Haemobartonella species are pleomorphic bacterial parasites found either singly or in chains on the surface of the red blood cells. Diagnosis of haemobartonellosis has previously depended on microscopic detection of organisms on a Wright-Giemsa stained peripheral blood smear. Although simple, the technique is unreliable, as organisms tend to detach from red cells in EDTA-anticoagulated blood samples, and precipitated stain and Howell-Jolly and refractile bodies on red cell surfaces are easily mistaken for red cell parasites. Because infection cannot be reliably diagnosed using microscopic methods, the nature of these organisms and the prevalence of diseases they cause have been widely disputed.

Based on their small size, negative Gram-staining, and association with the red blood cell of cats, Haemobartonella parasites were previously classified as rickettsia, until gene sequencing studies revealed otherwise. These organisms were found to be closely related to members of the genera Mycoplasma, and so were renamed as Mycoplasma haemofelis for the Ohio isolate or large form of feline Haemobartonella, and Mycoplasma haemominutum for the California isolate or smaller form. The organisms are commonly referred to as haemoplasmas in cats.

M. haemofelis is more pathogenic than M. haemominutum. Acute disease in the cat caused by M. haemofelis is associated with parasitemia and a severe, sometimes fatal, hemolytic anemia. The existence of chronic haemoplasma infections in cats that have low or transient levels of parasites in peripheral blood smears is also well recognized, but these infections are not readily detected by evaluation of blood smears. Only the polymerase chain reaction (PCR) technique is sensitive enough to detect such low levels of infection. M. haemominutum can cause chronic infections without causing anemia. Chronic M. haemofelis infection in cats is recognized as a secondary pathogen in conjunction with retroviruses, including FIV and FeLV or other debilitating diseases. Haemoplasma infected animals, even if treated with tetracycline, may remain chronic carriers after clinical signs have resolved.