The large and small forms of Haemobartonella felis are gram-negative, epicellular parasite of feline erythrocytes. The organisms are classified as mycoplasmas. The new name for the large form (Ohio isolate) is Mycoplasma haemofelis. The proposed name for the small form (California isolate) Candidatus"Mycoplasma haemominutum". In at least two studies of experimentally infected cats, M. haemofelis is apparently more pathogenic than M. haemominutum; all M. haemofelis inoculated cats became clinical ill whereas M. haemominutum inoculated cats were subclinically infected. Cats with chronic M. haemominutum infection had more severe anemia and longer duration of anemia when experimentally infected with M. haemofelis when compared to cats infected with M. haemofelis alone.

It was recently shown that naturally infected cats and fleas can be infected by M. haemofelis and M. haemominutum (Lappin et al, 2003). In addition, cats with experimentally induced M. haemominutum infections transfer the infection mechanically to fleas (Woods et al, 2003). We have just shown fleas to be competent vectors for M. haemofelis(unpublished data). Hemoplasmas have been transmitted experimentally by IV, IP, and oral inoculation of blood. Clinically ill queens can infect kittens; whether transmission occurs in utero, during parturition, or from nursing has not been determined. Transmission by biting has been hypothesized. Red blood cell destruction is due primarily to immune-mediated events; direct injury to red blood cells induced by the organism is minimal.

Clinical signs of disease depend on the degree of anemia, the stage of infection, and the immune status of infected cats. It appears that M. haemominutum infections are potentiated by feline leukemia virus coinfection (George et al, 2003). Clinical signs and physical examination abnormalities associated with anemia are most common and include pale mucous membranes, depression, inappetence, weakness, and occasionally, icterus and splenomegaly. Fever occurs in some acutely infected cats and may be intermittent in chronically infected cats. Evidence of coexisting disease may be present. Weight loss is common in chronically infected cats. Cats in the chronic phase can be subclinically infected only to have recurrence of clinical disease following periods of stress. A greater percentage of cats with fever are infected with M. haemofelis than cats without fever suggesting the organism can be associated with fever of unknown origin (Lappin et al, 2002).

The anemia associated with Mycoplasma spp. infections is generally macrocytic, normochromic but may be macrocytic, hypochromic if coinfections leading to chronic inflammation exist. Chronic non-regenerative anemia is unusual in Mycoplasma spp. infections. Neutrophilia and monocytosis have been reported in some Mycoplasma spp.-infected cats. Diagnosis is based on demonstration of the organism on the surface of erythrocytes on examination of a thin blood film or polymerase chain reaction (PCR). Organism numbers fluctuate and so blood film examination can be falsely negative up to 50% of the time. The organism may be difficult to find cytologically, particularly in the chronic phase. Thus, the PCR is the test of choice due to sensitivity. Primers are available that amplify a segment of the 16S rRNA gene common to both Mycoplasma spp. (Jensen et al, 2001).

Since hemoplasmosis and primary immune hemolytic anemia are difficult to differentiate, cats with severe, regenerative hemolytic anemia should be treated with glucocorticoids and antibiotics. Doxycycline has less side effects than other tetracyclines in cats and so is preferred. Doxycycline should be given at 10 mg/kg, PO, every 24 hours for at least 14 days. If administered for 28 days, more cats appear to stay persistently PCR negative. Generic tablets have been associated with esophageal strictures and should be liquefied or water should be given after pilling. Butter can also be coated on the tablet or rubbed on the nasal planum after pilling. If autoagglutination is evident, prednisolone is usually prescribed at 1 mg/kg, PO, every 12 hours for the first 7 days or until autoagglutination is no longer evident. Tetracyclines utilized to date appear to lessen parasitemia and clinical signs of disease but probably do not clear the organism from the body. In one study, experimentally infected cats treated with doxycycline have apparent clinical response but the organism could still be detected by PCR when the cats were given methylprednisolone acetate.

In cats intolerant of doxycycline, enrofloxacin should be considered. Enrofloxacin given at 5 mg/kg, PO, every 24 hours or at 10 mg/kg, PO, every 24 hours for 14 days is tolerated by cats and is equally effective or more effective than doxycycline. However, the 5 mg/kg dose should be used since blindness occasionally occurs in cats treated with higher doses. It is currently unknown whether other quinolones are effective. Azithromycin was not effective for the treatment of hemoplasmosis in one study (Westfall et al, 2001). Imidocarb dipropriate administered at 5 mg/kg, IM, every 2 weeks for at least 2 injections was used successfully in the management of five naturally-infected cats that had failed treatment with other drugs (Lappin et al, 2002). Blood transfusion should be given if clinically indicated. Potential arthropod vectors should be controlled. Cats should be housed indoors to avoid vectors and fighting. Clinic blood donor cats should be screened for Mycoplasma spp. infections by polymerase chain reaction. In a recent national prevalence study, it was shown that 9.8% of the cats used as blood donors were PCR positive for hemoplasmas (Hackett et al, 2003).

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