Hemobartonellosis in Cats

Class of 2001 (Cowgill) and Department of Pathology (Latimer), College of Veterinary Medicine, The University of Georgia, Athens, GA 30602-7388

Hemobartonella felis is a gram-negative, non-acid-fast mycoplasma that lives on the surface of the red blood cell. Measuring between 0.5 and 1.5 µm in diameter or length, this blood parasite exists as paired cocci, short rods, or small rings on the red blood cell plasma membrane.⁴H. felis infections are relatively common in cats in North America and produce extravascular hemolytic anemia. Feline leukemia (FeLV) positive cats with compromised immunity seem most susceptible to disease.¹Transmission of the parasite is thought to occur by blood-sucking arthropods such as fleas or by bite wounds. Queens can transmit the parasite to their offspring, although it is unknown if the transmission occurs transplacentally, at birth, or transmammary.³

A cat infected with H. felis may show mild anemia without clinical signs or may exhibit severe anemia with marked depression or subsequent death. Parasitemic episodes correlate with a decline in the packed cell volume (PCV). The erythrocyte life span also shortens with each bout of parasitemia. The primary method of red blood cell destruction is immune-mediated, although some direct damage may result from the presence of the parasite. Presumably, the host makes antibodies against exposed erythrocyte antigens or altered erythrocyte antigens that result from attached organisms. Due to the immune-mediated destruction of the red cell, these cats are Coombs’ positive or may occasionally demonstrate autoagglutination.

At necropsy, infected cats usually appear pale and emaciated. Splenomegaly and moderate icterus commonly are observed. Histologically, both erythroid hyperplasia of the bone marrow and extramedullary hematopoiesis are present. Erythrophagocytosis and splenic hemosiderosis also are observed. ³

The best method of diagnosis is observation of H. felis infected erythrocytes in the Romanowsky-stained blood film (Fig. 1). In addition, organisms may detach from erythrocytes where they can be observed scattered singly or in small aggregates in the background of the smear (Fig. 2). The presence of stain precipitate may interfere with the identification of parasitemia, especially if few organisms are present (Fig. 3). If stain precipitate is present, the ring form of H. felis is the most reliable morphologic evidence of parasitemia. Parasitized erythrocytes also may be seen in splenic and bone marrow aspirates. In addition, macrophages may contain the organisms within phagocytic vacuoles.⁵

H. felis infection usually is accompanied by a markedly regenerative anemia characterized by polychromasia and macrocytosis (Fig. 4). Metarubricytosis (nucleated erythrocytes) and Howell Jolly bodies are commonly seen in circulation during acute parasitemia (Fig. 5). As the disease progresses, bone marrow myeloid-to-erythroid ratio generally decreases.


Fig. 4. Cat, blood smear, Wright-Leishman stain. Polychromasia indicates regenerative anemia in a cat with hemobartonellosis. Also present are macrocytosis, a rubricyte, a metarubricyte, and a small lymphocyte.	Fig. 5. Cat, blood smear, Wright-Leishman stain. Polychromasia, three metarubricytes, and one erythrocyte with a Howell-Jolly body are present in the blood smear of a cat with hemobartonellosis.

Since the parasitemia is cyclic in nature, H. felis organisms may be absent from the blood smear at various time periods. A positive Coombs’ test with regenerative anemia, autoagglutination, or erythrophagocytosis is suggestive of hemobartonellosis; however, other diseases such as primary autoimmune hemolytic anemia and FeLV-induced hemolytic anemia also should be considered in the differential diagnosis (Fig. 6 & 7). Carriers of H. felis exist and may incidentally have organisms in the stained blood smear. If infected with FeLV, these cats may relapse and develop clinical signs of disease.³


Fig. 6. Cat, blood smear, Wright-Leishman stain. Autoagglutination in blood smear of a cat with hemobartonellosis. Erythrocyte aggregates did not disperse when blood was diluted with saline and examined as a wet mount preparation.	Fig. 7. Cat, blood smear, Wright-Leishman stain. Erythrophagocytosis by blood monocytes in a cat with hemobartonellosis.

When examining the blood smear for H. felis, one must take care to differentiate the parasite from other artifacts, organisms, and morphologic abnormalities. Stain precipitate may mimic or obscure H. felis, especially if few organisms are present in the stained blood film (Fig. 3). Stain precipitate is more variable in appearance, lies in a different plane of focus than the parasite, and also may be observed in the background of the smear. If stain precipitate is present, the ring form of H. felis is the most reliable morphologic evidence of parasitemia. The signet-ring appearance of Cytauxzoon felis (Fig. 8) may look somewhat similar to the smaller, ring-form of Hemobartonella felis. Howell-Jolly bodies are small, round, purple, nuclear remnants that are generally larger than H. felis (Fig. 5). Basophilic stippling, a dusting of the cytoplasm with fine gray granules, can also mimic H. felis (Fig. 9). Basophilic stippling, especially in lead toxicosis, may be accompanied by metarubricytosis, anisocytosis, and hypochromia. The basophilic inclusions in punctate reticulocytes of new methylene blue-stained blood films may also resemble the blood parasite (Fig. 10).²Therefore, Romanowsky-stained blood films are preferred to document parasitemia.


Fig. 9. Cat, blood smear, Wright-Leishman stain. Basophilic stippling of erythrocytes in a blood smear from a cat with lead poisoning.	Fig. 10. Cat, blood smear, new methylene blue stain. Inclusions within punctate erythrocytes are difficult to distinguish from H. felis organisms in this blood smear from a cat with hemobartonellosis.

In cats with hemobartonellosis, blood transfusions may be necessary if a rapid hemolytic crisis occurs (PCV declines to 12-15%). Otherwise, cats should be treated for 3 weeks with oral tetracyclines. During oral antibiotic treatment, cats should be monitored for fever, anorexia, and liver toxicity. To diminish the immune-mediated component of the disease process, an oral glucocorticoids, such as prednisolone, may be administered. The prognosis of uncomplicated hemobartonellosis is good, although many cats remain carriers as the parasite is incompletely eliminated from the blood.³

1. Carney HC, England JJ: Feline hemobartonellosis. Vet Clin North Am Small Anim Pract 23: 79-90, 1993.

2. Cowell RL, Tyler RD, Meinkoth JH (eds): Diagnostic Cytology and Hematology of the Dog and Cat. Mosby, St. Louis, 1999, pp. 271-272.

3. Greene CE: Infectious Diseases of the Dog and Cat. WB Saunders Company, Philadelphia, 1998, pp. 166-171.

4. Harvey JW: Atlas of Veterinary Hematology. WB Saunders Company, Philadelphia, 2001, p. 41.

5. Simpson CF, Gaskin JM, Harvey JW: Ultrastructure of erythrocytes parasitized by Hemobartonella felis. J Parasitol 64:504-11, 1978.