Genetic diversity of major surface protein 1a of Anaplasma marginale in beef cattle.

The present study was aimed to demonstrate genotypic diversity of Anaplama marginale in infected beef herds grazing within anaplasmosis endemic regions. The genotypic diversity was identified among different herds, within each herd, and also within single animals. The Israeli strains revealed unique characteristics of MSP1a repeats and, in addition to the published repeats, six new tandem repeats designated Is1-5, and Is9 were identified. The superinfections of individual Anaplama centrale vaccinated animals with two genotypically different A. marginale strains were detected. Six out of 43 vaccinated animals in the G herd were each infected with two A. marginale strains carrying two distinct genotypes; in this herd the follow-up during years 2003-2007 demonstrated that several animals carried different msp1a genotypes at different time points. Coinfection with two different genotypes of A. marginale in A. centrale vaccinated cattle was observed in another herd, as well. It appears that A. marginale is composed of a heterogeneous changing bacterial population that evolves in the host or, the genotypic diversity implies high transmission intensity by the vector, or both. Learning how this diversity is generated and identification of distinct A. marginale strains coupled with high sequence variation of MSP1a will aid in understanding Anaplasma transmission and disease development.