Anaplasma marginale is a tick-borne pathogen that causes the disease anaplasmosis in cattle (Bram, 1975; Ristic, 1968). This pathogen is classified within the Order Rickettsiales which was recently reorganized into two families, Anaplasmataceae and Rickettsiaceae, based on genetic analyses of 16S rRNA, groELS and surface protein genes (Dumler et al., 2001) (Table 15.1). Organisms of the family Anaplasmataceae are obligate intracellular organisms that are found exclusively within membrane-bound vacuoles in the host cell cytoplasm. Phylogenetic analyses consistently supported the formation of four distinct genetic groups of the organisms: (1) Anaplasma (96.1% similarity), (2) Ehrlichia (97.7%), (3) Wolbachia (minimum of 95.6% similarity) and (4) Neorickettsia (minimum of 94.9% similarity) (Dumler et al., 2001). The genus Anaplasma currently includes the three pathogens of ruminants, A. marginale, A. centrale and A. ovis, together with A. bovis (formerly Ehrlichia bovis), A. phagocytophilum (formerly E. phagocytophilum, E. equi and the HGE agent), and A. platys (formerly E. platys). Aegyptianella, also included in this genus, was retained as a genus incertae sedis due to lack of sequence information.
Anaplasma marginale is distributed worldwide in tropical and subtropical regions of the New World, Europe, Africa, Asia and Australia. Several geographical isolates of A. marginale have been identified in North and South America, which differ in morphology, protein sequence, antigenic characteristics and their ability to be transmitted by ticks (Smith et al., 1986; Wickwire et al., 1987; Allred et al., 1990; Rodriguez Camarilla et al., 2000; Palmer, Rurangirwa & McElwain, 2001; de la Fuente et al.