Molecular phylogeny and biogeography of North American isolates of Anaplasma marginale (Rickettsiaceae: Ehrlichieae)

Anaplasma marginale (A. marginale) is a tick-borne ehrlichial pathogen of cattle that causes the disease anaplasmosis. Six major surface proteins (MSPs) have been identified on A. marginale from cattle and ticks of which three, MSP1a, MSP4 and MSP5, are from single genes and do not vary within isolates. The other three, MSP1b, MSP2 and MSP3, are from multigene families and may vary antigenically in persistently infected cattle. Several geographic isolates have been identified in the United States which differ in morphology, protein sequence and antigenic properties. An identifying characteristic of A. marginale isolates is the molecular weight of MSP1a which varies in size among isolates due to different numbers of tandemly repeated 28-29 amino acid peptides. For these studies, genes coding for A. marginale MSP1a and MSP4, msp1alpha and msp4, respectively, from nine North American isolates were sequenced for phylogenetic analysis. The phylogenetic analysis strongly supports the existence of a south-eastern clade of A. marginale comprised of Virginia and Florida isolates. Analysis of 16S rDNA fragment sequences from the A. marginale tick vector, Dermacentor variabilis, from various areas of the United States was used to evaluate possible vector-parasite co-evolution. Our phylogenetic analysis supports identity between the most parsimonious tree from the A. marginale MSP gene data and the tree that reflected the western and eastern clades of D. variabilis. These phylogenetic analyses provide information that may be important to consider when developing control strategies for anaplasmosis in the United States.