseases of the dog and Cat, 3rd ed, edited by Greene, 2006, pg 255, Copyright Elsevier

Absence of organisms is thought to be due to transient sequestration of parasitized erythrocytes in the spleen with removal of organisms and release of erythrocytes with reduced lifespans. The severity of the anemia does not correlate to the degree of parasitemia, and is often worse in animals with underlying disease. Cats are often concurrently Coombs positive and can display cold agglutination (agglutination is often observed in blood tubes stored in the refrigerator). Thrombocytopenia may also be observed in some cats. Icterus is quite uncommon due to M. haemofelis infection alone. Most cats infected with M. haemofelis become asymptomatic carriers and re-develop milder versions of the disease when stressed.
      Diagnosis of M. haemofelis is accomplished by detection of the organism in blood smears. As mentioned above, organisms may not be detected because of the cyclic parasitemia, with dwindling parasite numbers. Note that the organism falls off erythrocytes in aged blood samples (> 24 hours) and can be mistaken for stain precipitate as they are found between and not on the erythrocytes, as expected. This can result in misdiagnosis. Newer PCR-based diagnostic tests are becoming more commercially available and are very useful in cats with low parasitemias or that are subclinical carriers.

Small coccoid-shaped forms (1-3) are seen on a few RBCs in this field. Notice the absence of polychromatophilic RBCs as well. One RBC also contains a Howell-jolly body....can you find it?

Mycoplasma haemominutum is a small red cell parasite that causes only minimal clinical signs of acute disease and negligible hematologic changes in infected cats. Single, and occasionally, multiple organisms are detected on each red blood cell, however they are far more difficult to see than M. haemofelis. The difference in virulence between these two red cell parasites (M. haemofelis and M. haemominutum) in the cat might be merely a dose dependent effect. It has been suggested that hemoplasma infections in cats can exacerbate the progression of FeLV-related diseases.

  Fleas infected with M. haemofelis can transmit parasites and produce disease in a susceptible cat, whereas the experimental transmission of M. haemominutum by fleas was not successful. Although treatment with doxycycline or enrofloxacin may control acute infections in the cat, none of the antibiotics tested to date consistently clear the parasites permanently from the animal, resulting in a chronic carrier state.