The red blood cell parasites known formerly as Haemobartonella and Eperythrozoon species have been traditionally classified as Rickettsia. However, based on strong phylogenetic evidence and 16S ribosomal RNA gene sequences, they have now been reclassified as hemotrophic mycoplasmas (hemoplasmas). These organisms-small, pleomorphic bacteria that parasitize red blood cells-are gram-negative; may be ring-shaped, rod-shaped, or spherical; and are found either individually or in chains across the red blood cell surface. First observed in Germany over 75 years ago, blood parasites have since been documented in laboratory and domestic animals, as well as in human beings. Lacking a cell wall and devoid of a nuclear structure, these parasitic blood organisms multiply through binary fission. They adhere to-but do not penetrate-the surface of the red blood cell. Delicate fibrils extend through a 15- to 25-nm clear zone that separates the parasite from the red blood cell membrane, thus attaching the parasitic organism to the host cell.

The 16S ribosomal RNA gene sequences are the basis of polymerase chain reaction (PCR) assays, which are used to detect infection in a wide range of vertebrate animals. Amplified and compared with sequences of known bacteria, the 16S ribosomal RNA of the hemoplasmas showed little similarity to other rickettsial organisms and a closer phylogenetic relation with Mycoplasma species. This has led to the proposition that the taxonomic classifications should be changed to reflect the phylogenetic affiliation of Haemobartonella and Eperythrozoon species with the genus Mycoplasma. Thus, H. felis (large form), H. canis, and H. muris are now called M. haemofelis, M. haemocanis, and M. haemomuris, respectively. E. suis and E. wenyonii are now called M. suis and M. wenyonii. Candidatus was added to the designation of new and incompletely described organisms of the cat (formerly H. felis [small form]), opossum, and alpaca, which are now called Candidatus Mycoplasma haemominutum, Candidatus Mycoplasma haemodidelphis, and Candidatus Mycoplasma haemolamae, respectively.

Hemoplasmas can be responsible for acute hemolytic anemia and a variety of chronic diseases in vertebrate hosts. The clinical infection spectrum ranges from asymptomatic to life-threatening (partially dependent on host susceptibility). Factors observed in animals with a predisposition to acute infections are age, immunosuppression, concurrent disease, or splenectomy. Both acute and chronic forms of hemoplasma infections are found in cats, dogs, pigs, mice, llamas, and primates in the presence or absence of the spleen. Route of infection and inoculum dose may also influence infection severity. Clinical features of acute disease have been extensively studied in such species as cats, pigs, and sheep. Chronic animal hemoplasma infections are well recognized and even those treated with effective antibiotics may remain chronic carriers after resolution of clinical signs. A recently developed, real-time quantitative PCR assay could become the assessment method for the antibiotic treatment response of hemoplasmas.

COMMENTARY: There have been many changes in nomenclature and information on these organisms (hemotrophic mycoplasmas) in the last 15 years, and I find that practitioners are sometimes still confused. This article helps clarify their names and their significance.

Hemotrophic mycoplasmas (hemoplasmas): A review and new insights into pathogenic potential. Messick J. VET CLIN PATHOL 33:2-13, 2004.