Feline haemoplasmas (Mycoplasma haemofelis and Mycoplasma haemominutum)

© Elizabeth S. Cowgill, DVM and Kenneth S. Latimer, DVM, PhD; http://www.vet.uga.edu/vpp/clerk/Cowgill/Index.htm; Macieira, DB (200) Prevalence and risk factors for haemoplasmas in doemstic cats naturally infected with feline immunodeficiency virus and/or feline leukaemia virus in Rio de Janeiro - Brazil. JFMS 10:120-129

Introduction

Hemobartonellosis is caused by Mycoplasma haemofelis, previously known as Haemobartonella felis. Cats infected with this organism typically develop regenerative anaemia. The related species Mycoplasma haemominutum (a less pathogenic strain) may also cause anaemia. A third species, M. turicensis has also been isolated from wild felids in Brazilian zoos. 

M. haemofelis (formerly 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. M. haemofelis infections are relatively common in cats in North America and produce extravascular haemolytic anaemia. M. haemofelis is also commonly recognised as a pathogen in conjunction with retroviruses, including FIV ( Feline Immunodeficiency virus), FeLV ( Feline leukaemia) and other debilitating diseases. FeLV-positive cats with compromised immunity seem most susceptible to disease. It has been suggested that  cats having a pre-existing retroviral infection may be at a greater risk for becoming infected with feline haemoplasmas. Further, cats that were experimentally infected with retroviruses and M. haemominutum developed more severe anaemia than cats infected with the parasite alone. Studies also suggest that M. haemofelis and M. haemominutum may act as cofactors, accelerating the rate of progression of feline retrovirus-related diseases. Thus, cats that are co-infected with haemoplasmas and retroviruses may be at greater risk for the development of lymphoma, leukaemia and immunodeficiency syndrome.

In Brazil, it has been shown that mycoplasmas are present in 57% of anaemic cats, and in FIV-positive cats, the prevalence of M. haemominutum was approximately 36% and M. haemofelis 37%. There is however, no direct relationship between FIV/FeLV status and mycoplasma infection.

Transmission

Transmission 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.

Pathological Changes

A cat infected with M. haemofelis may show mild anaemia without clinical signs or may exhibit severe anaemia 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.  

Diagnosis

The best method of diagnosis is observation of M. haemofelis-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 M. haemofelis is the most reliable morphologic evidence of parasitemia. Parasitised erythrocytes also may be seen in splenic and bone marrow aspirates. In addition, macrophages may contain the organisms within phagocytic vacuoles.

Fig. 1. Cat, blood smear, Wright-Leishman stain. Scattered erythrocytes contain delicate ring and rod forms of M. haemofelis. Fig. 2. Cat, blood smear, Wright-Leishman stain. M. haemofelis organisms are present on erythrocytes and scattered singly and in small aggregates in the background of the smear. Fig. 3. Cat, blood smear, Wright-Leishman stain. The presence of stain precipitate interferes with the identification of M. haemofelis. 

M. haemofelis infection usually is accompanied by a markedly regenerative anaemia 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 anaemia 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, M. haemofelis organisms may be absent from the blood smear at various time periods. A positive Coombs’ test with regenerative anaemia, autoagglutination, or erythrophagocytosis is suggestive of Hemobartonellosis; however, other diseases such as primary autoimmune haemolytic anaemia and FeLV-induced haemolytic anaemia also should be considered in the differential diagnosis (Fig. 6 & 7). Carriers of M. haemofelis 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.  

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 M. haemofelis, one must take care to differentiate the parasite from other artefacts, organisms, and morphologic abnormalities. Stain precipitate may mimic or obscure M. haemofelis, 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 M. haemofelis 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 M. haemofelis. Howell-Jolly bodies are small, round, purple, nuclear remnants that are generally larger than M. haemofelis (Fig. 5). Basophilic stippling, a dusting of the cytoplasm with fine grey granules, can also mimic M. haemofelis (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). Therefore, Romanowsky-stained blood films are preferred to document parasitemia.

Fig. 8. Cat, blood smear, Wright-Leishman stain. Signet ring morphology of Cytauxzoon felis. 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. 

Treatment and Prognosis

In cats with Hemobartonellosis, blood transfusions may be necessary if a rapid haemolytic crisis occurs (PCV declines to 12-15%). Otherwise, cats should be treated for 3 weeks with oral tetracyclines such as doxycycline (10 mg/kg PO oid). 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 glucocorticoid 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.

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